Hubble Legacy 30 Years of Discovers and images. 9781454936237

Table of contents :
Title Page
Copyright
Dediction
Table of Contents
Foreword
Introduction
Engineering & History
Science
Solar Systems
Stars
Nebulae
Galaxies
The Distant Universe
Beyond Hubble
Notes and Further Reading
Acknowledgments
Image Credits

Citation preview

STERLING and the distinctive s logo are registered trademarks of Sterling Publishing Co., Inc. Interior Text © 2020 Jim Bell Foreword © 2020 John M. Grunsfeld Cover © 2020 Sterling Publishing Co., Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (including electronic, mechanical, photocopying, recording, or otherwise) without prior written permission from the publisher. ISBN 978-1-4549-3623-7 (e-book) For information about custom editions, special sales, and premium and corporate purchases, please contact Sterling Special Sales at 800-805-5489 or [email protected]. sterlingpublishing.com For image credits, see page 205 Cover design by Scott Russo Interior design by Maria Mann

DEDICATION This book is dedicated to the tens of thousands of men and women who dreamed up, designed, built, tested, launched, upgraded, repaired, operated, and have continued to operate the incredible time machine known as the Hubble Space Telescope, and to the astronomically larger number of people around the world who have enjoyed and learned from all that Hubble has helped us to discover. BOOKS BY JIM BELL Asteroid Rendezvous: NEAR Shoemaker’s Adventures at Eros The Martian Surface: Composition, Mineralogy, and Physical Properties Postcards from Mars: The First Photographer on the Red Planet Mars 3-D: A Rover’s-Eye View of the Red Planet Moon 3-D: The Lunar Surface Comes to Life The Space Book The Interstellar Age The Ultimate Interplanetary Travel Guide The Earth Book ENDPAPERS: This magnificent shot from Hubble’s ACS instrument shows a star-forming region known as N159 within the Large Magellanic Cloud, a satellite galaxy of the Milky Way that is about 160,000 light years away. Intense ultraviolet energy and strong stellar winds from hot young stars embedded within N159 cause the surrounding hydrogen gas to glow and to be shaped into delicate filaments and other structures that can be resolved by Hubble’s excellent resolution. FOLLOWING PAGES: Hubble’s ACS false-color photo of the spiral galaxy known as NGC 3432, located about 45 million light-years away in the constellation Leo Minor (the Lesser Lion). It’s hard to tell that this is a spiral galaxy like our Milky Way because we’re viewing it “edge on” from our vantage point, like looking at a dinner plate from the side.

“Looking at these stars suddenly dwarfed my own troubles and all the gravities of terrestrial life.” —H. G. WELLS, THE TIME MACHINE (1895)

“You gotta embrace the chaos. You have to. That way, life might just astonish you.” —APRIL FROM HOT TUB TIME MACHINE (MGM/UA, 2010)

TABLE OF CONTENTS FOREWORD x INTRODUCTION 1

ENGINEERING & HISTORY SCIENCE

36

Solar Systems Stars

10

38

72

Nebulae

102

Galaxies

134

The Distant Universe BEYOND HUBBLE

166

188

NOTES AND FURTHER READING ACKNOWLEDGMENTS IMAGE CREDITS

196

204

205

This false-color composite of the Cigar galaxy, also known as M82, was made by merging images taken by the Chandra X-ray Observatory in blue, the Hubble Space Telescope in blue and green, and the Spitzer Space Telescope in red. M82 is located about 12 million light-years from Earth in the constellation Ursa Major.

FOREWORD John M. Grunsfeld, PhD, Astrophysicist/Astronaut, “Hubble Repairman” For millennia, humans have looked to the night sky to find meaning in the arrangement of the stars and the meanderings of the Moon, the planets, and the occasional comet. Modernity has not changed our fascination with the mysteries of the Universe; rather, our curiosity about the nature of the Cosmos has only increased. We know that the Universe was born in a “Big Bang” 13.8 billion years ago and has expanded to form the arrangement of galaxies seen today through telescopes that extend our vision far beyond what was available to our ancestors. Black holes dot the Universe, and planets circle nearly all the stars we see in the night sky. We owe this knowledge to the legions of astronomers who have dedicated their lives to studying the data we obtain from telescopes on Earth and in space, and to the ingenious engineers and technicians who create their astronomical observatories. One such observatory in space, the Hubble Space Telescope, holds a special place in the history of astronomical discovery. It is perhaps the most significant scientific instrument ever built, due to the breadth and depth of the discoveries made using it. As we celebrate the thirtieth anniversary of the launch of Hubble in 1990, we also celebrate the incredible journey of discovery by humankind in the pursuit of knowledge. The Hubble Space Telescope has made observations that helped provide answers to ancient questions like: Where did we come from? Where did the matter, the stars, planets, and galaxies, the chemical elements that we’re made of come from? Where are we going? What is the future trajectory of our solar system? What is the future of the Sun and the destiny of the galaxies in our Universe? Do black holes exist? (Yes!) Are there planets around nearby stars? (Yes!) Yet all this science is in some ways eclipsed by the most significant achievement by Hubble: the images taken by its cameras show us that the Universe is much richer and more beautiful than we ever could have imagined. These now-familiar images shown throughout this book have inspired people around the world, uplifting our spirits and driving our curiosity. These glorious thirty years of wonder and awe courtesy of the Hubble Space Telescope were anything but certain. In fact, after its launch on the space shuttle Discovery in 1990, the telescope was down and out with few expectations of success. A small flaw in the making of the 2.4-meter–diameter primary mirror (see page 4) portended a bleak future for the iconic telescope. The images were blurry, and astronomers worried that the mission might be a near-total loss. The investment in the telescope by the public ran into the billions of dollars, and so Congress was equally baffled and angry about the error in Hubble’s optics. To make matters worse, Hubble became the butt of jokes on late-night TV and fodder for editorial cartoonists. Fortunately for all of us, this was not the end of the story but the beginning of a marvelous journey that not only saved the telescope but extended its vision beyond anything that had been imagined when it was built.

OPPOSITE: NASA Hubble repair astronaut John Grunsfeld’s mirrored helmet visor reflects fellow astronaut Andrew Feustel working high above the Earth in the cargo bay of the space shuttle Atlantis during Hubble Servicing Mission 4 (SM4) in May, 2009.

NASA astronaut Story Musgrave, anchored to the end of the space shuttle Endeavour’s robotic arm, prepares to work on the initial repairs and maintenance of the Hubble Space Telescope during Servicing Mission 1 (SM-1) in December, 1993.

The brilliant designers of the Hubble Space Telescope, with the support of NASA leadership at the time, realized that the unique capability of the space shuttle to deliver it to orbit would also allow it to be serviced by astronauts on subsequent missions. And so began one of the most recognized and celebrated uses of the space shuttle—to repair and service the Hubble Space Telescope. The first servicing mission in

1993, aboard the space shuttle Endeavour, carried new instruments and corrective optics to “undo” the incorrect design of the primary mirror (see pages 4 and 5). It is not an understatement to say that the future of NASA and the human spaceflight program hung in the balance. Thankfully, the mission was a resounding success in spite of the many technical and human challenges. In all, five missions to Hubble were performed between 1993 and 2009, each one challenging the ingenuity and perseverance of the engineers, scientists, technicians, administrators, and astronauts involved. I had the privilege and honor of flying on three of those five missions, performing spacewalks to upgrade and repair the telescope. In 1997, I was assigned to my third spaceflight mission, which was also the third Hubble servicing mission. As an astrophysicist, this was the “holy grail” of space shuttle missions. We planned a record six spacewalks to do a major overhaul of several Hubble observatory systems, as well as install a new highresolution camera. Just two years later, Hubble presented us with a new challenge. By 1999, half the telescope’s gyroscopes had failed, threatening to shut down Hubble’s eye on the Universe. So in December that same year, we set off to rendezvous with Hubble on the space shuttle Discovery for a shortened rescue mission of only three spacewalks. On that mission, we installed new gyroscopes, a new spacecraft computer, and other equipment to keep Hubble operational. This mission included my first spacewalks, and the opportunity to get up close and personal with Hubble. On Christmas Eve, 1999, I had the pleasure of doing the final spacewalk with my partner Steve Smith, and on Christmas day we released the telescope to resume its mission of discovery, exploration, and observation of the heavens. I cannot imagine a greater gift to humanity than a working Hubble observatory. Following that successful mission, I was assigned as the Payload Commander to lead the spacewalks on a 2002 mission on the space shuttle Columbia. This mission was designed to complete the activities planned for the original third servicing mission interrupted by the gyroscope failures. On this mission we installed the Advanced Camera for Surveys, a new high-resolution digital camera for Hubble. This astounding camera would later take the data that would confirm that the Universe was accelerating, which led to the 2011 Nobel Prize in Physics, awarded to Adam Riess, Brian Schmidt, and Saul Perlmutter. However, before Hubble could make this contribution, we had to fix its main power-control unit. If we didn’t repair it, Hubble would eventually cease operations for good. But the repair was well beyond anything that had been attempted before in space; and if we didn’t get it right, Hubble would immediately fail. Thanks to the ingenuity and resourcefulness of the engineers at NASA’s Goddard Space Flight Center in Maryland and the amazing team of spacewalking instructors and operators at NASA’s Johnson Space Center in Houston, we were able to achieve the almost impossible and replaced the failed unit on the third spacewalk. I performed the repair with my spacewalking partner Rick Linnehan, a veterinarian by profession. Hubble dodged another bullet and was returned to service once again. Tragically, the space shuttle Columbia was lost returning to Earth after a successful science mission in 2003. This was a blow to all of NASA, the space community, and the public. It also cast doubt on whether a final fifth servicing mission to Hubble would be undertaken as planned. Unlike missions to the International Space Station, there is no opportunity to hang out in a “safe haven” on a mission to Hubble. After much debate, the final space shuttle was manifested for flight. Once again, I was tapped to lead the spacewalking team for a five-walk series. Scott Altman, the Shuttle Commander, was assigned to the mission as well as Mike Massimino, a mechanical engineer, both of whom had been on the previous Hubble mission with me. Together we were joined by first-time astronauts Megan McArthur, Drew Feustel (my spacewalking partner for the mission), Greg Johnson, and Mike Good. To help mitigate the risk of our crew’s being stranded in space, a second space shuttle, Endeavour, stood ready on a second launchpad in case we needed to be rescued.

Our goals for this mission on the space shuttle Atlantis were quite ambitious. We were to install the Wide Field Camera-3, new batteries, a complete set of new gyroscopes, and a new Fine Guidance Sensor, along with the repair of the failed Space Telescope Imaging Spectrometer and the incredible Advanced Camera for Surveys. Once again, the talented team of engineers, flight controllers, technicians, launch team, and support members performed flawlessly and we achieved all the objectives and more. When we released Hubble back to its orbit on May 19, 2009, Hubble was in the best condition of its time in space. With the repaired instruments and the new Wide Field Camera, the telescope was well equipped to unravel yet more mysteries of the Universe.

ABOVE: This power control unit, essentially the heart of the Hubble Space Telescope, was replaced during Servicing Mission 3B (SM-3B) by space shuttle Columbia astronauts John Grunsfeld and Richard Linnehan. OPPOSITE: To practice for Servicing Mission 4 (SM-4) in a simulated zero-gravity environment, NASA astronauts worked on a model of the Hubble Space Telescope underwater in the Neutral Buoyancy Laboratory at the Johnson Space Center in Houston.

When Hubble was first released into space, few could have imagined that it would last thirty years in orbit. The original estimate for its lifetime was fifteen years. If not for the servicing missions, Hubble never would’ve been able to achieve a fraction of its scientific exploration and discovery. From flawed mirror corrections to failed gyroscopes, broken cameras, faulty power systems, and more, the repair efforts from Earth and in space were crucial time after time. The five Hubble servicing missions will go down in history as some of the most daring and technically complex efforts of the space age. Nearly half the world’s population, those thirty years old or younger, have never known a world without the Hubble Space Telescope. When we gaze upon the wonderful images captured by Hubble, we don’t remember the early disappointments, the troubles, or the challenges. What we appreciate is the beauty of the Universe as revealed by Hubble. While the future of the telescope is increasingly uncertain, its contributions are crystal clear. The following pages are as much a celebration of these contributions as they are a record of the discoveries made by a telescope that has touched us all. OPPOSITE: Space shuttle Atlantis being loaded and fueled on Kennedy Space Center’s launch pad 39A prior to launch in

May, 2009. The shuttle and its crew of seven astronauts would successfully carry out Servicing Mission 4 (SM-4) that month, on the final space shuttle mission to service the Hubble Space Telescope. BELOW: Thumbs up: While helping repair the Hubble Space Telescope on the STS-109 mission (March 2002), Grunsfeld signals to crewmates inside space shuttle Columbia’s crew cabin that his third spacewalk is going well.

INTRODUCTION Imagine if you had a time machine, a special kind of time machine that only let you go back in time, way back in time where you could observe the events of the past, but not actually go there yourself. It sounds magical, but actually the world is full of such time machines—they’re called telescopes. And the most powerful of them so far, the one able to look the deepest back in time, isn’t actually on this planet but orbits some 330 miles above the surface. It’s called the Hubble Space Telescope, or sometimes just Hubble or HST for short. Serious scientific planning for a large, space-based telescope began more than thirty years before HST was approved for funding in the late 1970s. The landmark milestone in the “birth” of the telescope was a research paper written in the late 1940s by Princeton astronomer and physicist Lyman Spitzer. In it, Spitzer noted that if a large telescope could be launched beyond the Earth’s atmosphere, it would enjoy two distinct advantages over ground-based telescopes, even those located in mountaintop observatories. First, a space-based telescope would have the advantage of superior resolution compared to an earthbound telescope of the same size, whose focus is worsened and blurred by the constant “twinkling” of the Earth’s atmosphere. This effect, which astronomers call “the seeing,” usually prevents ground-based telescopes from reaching their theoretical (atmosphere-free) limit, even on clear nights (and of course, Earth-based telescopes can’t get any resolution at all when it’s cloudy!). A space-based telescope’s resolution could easily be up to ten times better or more, only limited by the physics of lenses or mirrors and the so-called “diffraction limit” of an optical system, as theorized by Spitzer. Since the diffraction limit is directly proportional to the diameter of the telescope’s optics, the bigger the space-based telescope, the finer its resolution will be. According to Spitzer, the second advantage of space-based telescopes was that they wouldn’t have to filter out what our atmosphere does to certain parts of the spectrum. Ultraviolet (UV) radiation, for example, is strongly absorbed by ozone and other gases in our atmosphere. This is very good for life on Earth, because high-energy UV radiation quickly breaks down organic molecules, so life wouldn’t be possible on Earth’s surface if it weren’t filtered out by our atmosphere. However, our ozone is not so good for astronomers, who want to study high-energy astrophysical processes and events that can only be detected and understood by studying UV radiation. Similarly, many important parts of the infrared spectrum of astronomical objects are filtered out by water vapor, CO2, and other gases in the Earth’s atmosphere, so those potentially diagnostic wavelengths aren’t accessible from ground-based telescopes either. From space, however, astronomers can study all the colors of the Universe.

Hubble’s WFPC2 instrument produced a false-color photo of a small region of turbulent clouds of gas and dust within the nebula known as M17 (also called the “Omega” or “Swan” Nebula), located about 5,500 light-years away in the constellation Sagittarius.

From Theory To Hubble Going from Spitzer’s concept to an actual telescope operating in space took a long time, in part because key technical problems had yet to be solved, and also because everyone (including the U.S. Congress) knew that it would be a very expensive project to fund. Cost concerns alone persuaded many astronomers to come out against the idea in fear that this one project could potentially eat up all, or most, of the federal

funding for astronomical research and instrumentation. Luckily, the U.S. National Academy of Sciences, which Congress and presidential administrations often consult when setting the nation’s scientific and technological research agendas, endorsed the idea of a large space-based telescope in the early 1960s, linking its mission to the still-new space agency NASA (the National Aeronautics and Space Administration). By the mid-1960s, both NASA and the British Science Research Council had launched and operated several small space-based telescopes, proving the scientific potential of looking at the Sun and other deep-space objects in the ultraviolet part of the spectrum (as well as the even-higher-energy X-ray and gamma-ray parts of the spectrum). Around the same time, Spitzer himself chaired a National Academy of Sciences committee exploring the concept of a Large Space Telescope (LST), perhaps up to 3 meters (almost 10 feet) in diameter. He worked tirelessly to convince his skeptical astronomical colleagues that even though it would be a very large investment, the potential scientific returns could be enormous. NASA pitched a plan to launch an LST around 1979, and to have it deployed and occasionally serviced by the agency’s new crewed space vehicle, the space shuttle. Sadly, the 1970s and early 1980s were a challenging time for NASA funding. The agency was scaled back in scope and budget after the costly Apollo Moon-landing missions, which had become an easy target for Congressional and Executive budget-cutters. Funds for a proposed LST were actually cut entirely from the federal budget by Congress in 1974. A national lobbying and letter-writing campaign by astronomers, along with another well-timed report from the National Academy of Sciences, stressed the need for a spacebased telescope and helped funding get restored, but at only half the expected levels. As a result, the LST designers were forced to cut down on the diameter of the telescope, from 3 meters to about 2.4 meters, as a cost-cutting maneuver. Another money-saving move was to enlist the partnership of the European Space Agency (ESA), which agreed to supply the solar panels and one of the telescope’s instruments in exchange for European astronomers getting 15% of the eventual study time on the observatory. Detailed design work on the telescope, and the spacecraft to transport it, finally began in 1978, with a launch scheduled for 1983. Designing, building, and testing such a complex machine required the combined experience and expertise of two major NASA research facilities—the Marshall Space Flight Center in Huntsville, Alabama, would build the telescope itself; and the Goddard Space Flight Center in Greenbelt, Maryland, would be responsible for the instruments and ground control center. Aerospace giant Lockheed would construct the spacecraft and integrate the telescope into it. Marshall then subcontracted the fabrication of the telescope’s mirror out to Perkin-Elmer, an optics company with a mirror-grinding facility in Danbury, Connecticut.

OPPOSITE: Astronomer Edwin Hubble peering through the guidescope of the 48-inch Schmidt camera telescope at Palomar Observatory, circa 1949.

The tasks proved to be technically daunting all around, with delays and cost overruns occurring in both the mirror fabrication and the spacecraft assembly and testing. NASA kept pushing back the launch date, to 1984, then 1985, then finally 1986 as problems cropped up and had to be solved. In the meantime, NASA

decided to name the telescope after the American astronomer Edwin P. Hubble (1889–1953), who had been a key scientist in the late 1920s and early 1930s discovery of galaxies beyond the Milky Way. Hubble was also one of the first scientists to realize that the motions of those distant galaxies revealed that the Universe is expanding, and thus (by inference, running the clock backwards) must have been born from a single unimaginable burst of matter and energy many billions of years ago—an event we now widely refer to as the Big Bang.

Hubble’s Dream Comes to Fruition The official purpose of the Hubble Space Telescope, according to NASA, was “to gather light from cosmic objects so scientists can better understand the Universe around us.” A critical facet of this very general goal was for the telescope to measure light not only in the visible part of the spectrum, but also in the ultraviolet part, which is not possible from ground-based telescopes because of the Earth’s atmosphere. More specifically, the telescope would be able to study these colors of light at extremely high resolution, acquiring observations that could enable new discoveries about planets, moons, asteroids, comets, stars, nebulae, galaxies, and the early Universe. Indeed, perhaps the single most important goal of Hubble would be to accurately determine the age of the Universe itself, improving on the work of the observatory’s namesake by measuring the rate of expansion of distant galaxies in much more exquisite detail. Things finally looked good for the revised launch date of the Hubble Space Telescope (HST) in late 1986, but the tragic explosion of the space shuttle Challenger shortly after it’s launch that January grounded the entire shuttle fleet. The nearly complete telescope had to be mothballed for more than three years while it waited for a ride. Finally, on April 24, 1990, the space shuttle Discovery lofted HST into space. It had taken a dozen years to get it there, and costs had ballooned from the original $400 million estimate to more than $4.7 billion along the way. Astronomers were nonetheless elated at the potential discoveries that awaited this historic new observatory. However, elation quickly turned to disappointment when it became clear that the telescope was badly out of focus. The first HST images of stars and galaxies were supposed to be stunningly crisp and detailed; instead, they were shockingly blurry and smeared out. The telescope’s resolution was something like ten times worse than what HST had been designed for, and not actually much better than what good groundbased telescopes could achieve at the time. It was both an engineering and a public relations catastrophe. An ensuing investigation revealed that the primary mirror had been manufactured to an incredibly precise —and incredibly wrong—shape. HST’s primary mirror was ground too flat, by about 2.2 micrometers, or about one fiftieth the diameter of a human hair. While it doesn’t seem like much, for such a large telescope the effect (called “spherical aberration”) was enormous and prevented the instruments from achieving tight focus. Eventually, investigators identified the culprit as a flawed piece of test equipment that was used to verify the proper shape of the mirror. Also implicated were flawed management and oversight processes at both Perkin-Elmer and NASA that had allowed such an enormous mistake to go unnoticed during the years of fabrication and testing. Fortunately, the telescope’s primary mirror was ground perfectly wrong, smooth down to the scale of just a few hundred atoms. So, just like for a near-sighted or far-sighted person, it was possible to design what was essentially a set of corrective eyeglasses that could be used to bring the telescope into proper focus. Work soon began on the design of a new instrument by Ball Aerospace called COSTAR, or “Corrective Optics Space Telescope Axial Replacement” (see page 21), to correct the spherical aberration. Because HST had been placed in low Earth orbit at an altitude where it was in position to be serviced by the space

shuttle, NASA was able to plan for and then launch COSTAR on a ten-day Endeavour shuttle mission known as “Servicing Mission 1” or “SM-1” in December 1993. Subsequent testing revealed the repair mission to be a total success: images were now as sharp as expected, and HST would finally be able to achieve both the sensitivity and resolution for which it had been designed.

OPPOSITE TOP: Technicians inspect the highly polished and finely ground aluminized surface of the Hubble Space Telescope’s 7.8 foot (2.4 meter) wide primary mirror. OPPOSITE BOTTOM: The Hubble Space Telescope’s primary mirror being ground at the Perkin-Elmer Corporation’s large optics fabrication facility in Danbury, Connecticut in 1979.

COLOR IN SPACE: How Hubble Works

The cameras on the Hubble Space Telescope produce spectacular color photos, but most of the time they are not “true color” (what we would see with our naked eyes), but instead are “false color,” composites of colors of the spectrum that we cannot detect with our eyes, but displayed in colors that we can see. An example is shown here, in a dramatic false-color composite of the famous Crab Nebula, the scattered remains of a massive nearby star that exploded in the year 1054 (see page 109). This composite was made by assigning different images taken by Earthbased and space-based telescopes across the electromagnetic spectrum (see individual photos at upper right) to the red, green, and blue hues that we can detect with our eyes. Different parts of the spectrum provide information on different parts of the nebula: Radio images (VLA) map magnetic fields; Infrared images (Spitzer space telescope) see through more dusty regions and map the innermost structures; Optical images (Hubble) map hydrogen in the nebula; Ultraviolet images (Astro-1 space telescope) map cooler, lower energy electrons; and X-ray images (Chandra space telescope) map the hottest electrons emerging from the rapidly spinning pulsar in the Crab’s heart.

HUBBLE’S INSTRUMENTS: A HISTORY Including COSTAR, HST has used a dozen different instruments during its thirty-year lifetime (so far) to achieve its spectacular science results. The telescope was launched with five original instruments: 1. A camera built by NASA’s Jet Propulsion Laboratory (JPL) with both wide-angle and higherresolution fields of view (called the Wide Field and Planetary Camera, or WFPC, pronounced “whiff-pic”). 2. Another camera called the Faint Object Camera (FOC), funded by ESA and designed to have extreme sensitivity for the dimmest, most distant objects observable. 3. A spectrometer called GHRS (Goddard High Resolution Spectrograph, built by NASA’s Goddard Space Flight Center, or GSFC). 4. A spectrometer called FOS (Faint Object Spectrograph, built by the University of California, San Diego). 5. An instrument called the High Speed Photometer (HSP), built by the University of Wisconsin and designed to study rapidly changing brightness variations in supernova explosions and other astrophysical events. All these instruments were eventually replaced with more advanced and capable versions during the five space shuttle servicing missions that visited Hubble (see “Hubble Servicing Missions,” page 25).

Hubble’s Legacy It is not an exaggeration to claim that the Hubble Space Telescope has completely revolutionized modern astronomical science. By enabling researchers to determine with exquisite precision the Universe’s rate of expansion, Hubble revealed that the Big Bang likely occurred exactly 13.799 billion years ago (plus or minus only about 20 million years!). By collecting light from the dimmest, most distant objects ever studied, Hubble provided critical data needed to understand the origin and evolution of galaxies, dating all the way back to the first galaxies formed during the very early lifetime of the Universe. Numerous other scientific discoveries and superlatives abound over Hubble’s thirty-year history: The first visible-light images of a planet around another star; the first and, as yet, most accurate measurements supporting the existence of Dark Matter; detailed and spectacularly beautiful images capturing the births and deaths of stars; the discovery that supermassive black holes are common in the Universe; the stunning realization that stupendous amounts of energy are released during impact events, like the 1994 collision of comet Shoemaker-Levy 9 into Jupiter (see pages 40 and 41); the discovery and analysis of new Solar System moons, rings, asteroids, and comets—and the list goes on. These and a slew of other key discoveries outlining Hubble’s legacy are highlighted in the entries of this book. It’s also not an exaggeration to say that another critical part of Hubble’s legacy is its responsibility, in part, for renewed popularization of—and public engagement with—astronomy and space science. Millions of people visit the websites of the Space Telescope Science Institute (STScI) annually and download huge numbers of spectacular high-resolution photos, posters, and screen savers. Hubble data are featured prominently in dozens of NASA and ESA press releases each year, and those stories are picked up and disseminated by hundreds of media outlets, both traditional and online. Hollywood and international television and film artists—especially in the science-fiction genre—routinely utilize Hubble photos for background and context. In my experience, much of the general public expect the Hubble Space Telescope to be involved when anything interesting is happening in space or in the night sky. After a difficult gestation period and a troubled youth, the telescope has grown up to be hugely successful and famous. Indeed, because of the way it is now ingrained heroically in our collective culture and psyche, I believe that to hundreds of millions of people around the world, not just in the U.S. and Europe, Hubble has become their telescope. Hubble is one of four so-called Great Observatories launched into space by NASA between 1990 and 2003, and is the only one designed specifically to observe the Universe primarily in ultraviolet and visible wavelengths of light. The other three Great Observatories are the Compton Gamma Ray Observatory, launched in 1991 on the shuttle Atlantis and designed to detect high-energy astrophysical events and processes; the Chandra X-ray Observatory, launched in 1999 aboard the shuttle Columbia and designed to study somewhat less energetic objects; and the Spitzer Space Telescope, launched on a Delta II rocket in 2003 and optimized to study the infrared (thermal) radiation emitted by astronomical objects. Following the Hubble model, NASA named each of these observatories after an accomplished astronomer who had helped to pioneer the study of the Universe in each telescope’s unique wavelength region. Naming the infrared space telescope after Lyman Spitzer also acknowledged his special role in the long struggle that led to the U.S. and international government investments that enabled these remarkable and historic observatories and their spectacular scientific advances.

In this Hubble WFPC2 photo, a nearly perfect ring of hot, blue stars pinwheels about the yellow nucleus of an unusual galaxy, PGC 54559, or Hoag’s Object.

Indeed, the legacy of the HST will live on long after the telescope itself eventually stops functioning and burns up during a planned “controlled de-orbit” into the Earth’s atmosphere, likely sometime in the 2020s. That legacy will include many thousands of images and other data sets processed and permanently archived by the now-highly experienced and dedicated science and engineering staff of the Space Telescope Science Institute in Baltimore, Maryland. Hubble’s amazing images and other data have already helped to revolutionize our view of the Cosmos, and there is no telling what new discoveries await deeper analysis of the archive. As well, the experience gained by STScI and other similar operations centers around the world have taught us how to control and “fly” large telescopes in space. The experience and expertise accumulated by the people who designed, built, operated, and made discoveries using the amazing time machine called the Hubble Space Telescope will also guide the next generation of astronomers and incredible new space-based observatories in the decades to come.

A MODERN-DAY TIME MACHINE 1990–2020 Why put a telescope in space? Isn’t it easier to build them and fix them on the ground, maybe on a high

mountaintop far away from city lights? And the largest telescopes in the world are something many times larger than the Hubble Space Telescope. So how could such a “small” observatory possibly compete? These are the kinds of excellent questions that have had to be answered since the idea of a space telescope was first seriously considered back in the 1940s. They are also the kinds of questions that have continued to drive the desire to repair, maintain, and upgrade Hubble during its 30-year history. While ground-based telescopes have continued to grow larger and to make stunning astronomical discoveries during the past three decades, Hubble has occupied several special “niche markets” that justify its continuing observations. For example, no telescope on Earth can observe the Universe in the ultraviolet, preventing us from understanding a large number of high-energy planetary and astrophysical processes that can only be detected and studied in that part of the color spectrum. As another example, Hubble views the Cosmos from far above the twinkling and shimmering interference of Earth’s atmosphere, enabling crisp and stable images to be taken at a resolution that is typically sharper by ten times or more when compared to telescopes as much as five times larger, but located on the ground. PAGES 10 AND 11: NASA Hubble repair astronaut Michael Good (foreground, on the shuttle’s robotic arm) helps astronaut Michael Massimino (inside Hubble) prepare to replace some of the telescope’s highly sensitive instruments during the space shuttle Atlantis’s Servicing Mission 4 (SM-4) in May, 2009. BELOW AND OPPOSITE: (a) Engineering drawing showing details of the Hubble Space Telescope’s major external components; (b) Exploded view of the major components and subsystems of the Hubble spacecraft.

Perhaps Hubble’s most important niche, however, is its proven ability to see farther back in time than any other machine humans have ever built. Because the speed of light is finite, looking out into space means, by definition, looking back into time (sunlight, for example, is 8.5 minutes old by the time it reaches us; light from the nearest star to the Sun shows how Proxima Centauri looked 4.2 years ago; and we see the Andromeda Galaxy as it was more than 2 million years ago). By being able to stare at small pieces of the sky for days or weeks at a time, with no clouds, haze, or city lights in the way, Hubble has enabled us to look back and see galaxies as they were billions of years in the past, to a time when the Universe was perhaps only a few hundred million years old and much smaller than it is today. Of course, the Hubble Space Telescope’s ability to also routinely study planets and stars and galaxies and other astronomical objects that have formed since those first few hundred million years provides a way to chronicle the origin and evolution of the entire Universe over time. Indeed, if you had a time machine, isn’t that what you’d use it for? BELOW: Part telescope, part spacecraft, this computerized view of Hubble reveals many of the components and subsystems that make up the most famous of NASA’s great space observatories. OPPOSITE: View from inside the space shuttle Atlantis of the back of Hubble’s solar panels (in the lower windows). The telescope was held in the shuttle’s cargo bay while astronauts worked to repair and upgrade it during Servicing Mission 4 (SM-4) in May 2009.

LAUNCH AND DEPLOYMENT! April 1990 The Hubble Space Telescope was designed to be launched and serviced by NASA space shuttles. Indeed, the maximum size of the telescope’s primary mirror was determined by the width of the shuttle’s cargo bay, into which the telescope would have to be packed for its ride into space. More than twelve years after construction of the telescope began, and after waiting for the shuttle program to recover from the tragic loss of the Challenger in 1986, Hubble rocketed up to an orbit 380 miles (612 km) above the Earth on April 24, 1990. The five-person crew of STS-31 (actually the thirty-fifth shuttle mission launched) was commanded by Air Force Colonel Loren Shriver and piloted by Marine Corps Major General Charles Bolden, Jr., who would later become the NASA Administrator from 2009 to 2017. The 1990 launch would mark the highest orbit achieved to this date by the space shuttle program.

INSET: Discovery space shuttle astronauts Steven Hawley, Kathryn Sullivan, Bruce McCandless, Charles Bolden, and Loren Shriver (left to right) pose at NASA’s Johnson Space Center in Houston with a model of the telescope that they would help to deploy in space a month later.

Once the flight reached an altitude designed to maximize the telescope’s lifetime by getting high above the vast majority of Earth’s atmospheric friction, Mission Specialists Bruce McCandless (a naval officer and aviator), Steven Hawley (an astronomer), and Kathleen Sullivan (a geologist and Naval reserve oceanography officer) began the process of freeing Hubble from the shuttle. This involved tricky maneuvers with the shuttle’s Canadian robotic arm (“Canadarm”) to release it from the cargo bay. At one point, the observatory’s solar panels got stuck while unfurling, forcing McCandless and Sullivan to don their space suits in preparation for a potential extravehicular activity (EVA, or spacewalk) to free them. However, ground controllers were able to get the panels to unfurl without a potentially dangerous EVA. Deploying Hubble was the shuttle’s primary mission, but it also carried several secondary payloads and conducted other experiments after the telescope was successfully released. Two of those additional payloads included a super high-resolution IMAX camera that took spectacular movie coverage of Hubble’s deployment, much of it featured in the 1994 film Destiny in Space. Discovery orbited the Earth 80 times during the five-day mission—Discovery’s tenth voyage into space—before landing safely at Edwards Air Force Base in California on April 29, 1990.

Launch of the space shuttle Discovery on April 24, 1990, carrying the Hubble Space Telescope into Earth orbit.

The Hubble Space Telescope’s front lens cover reflects the ocean and clouds below in this IMAX photo taken right after the astronauts released the telescope from the shuttle’s cargo bay on April 25, 1990.

HUBBLE GETS SPECS December 1993 To the shock and dismay of NASA and the global astronomy community, the 95-inch (2.4m) diameter primary mirror of the Hubble Space Telescope had a fundamental engineering flaw—it was ground to the wrong shape and couldn’t focus properly. The first images that came back from the new telescope in 1990

showed blurry stars and fuzzy galaxies. But while the telescope could still achieve some useful science because of its space-based platform, the fact that Hubble was out of focus was a major engineering mistake and a public relations nightmare for NASA. Fortunately, although the mirror was ground to the wrong shape, it was perfectly ground to the wrong shape. Which meant it was straightforward to design a corrective lens to bring images into the proper focus. The job of installing Hubble’s “eyeglasses” fell to the crew of the Endeavour space shuttle, during what NASA called Servicing Mission 1, or SM-1, in late 1993. Engineers spent several years designing and building an instrument called COSTAR, for Corrective Optics Space Telescope Axial Replacement, which the crew would swap out with one of the original instruments to enable the telescope’s mirrors and lenses to finally be in focus. Endeavour launched on December 2, 1993 carrying COSTAR, another replacement instrument called the Wide Field/Planetary Camera 2 (WFPC2, or “whiff-pick 2”), and a crew of seven astronauts, including Commander Richard Covey and Pilot Kenneth Bowersox. Four of Endeavour’s Mission Specialists, astronauts Thomas Akers, Jeffrey Hoffman, Story Musgrave, and Kathryn Thornton, would alternate in pairs for a series of five spacewalks to install COSTAR and WFPC2, and to service several of Hubble’s other critical systems, while ESA’s first Swiss astronaut, Claude Nicollier, controlled the shuttle’s robotic arm. The mission was a spectacular success. Images taken with the new COSTAR system showed that the telescope was finally achieving the resolution and clarity that it had been designed to provide. The decision to deploy Hubble close to Earth (as opposed to a darker location farther away) meant that it could be saved by the shuttle crew. Now, the full capabilities of the world’s first large space telescope could be used to achieve great science.

Space shuttle Endeavour astronauts Kathryn Thornton and Thomas Akers working to unstow and prepare COSTAR for installation on the Hubble Space Telescope during the third spacewalk of shuttle Endeavour mission STS-61 on December 6, 1993. Thornton, at top, is holding COSTAR while Akers works below. OPPOSITE: Spiral galaxy M-100, in the constellation Coma Berenices, as imaged by the Hubble Space Telescope with its original optics in 1990 (upper left), and after installation of COSTAR in 1993 (upper right).

INFRARED EYES February 1997 On its twenty-second flight into space, and its first trip back to the observatory since launching it in 1990, the shuttle Discovery took a crew of seven astronauts to the Hubble Space Telescope for NASA’s Servicing Mission 2, or SM-2. Now nearly seven years old, many of the electronics and other systems were starting to show evidence of wear and tear in the hostile environment of space. In addition, advances in the technologies of imaging and spectroscopy (the splitting of light into its constituent colors, as if by a prism) meant that several of the original instruments could be replaced with more sensitive and capable new instruments. These new instruments would allow Hubble to detect fainter objects (to see farther back in time) and to do a much better job of determining their composition. The crew of Discovery’s STS-82 mission was commanded by Kenneth Bowersox (on his second flight to Hubble) and piloted by Scott Horowitz. Four mission specialists (Gregory Harbaugh, Mark Lee, Steven Smith, and Joseph Tanner) would pair up to conduct four planned EVAs, or spacewalks, to install the NICMOS (Near-Infrared Camera and Multi-Object Spectrograph) and STIS (Space Telescope Imaging Spectrograph) instruments and to repair and upgrade other critical systems, while Steven Hawley (who had helped to deploy the telescope from Discovery back in 1990) operated the Canadarm from inside the shuttle. Among the critical repairs and replacements performed by the crew was a swap-out of a failed taperecorder data storage system for a much larger-capacity solid-state hard drive like those on modern computers, and the replacement of one of four reaction wheels, which help keep the observatory pointed at distant objects of study, but had failed.

ABOVE: Space shuttle Discovery astronauts Joseph Tanner (foreground) and Gregory Harbaugh (background) used the Canadarm as a “cherry picker” to replace critical components of Hubble’s electronics and instrument systems during their February 1997 STS-82 mission. OPPOSITE: Hubble floats free about 340 miles (540 km) above the Earth after being released from the cargo bay of the space shuttle Discovery at the end of Servicing Mission 2 (SM-2) in February, 1997.

HUBBLE SERVICING MISSIONS (SMs)

SM-1 • December 2–13, 1993: HSP was replaced with COSTAR, and WFPC was replaced with a higher-resolution JPL camera called WFPC2.

SM-2 • February 11–21, 1997:

The crew of the shuttle Discovery replaced GHRS with another GSFC spectrometer called STIS, for Space Telescope Imaging Spectrograph, and also replaced FOS with an instrument called NICMOS (Near-Infrared Camera and Multi-Object Spectrograph, designed by the University of Arizona). The upgrades provided HST with more powerful imaging capabilities and extended the range of the telescope’s sensitivity into the infrared.

SM-3A • December 19–27, 1999: The crew of the shuttle Discovery replaced HST’s aging gyroscopes and installed a faster new main computer.

SM-3B • March 1–12, 2002: Another new camera, called ACS (Advanced Camera for Surveys) led by Johns Hopkins University, was installed by the crew of the shuttle Columbia. ACS uses three independent sensors to capture images from the ultraviolet to the near-infrared part of the spectrum with ten times more sensitivity than HST’s previous cameras for imaging of extremely faint objects.

SM-4 • May 11–24, 2009: The final servicing mission was carried out by the crew of the shuttle Atlantis, and included replacements and upgrades of aging systems like batteries and computers, as well as the installation of two new instruments: JPL’s WFC3 (replacing WFPC2 with improved capabilities) and the University of Colorado’s Cosmic Origins Spectrograph (COS), which included an updated corrective lens system that replaced COSTAR. As the last planned servicing mission for the telescope, SM-4 was designed to enable HST to operate as long as possible and to

hopefully avoid a gap in observing capabilities while NASA’s replacement mission, the James Webb Space Telescope (known as JWST; see page 190) was being built and tested.

ABOVE: This false-color view of massive stars and hot ionized gas in the central part of our Milky Way galaxy is a composite of infrared images acquired with the NICMOS camera (installed on Hubble by the SM-2 astronauts in 1997) merged with other infrared images taken by NASA’s Spitzer Space Telescope.

During the second EVA, astronauts Harbaugh and Tanner noticed that Hubble’s protective thermal insulation was cracking and wearing down from continued exposure to the harsh sunlight and radiation of space. Since the crew had brought extra insulation from Earth, astronauts Lee and Smith were sent out on an unscheduled fifth spacewalk to replace the insulation near key electronics and instrumentation, hoping to extend the lives of those systems. The crew also used Discovery’s boosters to raise the orbit of Hubble, which had slowly spiraled closer to Earth since its initial deployment. This further extended the life of Hubble by decreasing the effects of atmospheric friction, which causes the orbits of satellites like Hubble to slowly spiral down. After completing almost 150 orbits of Earth and spending in excess of thirty-three hours outside the shuttle repairing and replacing components on Hubble, the crew of Discovery returned home on February 21, 1997.

OPPOSITE: Hubble repair astronauts Steven Smith and John Grunsfeld work on the end of the space shuttle Discovery’s robotic arm to replace the telescope’s gyroscopes during Servicing Mission 3A (SM-3A) in December, 1999.

BRAIN TRANSPLANT December 1999 NASA had scheduled the next “regular service call” to visit the Hubble Space Telescope for June 2000.

However, between 1997 and 1999 three of the telescope’s six gyroscopes failed. The “gyros” provide the fine guidance and pointing capability needed to target the observatory precisely. While Hubble could operate with just three, if one more failed the telescope would have to be shut down until a repair mission could be mounted. So NASA decided to be proactive and schedule the first part of the third Servicing Mission, known as SM-3A, as soon as possible to replace the failed gyros. The shuttle Discovery lifted off on December 20, 1999, for its third mission focused on Hubble and its twenty-seventh trip into space. The seven-person crew of STS-103 included Commander Curtis Brown, Pilot Scott Kelly, and five Mission Specialists. Four of them (Michael Foale, John Grunsfeld, Claude Nicollier, and Steven Smith) worked in pairs during three spacewalks outside the shuttle’s cargo bay, while the fifth, Jean-François Clervoy, operated the Canadarm to capture and manipulate the telescope and assist the astronauts. In total, the astronauts spent more than twenty-four hours outside the shuttle performing work on Hubble. The astronauts replaced all six of Hubble’s gyroscopes with newly designed versions that were expected to have a longer lifetime than their predecessors. In addition, the crew replaced one of the three Fine Guidance Sensor (FGS) units—detectors that are used to keep the telescope steadily pointed on targets by locking the system onto a network of specific guide stars at very accurately known positions in the sky. FGS is also a science instrument, in a sense, in that it can monitor the movement of stars relative to each other over time, or wobbles in their motion caused by the slight gravitational tug of any orbiting planets. In addition to replacing some other electronics components and some of the telescope’s external insulation, the STS-103 astronauts also gave Hubble a brain transplant, replacing the observatory’s aging 1980s-era computer with a new one more than twenty times faster and with six times the onboard memory capacity. With its faster processing power and enhanced memory, an upgraded Hubble was able to collect more data during each observing session, to run more sophisticated software that could detect or mitigate instrument or telescope anomalies, and to significantly simplify the data-processing work that had previously proved to be burdensome to ground controllers.

POWER TRIP March 2002 The fourth shuttle mission to the Hubble Space Telescope was actually the second part of what had originally been scheduled as a third Servicing Mission in June 2000 (see page 29). However, the early failure of three of the observatory’s gyroscopes caused NASA to mount the third mission, known as SM3A, earlier than planned, leaving it to the crew of the shuttle Columbia to conduct SM-3B to complete the rest of the regularly scheduled work. Columbia launched from the Kennedy Space Center on March 1, 2002 with a crew of seven astronauts, commanded by Scott Altman. The pilot was Duane Carey, and there were five Mission Specialists, all tasked to servicing Hubble. Following the now-familiar pattern established by earlier Servicing Missions, the SM-3B Mission Specialists included two pairs of astronauts who alternated shifts during the mission’s five spacewalks (John Grunsfeld and Richard Linnehan on EVAs 1, 3, and 5, and James Newman and Michael Massimino on EVAs 2 and 4), while Mission Specialist Nancy Currie operated the Canadarm from

within Columbia. The crew replaced the observatory’s aging solar panels during the mission and a power-control unit with a new one that provides 30% more power than before, and thus the capability to operate longer and to accommodate more capable future instruments. The crew also replaced the Faint Object Camera—the last of Hubble’s original 1990 instruments—as part of the mission. The new, more capable imaging system was called the Advanced Camera for Surveys (ACS), and it represented a major advance for Hubble, providing three independent sensors that captured the ultraviolet to the near-infrared part of the spectrum. The ACS had ten times more sensitivity for imaging of previously undetectable objects like galaxies that had formed in the early Universe. This new capability was demonstrated by spectacular long-exposure images like the Hubble Ultra Deep Field (see page 182). The upgrade to ACS would once again revolutionize the telescope’s science potential by extending Hubble’s reach even deeper back in time. Additional work performed by the crew during nearly 36 hours of spacewalks was the resurrection of the NICMOS (Near-Infrared Camera and Multi-Object Spectrograph) instrument. Originally installed during SM-2 in 1997, NICMOS was able to observe infrared (heat) energy by cooling its detector to around 60 degrees above absolute zero, using a solid block of nitrogen ice. But that nitrogen evaporated over time, leaving NICMOS much less sensitive. The new cryocooler installed in the instrument by the crew of Columbia brought NICMOS back to life, however, and restored Hubble’s high-quality infrared imaging capabilities. This proved especially useful for the study of dense nebular clouds of gas and dust, the interiors of which are opaque to visible light, but relatively transparent to infrared light.

OPPOSITE: The crew of the space shuttle Columbia successfully delivered and installed new solar panels on Hubble during Servicing Mission 3B (SM-3B) in March, 2002. The new panels are seen here folded up inside the shuttle’s cargo bay prior to being released and installed on the telescope.

Columbia space shuttle astronauts Michael Massimino (riding the Canadarm) and James Newman (background) replacing one of the Reaction Wheel Assemblies in the Hubble Space Telescope during the STS-109 mission’s second spacewalk.

A FINAL TUNE-UP May 2009 The next scheduled maintenance of Hubble had been set for February 2005, but the tragic loss of the shuttle Columbia and crew in 2003 resulted in NASA’s cancellation of all future Hubble Servicing Missions. After public and Congressional outcry for the preservation of Hubble as long as possible until the follow-on

James Webb Space Telescope (JWST) could be deployed in the early 2020s, NASA reversed their decision, and a fifth and final Servicing Mission, known as SM-4, was scheduled for May 2009. The shuttle Atlantis took a crew of seven astronauts back to Hubble, including physicist John Grunsfeld, flying on his third mission to the telescope (see page xiii). STS-125 was commanded by Scott Altman, piloted by Gregory Johnson, and included three other Mission Specialists besides Grunsfeld who would alternate work during the mission’s five spacewalks (Andrew Feustel, Michael Good, and Michael Massimino), as well as Mission Specialist Megan McArthur operating the Canadarm. The mission would be Atlantis’s 30th trip into space, and it would be the last flight of any space shuttle to not visit the International Space Station. The primary goals of SM-4 were to swap out WFPC2 for a new camera, WFC3 (Wide Field Camera 3), and to install the Cosmic Origins Spectrograph (COS), which would replace COSTAR, as corrective lenses were now built into all the newer Hubble instruments. Other upgrades included replacing another Fine Guidance Sensor, installing new batteries, six new gyroscopes, and some failed electronics components, as well as repairs on several other instruments. COS and WFC3 would once again dramatically increase Hubble’s sensitivity compared to earlier instruments, keeping the observatory at the cutting edge of spacebased astronomical discoveries by extending its reach to much fainter, more distant galaxies formed even farther back in time. Additionally, the astronauts installed a mechanical device called the Soft Capture and Rendezvous System onto the bottom of the telescope, to enable a future robotic or crewed vehicle to more easily grapple the telescope and help guide it toward an eventual controlled atmospheric re-entry. SM-4 was the 126th space shuttle mission out of 135 total, and the last shuttle mission to visit Hubble because of the retirement of the space shuttle fleet in 2011. Part of the legacy of that Atlantis crew was to help Hubble operate effectively for as much of the decade or more to come until JWST would go operational. In that sense, SM-4 was a phenomenal success: as of 2019, ten years after the shuttle’s last visit, Hubble continues to collect amazing images and other data sets, and to enable spectacular continuing astronomical discoveries.

OPPOSITE: This stunning view of huge star-forming pillars of gas and dust and so-called “Herbig-Haro Objects” (see page 96) streaming out geysers of hot gas is part of the Carina Nebula. This false-color composite image was created from multiple color filters in the WFC3 instrument, which was installed on Hubble by the SM-4 astronauts in 2009. ABOVE RIGHT: Technicians at NASA’s Kennedy Space Center in Cape Canaveral, Florida, inspect Hubble’s new WFC3 instrument before it was launched on the space shuttle Atlantis in 2009.

PAGES 36 and 37: False-color Hubble WFC3 photo of a section of the Veil Nebula, the outer shell of the famous supernova remnant known as the Cygnus Loop, located about 1,500 light-years away in the constellation Cygnus.

PAGES 38–39: A ground-based photo of Comet Hyakutake (also known as C/1996 B2), which blazed across the night sky during its close pass by the Earth in March 1996. INSET: Shortly after the comet was discovered, Hubble was pressed into service to take photos and other unique measurements of “The Great Comet of 1996.” This WFPC2 false-color view, which covers an area just 2,000 miles (3,200 km) across, zooms in on the comet’s small, bright icy nucleus and its dusty, gassy surroundings.

This is the famous “String of Pearls” image, where all the fragments of Shoemaker-Levy 9 are visible.

STRING OF PEARLS October 1993 We don’t have to look farther than our own Moon to know that the Solar System was once a violent place. Its surface preserves the scars from millions of ancient impacts by large asteroids and comets over time. We see the same sorts of battered surfaces all across the Solar System—on other planets, on rocky and icy moons, even on small asteroids and comets themselves. But large impacts are rare events these days, so when astronomers know in advance that a large impact is coming, it’s a once-in-a-lifetime opportunity. Such an opportunity came to pass in the spring of 1993, when a bizarre comet was discovered by the observing team of Carolyn and Eugene Shoemaker and David Levy. It was the team’s ninth periodic comet discovery, so they called it P/Shoemaker-Levy 9, or just SL-9. Rather than the typical single bright head or nucleus (the solid icy/rocky part of a comet, usually surrounded by a diffuse cloud of water vapor) followed by a long dusky tail, however, SL-9 had multiple bright spots and a fuzzy elongated “tail” passing through them. Orbit-tracking by other astronomers later revealed that the comet was actually in a two-year orbit of Jupiter rather than the Sun. The elongated nature of SL-9 was apparently the result of a single larger comet having been ripped apart by a previous close encounter with Jupiter. Stunningly, astronomers realized that SL-9’s “train” of mini-comets would crash headlong into the cloudy atmosphere of the Solar System’s largest planet in July 1994! It was the first time in history that a large Solar System impact event could be predicted in advance. Astronomers and telescopes around the world began to mobilize for the astronomical event of the millennium. Of course, this meant that Hubble had to get involved as well. Even with the still-out-of-focus nature of the original 1990 instruments, Hubble images and other data during the second half of 1993 still revealed details of the structure, composition, and motions of SL-9’s famous “string of pearls” that would soon slam into Jupiter. During early 1994, after astronauts had installed the COSTAR corrective optics (see page 5) on the telescope, the images and other data being obtained on SL-9 got even better. Hubble images of SL-9 revealed that there were to be at least 20 large fragments in the comet’s train.

While perhaps small in an absolute sense, the larger fragments were estimated to each be more than a mile and a quarter (>2 km) in diameter, still fairly large in terms of average comet nucleus sizes. Several nuclei that looked like diffuse bright clumps in ground-based and early Hubble telescopic images were revealed in COSTAR-corrected Hubble images to be multiple smaller nuclei traveling close together. Hubble’s images helped to build great public and professional excitement for the “Jovian” fireworks to come in the summer of 1994.

IMPACT ON JUPITER July 1994 Although the fragments of comet Shoemaker-Levy 9 (see String of Pearls, pages 40 and 41) were tiny compared to Jupiter, some astronomers and planetary scientists thought it was likely that when they impacted the planet in 1994 they would create significant atmospheric disturbances because they were traveling so fast. The mile-wide boulders of ice and rock were set to impact the planet’s top layer of clouds at around 134,000 mph (216,000 kph), which would release amounts of energy equivalent to many thousand times the destructive power of all the world’s nuclear bombs combined. But other astronomers were skeptical of this, predicting that the mostly low-density, weak, icedominated fragments would break up and vaporize harmlessly deep below Jupiter’s thick cloud layers. Some astronomers in this camp pointed to the Tunguska event in 1908, when what was likely a lowdensity, weak, cometary nucleus exploded high above Siberia, creating a shock wave that leveled trees but didn’t cause a discernible surface impact crater. HST had a front-row seat for the show, of course. It’s the world’s highest-resolution observatory, with imaging and spectroscopic capabilities that far surpassed those available to ground-based observers at the time. Astronomers worldwide competed for the right to collect images and other data using Hubble during and after the impact events. Between July 16 and July 22, twenty-one distinct impact events were directly or indirectly observed on Jupiter as the fragments of SL-9 slammed into the giant planet’s cloud tops, one by one. The impacts themselves actually occurred on the side of Jupiter facing away from the Earth, so astronomers were not expecting to see them directly. They were expecting to observe their after-effects as the impact sites rotated into view (Jupiter rotates once every ten or so hours). Surprisingly, Hubble and ground-based telescopes observed enormous and dramatic fireball explosions rising high above the edge of the planet from many of the impacts, far exceeding most expectations. The after-effects of the impacts were just as stunning. Giant semi-circular dark spots—some larger than the Earth—appeared at the locations of the impact sites, thought to be caused by the vaporization and dispersal of cometary material, as well as potentially by the dredging up of carbon- and sulfur-bearing molecules from deeper atmospheric layers. Jupiter’s impact scars were so enormous that they were visible even from small telescopes and persisted on the giant planet for months before eventually fading away.

OPPOSITE: Hubble’s WFPC2 natural-color image of the southern hemisphere of Jupiter and the locations of the fragment “G” (larger blemish) and “D” (smaller spot) impacts from comet P/Shoemaker-Levy 9 on July 18, 1994. The observations were made only about 1 hour and 45 minutes after the “G” impact, which by this point had generated circular atmospheric structures in Jupiter’s clouds comparable to the Earth in size.

OPPOSITE: This non-Hubble photo shows the brilliant “evening star” Venus hanging in the twilight sky above Clear Creek Canyon Observatory in central Arizona, USA. OPPOSITE, INSET: This false-color Hubble WFPC2 ultraviolet wavelength image of Venus was taken on January 24, 1995, when the planet was very close to its greatest elongation (angular distance in the sky) from the Sun. Venus orbits closer to the Sun than Earth, and goes through phases like the Moon—from a full disk to a crescent. At greatest elongation, Venus appears quite similar to the same phase as a first-or last-quarter Moon.

VENUS HIDING

January 1995 The instruments and systems of the Hubble Space Telescope are optimized for observing extremely faint and extremely distant objects. This means that the team operating the telescope must generally avoid relatively intense levels of light from very near and bright objects, like the Sun, Earth, and Moon, which would shine into the telescope and damage its very sensitive detectors. Specific “exclusion zones” were developed around each of these bright objects, and software and other systems were developed to prevent the telescope from being pointed closer than a certain “avoidance angle” to them. Hubble’s avoidance angle for the Sun is around 50 degrees, meaning that the telescope cannot be pointed to any object within that angular distance from the Sun in the sky. The Solar System’s first planet, Mercury, for example, never gets more than about 28 degrees from the Sun as viewed from the vicinity of the Earth, so Hubble can’t ever observe Mercury. Venus, the second planet, is also always relatively close to the Sun in the sky, but its greatest angular distance (also known as “greatest elongation”) is more like 47 degrees—tantalizingly close to Hubble’s nominal 50-degree limit! Indeed, Venus at greatest elongation is so close to being an allowable Hubble observation that planetary scientists appealed to the Space Telescope Science Institute for a little leniency to allow the telescope to acquire unique and scientifically interesting ultraviolet observations of the planet. In January 1995, the Institute granted their wish, and Hubble took some stunning UV images and spectra (graphs of the intensity of light from an object after it has been split into many hundreds of individual colors) of Venus. Planetary atmospheric scientists used Hubble’s UV data to estimate the amount of sulfur dioxide (SO₂) in Venus’s cloud tops, which was impossible to gather from ground-based telescopes, but which had been done from the late 1970s through the early 1990s from a smaller, older space telescope called the International Ultraviolet Explorer, or IUE, as well as from the NASA Pioneer Venus orbiter mission. Curiously, SO₂ appeared to have decreased since the earlier observations, leading some astronomers to suggest that perhaps we were observing the slow decay of atmospheric SO₂ injected by one or more active volcanoes on the surface of Venus. Hubble’s UV images revealed interesting dark markings within the clouds of Venus, similar to dark markings first seen by Pioneer Venus. The detailed nature of the so-called “UV absorber” that caused these dark markings on Venus remains an unsolved mystery to this day.

BETA PIC AND THE WARPED DISK January 1995 In 1983, astronomers used high-powered ground-based telescopes to discover the first “debris disk” around another star. In this case, it was the disk of gas and dust orbiting the nearby star Beta Pictoris, only about 63 light-years from our Solar System. Beta Pic, as it’s informally called, is a very young star (less than 30 million years old) that is about 75% larger than our Sun. The discovery spoke directly to the leading hypothesis for the formation of our solar system, in that the planets, moons, asteroids, and comets were all formed from a similar circumstellar disk of gas and dust around our young Sun. So Beta Pic was possibly

showing us what our early solar system was like! Hubble and the WFPC2 camera got in on the action in 1995, capturing higher-resolution photos of the region around Beta Pic by carefully avoiding imaging the star itself, which would have swamped the much fainter reflected starlight from the disk. The images revealed unprecedented detail in the innermost part of the disk. In particular, they showed evidence for a slightly unexpected warping or bending of the innermost part of the dusty ring around the star. One hypothesis to explain the warping of the Beta Pic disk is that there is a large, Jupiter-sized (or larger) planet orbiting the star—within the innermost blacked-out portion of the photo—that exerts a gravitational tug on the disk, warping its shape as the planet orbits the star. The idea that there are planets orbiting other stars has been around for centuries; and by the late twentieth century, astronomers were beginning to find proof of the existence of these “extrasolar” worlds, or “exoplanets,” and especially of the existence of giant planets like Jupiter. Spurred by this hypothesis, astronomers followed up the Hubble observations with those from larger ground-based telescopes and also by using more advanced instruments on Hubble as they were installed in subsequent servicing missions. In 2006, the ACS camera was used to discover a fainter secondary debris disk around Beta Pic, slightly tilted relative to the primary disk, consistent with the presence of perhaps a second giant planet (or more) in the system. By 2008, astronomers had enough data from ground-based studies to confirm the discovery of a large planet, seven times the mass of Jupiter, orbiting Beta Pic; this very large planet is potentially responsible for the warps in the star’s primary ring. The hunt for additional smaller planets in that alien solar system continues in earnest today. OPPOSITE: An artist’s rendering of the Star Beta Pictoris, including its near stellar environment. BELOW: Hubble’s WFPC2 image of the inner region of the disk of gas and dust circling the nearby star Beta Pictoris. The telescope was pointed to avoid imaging the star itself, which would be in the central blacked-out region in this view. For scale, that blacked-out region is about the size of our solar system out to Neptune. BOTTOM: Enhanced false color showing regions where the disk is denser (reds, whites).

ASTEROIDS: VERMIN OF THE SKIES! March 1998 The Hubble Space Telescope was designed primarily to study faint and fantastically distant objects, to provide a window into the early Universe. However, to see objects so far away requires the telescope to look beyond objects that are nearby. And sometimes those pesky nearby objects get in the way.

Astronomers often need to take photographs at very long exposures, carefully tracking faint objects across the sky as they move. And those long-exposure observations make tracking even more complex because Hubble is in orbit around our spinning planet. So if some nearby planet, asteroid, or comet, moving at a different rate across the sky than more distant stars and galaxies, happens to pass through the field of view during one of these long exposures, the result can be messy streaks that “contaminate” the long-exposure data of the more distant objects. Indeed, early twentieth-century astronomer Walter Baade is widely credited as voicing his frustration on behalf of all deep-object astronomers regarding the contaminating influence of asteroids on long-exposure photos, calling them the “vermin of the skies.” However, one person’s trash is another person’s treasure. Hubble researchers combing through the archive of long-exposure photo/s have found hundreds of examples of such asteroid trails. Because the direction of the telescope’s lens and the orbital path of the observatory are known in precise detail, astronomers are able to calculate the location, the orbital path, and even some of the basic properties of these asteroids, most of which have never been detected before because they are too faint to be seen by ground-based telescopes. Most of the new asteroids accidentally discovered in such Hubble images orbit within or close to the same plane (called the “ecliptic”) as most of the rest of the planets, moons, and asteroids in our Solar System. Hubble data in the 1990s let astronomers make predictions of the total number of much smaller, fainter asteroids than seen before, which provided critical information needed to design and carry out a number of ground-based observations that have been conducted since, with much larger telescopes that have, over time, more fully cataloged much fainter “vermin.”

OPPOSITE: The graceful blue curve in this Hubble WFPC2 image is the trail of the asteroid u2805m01t4, a small (about 1 km diameter) rocky body that orbits in the main asteroid belt between Mars and Jupiter. While often appearing as interlopers in long-exposure Hubble images, some astronomers use the presence of such trails to discover and categorize new asteroids in our Solar System.

False-color photo of Jupiter’s Northern Lights, imaged by Hubble’s STIS ultraviolet instrument on November 26, 1998. These are the same kinds of shimmering curtains of light that can often be seen in the skies of Earth’s polar regions, formed by the same kinds of interactions between the solar wind and a planet’s magnetic field—except the Northern Lights on giant Jupiter are many times the size of Earth.

JUPITER’S NORTHERN LIGHTS November 1998

The Aurora Borealis and Aurora Australis (or Northern and Southern Lights) on Earth are a spectacular and beautiful manifestation of the interaction between the solar wind—the steady stream of high-energy particles emitted by the Sun—and our planet’s strong magnetic field. The solar wind is generally diverted past our planet (helping to protect life here), except for at places near the poles where the Earth’s magnetic field interacts most strongly with the surface. There, trapped solar-wind particles collide with atmospheric atoms and molecules, emitting prodigious amounts of ultraviolet radiation, along with beautiful hues of green, yellow, and red visible light, as they lose energy. But Earth is not the only planet in the Solar System with a strong magnetic field. In fact, all the giant planets of the outer Solar System have even stronger fields than Earth, and Jupiter is the strongest of them all. As it does on Earth, when the solar wind interacts with Jupiter’s strong magnetic field, it produces beautiful and enormous (many times larger than our entire planet) auroral displays in the planet’s polar regions. The unique ultraviolet imaging and spectroscopic capabilities of Hubble make it a perfect platform for studying the enormous and powerful aurorae of the giant planets. Jupiter’s auroral features include an enormous oval-shaped region of strong ultraviolet emissions centered on its magnetic north pole, as well as a series of rapidly changing, more diffuse emissions within that main oval region. An additional unique set of features appears as small bright spots outside the oval aurora. Called “magnetic footprints,” they represent the places where electrical currents flow from Jupiter’s major moons Io, Europa, Ganymede, and Callisto, along strong magnetic-field line, and into the planet’s polar atmosphere. Magnetic footprints result from the very strong nature and enormous size of Jupiter’s magnetic field—all four major moons are deeply embedded inside the field. If we could see Jupiter’s magnetic field with our naked eyes, it would appear nearly three times the size of the Moon in our sky. Our Moon doesn’t produce a magnetic footprint in Earth’s polar atmosphere because it spends most of its time outside Earth’s magnetic field, which is much smaller and weaker than Jupiter’s.

OPPOSITE: Hubble’s WFPC2 color photo of Mars captured on April 27, 1999. The color is a composite of separate red, green, and blue filter wavelengths chosen so the result approximates the “natural color” view we would see with our own eyes. Major features visible in the photo include the bright water ice polar cap (top), dark brownish sandy terrains and bright reddish dusty terrains, white water ice clouds along the morning (right) limb (edge of the planet), and a large cyclonic storm system near the north pole.

MARTIAN WEATHER UPDATES April 1999

Shortly after it was launched in 1990, Hubble began observing Mars during its closest approaches to the Earth. But it wasn’t until after the telescope’s focus problem was fixed in 1993 (see page 21) that images of the Red Planet began to far exceed the best resolution possible from ground-based telescopes. Indeed, even after several NASA robotic spacecraft began taking photos and other data from Mars’s orbit starting in the mid-1990s, Hubble data were still filling important gaps in Mars science. Hubble’s advantage when viewing planets like Mars is that it does it similarly to how geostationary weather satellites view the Earth—all at once for an entire hemisphere. In contrast, most Mars orbiters since the mid-1990s have been in very low polar orbits around the planet, able to view only a small fraction of the planet at a time—so they have to build up global coverage over the course of many orbits around the planet. Hubble images allow morning, midday, and afternoon weather patterns to be observed across the planet, while orbiters generally monitor only a single time of the day. In another example of Hubble’s scientific importance, until late 2014 it was providing the only highresolution ultraviolet data needed to study the daily and seasonal variations of ozone and water vapor on Mars. Those gases are minor components of Mars’s atmosphere, but they provide important information on photochemical processes—chemistry induced by ultraviolet sunlight—as well as on the ways that water moves between the surface and the atmosphere. During the 1999 Mars imaging campaign, for example, Hubble data enabled the discovery of new kinds of wintertime equatorial clouds, as well as large-scale cyclone-like storms in the north polar region. Hubble images of Mars have also helped to reveal some of the processes at work during the formation, growth, and decay of the planet’s famous dust storms. Sometimes, for instance during the 2001 Hubble imaging campaign of Mars, local dust storms grew into planet-encircling events that completely obscured the normal bright and dark surface markings from view. (It can take weeks to months for the dust to clear, and afterward we often can see that the post-storm surface markings have gone through dramatic changes.) Ground-based telescopes have been used to study the changing surface markings on Mars for centuries, but Hubble represented a capstone to that era, providing a critical link between the classical and spacecraft eras of Mars exploration.

SATURN: Crown Jewel of the Solar System January–March 2004 The Hubble Space Telescope has taken many images and other data sets focused on the study of the Solar System’s so-called “crown jewel,” the spectacularly ringed planet Saturn. Especially important has been the monitoring of Saturn’s atmosphere and aurora in the 1990s and early 2000s, after the first reconnaissance by the Voyager missions in the early 1980s, prior to NASA’s Cassini mission, which orbited Saturn from 2004 through 2017. Saturn’s cloud layers consist of darker “belts” and lighter “zones,” similar in some ways to those on Jupiter. However, ground-based and Hubble images have revealed that Saturn’s atmosphere is generally less active than Jupiter’s, with fewer storms and other disturbances. Occasionally, astronomers going back to the nineteenth century have spotted small Jupiter-like, oval-shaped storms in Saturn’s atmosphere. And around Saturn’s northern summer solstice, which happens every 30 Earth years or so, a large equatorial

storm system called the “Great White Spot” has been documented. Hubble studied the Great White Spot and other large equatorial storm systems in detail in the 1990s using its WFPC2 instrument. The storm’s white clouds were found to be ammonia ice crystals that “snow out” of the gaseous atmosphere when deeper, warmer atmospheric layers cool after they rise to higher levels. Two specific occurrences of these kinds of storms that were studied using Hubble in 1990 and 1994 turned out to be two of the three largest storms observed on Saturn in the past few hundred years. The behavior of the atmosphere captured in Hubble images at the highest possible resolution helped to design the filters and observation planning for the much more up-close imaging that was conducted during the Cassini mission. Like Jupiter, Saturn also has a strong magnetic field that interacts with the impinging solar wind (see Jupiter’s Northern Lights, page 50), which creates magnificent auroral displays in the planet’s north and south polar regions. And like on Jupiter, Saturn’s Northern and Southern Lights are primarily organized into an oval-shaped pattern centered on the planet’s magnetic north and south poles. They are also dynamic and ever-changing on timescales from minutes to hours. Another similarity to the Jupiter system is Saturn’s magnetic footprint—an electrical connection between a moon and a giant planet’s atmosphere. For example, such a link was discovered by the Cassini mission from the planet to its small moon Enceladus.

OPPOSITE: Composite view of a visible light, natural-color photo of Saturn taken by Hubble’s ACS instrument on March 22, 2004, with a superimposed false-color ultraviolet image of the planet’s dynamic Southern Lights auroral display, photographed with the STIS instrument on January 28, 2004.

THE FIVE MOONS OF PLUTO February 2006 The Solar System’s ninth planet, Pluto, was discovered orbiting just beyond Neptune by Lowell Observatory astronomer Clyde Tombaugh in 1930. During the 1990s, ground-based astronomers discovered that Pluto was just the tip of the iceberg—the nearest and brightest member of a whole new

population of comparably sized, smaller icy bodies known as Kuiper Belt Objects (KBOs) that orbit beyond Neptune, between about thirty and one hundred times the distance from the Earth to the Sun. The realization that Pluto was just one member of a larger population of KBOs caused some (but far from all) astronomers to propose that Pluto is not a full-fledged planet but should be called a “dwarf planet” instead (See Pluto, Revealed, page 62). Pluto has one large moon, Charon, discovered in 1978. Hubble got involved in observations of Pluto in 2005, when planetary astronomers realized that the combination of Hubble’s outstanding resolution and sensitivity would allow them to conduct the most detailed search possible for additional, fainter moons. The so-called Pluto Companion Search team found preliminary evidence for two additional moons in their initial set of observations. But it took a second set of follow-on images using the ACS instrument’s High Resolution Camera (HRC) in February of 2006 to confirm that they really are there, and to determine their basic orbital parameters. Originally called S/2005 P1 and S/2005 P2, these newly discovered moons of Pluto are indeed quite small and irregularly shaped, both with their longest dimension around 30 miles (about 50 km) across. The new moons were subsequently named Hydra and Nix, after the mythological Greek nine-headed serpent and goddess of the night, respectively. Hubble images showed that they are both in circular orbits in the same plane as Pluto’s equator, and that they orbit in a 2:3 “resonance” around Pluto (i.e., the closer moon Nix orbits Pluto exactly three times for every two orbits of Hydra). Even more sensitive Hubble observations of the Pluto system made in 2011 and 2012 revealed two more new moons. Eventually named Kerberos and Styx (after the mythological Greek dog that guards the underworld and the goddess of the underworld’s river of the same name), these moons are also quite small and irregular (both less than 12 miles or 20 km across), and they orbit in resonance with the other three moons.

OPPOSITE: Hubble’s ACS/HRC image of two of the four “new” moons of Pluto discovered by astronomers using the space telescope. Pluto turns out to have five moons in all, a thin atmosphere, and (as revealed by the NASA New Horizons mission in 2015) interesting past and possibly present internal geologic activity.

Hubble’s ACS high-resolution photo of the region around Fragment G of the broken-up comet known as 73P/Schwassmann– Wachmann 3, taken on April 18, 2006. The comet, originally discovered back in 1930, began breaking apart as it passed closer to the Sun on its elliptical orbit in 1995. It is still disintegrating, now existing as a chain of over 33 separate fragments stretching across several degrees of the sky.

A COMET UNDONE April 2006 Comets are icy, rocky solar system objects usually orbiting on highly elliptical (oval-shaped) paths that also occasionally take them through the inner solar system closer to the Sun. As they are warmed, their ices often sublime from their surfaces and interiors, jetting water vapor and dust into space to form a fuzzy “head” or “coma” and/or long “tails” of gas and dust in the wake of their solid nucleus. Most comet nuclei

are fairly small, only a few to perhaps 10 km across, and are thought to represent the primitive remains of the originally condensed, ice-dominated small bodies of the outer solar system that became the building blocks for the giant planets along with their moons and rings. Astronomers have discovered a small population of “periodic” comets, ones that return to the inner Solar System at predictable times on predictable paths. The most famous, of course, is Halley’s Comet. But a few hundred others are also known to come back to Earth’s general neighborhood anywhere from every few years to every century or two. A dozen or so of these debris-shedding comets are the sources of the most famous meteor showers that occur every year. Hubble has helped planetary astronomers study the composition and morphology of many comets during its 30-year history, including the famous comet SL-9 that impacted Jupiter in 1994 (see page 40). Another dramatic comet captured by Hubble is called 73P/Schwassmann–Wachmann 3 (SW-3), which orbits the Sun approximately between the orbits of the Earth and Jupiter, returning to the inner Solar System every 5.3 years. During its 1995 return, ground-based astronomers noticed that it had begun to break apart into at least four large pieces, as the Sun’s heat continued to evaporate its surface and internal ice deposits. By the time Hubble was trained on SW-3 in 2006, the number of observed ground-based fragments had increased to eight. At a higher resolution than possible from the ground, Hubble discovered that each of those fragments is itself composed of dozens of smaller fragments. In fact, SW-3 appears to be slowly disintegrating. During upcoming close passes by the Sun maybe in the not-too-distant future, it will likely transform back into its original form of more than 4.5 billion years ago—a ghostly cloud of fine-grained vapor and dust.

PLUTO, REVEALED February 2010 While Hubble has been able to achieve unprecedented resolution for a telescope, there are practical limitations for how much detail it can record, based on a combination of the size of the object being observed and its distance from the telescope. Pluto, for example, is a relatively small world (only around 1,500 miles, or 2,400 km, across), and has been more than 30 times as far away from the Earth as the Earth is from the Sun for most of Hubble’s lifetime (the distance from the Earth to the Sun is defined as 1 Astronomical Unit [1 AU]; therefore, Pluto has been more than 30 AUs from the Earth for most of Hubble’s lifetime). So, even with Hubble’s highest-resolution cameras, Pluto is only a few pixels across.

ABOVE: This much more detailed view of Pluto came from the July 2015 flyby of the planet and its five moons by NASA’s New Horizons spacecraft. The bright heart-shaped feature, seen also by Hubble in the 180° view opposite, is an icy, nitrogenand-methane rich plains region now known as Sputnik Planitia. OPPOSITE: The most detailed global view of Pluto as of 2010, constructed from multiple Hubble ACS photos originally taken in 1994, 2002, and 2003. Even at this coarse resolution, Pluto showed evidence for bright and dark markings and color variations hypothesized to be a result of ultraviolet solar radiation breaking down methane ice on the planet’s surface.

But astronomers are clever and particularly good at squeezing every ounce of possible information from images and other data pushed to the limits of resolution. By taking multiple images of objects and shifting the telescope’s pointing, or direction, by small amounts between images (a process called “dithering”), it’s possible to essentially enhance the maximum possible resolution of Hubble images, even for objects only a few pixels across like Pluto. The catch is that the process requires lots of careful calibration and understanding of the camera being used, and lots and lots of computer time to process the images. Using such methods with Hubble ACS images of Pluto taken in 1994, 2002, and 2003, planetary astronomers were able to synthesize multiple views of Pluto into a higher-resolution global view of bright and dark markings on its surface. The results were exciting—Pluto actually has variations from place to place on its surface, likely due to changes in geology or composition (or both). And some of those places change in color and brightness over time, possibly from changes in the planet’s thin atmosphere. Hubble’s results were folded in to the planning for the imaging and other observations that were going to be carried out by the NASA New Horizons mission, which launched in 2006 and flew through the Pluto system in 2015. When New Horizons finally photographed Pluto up close, the Hubble results and predictions were vindicated: bright and dark markings generally matched what the highly processed ACS images predicted, even including the large heart-shaped area known as Tombaugh Regio, seen as the bright yellowish Hubble feature in the 150° and 180° longitude ACS images. Pluto turns out to have fascinating geologic and atmospheric processes, a world worthy of once again being called a planet.

BLUE MOON May 2012 In general, the operations staff of the Hubble Space Telescope needs to avoid pointing the observatory at the Sun, Earth, or Moon because the intense brightness of those astronomical bodies could potentially damage some of the telescope’s super-sensitive detectors and other instruments. However, while pointing at

the Sun is never allowed because it would create too much internal heat, it is possible to point the observatory at the Earth (for calibration purposes) or even the Moon, if the most sensitive systems are turned off. Hubble has been used to obtain scientific observations of the Moon a number of times. In 1999, Hubble’s WFPC2 and STIS instruments were used to collect images and spectra of the 58-mile-wide (93 km) crater Copernicus for both calibration purposes and scientific investigations of the composition and mineralogy of that part of the lunar surface. In 2005, a group of planetary scientists imaged the regions around the Apollo 15, Apollo 17, and Aristarchus Plateau regions of the Moon using visible-wavelength and ultraviolet filters in the highresolution channel of the ACS instrument. The Apollo missions had brought back samples from the Moon, so the chemistry and mineralogy of those specific places is well known. But can the known properties of the Apollo sites be used to infer the properties of other places on the Moon from where we don’t have samples? Using Hubble’s green, blue, and especially ultraviolet imaging capabilities, the researchers were able to establish a relationship between the color of the Moon’s surface and the amount of titanium—an important component of lunar volcanic rocks—in the Apollo samples, to use that relationship to predict the amount of titanium in other volcanic deposits. Another very cool example of Hubble Moon imaging involved a set of observations taken in January 2012 as a test of observations planned for the June 2012 transit of the planet Venus across the disk of the Sun. Since Hubble can’t observe the Sun directly, astronomers came up with the idea of monitoring sunlight reflected off the Moon during Venus’s transit as a way to possibly detect the spectroscopic signature of the atmosphere of the planet “imprinted” on the reflected sunlight. Data processing and analysis is still ongoing for those observations, but similar techniques are already being used to search for atmospheric-composition signatures in giant planets that transit in front of other nearby stars.

Hubble’s ACS image of the lunar impact crater Tycho, taken on January 11, 2012. Tycho is about 50 miles (80 km) wide and is surrounded by a system of bright radial streaks or rays formed by the ejection of material during the impact event, some 100 million years ago. The scene here is about 435 miles (700 km) across, and features as small as about 560 feet (170 m) across can be resolved.

THE MYSTERY OF FOMALHAUT B January 2013

Our Sun and everything else in our Solar System formed some 4.6 billion years ago from a spinning, relatively flat disk of gas and dusty-rocky-icy debris. Part of the support for this hypothesis comes from images of young nearby stars that have relatively flat and dusty-rocky-icy disks around them (see Beta Pic and the Warped Disk, page 46). Many such disks have now been discovered and studied by both Hubble and ground-based observatories. In 1998, astronomers used a millimeter-wave telescope (which detects deep infrared heat energy) to discover a warm dusty belt around the nearby star Fomalhaut (alpha Pisces). Fomalhaut is only about 25 light-years from the Sun and is a young star (around 450 million years old). The belt is toroidal (donutshaped) and orbits in the same zone of that solar system as the Kuiper belt in ours. Fomalhaut’s debris belt is presumed to be a zone where planets are forming, and the relatively emptier zone between it and the star is presumed to have been “cleared” by one or more planets orbiting there. In 2008, Hubble captured an image of a faint object that was suspected of being a Jupiter-sized planet orbiting just within Fomalhaut’s debris belt. Comparisons with both earlier and later images revealed that it is indeed likely to be a planet, moving in a 1,700-year-long inclined, elliptical orbit around the star. Analysis of the planet’s brightness (the first extrasolar planet to be directly imaged in visible wavelengths of light) and of its gravitational influence on the nearby debris belt led astronomers to conclude that the planet, named “Fomalhaut b,” is probably somewhere between the mass of Neptune and about three times the mass of Jupiter. Some follow-up observations with other ground-based and space-based telescopes have led to some astronomers questioning whether Fomalhaut b is really a Jupiter-class planet or not. Infrared observations from the Spitzer Space Telescope, for example, could not detect the kind of heat signature expected for a giant planet at that distance from its star. The Spitzer observations suggested to some astronomers that Fomalhaut b might be a clumpy or rubbly dust cloud, or perhaps a smaller rocky-icy planet surrounded by rubbly debris and dust. Higher-resolution images from future ground-based and space-based telescopes should help to resolve the great mystery of Fomalhaut b.

False-color composite Hubble STIS image of the dusty and rocky protoplanetary disk around the nearby star Fomalhaut, showing the orbital motion of what is presumably a Jupiter-sized planet orbiting just inside the disk. The light from the star was blocked to allow imaging of the much fainter disk and planet, known as Fomalhaut b.

HYPERACTIVE ASTEROID-COMETS September 2013 Most of the currently known asteroids in our solar system (approaching 800,000 objects) orbit in the Main

Asteroid Belt between Mars and Jupiter. This is a transitional zone for our Solar System: the small bodies that formed around Mars and inside its orbit are primarily rocky bodies, while small bodies that formed around Jupiter and outside its orbit are primarily icy. It should not be surprising then to find transitional objects—made of rocky-icy mixtures—in this transitional zone. It was nonetheless something of a surprise to the astronomical community when a typical main belt asteroid discovered back in 1979 (called 7968 Elst–Pizarro) showed a cometlike tail when it was observed again by ground-based astronomers in 1996 at its closest point to the Sun (but still well beyond Mars). Since then, more than thirty other thought-to-be-asteroids have been observed to exhibit seemingly cometlike behavior. Astronomers call these objects “main belt” comets, if it’s clear that the sublimation of ices is involved; or, alternately, active asteroids if the main material involved is fine-grained rocky dust. Hubble got in on the active asteroid action with high-resolution images of an asteroid called P/2013 P5. Discovered by the Pan-STARRS ground-based telescope survey project in August of 2013, astronomers noted that instead of a crisp point of light, the object instead showed a fuzzy, cometlike appearance. Hubble was swung into action and quickly obtained higher-resolution images of P/2013 P5 that showed six distinct “tails” extending out from a central bright region. Repeat Hubble imaging just a few weeks later showed the tails in a very different orientation, suggesting that the asteroid is changing rapidly. More detailed analysis of the multi-tailed small body is consistent with P/2013 P5 being a rapidly spinning, “rubble pile” asteroid, meaning that the asteroid could be a collection of larger boulders and other rocky fragments barely held together by the very low gravity of such small bodies. As the asteroid spins, centripetal forces could be causing some of the rocky fragments to occasionally be ejected, lifting dust and rocks off the surface. The tiny radiation pressure force of sunlight could then stretch the lifted dust into taillike streams. Whether these kinds of active objects are called asteroids or comets is less important than what they help us to understand about the composition, physical properties, and interior structures of small Solar System worlds that are the primitive leftover building blocks of the planets.

OPPOSITE: Hubble’s WFC3 visible-light images of the approximately 1,600-foot (500-m) wide active cometlike main belt asteroid P/2013 P5. Hubble images on September 10 (left) and September 23 (right), 2013, showed dramatic changes in the six different “tails” exhibited by this small body. The tails are presumed to be caused by the shedding of dust from the object’s surface.

OUTER PLANET LOVE January 2015

The Hubble Space Telescope can resolve fine details of the surfaces and atmospheres of planets and moons in our solar system, but only infrequently. Because the primary science goals of the observatory and instruments are focused mostly on more cosmological questions that pertain to the study of the much more distant Universe, only about 5% of Hubble’s time has historically been devoted to Solar System observations. This makes it difficult to study the most time-variable planetary phenomena, like the dramatically changing atmospheres of the giant planets of the outer Solar System. In 2014, a group of planetary scientists began to collect Hubble WFC3 multi-color images of all four giant planets—Jupiter, Saturn, Uranus, and Neptune—in an attempt to better document and monitor storm activity, wind speeds, and changes in atmospheric structures and chemistry over time (in this case for two consecutive rotations, at least once every Earth year). The project, called the Outer Planet Atmospheres Legacy (OPAL) program, continues to this day. When combined with earlier Hubble images, as well as earlier or concurrent ground-based and spacecraft images, the record of weather patterns on the giant planets can be extended back many decades. For example, Hubble images have provided critical measurements needed to understand the details of the gradual shrinking of Jupiter’s famous Great Red Spot. Detailed astronomical measurements of that enormous storm system in the planet’s southern hemisphere go back more than 150 years. During that time, the storm system has steadily changed from much more oval-shaped (spanning nearly 40° of longitude) to more circular (now spanning less than 15° of longitude), and it is also drifting westward faster over time, relative to Jupiter’s 10-hour planetary rotation rate. Images and spectra from OPAL and other Hubble Jupiter-monitoring observations also show that the color of the Great Red Spot is slowly changing—becoming less red—over time, potentially due to slight changes in the distribution of clouds and hazes within the storm. If these trends continue, the Great Red Spot could instead become the Modest Beige Spot by mid-century, and perhaps even disappear completely before the year 2100.

This global map of the colorful cloud tops of Jupiter is a composite of red, green, and ultraviolet Hubble WFC3 instrument images taken at several different Jupiter rotational aspects on January 19, 2015. The original images of different whole-disk views, or “faces,” of Jupiter were converted into maps spanning 80°N to 80°S and covering all longitudes, similar to maps of the Earth made by “unrolling” our spherical geography onto a flat plane.

URANUS AND NEPTUNE: Dynamic Duo November 2018 The planets in our solar system that we know the least about are Uranus and Neptune, partly because of their enormous distances from us—they orbit some nineteen and thirty times Earth’s distance from the Sun, respectively. We know less about them also because each has been visited by only one spacecraft mission. Voyager 2 flew by Uranus in 1986 and Neptune in 1989, providing our only up-close but fleeting glimpses of those distant worlds. Given its high-resolution imaging capabilities, Hubble is capable of resolving atmospheric storms and other features on both of those planets, providing a record of changes in their atmospheres since the Voyager 2 flybys. Dedicated campaigns like the OPAL program (see Outer Planet Love, page 70) helped guarantee that at least annual high-resolution snapshots were taken of Uranus and Neptune to study their changes over time. Uranus has changed dramatically since 1986, when it appeared to be a relatively bland, hazy blue-green cue ball in Voyager 2’s visible wavelength images. The seventh planet is tilted almost 90° on its side, so it essentially rolls around the Sun rather than spins. Uranus’s extreme tilt gives it extreme seasons. When Voyager 2 flew past, it was near the southern summer solstice, which turned out to be the “hazy season” for that planet because the sunlight shines only on the southern hemisphere while the north is in complete darkness. By the mid-2000s, Uranus had moved into northern springtime/southern autumn, with a much more even distribution of sunlight between the north and south. The result has been a much more active atmosphere, punctuated with white cloud layers, small storm systems like on Jupiter and Saturn, and hazy “polar hood” clouds at high latitudes. Neptune, too, has undergone major atmospheric changes since the 1989 Voyager 2 flyby. The azure blue eighth planet’s tilt and seasons are much more Earthlike—numerous white clouds and dark storm systems were seen in the Voyager 2 images. One, dubbed the Great Dark Spot, looked similar in form but not color to Jupiter’s Great Red Spot. Hubble photos, along with other ground-based telescopic images since 1989, have shown that the Great Dark Spot has disappeared and been replaced by other dark storm systems, and that associated white “companion clouds” have come and gone over time. Planetary atmospheric scientists are still trying to figure out exactly how these storm systems grow and evolve.

OPPOSITE: Hubble’s WFC3 color composite photos of Uranus (top) and Neptune (bottom), taken in November 2018 as part of the Outer Planet Atmospheres Legacy (OPAL) observing campaign. Uranus has developed many more active storms and clouds since the 1986 Voyager 2 flyby, whereas Neptune’s storms and clouds have remained as active as during the 1989 Voyager 2 flyby but have changed significantly in size and location.

MONSTER STELLAR OUTBURST September 1995 The Sun and other stars emit prodigious amounts of energy and radiation, and this energy propagates

outward in the form of light, heat, and high-energy particles. Occasionally, the Sun erupts filaments of hot energized gas out into space. Such solar flares are common in many kinds of stars, and they can extend far beyond a star’s visible surface or photosphere. In 1837, astronomers noticed that the young giant star Eta Carinae (a star more than 100 times the mass of our Sun, located about 8,000 light-years away in the southern-hemisphere constellation Carina) suddenly brightened dramatically from a nondescript dim star to brighter than the bright stars of Orion. The star slowly dimmed to around the limit of naked-eye detection over the rest of the nineteenth century, but has brightened somewhat again during the twentieth and twenty-first centuries. Even though Eta Carinae became as bright as a supernova, it hadn’t actually exploded. Ground-based astronomers in the 1940s noticed that the star became surrounded by an oblong nebula of gas, presumably ejected in an outburst from the star during the rapid brightening a century earlier. In September 1995, Hubble got in on the action. The observations were challenging, however, because the star is more than 100,000 times brighter than the nebula surrounding it. Multiple photos were taken through red and ultraviolet filters with a wide range of exposure times—some short exposures were designed to get good exposures on the bright central star, and other long exposures intended to tease out details in the fainter nebula. The resulting composite image is stunning, and represents one of the highest-resolution photos of a star and its nearby environment ever produced from Hubble data. Dark lanes and streaks of cooling, condensing dust mingle with the glowing hot gas (illuminated from the inside by the star). The photo immediately gives the sense that a violent event is going on in Eta Carinae. Indeed, the nebula is growing over time, expanding outward at around 1.5 million miles per hour (2.4 million kph). Eta Carinae is a monster of a star, radiating around five million times more energy than our Sun. Exactly how and why it is continuing to erupt and shed material in this way is a great mystery, and is the subject of intense research.

PAGES 72–73: This giant cluster of about 3,000 stars is called Westerlund 2 and is located about 20,000 light-years away in the constellation Carina. This merged ACS and WFC3 false color composite was acquired in September 2013 and displays light from two infrared and one green wavelength filter. OPPOSITE: Hubble WFC3 false-color composite of the huge, billowing clouds of gas and dust streaming out of the supergiant star Eta Carinae, located around 8,000 light-years away in the constellation Carina. The star began erupting the gas and dust in 1837, and in the 1840s it briefly became the second brightest star in the night sky. This false-color composite was acquired in July 2018 and displays light from red, blue, and ultraviolet filters.

ECHOES OF LIGHT

February 2004 In early 2002, ground-based astronomers discovered a previously undetected and rapidly brightening star in the southern-hemisphere constellation Monoceros (the Unicorn). As the 838th variable star discovered in that constellation, the star was named V838 Monocerotis, or just V838 Mon. Follow-up observations revealed that the star had become surrounded by a fuzzy halo of light that appeared to be growing larger over time. Hubble began observing V838 Mon in much greater detail in May 2002 using blue, red, and infrared filters in the ACS instrument’s Wide Field Camera channel. Hubble images revealed a central red giant star surrounded by roughly circular and wispy arcs of gas and dust. Subsequent Hubble images show that the nebular “shell” surrounding the star is increasing in size, now many times larger than the angular diameter of Jupiter. It is natural to think of the structure surrounding V838 Mon as an expanding spherical shock wave from a central explosion, but it’s an illusion. V838 Mon is a young star, less than five million years old. As such, it is still surrounded by much of the nebular gas and dust from which it initially formed. That residual gas and dust was invisible before the star increased dramatically in brightness. But as that more-intense light shines on the surrounding nebular material, it lights it up, and some of that light is reflected off that nebula toward us. Because that reflected light has had to travel farther than the direct light from the star, it gets to us later. So, over time, the nebula appears to be expanding, as later and later reflected light echoes make their way to us. However, it’s only the light from the extreme brightening of V838 Mon that is propagating outward, not the nebula itself. V838 Mon is not exactly in our neighborhood, but is about 20,000 light-years away, which is about 20% the diameter of the Milky Way galaxy. The fact that it got so bright to be seen from so far away underscores the intensity of the star’s outburst—for a brief time, V838 Mon shone around a million times brighter than our Sun and was one of the brightest stars in the entire galaxy. The reason for the massive outburst of light from V838 Mon is still uncertain, though many hypotheses have been proposed. Was it a strange kind of stellar explosion? A nuclear chain reaction initiated by the progenitor star colliding with another star, or swallowing one or more giant planets? Future observations and computer simulations may provide more clues.

OPPOSITE: Hubble’s ACS false-color photo of the red supergiant star V838 Mon, which began to brighten dramatically in 2002. The light emitted during the star’s outbursts creates a “light echo” in nebular gas surrounding the star, which gives the illusion of an expanding spherical shell of debris around the star.

SPIRALING NEBULA September 2004 Most stars exist in binary systems, with their companion stars orbiting their common center of mass.

However, the stars in binary star systems are usually not equal in mass, so they can go through different life histories even though they are bound together by gravity. The divergent evolution of paired (or more) multi-star systems can often lead to interesting phenomena as their stars age. A prime example is the star LL Pegasi (LL Peg), in the northern-hemisphere constellation Pegasus. LL Peg is a “carbon star,” a kind of red giant that contains more carbon than oxygen in its visible outer layers. The carbon-rich atmospheres of such stars have a high amount of dust or soot, giving them a striking red coloration. Some of them, like LL Peg, are mostly hidden from view inside their dusty, sooty surroundings. LL Peg turns out to be a binary system, however, and the main evidence for the existence of both stars comes from infrared observations that reveal the ways that the two stars interact with the dusty nebular cloud emitted by the primary carbon star. Specifically, Hubble observations reveal that the dusty LL Peg system is unique because it includes a thin and nearly perfect spiral brightness pattern that winds gracefully around the central star. The spiral nature of the nebula suggests that it is formed by some sort of regular, periodic motion. Indeed, measurements of the velocity of the material in the spiral have been used to determine that each ring of the spiral takes approximately 800 years to form. The best hypothesis to explain the 800-year spacing of the spiral rings is that LL Peg has a fainter (not directly observed) binary star companion that is shedding material and plowing through the sooty nebula created by the primary carbon star. So 800 years represents the orbital period of the binary companion around LL Peg. OPPOSITE: Hubble’s ACS photo of the remarkable spiral-shaped planetary nebula (just left of center) known as AFGL 3068 around the star LL Pegasi, about 4,200 light-years away. The bright star just right of center is much closer to us and unrelated to the spiral nebula. INSET: Interpretation of Hubble’s data on the LL Pegasi system is significantly enhanced by images and other data from complementary telescopes observing in other wavelengths. For example, this image, taken by the Atacama Large Millimeter Array (ALMA) radio telescope facility in Chile, reveals stunning additional details within the spiral-shell pattern imprinted in the gas and dust surrounding the star.

LL Peg is a giant variable star (called a “Mira variable”) some 600 to 900 times the size of the Sun, with more than 10,000 times its brightness. As stars like this pulsate near the ends of their lives, they emit prodigious amounts of gas and dust that form a structure known as a planetary nebula (presumably because the material expands into the solar system immediately surrounding the star). Eventually, the central stellar

remnant left after all of the giant star’s outer layers are shed will become a white dwarf star. A similar fate awaits our own Sun, some five billion years or so from now.

PAGES 80–81: This spectacular Hubble photo captures the scattered debris from an enormous supernova explosion known as Cassiopeia A (Cas A). About 350 years ago, a massive star exploded at the end of its life, scattering its remains into surrounding space. Because this supernova remnant is relatively close to us in the Milky Way (only about 10,000 light years away), Hubble photos have been able to track the movement of fragments of hot gas and dust, some of which are moving at more than 30 million mph (48 million kph)!

GIANT PUFFING BUBBLE January 2005 Another spectacular example of a carbon star (see Spiraling Nebula, page 78) is the cool red giant star known as U Camelopardalis, or U Cam for short, located about 1,500 light-years away in the constellation Camelopardalis (the Giraffe), near the North Celestial Pole. U Cam, like many red giant stars nearing the end of the stable hydrogen-burning phase of their lives, undergoes occasional pulsations that expel the outer layers of the star’s atmosphere into surrounding space. Ground-based and Hubble imaging have revealed that in one of these spasms, relatively recently (perhaps only 700 or 800 years ago), U Cam puffed out a thin and fairly spherical shell of gas and dust that has since expanded into a tenuous bubble that is more than 4,000 Astronomical Units (the average distance between the Earth and the Sun) across. Based on stellar evolution models and observations of other carbon

stars, it is likely that U Cam blurts out such bubbles every few thousand years, and that it will continue to do so until its supply of hydrogen is completely exhausted and it begins to fuse helium in its deep interior. U Cam and other carbon stars occasionally cough up bubbles like this because they go through brief “shell helium flashes” as they begin to run out of hydrogen. These flashes occur when increasing helium abundance and pressure over time eventually lead to helium fusion in a layer around the star’s core, which makes it get hotter. As it heats, it expands in size, dredging up deeply formed elements (like carbon), lowering the interior pressure and ceasing the helium fusion. However, during the expansion, significant amounts of mass can be rapidly lost to space. Eventually, so much of the star’s original atmosphere is lost in repeated shell helium flashes that the remnant becomes a hot white dwarf star (see page 106). The intense ultraviolet radiation emitted by such remnants can ionize the surrounding gas and dust, causing the dim shells of previously blown-off layers of the star to glow in the beautiful colors of a planetary nebula. Such stars thus “seed” interstellar space with the sooty carbon and other heavy elements created during shell helium flashes, providing raw materials for giant molecular clouds, some of which will collapse to form future generations of stars that will fuse still heavier elements either in their interiors or via their explosive deaths.

OPPOSITE: Hubble’s ACS High Resolution Camera visible and near-infrared color composite of the enormous spherical bubble of gas and dust surrounding the red giant star U Camelopardalis. The central star is heavily overexposed in this photo to bring out details in the much-fainter bubble surrounding it.

Hubble’s ACS blue, green, and near-infrared composite photo of the globular cluster known as Messier 9. Hubble’s resolution enables individual stars right into the center of the cluster to be identified. In this color composite taken on May 31, 2006, redder colors signify cooler stars, while bluer colors represent hotter ones.

TOTALLY GLOBULAR May 2006 Our Milky Way, like many other galaxies, is surrounded by a swarm of densely packed collections of stars

called “globular clusters” because of their roughly spherical shape. These clusters are each composed of many millions of stars that all orbit their common center of mass, and the clusters as a whole orbit around the center of mass of the galaxy. Globular clusters represent some of the most closely spaced collections of stars in the Universe. This cluster, Messier 9, was first discovered by French astronomer Charles Messier in 1764, and was the ninth out of 110 non-stellar objects listed in his famous Catalogue of Nebulae and Star Clusters. Messier’s telescope technology could only reveal the object as a smudge, so he classified the cluster as a nebula (Latin for “cloud”). Later eighteenth-century astronomers could resolve some individual stars away from the object’s center, revealing it in fact to be a tightly packed star cluster. The Hubble Space Telescope’s superior resolution can now identify individual stars all the way to the center of the cluster. Messier 9 is about 25,000 light-years from Earth, near the center of the Milky Way. More than 250,000 individual stars can be identified in the cluster from Hubble images, and the cluster as a whole shines with a total brightness of about 120,000 times that of our Sun. Still, because of its great distance from our Solar System and relatively small apparent angular diameter in the sky (about a third the width of the full Moon), Messier 9 cannot be seen with the naked eye—it requires at least a small telescope to view. The stars in Messier 9, like those in most globular clusters, are ancient, thought to have formed in the Universe’s first generation of stars more than 12 billion years ago. These so-called “Population II” stars have very different compositions than younger stars like our Sun (a “Population I” star). Specifically, Population II stars are almost entirely composed of hydrogen and helium and other light elements formed in the Big Bang and the very earliest stages of the growth of the Universe. Population II stars lack the heavier elements like carbon, oxygen, and iron contained in more recent Population I stars because those heavier elements can only be formed when older stars die in supernova explosions or other violent end-oflife spasms that scatter such heavier elements into space.

COSMIC PEARLS December 2006 In late February 1987, astronomers around the world recorded the sudden appearance of a new bright star in the Large Magellanic Cloud, a small galaxy that orbits the Milky Way. Dubbed Supernova 1987A (SN 1987A), the star quickly became one of the few hundred brightest stars in the sky before slowly fading back into the background over the following weeks and months. At a distance of about 168,000 light-years from Earth, SN 1987A was the closest exploding star to us since “Kepler’s Supernova” was observed in 1604, so it provided the first chance to study in detail these kinds of dramatic stellar death throes using modern instrumentation. A key component brought to bear on the study of this rare natural phenomenon was the instrument suite on the Hubble Space Telescope. While the explosion of the dim progenitor blue giant star occurred before Hubble was launched, the aftereffects of the supernova continue to persist and evolve over time, providing a unique window on the effects of shock waves on interstellar gas, as well as the formation of new elements from stellar explosions. Before the supernova explosion, the massive progenitor star (perhaps 20 times the mass of our Sun) is

thought to have gone through the typical near-end-of-life star cycle, which included shedding significant amounts of its mass into nearby space as it became a red giant, then a blue supergiant. Evidence of that evolutionary history appears dramatically in Hubble images taken years after the explosion, as the supernova’s shock wave plows through the layers of gas and dust released many tens of thousands of years earlier as the star grew substantially larger. Specifically, the shock wave appears to be heating and ionizing that circumstellar material, causing blobs of it to “light up” as a series of brighter dots within bright rings about a light-year across and centered on the location of the explosion. Hubble is observing these rings brighten over time as the shock wave penetrates deeper into the surrounding gas and dust. At some point—although the specific triggering event is the subject of much debate—the blue supergiant’s core density got so high that it collapsed violently, releasing enormous amounts of energy and triggering both the death of the star and the supernova observed from Earth. Mysteriously, neither Hubble nor other observatories in other diagnostic regions of the spectrum have observed the compact stellar remnant (like a neutron star: the super-small, super-dense central star that has been stripped of protons and electrons so that only neutrons remain) that is expected to exist in the aftermath of a supernova explosion like this. The remnants of SN 1987A are expected to continue to brighten and evolve for decades, so the rest of the story is yet to be told.

Hubble’s ACS false-color photograph of the expanding ring-shaped remnant of Supernova 1987A, the closest massive stellar explosion to our Solar System in nearly 400 years.

Hubble’s ACS instrument blue, green, and infrared color composite of the massive stars WR25 (brightest star, at center) and Tr16-244 (third brightest star, just to upper left of WR25) in the star cluster known as Trumpler-16, located in the southernhemisphere constellation Carina. The bright reddish star left of center is much closer to the Earth and not associated with the other stars in the cluster.

MAMMOTH STARS: Live Fast and Die Young November 2008

What are the upper limits on the size, mass, and brightness of stars? Astronomers have learned that stars much larger and much more massive than our Sun live shorter lives and usually die in spectacular deaths (see Cosmic Pearls, page 86). But how big can they get before they reach their catastrophic final destiny? One way astronomers can search for the largest and most luminous stars is by searching for the sources that are lighting and heating nearby clouds of gas and dust. One of the largest such nearby regions is the Carina Nebula, an enormous star-forming area of gas and dust that covers a region of the southern sky more than 16 times the size of the full Moon. The Carina Nebula is intrinsically large (nearly 500 light-years across), and spans so much of the night sky because it is located only about 8,500 light-years from our Solar System. Detailed studies of bright stars in the Carina Nebula by the Hubble and other observatories have identified two specific stars called WR25 and Tr16-244 that are lighting up part of the nebula because of their super-hot nature. WR25 is a young (only a few million years old) supergiant star that is among the most luminous in the entire Milky Way galaxy, shining with a brightness somewhere between 1.5 million and 6 million times that of our Sun. The uncertainty around its precise brightness is partly related to how its brightness is diminished by being deeply embedded in a nebula of gas and dust. Along with Tr16-244 (another superhot, super-luminous young star) and others, WR25 is a member of a young star cluster known as Trumpler16. These hot young stars emit enormous fractions of their energy in the ultraviolet part of the spectrum, which heats and ionizes the surrounding gas and dust of their still-evolving stellar nursery, creating the spectacular colors and structures of the Carina Nebula. This is also partly why Hubble is so important to understanding them, because ultraviolet observations cannot be made from the surface of the Earth. High-resolution imaging, from Hubble especially, reveals that WR25 is part of a binary system and that Tr16-244 is part of a triple-star system. Surveys have revealed that such massive multi-star systems are typical among young compact star clusters like Trumpler-16, and that the exchange of stellar material among companions orbiting each other can be an important component of the evolution and ultimate demise of such stars. Colossal stars like WR25 and Tr16-244 live fast and die young, making them a challenge to study but also important benchmarks in understanding the details of stellar evolution.

YOUNG, DUSTY, AND GASSY March 2009 Stars form when giant clouds of gas and dust slowly begin to shrink and compress under their own gravity, forming compact regions at their centers where high pressure and temperature conditions can begin to initiate nuclear fusion. Various kinds of “protostars,” the precursors to full-fledged hydrogen-burning stars, form along the way, as the “protostellar” cloud evolves. One important class of protostars are known as T Tauri stars, named after the first one studied in detail in the constellation Taurus. T Tauri stars are young stellar objects (perhaps 10 million years old or younger) that are forming within still-contracting giant molecular clouds. They are variable stars with central temperatures too low for hydrogen fusion, though they still emit enormous amounts of energy and radiation from the gravitational contraction of the cloud. T Tauri stars emit more than a thousand times the Sun’s output of X-ray and radio energy, and they have powerful “stellar winds” that eject high-energy particles into their surrounding

neighborhood. After about 100 million years of such a violent youth, T Tauri stars usually settle into a more peaceful stellar life cycle like that of our own Sun, which is a traditional “main sequence” star. Hubble has observed numerous T Tauri stars and their surroundings in great detail, which provide rich information on the infancy of stars like our own Sun. A wonderful example is Hubble’s imaging of the star V1331 Cygni, a young stellar object about 1,800 light-years away in the northern-hemisphere constellation Cygnus (the Swan). V1331 Cyg is still surrounded by the disk-shaped remnants of the giant cloud of gas and dust from which it is still forming. However, we are lucky because we’re looking right down one on of the poles of the star, where a jet of gas streaming out of the star’s powerful magnetic field is clearing the surrounding gas and dust from view (see Space Geysers, page 96). Most other T Tauri stars are far more hidden from view because we see them edge-on, where they shine more dimly through their surrounding collapsing disks of gas and dust. High-resolution imaging by Hubble over many years shows changes in the arcs and clumps within the circumstellar disk around V1331 Cyg. Continued studies might some day reveal evidence for smaller objects—planets—forming within that young new solar system.

OPPOSITE: Hubble’s WFPC2 blue, green, and near-infrared color composite of the T Tauri protostar V1331 Cygni. Viewed from above one of its poles from our perspective, the star itself, and the surrounding disk of gas and dust from which it is still forming, can both be clearly seen. The helical, snail-shell–shaped arcs of the nebula reflect turbulence and the flow behavior of hot gases occurring as that material continues to fall into the growing central star.

Hubble’s ACS Wide Field Channel color composite image of the core of the globular cluster Omega Centauri. Photos of the cluster were taken from 2002 to 2009 to track the relative motions of the stars over time and to search for evidence of a black hole at the cluster’s center (evidence for which is still circumstantial).

MASSIVE OMEGA CENTAURI July 2009 One of the brightest and largest—and therefore most famous—globular clusters (see Totally Globular, page 85) is known as Omega Centauri. Located about 17,000 light-years from us in the southern-hemisphere constellation Centaurus, Omega Centauri is a collection of nearly 10 million stars that covers an angular size in the sky about the same as the full Moon. The cluster is visible to the naked eye from dark, rural areas. With a combined mass of about 4 million times the mass of our Sun, Omega Centauri is the most massive of the 150 or so globular clusters associated with our Milky Way galaxy. Hypothetical inhabitants of any planets within this cluster would experience night skies more than a hundred times brighter than Earth’s sky. Many globular clusters contain stars of essentially the same mass and age, all formed from the same initial (enormous) cloud of gas and dust. But Omega Centauri is different. Hubble’s high-resolution observations revealed its stars exhibiting a wide variety of colors, implying a wide variety of sizes and ages. White and yellow stars are typically middle-aged stars with masses not very different from our Sun. Bright blue stars are old, massive, hot stars approaching their violent (explosive) ends. Bright reddish stars are cooler and less-massive giant stars heading toward gentler endings, and dimmer reddish stars are even cooler dwarf stars that are destined to continue burning their hydrogen well into the future. Many of the stars within the Omega Centauri cluster are ancient—between 10 and 12 billion years old, dating back to the earliest few billion years of our Universe. This fact, plus the relatively younger (Sunlike) stars within the cluster, suggests to astronomers that Omega Centauri is the ancient remnant of a small dwarf galaxy that was gravitationally ripped apart by the Milky Way galaxy long ago, tearing gas, dust, and many stars out of that putative precursor galaxy, and leaving a dense and mixed population of stars behind. Hubble observations can isolate individual stars in the cluster and have been used over time to determine

their relative motions. This has resulted in a significant debate, because some interpretations of the observed stellar motions have been used to infer that there is a black hole (an extremely massive star, from which light cannot even escape; see A Monster Black Hole Gazes Back, page 136) more than 10,000 times as massive as our Sun in the center of Omega Centauri, while other interpretations have been inconsistent with that hypothesis. Additional high-resolution monitoring of the motions of Omega Centauri’s stars over time will likely be required to resolve the debate.

Hubble’s ACS visible-wavelength composite photo of southern-hemisphere variable star RS Puppis, the brightest Cepheid variable star in our skies. RS Puppis is 200 times larger than our Sun, about 15,000 times as luminous, and is embedded within a nebula of gas and dust that shimmers from the star’s reflected light.

VARIABLE STARS: Extragalactic Candles March 2010 While the overall brightness (luminosity) of most stars varies in slow, predictable ways during their long lifetimes, some also vary dramatically in this regard over relatively short periods of time. These “variable stars” are highly interesting to astronomers because their variability can often provide important insights into the physics of stellar interiors and the process of stellar evolution in general. One class of variable stars, however, provides a completely different and critically important piece of information sought by astronomers: distance. Called “Cepheid variables,” after Delta Cephei, the first such star studied in detail in the eighteenth century, these kinds of stars, which are close to the end of the hydrogen-burning phase of their lives, pulsate in a very regular way with a very predictable period, like the swinging of a pendulum. But more importantly, Harvard astronomy researcher Henrietta Swann Leavitt and colleagues discovered in the early twentieth century that the luminosity of Cepheid variables is directly proportional to their pulsation period. That is to say, if you could monitor the time it takes any given Cepheid variable star to brighten, then dim, then brighten back to its original luminosity again, you could know the star’s intrinsic luminosity. Comparing that with its observed brightness from Earth (which decreases with the square of the distance from us) tells you the distance to that star. Cepheid variables are often referred to as “standard candles” for astronomers, because they provide known brightness levels that can be scaled to estimate absolute distances to astronomical objects. Hubble has observed many Cepheid variables as a way to chart the size of the Universe. A close-to-home example is RS Puppis, a classic Cepheid only about 6,500 light-years away with a pulsation period of about forty days. Hubble has produced time-lapse animations of “light echoes” (see Echoes of Light, page 78) shimmering by reflected, pulsating starlight through the dark gas and dust nebula surrounding RS Puppis. Particularly powerful are Cepheids observed in other galaxies, which provided the earliest reliable ways to estimate the distances to other galaxies and therefore some of the earliest evidence for the vast scale of the Cosmos. Hubble’s exceptional resolution and accuracy provides a way not only to very precisely characterize the period of Cepheid variables, but also a way to detect the most distant ones in the Universe, thereby expanding our knowledge of absolute distances to extragalactic objects significantly.

SPACE GEYSERS: The Nebulosity of Youth April 2011 The Universe can be a violent place, with exploding stars, colliding galaxies, merging black holes, and other astrophysical events providing the potential for enormous releases of energy and radiation into surrounding space. Of course, stars are a major source of the most energetic astrophysical events ever observed—not just dying stars, but newborn ones as well.

In fact, an entire class of violently created astronomical objects has been associated with some newborn stars. Called Herbig–Haro Objects after the first astronomers to study them in detail, these objects are turbulent patches of ionized gas that are jetted at high velocity from young stars. As that ionized gas collides with gas and dust in the surrounding nebular cocoon that still surrounds such emerging young stars, it can heat and ionize that material as well, creating colorful and dynamic nebulosity in the young star’s neighborhood. The exceptional spatial resolution and spectroscopic capabilities of Hubble have provided important new insights on many of the more than 500 currently known Herbig–Haro objects. A prime example is Hubble’s WFC3 imaging of HH 110, a geyser of hot gas streaming out of a newborn star near the Orion Nebula, about 1,500 light-years away. The star itself is still mostly shrouded in the dense cloud of gas and dust from which it was formed. Jet-like structures like HH 110 emerge when new stars form from a collapsing cloud of gas and dust. While the exact process that forms these jets is not completely understood, the basic model is that nebular material streaming into the newly formed central star is diverted and accelerated to high velocities by the young star’s intense magnetic field. Those field lines are concentrated at the star’s polar regions, so the heated and accelerated gas and dust are “collimated” (focused, or aligned) into tight jets streaming away from the star. Many Herbig–Haro objects occur as a bipolar pair of such jets, streaming away from the star’s north and south poles. HH 110 is a rarer example of just one jet visibly streaming from a newborn star. (The other is either hidden in the surrounding nebula, or perhaps associated with another nearby Herbig–Haro object, leaving either it or HH 110 and diverted by the surrounding nebula.) Objects like HH 110 are transient, lasting only perhaps a few tens of thousands of years, and change rapidly. Hubble’s resolution thus allows us to track the evolution of these features over time, providing new insights into the process of early star formation.

Hubble’s WFC3 false-color composite photo of Herbig–Haro 110, a geyser of hot gas flowing from a newborn star. The observations were made on April 25, 2011, using near-infrared filters chosen to maximize the contrast between hot gases, nearby nebular dust, and background stars and galaxies.

BLACK HOLE SUNS March 2006–January 2014 Not all globular star clusters have central cores that are massively packed with stars (see Totally Globular, page 85). Some, like the cluster known as NGC 3201 in the southern-hemisphere constellation Vela (the Sails), consist of hundreds of thousands of stars co-orbiting a common center of mass but spread out much

more widely than other clusters. NGC 3201, which is about 16,000 light-years away and has a total mass of stars more than a quarter million times that of the Sun, can provide unique information on why some star clusters form and evolve differently than others. NGC 3201 is one of about 150 globular clusters that are gravitationally bound into orbit around the center of the Milky Way galaxy. Like most of the others, it is an ancient collection of stars, perhaps exceeding 10 billion years old, or more than 75% of the age of the Universe. But it’s an oddball cluster because it has a much faster velocity orbiting the Milky Way than the others, and it is also orbiting the galactic center backwards compared to almost all of the others. These properties point to some kind of unique origin and/or history of this cluster. Some astronomers hypothesized that NGC 3201 may have formed completely separately from the Milky Way and was later captured by our galaxy’s gravity. A problem with this hypothesis is that the chemical compositions of the stars within NGC 3201 are generally similar to those of other globular clusters orbiting the Milky Way, implying that they all formed in a similar environment and in association with the Milky Way. So the reason for the unusual speed and orbital direction of NGC 3201 remains unclear. The high-resolution capabilities of Hubble, combined with imaging and spectroscopic facilities at ground-based observatories, are helping to reveal some important clues about NGC 3201. For example, observations of the individual motions of certain stars over decades of Hubble and ground-based observations has revealed evidence for at least one black hole among the cluster’s stars—the first time such direct evidence has been found inside a globular cluster. The origins of many of the enigmatic and surprising properties of globular cluster NGC 3201 remain unclear, and so follow-on observations from Hubble and other ground- and space-based observatories will be needed to resolve these mysteries.

A Hubble combined ACS + WFC3 color-composite image of the ancient globular star cluster NGC 3201, generated from multiple-filter ultraviolet to near-infrared photographs taken between March 2006 and January 2014. NGC 3201 is in the southern-hemisphere constellation Vela, and is too dim to be seen with the naked eye.

NEW HOT YOUNG STARS May 2015

The center of the Milky Way is a busy place. We know that, like most galaxies, ours harbors a supermassive black hole (more than 4 million times the mass of the Sun; see A Monster Black Hole Gazes Back, page 136). High-resolution images from ground-based facilities and Hubble have been able to reveal details of other astronomical objects orbiting near that black hole at the galactic center. These include three extremely dense and compact young star clusters. One of those clusters, known as the Arches cluster (located in the constellation Sagittarius, the Archer), consists of about 150 massive stars that are among the brightest in the entire galaxy (plus many thousands of less-massive stars). Despite its spectacular brightness, the Arches cluster is invisible to the naked eye from Earth because it is dimmed substantially by the dense gas and dust near the galactic center. The stars in the Arches cluster are packed more closely together than anywhere else in the galaxy. Indeed, the proximity of the stars is so extreme that if it were duplicated around our own Sun, there would be more than 100,000 stars in the same volume as the space between the Sun and Alpha Centauri, the closest other star to us at 4.4 light-years away.

ABOVE: Hubble’s ACS instrument near-infared false-color composite photograph of the Arches Cluster, located some 25,000 light years away and very close to the center of the Milky Way galaxy. The Arches cluster is one of three young, massive star clusters near the galactic center that have some of the largest number of stars per unit volume in the known universe. OPPOSITE: This Hubble WFC3 false-color photo mosaic reveals a tapestry of millions of stars deep within the heart of our Milky Way galaxy. This central region of the galaxy is about 27,000 light years away and is so packed with stars that it would be like cramming a million Suns into the space between our star and its nearest neighbor, Alpha Centauri. Lurking in the very center of this dense star cluster is a black hole estimated to have a mass more than 4 million times that of our Sun.

It is unlikely that there are any inhabitants to enjoy the spectacularly bright and star-packed night skies on planets orbiting the stars in the Arches cluster—and if there are, they won’t be around for much longer in astronomical terms. Most of the stars in the cluster are extremely young (only a few million years old or less), and their intense luminosity will burn through their hydrogen fuel supply in at most a few million years more. Such hot, massive stars are destined to die in spectacular supernova explosions, which will seed their surroundings with gas, dust, and heavier elements that can go on to form new hot, young stars. But the Arches and its neighboring clusters near the galactic center (the Quintuplet and Central clusters) are not likely to be around for many more generations of new star formation. The Arches cluster is only about 100 light-years from the galaxy’s central supermassive black hole known as Sagittarius-A*. Close

encounters and gravitational interactions between the cluster stars and the black hole are likely to rip all three clusters apart in ten million years or less.

PILLARS OF CREATION April 1995 Certain photographs have become icons associated with their photographers. Think: Ansel Adams’s view

of Half Dome or Annie Leibovitz’s John and Yoko. If Hubble were to have a single representative iconic photo, it would be this one, its stunning portrait of towering columns of gas and dust embedded within the Eagle Nebula, in the constellation Serpens (the Snake). The scene is so striking, so beautifully colorful and composed, and so rich with scientific importance and meaning that it has been dubbed “The Pillars of Creation.” Relatively small and dim (barely at the limit of naked-eye vision), the Eagle Nebula was first characterized telescopically in the mid-1700s. A notable characteristic even in early observations is the occurrence of a number of dark silhouettes within the nebula, set in stark contrast to the bright reddishwhite nebular gas and numerous bright blue, white, and reddish stars scattered throughout. The nebulosity is actually associated with a loosely bound cluster of more than 8,000 stars in various stages of formation and evolution. One of those dark silhouettes seen previously only as a smudge in the nebula is what Hubble’s superior visual resolution was able to bring into sharper and more dramatic clarity. Rising up like stalagmites from the floor of an eerie cave, the pillars of cool interstellar hydrogen gas and carbon-rich dust protrude some two to four light-years beyond the interior wall of the rest of the molecular cloud. The three-dimensional serpentine outlines of the pillars are being carved and shaped by strong ultraviolet radiation and stellar winds from newly formed hot young stars embedded nearby in the nebula (off the top edge of the photo here). Denser regions of gas and dust near the tips of the pillars act in some ways like a rock in a stream, protecting some of the gas and dust “downstream” from being eroded away by the strong stellar winds. “Creation” is an apt term to describe the environment in this part of the galaxy. The dark molecular clouds contain the remains of previous generations of older stars, which are the building blocks of new stars. Almost certainly happening around most of those newly forming stars deep within the pillars and elsewhere in the nebula, a tiny fraction of gas and dust is condensing as rock and ice that doesn’t fall into its parent star, creating new planets and therefore new potential habitats for life.

PAGES 102–103: This spectacular Hubble WFC3 photo shows part of the Lagoon Nebula (also known as M8 and featured on the cover), a vast star-forming region located 4,000 light years away in the constellation Sagittarius. Much of the action is centered around the teal part of this false-color composite, where a powerful young star named Herschel 36 (which is more than 200,000 times brighter than our own Sun) is bursting out of its natal cocoon, ionizing and eroding the surrounding gas and dust with its powerful ultraviolet radiation and high-speed stellar winds. OPPOSITE: Hubble’s WFPC2 false-color photo of the Eagle Nebula, also known as Messier 16, a nearby star-formation region about 6,500 light-years away in the constellation Serpens. This color composite displays light from ionized sulfur (red), hydrogen (green), and doubly ionized oxygen (blue).

HOURGLASS NEBULA July 1995 Hubble’s increasing resolution and sensitivity over time have allowed it to reveal intricate details of distant astronomical objects, enabling astronomers to tease out their origin and evolution histories. Hubble photographs of planetary nebulae, for example (see Giant Puffing Bubble, page 82), show unprecedented detail compared to ground-based observations, and allow the physics of these beautiful structures to be modeled and understood. Hubble’s photos of the once poorly resolved nebula known as MyCn 18, for example, showed that the structure surrounding the central stellar remnant has a surprising and elegant hourglass shape. Henceforth known as the Hourglass Nebula, Hubble data show subtle details in the physical and compositional characteristics of this glowing relic of a dying Sunlike star. As average stars with masses not too different from the Sun age and begin to use up their supply of hydrogen fuel, they can begin to pulsate, expanding dramatically into red giant stars. During this process, the star sheds huge amounts of gas and dust from its outer atmospheric layers into the surrounding space. Once the hydrogen is all used up, some of these stars contract into hot white dwarf stars. High-energy radiation from these stellar remnants can then ionize their surrounding gas and dust, causing the previously shed shells of the star to glow in spectacularly colored structures. Astronomers call these structures planetary nebulae, even though they don’t have anything to do with planets. Such a fate awaits our Sun, when it expands and sheds its outer layers some 5 billion years from now. But what causes the hourglass shape and the subtle arced patterns in the walls of this stellar remnant? Theoretically, some astronomers have shown that such a shape could occur by fast stellar winds streaming through a nebular cloud that’s denser near the star’s equatorial axis than along its polar axis. The cloud can therefore expand more at higher latitudes, producing an hourglass-like pattern. However, the details of the nebula as revealed in Hubble photos do not fit nicely into this theoretical model. The hot central star, for example, is offset from the symmetrical center of the hourglass, and a second, smaller hourglass structure appears embedded within the nebula, closer to the star. As yet, no unique explanation for the arc-like patterns in the hourglass walls has emerged. One intriguing hypothesis —that the central star has an unseen but gravitationally important companion star—continues to be studied in detail.

OPPOSITE: Hubble’s WFPC2 false-color composite image of the Hourglass Nebula, a planetary nebula structure some 8,000 light-years away, in the southern-hemisphere constellation Musca (the Fly). The colors here are a combination of the light of ionized nitrogen (red), hydrogen (green), and doubly ionized oxygen (blue).

Hubble’s WFPC2 false-color photo of the Crab Nebula, the scattered remnants of a supernova that exploded in the year 1054. This color composite is made from individual images taken in October 1999 through filters designed to detect ionized sulfur (red), neutral oxygen (green), and doubly ionized oxygen (blue).

THE CRAB October 1999 In the summer of 1054 CE, Chinese and Japanese astronomers noted the appearance of a bright new “guest

star” in the night skies, in the constellation known in the West as Taurus. The star outshone everything in the sky except the Sun and the Moon. It was even visible in the daytime sky for several months, before eventually fading from view with the naked eye in the night skies after a few years. Unbeknownst to them, they had observed the first supernova explosion—the violent death of a high-mass star—in recorded history. The first telescopic detections of a nebula coincidentally around the same location as the 1054 guest star were reported in the early to mid-eighteenth century. By the early twentieth century, the first spectroscopic observations of the nebula revealed that it was expanding. By running the clock backward, astronomers determined that it must have formed around 900 years earlier. The association with the guest star of 1054 was not a coincidence: the nebula (dubbed “the Crab” in 1840 because of its shape) was created by the same event as the guest star. Not until the mid-twentieth century, when the life cycles of stars were finally understood, was it realized that the daytime guest star was a supernova—a massive star that had exhausted its supply of energy from nuclear fusion and collapsed in on itself in a catastrophic explosion. The Crab is the still-expanding remnant of that explosion, now nearly 11 light-years across. The Crab is big and relatively close (only about 6,500 light-years away), and spans about a quarter of the size of the full Moon in the sky along its long axis. Hundreds of images were taken by HST through numerous filters in 1999 and 2000 to build the highest-resolution mosaic ever made of the Crab Nebula. Orange filaments of gas represent the hydrogen-rich remains of the original progenitor star (estimated to have been eight to ten times the mass of our Sun), and the bluish inner glow is residual gas being ionized by the surviving central stellar remnant of the explosion. That surviving remnant is the ultra-dense (perhaps only 20 miles [30 km] wide, but with the mass of the Sun) core of the exploded star, spinning on its axis 30 times per second and emitting powerful pulses of gamma rays, X-rays, and radio waves. The Crab’s central star was one of the first “pulsars” to be discovered. Pulsars are rapidly rotating neutron stars whose powerful magnetic fields concentrate their radiation into narrow lighthouse-like beams that sweep across the sky.

CLOWNFACE NEBULA January 2000 Planetary nebulae (“nebulae” is plural for “nebula,” which is Latin for “cloud”) exhibit a wide variety of sizes, shapes, and colors when photographed by Hubble and other astronomical observatories. These variations reflect the compositions of the precursor Sunlike stars that initially cast the nebula’s gas and dust out into space when they became red giants, and also display the subsequent level of activity of the whitedwarf stellar remnant left behind later. A common attribute of most planetary nebulae is that they are round in nature, reflecting the relatively spherical expansion and shedding of the outer layers of the red-giant precursor star. Sometimes they are composed of multiple spherical shells, reflecting numerous episodes of mass loss into space. Those spherical structures are sometimes also warped into other shapes by the effects of stellar winds coming from the eventual white-dwarf star at the center of the nebula (see Hourglass Nebula, page 106). HST imaging of another famous planetary nebula, dubbed the Eskimo or Clownface Nebula, has

revealed new structures that appear unique to the specific circumstances involved in its creation and evolution. Discovered in the late eighteenth century by astronomer William Herschel (who also discovered the planet Uranus), the spherical nature of the Eskimo was long known. Subsequent ground-based data improved photos of the nebula’s morphology, but studying subtle details of the structure required the superior resolution of Hubble. Indeed, photos of the Clownface Nebula were among the first observations made after the December 1999 space shuttle Discovery crew’s Servicing Mission 3A upgraded many of Hubble’s capabilities. The resulting WFPC2 photos reveal the “fur parka” (from the Eskimo moniker) to contain fascinating details, such as light-year-long, cometlike streaks streaming radially away from the central star. The bright central region was also resolved into several bubbles (one in front of the other) of material being blown into space by the high-speed stellar wind. The Clownface Nebula is about 5,000 light-years away and appears to have formed about 10,000 years ago, when a dying precursor Sunlike star began shedding its outer layers into space. That material is streaming away from the star at more than 72,000 mph (115,000 kph), and is now being crashed into and ionized by the more recent 900,000 mph (1.5 million kph) stellar wind from the high-energy white dwarf.

OPPOSITE: Hubble’s WFPC2 false-color photo of NGC 2392, also known as the Clownface Nebula or the Eskimo Nebula. This color composite was created by merging images taken through filters designed to be sensitive to nitrogen (red), hydrogen (green), oxygen (blue), and helium (violet).

OPPOSITE: Hubble’s ACS/Wide Field Camera false-color photo of the Cat’s Eye Nebula (also known as NGC 6543). This composite photo was generated from images taken through filters designed to be sensitive to ionized nitrogen (red), and two different wavelengths for doubly ionized oxygen (green and blue).

THE CAT’S EYE May 2002 One of the earliest discovered—and so most extensively studied—planetary nebulae is known as the Cat’s

Eye. It was first observed in 1786 by astronomer William Herschel, who could discern few details of the faint smudge of cloudiness or “nebulosity” using the telescope technology of the day. In 1864, the Cat’s Eye became the first nebula of its kind to be measured spectroscopically, where splitting its light into dozens of different colors revealed it to be made of tenuous ionized gas. Subsequent improvements in ground-based instruments and resolution allowed more details to be teased out of the nebula’s structure (and gave the nebula its feline-invoking name), but it would take repeated higher-resolution imaging by Hubble to truly begin to understand this enigmatic astronomical object. Between 1994 and 2012, Hubble made hundreds of observations of the Cat’s Eye, covering a wide range of colors and imaging/spectroscopic observing modes to reveal details of its expansion over time. The nebula is about 3,000 light-years away toward the northern-hemisphere constellation Draco (the Dragon), and its bright inner core subtends an angle on the sky only about 1% the size of the full Moon. It is a young object, estimated to be perhaps only about 1,000 years old. Its fine-scale details, including changes in its morphology (shape), have been observed over time in Hubble photos. For example, the central region of the Cat’s Eye is surrounded by onion skin–like concentric spherical shells (bubbles) of dust that were periodically ejected from the central star at something like 1,500-year intervals. High-speed jets of gas (reddish-orange in the photo here) spread radially outward through those shells, and in places shock waves from the jets appear to be creating bunched-up knots of gas. Surprisingly, even though it looks like enormous amounts of material have been expelled into space during the death throes of the central star, the total mass of all the nebular gas and dust is probably only around 1% the mass of our Sun. The Cat’s Eye is just one of many planetary nebulae that show concentric, bulls-eye-like patterns around a hot central stellar remnant. Numerous hypotheses exist for how such patterns form, including periodic magnetic-field activity like our Sun’s sunspot cycles, periodic pulsations leading to expansion and contraction of the precursor red giant star, the effects of waves induced by outflowing stellar winds, and the effects of a binary companion star in a periodic orbit around the central star. No single answer has yet been found, and so the search for more clues with Hubble and other observatories continues.

THE HELIX (a.k.a. “The Eye of God”) November 2002 One of the closest, largest, most colorful, and most famous planetary nebulae in the sky is the Helix Nebula, also known as NGC 7293 (and sometimes colloquially as “The Eye of God”). While it is too dim to see with the naked eye, it’s among the brightest of all planetary nebulae and was one of the earliest to be discovered and studied with telescopes. The Helix is only about 700 light-years away (in the constellation Aquarius), and it subtends an angle in the sky almost the size of the Full Moon. This makes it an ideal object for detailed high-resolution photographic study by Hubble. Indeed, HST photos have revealed a complex combination of features and structures in the Helix that are unlike those seen in any other planetary nebula. The nebula’s actual 3-dimensional structure is not in the shape of a slightly elongated donut, as the mosaic would suggest. Rather, the central star is surrounded by a donut-shaped gas and dust disk, which is itself surrounded by a second disk tilted almost perpendicular to

the inner disk. Farther out, more dusty rings, gaseous arcs, and shockwave fronts surround those features. Parts of the outer rings are flattened, suggesting that they are colliding with interstellar materials as the nebula moves through space in its galactic orbit. The Helix was the first planetary nebula recognized to exhibit “cometary knots,” or clumps of nebular gas and dust with bright ionized “heads” pointed toward the central star and “tails” of darker molecular gas and dust pointing radially away. These are not actually comets (the heads are the size of our solar system!), but the analogy to gas and dust streaming away from a central stellar wind is irresistible. It is estimated that the Helix has more than 20,000 such cometary knots along the periphery of its inner disk. The origin of the Helix Nebula’s complex structure is not fully known, but one possibility is that the nebula’s central star has an as-yet undiscovered binary companion star exerting a strong gravitational influence on the system. One of the gas-and-dust disks might also be associated with the dying central star, while the other is in the plane of the orbit of the binary pair. Whatever its origin, it changes rapidly—the Helix is estimated to be only about 10,000 years old.

OPPOSITE: The Helix Nebula is so large in the sky that it would take an impractical number of Hubble photos to cover it all. Thus, to study the entire object, a mosaic of six Hubble ACS photos was combined with a wider-field mosaic from the Cerro Tololo Inter-American Observatory’s 4-meter telescope in Chile using comparable color filters.

Hubble’s ACS mosaic of the Orion Nebula, located in the northern-hemisphere constellation of Orion, just below the belt of the famous hunter. Five hundred twenty separate Hubble photos (taken through five different color filters between January 2004 and October 2005), filled in with a ground-based wide-field mosaic of periphery images, were used to create this entire mosaic, which covers approximately the same angular size of the sky as the full Moon.

MAJESTIC ORION October 2004

Among the brightest and most famous nebulae in the night sky is the Great Nebula of Orion, also known as M42. At only about 1,300 light year light-years away, the Orion Nebula is the nearest region of massive star formation. As such, it is one of the most well-studied nebulae, and its relative proximity has made it a popular nebular target for high-resolution imaging and spectroscopic observations by the Hubble Space Telescope, dating all the way back to 1990. The Orion Nebula is a textbook stellar nursery—a giant cloud of gas and dust hundreds of thousands of times the mass of our Sun, where thousands of new stars are being born. The bright region in the center is home to four of the hottest, most massive new stars. Called “The Trapezium” because they appear to outline a trapezoidal shape, these hot young giant stars (each estimated to be about 15 to 30 times the mass of the Sun, and about 300,000 years old) are emitting prodigious amounts of ultraviolet radiation, ionizing their surroundings and creating a deep and turbulent cavity in the nebula via “photoevaporation,” an acceleration of the gas and dust to such high speeds that they escape from the gravity of the nebula. Via such processes, the Trapezium stars are actually impeding the growth of hundreds of smaller stars in their neighborhood. Some even younger stars (perhaps only 10,000 years old) in the Orion Nebula are so young that they are still partially embedded within the flat, spinning disks of gas and dust from which they formed. Such protoplanetary disks (called “proplyds”) are thought to represent the typical formation environments of solar systems, including our own. The piece of the Orion Nebula at the upper left in the mosaic is actually a separate “mini” Orion Nebula (called M43) that is being lit up from the inside by just one massive Trapezium-like star. Some of this neighboring nebula appears to be influenced by the Trapezium cluster as well, exhibiting shockwave fronts and knots formed when the high-energy Trapezium stars’ stellar winds collide with the nebular gas and dust. An unexpected surprise from the super-sensitive Hubble imaging of more than 3,000 stars in the Orion Nebula was the detection of hundreds of faint red brown-dwarf stars (many seen in the bottom half of the mosaic). Brown dwarfs aren’t really stars, because they are not glowing by nuclear fusion; rather, they are warm “super-giant planets,” about 15 to 80 times the mass of Jupiter. They can be considered failed stars in a sense, but they’re nonetheless important transitional objects between the realms of the planets and the stars.

COLORFUL CARINA April 2007 The largest and brightest nebula in our night skies is also one of the least well known, because of its position in the far southern skies. The Carina Nebula (also known as the Grand Nebula or NGC 3372) spans an angle in the sky four times as large as the full Moon, and it shines fifteen times as brightly as the famous Orion Nebula (see page 117). Yet most of the population of our planet (90% of whom live in the northern hemisphere) are completely unaware of this impressive astronomical marvel because it rarely (if ever) rises above their local horizon, being at almost 60° South declination. Regardless, the Carina Nebula is one of the most dramatic and nearby (at only around 8,500 light-years

away) examples of a giant molecular cloud being studied by the HST and other observatories. Carina exhibits many examples of new stars being born and older stars dying off. The structure of the nebula is very complex, with bright regions dominated by light emitted by ionized gases, dark regions dominated by opaque clouds of carbon-rich dust, and numerous embedded stars and star-forming regions in various phases of their life cycles. The super-luminous, super-giant star Eta Carinae, for example, with its enormous billowing bilobed explosion nebula (see Monster Stellar Outburst, page 74) is a major source of heating and ionization of the Carina nebula’s gas and dust. Numerous other hot young stars are embedded within the nebula, including many in at least eight known clusters that have an enormous influence on their surroundings (see Mammoth Stars, page 89). Other important objects within the Carina Nebula include enormous “pillars” of gas and dust that can be several light-years tall and that house some of the youngest new stars in the nebula; small (but still SolarSystem–sized) protoplanetary disks called “proplyds” of gas and dust where new stars (and perhaps planets) are still in the process of forming; numerous Wolf–Rayet stars, which are among the hottest stars in the known Universe (with temperatures between 54,000°F [30,000°C] and 360,000°F [200,000°C]) that are emitting strong stellar winds and nearing the end of their (short) hydrogen-burning lifetimes; and finally numerous examples of isolated dark and relatively small nebulae containing dense regions of dust and gas known as “Bok globules,” which appear to be areas where small numbers of binary or multiple star systems are in the process of forming.

OPPOSITE: This image is part of an enormous false-color mosaic of the Carina Nebula made from a combination of Hubble ACS images taken in 2005 and ground-based images from the Cerro Tololo Inter-American Observatory in Chile taken from 2001 to 2003. Colors correspond to emissions from sulfur (red), hydrogen (green), and oxygen (blue). PAGES 120 and 121: A much wider view of most of the rest of the Carina Nebula, spanning a field of view of nearly 40 lightyears across.

CELESTIAL LANDSCAPE March 2006–July 2008 Astronomers who use the Hubble Space Telescope, as well as the engineers and programmers who interact with it, are photographers. Collectively, they must all think about how to point and shoot the observatory’s technologically sophisticated cameras (and other instruments); how to accommodate lighting and other environmental conditions; how to frame a shot or to provide visual context for its interpretation. The question then becomes: Where is the intersection of science and art when photographing the Cosmos with such a high-tech, team-driven system? One answer comes from a special program initiated in 1998 called “Hubble Heritage.” The goal of Hubble Heritage is to acquire or create (from archival images) a new, previously unseen Hubble photo every month that showcases some of the most visually stunning places in the Universe, as seen through the eyes of the most advanced telescope in history. The team of scientists and engineers at the Space Telescope Science Institute in Baltimore takes seriously the goal of creating photos that touch on the science, but that also present the Universe from an artistic perspective. The Hubble Heritage program celebrated its tenth anniversary in 2008 by releasing a spectacular sciencemeets-art landscape photo mosaic of part of NGC 3324, a star cluster in the southern-hemisphere

constellation of Carina, near a portion of the enormous Carina Nebula (see Colorful Carina, page 118). The scene evokes a depth and framing that is at once familiar from classical landscape photography—sunlight, blue sky, clouds, foreground “hills and valleys”—but at the same time is alien, in that the scale is enormous (with features that are light-years tall), the objects are fantastically far away (some 7,200 light-years distant), and the landscape imaged is one of gas and dust, not dirt, plants, and rocks. There’s plenty of scientific interest in places like this. The nebular gas and dust in this region are being heated, ionized, and “lit from within” by the intense ultraviolet radiation and stellar winds of several hot young stars (well out of the field of view here) that are forming deep within the dark molecular cloud. These stars are “evaporating” the nebular gas and creating new 3-D topography—towering hills, steep valleys, deep cavities—within the walls of the nebula. And yet, true to the aims of the Hubble Heritage project, the scene is also undeniably artistic and evocative.

Hubble’s composite image of a picturesque part of the nebula NGC 3324, within the enormous Carina molecular cloud complex. The mosaic is a combination of ACS photos taken in March 2006 and WFPC2 photos taken in July 2008, using filters designed to detect sulfur (red), hydrogen (green), and oxygen (blue).

OYSTER NEBULA November 2008 Some planetary nebulae have complex or peculiar shapes because of their equally complex interactions between stellar winds and nebular gas and dust, or because they’re affected by the gravitational and/or radiated energy effects of one or more companions to the central star (see Hourglass Nebula, page 106, and the Cat’s Eye, page 113). However, in the curious case of planetary nebula NGC 1501, nicknamed the Oyster Nebula, the gas and dust are relatively regular and well behaved—it is the central star that appears to be complex and peculiar. And beautiful. And easily visible in Hubble photos, but also studied for decades from large ground-based observatories. The central star—the “pearl” that gave the Oyster Nebula its nickname—is the leftover hot

and luminous remnant of an old red giant star that shed its outer atmospheric layers as its hydrogen supply ran out. The star must have shed those outer layers somewhat slowly and gracefully, as the resulting threedimensional almost egg-shaped blob of expanding gas and dust has a simple ellipsoidal (egg-like) shape. But lots of texture—lobes and filaments and knots and bumps—has been superimposed on that simple shape, almost certainly by the intense stellar wind generated by the central star since it began collapsing toward its eventual white dwarf destiny. This high-energy bombardment travels faster than the previously shed gas and dust, which, when it collides, results in shock waves that cause the gas and dust to ionize and expand, changing the shape of the former ellipsoidal envelope. The details of the nebula’s structure have been mapped out with the same kinds of tomography methods (using light or sound waves to map internal structure) used in medical imaging machines like MRI instruments, made possible by a combination of Hubble and other ground-based imaging and spectroscopy data. Unlike most hot planetary nebula remnant stars, the central star in the Oyster Nebula is a variable star, pulsating irregularly from brighter to dimmer over the course of minutes to hours. The reason for these irregular pulsations is not understood, nor do we know how this kind of variable star relates to other more common kinds of variable stars not associated with a planetary nebula. It is easy to speculate that the complex, multilobed structure of the Oyster Nebula has something to do with the irregular heartbeat at its center, though a solid explanation has yet to emerge. Future, more detailed studies of the Oyster and its pearl appear necessary to pry open its mysteries.

OPPOSITE: Hubble’s WFPC2 false-color photo of the Oyster Nebula, also known as NGC 1501, in the faint northernhemisphere constellation Camelopardalis (the Giraffe). The Oyster is about 5,000 light-years from us, and it glows from the high-energy ionizing radiation emitted by the “pearl”—a hot dying star at the nebula’s center.

OPPOSITE: Hubble’s WFC3 false-color composite of the Butterfly Nebula, acquired on July 27, 2009, using a combination of ultraviolet to visible wavelength filters designed to highlight the presence of oxygen, helium, hydrogen, nitrogen, and sulfur in the nebula.

BUTTERFLY EFFECT July 2009 Not all planetary nebulae exhibit the typical circular or spherical shape that initially gave the class of

astronomical objects its name. Some, like the object known as NGC 6302, more commonly known as the Butterfly Nebula, appear as distinctly non-circular, though still highly symmetrical. The Butterfly Nebula is about 3,400 light-years away, associated with a dying star in the constellation Scorpius. Though relatively close, it is also somewhat small (spanning about 10% the angular size of the full Moon), amplifying the importance of high-resolution observations made by the HST to tease out details of its structure and history. Indeed, the Butterfly was one of the first objects imaged with Hubble’s new WFC3 and COS instruments after the final space shuttle servicing mission in May 2009 (see A Final TuneUp, page 35). It served as both a beautiful and scientifically compelling target to test out the telescope’s newest camera and spectrometer. The “wings” of the Butterfly Nebula are a 2-light-year–wide pair of high-speed (more than 600,000 mph or 965,000 kph), high-temperature (about 36,000°F or 20,000°C) lobes of gas being jetted out in the death throes of one of the hottest stars in the galaxy. The jets expand into an hourglass shape as they move away from the thicker layers of dust and gas that surround the central star’s equatorial plane. Despite that central star’s extreme energy output (at a surface temperature of around 400,000°F/220,000°C), it’s mostly hidden from view behind the dark and dense donut-shaped ring of dust and gas at the center of the nebula. It took Hubble observations with the much more sensitive WFC3 instrument to finally detect the Butterfly’s central star directly. The star that formed and continues to modify the Butterfly Nebula likely started as a “normal” star around five times the mass of our Sun. As it began to use up its hydrogen, it would have expanded into a red giant star with a diameter about a thousand times that of our Sun (out to the orbit of Saturn, if that star had lived in our Solar System). The expansion cast off layers of the star’s atmosphere into surrounding space at low speed. Later, as the star contracted and heated up, more intense stellar winds plowed through and ionized that earlier-shed material, forming the turbulent knots, edges, and walls revealed in such exquisite detail in Hubble photos.

SPIRAL SCULPTURE July 2012 Most stars in the galaxy are part of binary (or more) star systems, and many within the same systems are very different from each other in mass. This makes for interesting and divergent lifespans, as the evolutionary path and ultimate destiny of a star is intimately tied to its mass. For example, as a moremassive star in a binary system approaches the end of its hydrogen-burning phase, it will go through the kinds of end-of-life experiences typical of its class. But the physical manifestation of those experiences can be significantly influenced by its lower-mass companion. This is the hypothesis that might explain the strange physical appearance of the spiral-shaped planetary nebula known as NGC 5189. Within it, colorful clouds of gas and dust are being ionized by an extremely high-temperature white dwarf central star. The previously shed gas and dust from the star’s red giant phase created the raw materials that would become a planetary nebula. But the nebula is not circular or spherical like many others (see The Helix, page 114), or even symmetrically bilobed (see Butterfly Effect, page 127). Instead, the planetary nebula surrounding NGC 5189’s central star is made up of two complex nested

structures, tilted with respect to each other and expanding away from the central star in different directions. The fusion of those two 3-dimensional structures onto the 2-dimensional plane of the sky creates the illusion of an S-shaped spiral that is stwisting through the overall structure. Adding to its beauty and complexity, winding through the structure is a bright golden ribbon of ionized gas with filaments and “cometary” knots pointing radially away from the central star. That ionized gas and those filaments and knots are a manifestation of the high-energy erosion and sculpting of the nebula’s gas and dust by the intense stellar wind from the central star. The doubled-up structure of NGC 5189 suggests two separate bipolar flows emerging from the center, hinting at not one but maybe two stars shaping the nebula. And while a second central star has not yet been identified by Hubble or other telescopes, its gravitational influence, and possible outflows of its own— assuming it is of comparable age to its companion—might solve the mystery of the nebula’s unique spiral structure.

OPPOSITE: Hubble’s WFC3 false-color photo of the spiral-shaped planetary nebula known as NGC 5189, located about

1,800 light-years away in the constellation Musca (the Fly). The color composite here shows variations in the abundances of ionized oxygen (red), nitrogen (green), and sulfur (blue) in the nebular gas and dust.

Hubble’s WFC3 false-color photo of the “head” of the famous Horsehead Nebula, a turbulent cloud of gas and dust embedded deep within the Orion Nebula, some 1,500 light-years from Earth. This composite photo was made using filters sensitive to infrared (heat) energy emitted by the ionized nebula.

HORSE OF A DIFFERENT COLOR November 2012 Human vision is tuned to what we call “visible light.” The colors that our eyes—and the eyes of most other creatures on this planet—have evolved to see are (unsurprisingly) close to those primarily output by our

local star, the Sun: from red to violet, the colors of the rainbow. But the rainbow has lots of colors bluer than blue and redder than red.

INSET: This visible light ground-based telescope photo of the Horsehead shows the nebula in its more iconic form, as a shadowy silhouette set against a backdrop of brightly glowing ionized gas and dust.

Hubble is especially tuned to detect the bluer-than-blue colors, known as “ultraviolet,” because they’re absorbed by Earth’s atmosphere and thus can’t be detected by ground-based telescopes. Some redder-thanred colors, known as “infrared,” are difficult or impossible to detect from ground-based telescopes, which is why Hubble’s Wide Field Camera 3 was designed to take photos through some infrared filters as well. Infrared vision can reveal features and processes in astronomical objects complementary to visible and ultraviolet vision, which is why many telescopes, like the future James Webb Space Telescope (see page 190), are optimized to detect infrared light. Many common and famous astronomical objects look quite different in infrared than they do in visible light, a prime example being the famous Horsehead Nebula (see page 130). A small region of turbulent ionized dust and gas within the much-larger Orion Nebula (see Majestic Orion, page 117), the Horsehead Nebula is well known from introductory astronomy textbooks because of its iconic dark shadowy silhouette of a horse’s head set against the bright colors of Orion’s ionized gas and dust. However, in infrared the Horsehead takes on an entirely new color, glowing in ethereal transparent hues against the background stars, seemingly emerging from foamy whitecaps on an interstellar sea. Of course, such a poetic description belies the scientific reality of the situation: the Horsehead is an enormous pillar of hydrogen gas laced with visually opaque dust and organic molecules. New stars and solar systems are being born within a giant molecular cloud (and slowly eroding/dissipating it), and infrared vision can pierce some of its dusty coating to give us a closer look at the spectacle within.

Hubble WFC3 false-color composite photo of the Monkey Head Nebula, also known as NGC 2174. Infrared filters were used to collect light corresponding to different ionized chemical elements in the nebular gas. The part of the nebula photographed here is about 5 light-years across.

MONKEY HEAD February 2014 A common form of illusion is called “pareidolia,” the tendency to see something known or familiar to an observer in inanimate objects or abstract patterns. Famous examples include the Man in the Moon, animals in clouds, and the Face on Mars. The human eye–brain combination seems geared to recognizing familiar patterns from complex and colorful abstract forms. A prime example from deep space is a nebula around 6,500 light-years away in the constellation Orion.

The nebula is officially known as NGC 2174 but is more commonly called the Monkey Head because of what people see in the gas-and-dust cloud at first glance. But that’s where the comparison ends. The Monkey Head Nebula is a massive interstellar molecular cloud that is being simultaneously ionized and evaporated by some of the hot new stars created from its raw materials. The sharp-edged features along the right-hand side of the nebula seen in the photo opposite are being pummeled by high-energy stellar winds from embedded new stars in the left part of the nebula. Young hot massive stars forming from this molecular cloud (up to 30 times the mass of our Sun and more than 5 times its temperature) emit prodigious amounts of high-energy ultraviolet radiation into the space surrounding them. This energy ionizes the nearby gas and dust and causes it to glow, while at the same time intense stellar winds coming off such stars shape and erode the nebula. In some places, like in the knotty, clumpy “globules” of dark nebular materials along the right side of the Monkey’s profile, pressure and shock waves from the intense stellar wind front could be collapsing the nebular gas and helping to trigger the formation of new stars. Out with the old, in with the new. Such new stars deeply embedded in their nebular nurseries are hidden from view at visible wavelengths but can be detected in the infrared from the heat they’re giving off. Over time, those stars will evaporate and/or blow away their dusty cocoons and shine forth in visible light as well.

A MONSTER BLACK HOLE GAZES BACK December 1998 The unprecedented resolution and other capabilities of the Hubble Space Telescope have revolutionized our

understanding not only of astronomical objects in the Milky Way galaxy, but also some of the hundreds of billions of galaxies beyond our own. A prime and important example is Hubble’s imaging and spectroscopic observations of the giant elliptical galaxy known as M87, a behemoth collection of hundreds of billions (possibly as many as several trillion) of stars some 50 million light-years away. M87, also known as NGC 4486, is near the center of the Virgo cluster, a collection of about 2,000 galaxies loosely bound together by their mutual gravity. That cluster is part of a larger gathering of about 100 such clusters called the Virgo supercluster (of which our Milky Way is a member). Ground-based astronomers have been studying M87 since the early twentieth century, noting a fuzzy linear feature protruding from the galaxy’s center. Also of note is that M87 and its fuzzy feature are among the brightest sources of radio emission in the entire sky. So something strange and powerful is going on at M87. Hubble’s superior vision revealed that linear feature to be a strong jet of electrons and other subatomic particles streaming out of the center of M87 at close to the speed of light. While the billions of stars and globular clusters that comprise M87 can only be seen by HST as an unresolved yellowish blur of light, the jet itself reveals structure and details that help clarify its origin. The leading hypothesis was that the jet is a high-energy outflow propelled by the intense magnetic fields around a supermassive black hole—a star more than 2 billion times the mass of our Sun—at the center of M87. As gas and dust swirl into that maelstrom, they are ionized and concentrated by the black hole’s strong magnetic fields. Those tightly twisted field lines shoot this plasma out along the polar axes of the central star, similar to the bipolar jets coming from some planetary nebula central stars (see Butterfly Effect, page 127). PAGES 134 and 135: This false-color composite, composed from multiple filter observations from Hubble’s WFPC2, ACS, and WFC3 instruments, captures the spectacular spiral galaxy Messier 106, located about 20 million light years away in the constellation Canes Venatici. OPPOSITE: Hubble’s WFPC2 false-color photo of the galaxy known as M87, a member of the “nearby” (50 million lightyears away) Virgo cluster of galaxies. The jet of high-speed particles streaming out of the black hole thought to be at the center of this galaxy appears bluish in this ultraviolet, green, and infrared color composite. BELOW: In April 2019, the Event Horizon Telescope radio astronomy project announced the first-ever direct detection of a black hole. The dark spot here is actually the shadow of M87’s black hole, projected onto its superheated disk of falling gas and dust.

The philosopher Friedrich Nietzsche once wrote: “ . . .if you gaze long enough into an abyss, the abyss will gaze back into you.” Sure enough, in 2019 a team of astronomers announced, after years of gazing and

number-crunching, that they had directly imaged the black hole at the center of M87 using a worldwide network of radio telescopes. So, in a sense, the monster black hole at the heart of M87 gazed back.

WHEN GALAXIES COLLIDE April 2002 When planets, asteroids, or comets collide, the results can be spectacular. The same can be said for when stars merge or crash together—the ensuing release of energy is extremely impressive. But when entire galaxies collide, the results can be truly, astronomically incredible. A beautiful and illustrative example of this comes from Hubble photos of a galaxy known as UGC 10214, a spiral galaxy about 420 million lightyears away in the constellation Draco (the Dragon). It has a long, curious tail, which has earned it the

nickname the Tadpole. The Tadpole is a distorted spiral galaxy trailed by a 280,000-light-year–long streamer of bluish stars that are too distant to be resolved, even with Hubble’s amazing vision. That star trail appears to be the wreckage left behind by a galactic “hit-and-run” driver—a smaller, bluer, more elliptical galaxy passing through the upper left corner of the Tadpole in an attempt to escape. And it’s not clear that it will—the Tadpole’s stronger gravity has already ripped much of the smaller galaxy apart. Its eventual fate might be total disintegration, as its stars are captured by the Tadpole’s gravity, or scattered to wander through intergalactic space. While it’s easy to imagine such a collision as a catastrophic event, the direct carnage is likely to have been minimal—the spaces between individual stars within the “colliding” galaxies is vast. Rather, gravity is what wreaks havoc on the systems, tearing apart the galaxies’ elegant pre-existing structures. On the other hand, gravity also acts as an agent of creation in such collisions because it forces clumping, merging, and mingling of interstellar clouds of gas and dust that subsequently collapse to form new stars. These massive newly formed stars, up to ten times hotter and a million times brighter than our Sun, are partly what makes the trail of collisional debris (like the Tadpole’s tail) glow bluish in this color composite. Millions of years from now, the stars in the tail of the Tadpole, and in whatever remains of the smaller colliding galaxy after it passes all the way through, could become gravitationally bound into clusters or mega-clusters (the tail is already clumpy with groups of stars). These could end up being globular clusters or small “satellite” galaxies that orbit within a diffuse “halo” around the main Tadpole, similar to the swarm of globulars and satellites that orbits the Milky Way. Indeed, the presence of these satellites around our own galaxy means that perhaps the Milky Way has also been the victim of such hit-and-run collisions in the past.

OPPOSITE: Hubble’s ACS photo of UGC 10214, nicknamed the Tadpole Galaxy because of its long, tail-like streamer of stars. The tail appears to be the ripped-off remains of a smaller galaxy that is in the process of colliding with the Tadpole. This false-color composite was created from images taken through blue, orange, and near-infrared filters.

RADIO GALAXIES March 2003 Astronomers believe that the first kinds of galaxies that formed in the early Universe, perhaps 10 to 12

billion years ago, were elliptical galaxies—massive clusters of stars formed from the collapse of huge masses of interstellar hydrogen and helium formed in the Big Bang. Many of these ancient elliptical galaxies emit enormous amounts of radio wavelength energy, suggesting that they harbor supermassive black holes deep within their cores. One such example of a bright “radio galaxy”—the fourth-brightest source of radio emissions in the sky —is the giant elliptical galaxy known as NGC 1316, also called Fornax A because it is located in the southern-hemisphere constellation Fornax (the Furnace). HST photographed Fornax A at high-spatial resolution in 2003 and revealed evidence for mysterious dark lanes of dust and bright clusters of star formation that are consistent with the unique and unusual history of this galaxy. Specifically, astronomers believe that the distinctive features of Fornax A are consistent with it being a merger of two or more separate gas-rich galaxies that collided billions of years ago. One of the brightest elliptical galaxies in a cluster of similar astronomical objects called the Fornax Cluster, Fornax A is about 75 million light-years away. While the parts of Fornax A visible to Hubble span about a third of the angular size of the full Moon, the radio “lobes” of the galaxy extend much farther out. Ground-based imaging of these extended regions of Fornax A reveal a variety of ripples, loops, and plumes of gas and dust that hint at a violent past. Indeed, the dark patches of dust seen in the Hubble images of Fornax A have been interpreted to be the remnants of giant molecular clouds associated with the precursor galaxies that might have been swallowed up by Fornax A. Hubble’s incredible resolution allows astronomers to study the halo of faint globular star clusters— millions of stars bound to each other gravitationally—that swarm around elliptical galaxies like Fornax A. Some of these clusters are extremely massive, others less so. The clusters within the inner regions of Fornax A generally tend toward the massive side, suggesting that the lower-mass clusters were more easily scattered out of the galaxy during the hypothesized earlier galactic collision. While it’s hard to know for sure exactly what happened, Fornax A is different enough from typical giant elliptical galaxies that many astronomers are trying to solve the mystery.

Hubble’s ACS Wide Field Camera composite photo of the giant elliptical galaxy NGC 1316, also known as Fornax A because it is a bright radio source in the southern-hemisphere constellation Fornax. This false-color composite was created from images taken through blue, green, and infrared filters.

SOMBRERO GALAXY June 2003 Among the most visually stunning and beautifully symmetrical galaxies photographed by the Hubble Space

Telescope is the object called Messier 104 or NGC 4594. Known colloquially by its nickname the Sombrero Galaxy, this photogenic collection of hundreds of billions of unresolved stars is located about 28 million light-years away, in the constellation Virgo. The Sombrero’s most famous feature is the “brim” of the Mexican hat—a 50,000-light-year–wide lane of dust that girds the circumference of the galaxy, surrounding a more diffuse and elliptical halo of stars that orbit the brilliant white galactic center. In visible light, like in this Hubble photo, the dark band is opaque and prevents additional details from being discerned within the equatorial plane of the Sombrero. However, in infrared observations from the Spitzer Space Telescope, the Sombrero’s band was revealed to be a much more massive ring of dust that encircles the halo of stars. The Sombrero is a giant elliptical galaxy, estimated to contain the mass equivalent of 800 billion of our Suns. Hubble can resolve some 2,000 ancient (10 to 13 billion years old) globular star clusters orbiting the Sombrero, which is nearly 10 times the number of such clusters that orbit our Milky Way galaxy. The brilliant white core of the Sombrero is a prodigious emitter of X-rays, causing astronomers to speculate that a billion-solar-mass black hole—among the largest in the Virgo cluster of nearby galaxies—lurks in the Sombrero’s heart. We see the Sombrero nearly edge-on in the sky, and some early astronomers thought the object was likely a planetary nebula with a dusty disk surrounding a young star. But early in the twentieth century, it was discovered that the Sombrero, like so many other “nebular” objects, is moving away from us at an enormous velocity. This became an important clue that the Sombrero is actually a distant galaxy of its own, appearing to recede from us because of the expansion of the Universe. Spectroscopic observations have revealed that the Sombrero’s nearly edge-on symmetrical ring that encloses the bulge of the galaxy is composed mostly of cold atomic hydrogen, other cold molecular gases, and dust. Hubble’s exceptional resolution reveals knots and clumps in this dark, dusty ring, and infrared data indicate that, despite the extreme age of this galaxy, there is likely still significant new star formation going on within those knots and clumps.

Hubble’s ACS High Resolution Camera natural-color composite (through red, green, and blue filters) mosaic of the Sombrero Galaxy in the constellation Virgo. The Sombrero is 28 million light-years away but still spans an angular size in the sky of about 20% the size of the full Moon.

This false-color photo of part of a region of the Small Magellanic Cloud known as the “Wing” is a composite of green and blue colors from Hubble ACS Wide Field Camera images, purple colors from X-ray data from the Chandra X-ray Observatory, and red colors from infrared data from the Spitzer Space Telescope.

SMALL MAGELLANIC CLOUD July 2004 The Milky Way, like many other galaxies, travels through space along with a number of smaller companion

or “satellite” galaxies. Two of these are large and bright in the southern sky, bright enough to be seen with the naked eye. In fact, these two satellite galaxies have been named the Large and Small Magellanic Clouds, after the first European explorer to notice them, Ferdinand Magellan. The Large Magellanic Cloud covers an angle of more than 10° in the sky, or more than 20 times the size of the full Moon. The Small Magellanic Cloud is about half that angular size in the sky. The Magellanic Clouds are dwarf irregular galaxies, most likely disrupted into their irregular shapes by gravitational encounters with their large neighbor, the Milky Way. Because they are relatively nearby (less than 200,000 light-years away), it is possible to use the outstanding spatial resolution and other capabilities of Hubble to study extragalactic processes within them in greater detail than in more-distant galaxies. A picturesque and scientifically compelling example comes from HST’s imaging of a star-forming region in the Small Magellanic Cloud called the “Wing” (because of its shape). This region of the Small Magellanic Cloud is generally less dusty and gaseous, with fewer stars per unit volume, than the Milky Way—characteristics typical of irregular dwarf galaxies. The stars in the Wing also have higher ratios of hydrogen and helium compared to heavier elements than typical spiral-galaxy stars. Astronomers label these kinds of stars as having “low metallicity,” since many of them consider elements heavier than hydrogen and helium to be “metals.” A surprising discovery, made by Hubble’s space-telescope cousin, the Chandra X-ray Observatory, is that some of these young stars, even some that are modestly average in mass like our own Sun, are emitting strong X-ray radiation into space. This was the first time X-ray emissions from young stars had been detected outside the Milky Way galaxy. Hubble’s ultraviolet and visible studies of these stars, combined with Chandra X-ray data and infrared data from the Spitzer Space Telescope, paint a picture of some of these X-ray–emitting stars being extremely young, perhaps a few million years old or less, with levels of magnetic field activity that might be somehow related to their low metal contents. Not only are such stars seemingly typical in dwarf galaxies like the Magellanic Clouds, they could be typical of young stars everywhere in the early Universe’s first generation of stars.

BARRED SPIRAL September 2004 One of the reasons the Hubble Space Telescope is named after early twentieth-century astronomer Edwin Hubble is to recognize his pioneering work in the classification of galaxies. Hubble classified galaxies in four main styles: elliptical, spiral, barred spiral, and irregular. And, aside from irregular galaxies, there is a continuous blend of subtypes among the three other styles. A textbook example of a barred spiral galaxy in Hubble’s classification scheme is NGC 1300, a galaxy roughly 61 million light-years away in the southern-hemisphere constellation Eridanus (the River). NGC 1300 has a remarkably straight central bar of stars that passes through the galaxy’s bright white central core. Our own Milky Way galaxy is thought to have a bar like this close to its center. A particularly photogenic aspect of NGC 1300 is the way its spiral arms curl gently, and symmetrically, outward from the central bar. The scene evokes both elegance and the fundamental physics of the laws of gravity at the same

time. While the bar and central regions of NGC 1300 are dominated by the yellowish to red colors of middleaged to older stars, the galaxy’s outer spiral arms contain numerous clusters of younger, bluer stars, as well as clumps and knots of nebulosity where even younger stars are being formed. Hubble’s photo of NGC 1300 provides unprecedented detail on the structure of the central bar and outer spiral arms of this classic galaxy type. This includes subtle details of clumpy dust lanes within the outer parts of the spiral arms and central bar where mostly older, orange-red stars appear to reside. Those dust lanes dissolve into a tight central spiral structure of stars close to the bright central nucleus of the galaxy, which is a spiral within a spiral. Some models of the evolution of such galaxies suggest that these central spirals could help to feed supermassive black holes in their centers (though evidence for such a black hole within NGC 1300 has yet to be found). Barred spiral galaxies appear to be more common in the Universe today than they were when the first galaxies formed some 10 to 12 billion years ago, suggesting that the development of bars might be an evolutionary trait that some galaxies evolve into as they mature.

Hubble’s ACS Wide Field Camera view of the textbook barred spiral galaxy NGC 1300, located some 61 million light-years away in the constellation Eridanus. The galaxy is about 110,000 light-years in diameter. This photo is a natural-color composite created from ACS images taken through red, green, and blue filters.

Hubble’s ACS false-color photo of the spectacular nearly face-on spiral galaxy called the Whirlpool (left of center), with its smaller companion galaxy NGC 5195 (upper right). The color composite was created from images taken through nearinfrared, green, and blue filters.

THE WHIRLPOOL January 2005 A spiral is one of nature’s purest, and also most common, forms. From the growth of a snail’s shell to the bands of clouds in a hurricane to the wound-up lanes of stars in massive galaxies, spirals are ubiquitous. And among spiral-shaped galaxies, few can match the beauty and elegance of the Whirlpool galaxy, so named for its elegant swirling structure. The Whirlpool galaxy was discovered by noted astronomer Charles Messier in 1773, while he was

surveying the skies for non-stellar objects. He designated the “nebula” as Messier 51 in his famous catalog of astronomical objects (see page 85). By the mid-1800s, larger telescopes had enabled astronomers to discover that this object had the first spiral structure ever observed in a nebula. It wasn’t until the 1920s when Edwin Hubble and others began to catalog Cepheid variable stars (see page 95) in some of these spiral nebulae that astronomers finally realized that objects like the Whirlpool are actually galaxies of their own, “island universes” roaming the vast and expanding cosmic sea. The Whirlpool has been observed frequently by HST, partly because it’s relatively close to us as galaxies go (only about 31 million light-years away, in the constellation Canes Venatici, or the Hunting Dogs), and so Hubble’s exceptional resolution can resolve unprecedented details in the spiral arms and other parts of the galaxy’s structure. Hot young star-forming regions, for example (seen in this Hubble ACS composite photo as hot pink in color), are concentrated within the galaxy’s spiral arms. The motions of the gas and the many visible clumps and lanes of dust in those arms as they orbit the galactic center create compressive forces that trigger the gravitational collapse of hydrogen gas, which then forms clusters of new stars that show up as bright blue in this image. In contrast, the central region of the Whirlpool glows with the yellowish hues of older stars. For the past few hundred million years, the Whirlpool has had a companion, a smaller, dustier, and more elliptical galaxy called NGC 5195 that has been passing close to and behind (from our perspective) the Whirlpool, perhaps playing a gravitational role in stretching and shaping the giant galaxy’s spectacular arms. This dwarf galaxy is heading away from the Whirlpool, but still appears to be giving one last tug on the larger galaxy’s longest arm.

STARBURST GALAXIES May 2005 Some galaxies are dominated by old, highly evolved stars, but in others dramatic numbers of new stars are still being born. The most prodigious of these galactic-scale stellar nurseries are called starburst galaxies, where new stars are bursting on the scene almost constantly. A prime example, studied in detail by Hubble, is the beautiful face-on starburst galaxy known as Messier 94 (M94), located about 16 million light-years away from Earth in the small northern-hemisphere constellation Canes Venatici (the Hunting Dogs). New stars are forming within M94 at a frantic pace, faster than in most other known spiral galaxies. Many of them are forming within a bright bluish “starburst ring” around the periphery of the galaxy’s magnificent spiral arms, but deep within the galaxy’s core is a second, smaller ring of intense star formation. Each bright blue dot in this stunning photograph represents a cluster of hot, massive, young stars that are emitting huge amounts of ultraviolet radiation, ionizing or totally evaporating the residual gas and dust around them, from which they came. We’re still not sure why so many new stars form within the inner and outer rings of this galaxy and so few are created within the central part of it outside the core (which is dominated by older, yellowish stars and darker lanes of dust and gas). One hypothesis is that M94’s inner starburst ring is somehow spurred by the galaxy’s central bar of stars. A rotation of the compact propeller-like cluster of stars could be creating

waves of compression that cause the gas and dust near the galaxy’s center to more easily collapse into new stars. The origin of the bright blue outer starburst ring is also the subject of much attention. Some astronomers believe that a close gravitation encounter with another galaxy, possibly even a merger with a smaller satellite galaxy, supplied the energy and materials that created a spike in new star formation. Others think that pressure waves from the galaxy’s central bar might be propagating outward, like waves on a pond, triggering the collapse of clouds of gas and dust in the outer spiral arms, where those waves “break.” While the detailed reasons for M94’s spectacular starburst behavior are still not resolved, one thing is certain: Hubble photos are revealing not just its beauty, but also the inner workings of this spectacular starmaking spiral.

OPPOSITE: Hubble’s WFPC2 color mosaic of the center of the spectacular starburst galaxy M94, located about 16 million light-years away from us in the constellation Canes Venatici. This false-color composite was created from separate ultraviolet, visible, and near-infrared filter images. PAGES 152 and 153: Hubble WFPC2 photo of the grazing impact of two spiral galaxies, NGC 2207 (left) and IC 2163 (right).

CIGAR GALAXY March 2006 Starburst galaxies are enormous collections of stars, gas, and dust where new stars are being formed at rates dramatically higher than average galaxies (see Starburst Galaxies, page 150). While starburst galaxies are often recognized by clusters of bright bluish young stars being born in their spiral arms and/or cores, it’s easiest to see this if the galaxies are closer to Earth or “face-on,” so the details of their structure can be more easily discerned. However, galaxies in other orientations or those with other unusual or confusing features are hard to identify as starburst regions. A prime example, studied in detail by Hubble, is the galaxy known as Messier 82 (M82). Known as the “Cigar Galaxy” because of its long elliptical shape, M82 is located about 12 million light-years away in the constellation Ursa Major (the Great Bear). While first thought to be an elliptical or irregular galaxy, more recent infrared observations have revealed M82 to be a spiral starburst galaxy that we see almost edge-on. Dusty lanes and extended clumpy filaments of hydrogen being blasted out of its central core obscure parts of the bright bluish galactic disk, frustrating earlier attempts at confirming its true structure. M82 is also big, spanning nearly a third of the diameter of the full Moon in the sky. It’s also intrinsically

bright, with more than five times the total luminosity of the Milky Way, and its central core region is stunningly energetic, accounting for much of the starburst activity. The intense stellar winds and magnetic fields from those young stars ionize and compress the surrounding gas and dust, helping to form even more new stars. Star formation in the central core of M82 is ten times faster than in the center of our own galaxy. Ultimately, though, M82’s pace of star formation is not sustainable—eventually, the “raw materials” will be used up. Only after a significant number of these new stars die and scatter their remains back into space will the raw materials for more new stars become available once again. M82 is part of a cluster of galaxies in Ursa Major known as the M81 cluster, named after its largest member. Indeed, the giant spiral galaxy M81 is a near neighbor of M82, and appears to have gravitationally interacted with M82 hundreds of millions of years ago, warping M81’s disk and sending compressional waves through its structure. These waves might have also been the trigger for an extreme but temporary epoch of star formation within M82.

Hubble’s ACS false-color photo of the starburst galaxy known as Messier 82, also known as the Cigar Galaxy. Starburst galaxies undergo exceptionally high rates of new star formation, and M82 is no exception—new stars are forming there at a rate more than 10 times that of our own Milky Way galaxy. This color composite was created from images taken through near-infrared, green, and blue filters.

Hubble’s WFC3 photo of the spiral galaxy NGC 2841, a classic example of a “flocculent” (fluffy) spiral galaxy with patchy and dusty spiral arms and a relatively low rate of new-star formation compared to other types of spiral galaxies. NGC 2841 is located about 46 million light-years away in the constellation Ursa Major.

A FLOCCULENT SPIRAL January 2010 Spiral galaxies share a number of distinguishing features. These include a relatively flat disk of young and old stars, interstellar gas, and dust organized into multiple spiral arms, and a central bright bulge. They can also possibly feature a bar of mainly older stars, a supermassive black hole at their center, and a nearspherical halo of stars, including multiple globular clusters surrounding the central bulge and arms. What separates one spiral galaxy from another is their relative size, mass, and brightness of these parts.

A specific kind of spiral galaxy that interests astronomers is known as a “flocculent” (or fluffy) spiral galaxy because they exhibit patchy, discontinuous spiral arms. Star formation in these kinds of spirals is thought to be this way because it’s triggered by the essentially random nature of shock waves and interstellar gas compression caused by individual supernova explosions and high-intensity stellar wind outbursts. These events cause localized compression and collapses of interstellar gas and dust, leading to the patchy nature of a flocculent galaxy’s spiral arms. An ideal example of a flocculent galaxy is NGC 2841, located about 46 million light-years away from us in the constellation Ursa Major (the Great Bear). Spiraling just outward from the galaxy’s bright central core, Hubble’s high-resolution WFC3 images reveal opaque lanes of dust, intermingled with the yellowishwhite glow of primarily older stars. Farther out from the core, the bluish glow of brighter young star clusters traces out of the galaxy’s spiral arms. These hot young stars have apparently cleared out (via strong ultraviolet radiation and stellar winds) much of the typical gas and dust normally seen in new star-forming regions within spiral galaxy arms, which could explain why NGC 2841 currently has a very low rate of new star formation and doesn’t exhibit the pinkish glowing nebulae of other spirals, like the Whirlpool Galaxy (see page 149). Infrared imaging of NGC 2841 by the Spitzer Space Telescope is able to penetrate much of the dark dust that blocks Hubble’s view of the inner regions of this galaxy’s core and reveals that the innermost spiral arm actually forms a complete ring around the galaxy’s nucleus.

ACTIVE GALACTIC NUCLEI July 2010 While the dense, star-packed cores of all galaxies are active places, it takes an extreme amount of activity for a galaxy’s central core to qualify as an “active galactic nucleus,” or AGN. Specifically, AGNs are places where there is so much energy and luminosity that it’s impossible for stars alone to cause the observed level of activity. Rather, AGNs are places where astronomers believe that huge amounts of gas and dust, perhaps even entire stars, are falling into supermassive black holes—monsters with a mass from a million to ten billion times that of our Sun. The nearest active galactic nucleus to the Milky Way is about 11 million light-years away in the center of a galaxy known as Centaurus A in the southern-hemisphere constellation Centaurus. Centaurus A is a giant elliptical galaxy with a visible bulge and a central disk crisscrossed with dark lanes of dust measuring more than 60,000 light-years across and covering an angular area of the sky just a little smaller than the full Moon. Centaurus A is also a strong emitter of X-rays and radio wave jets, some that stretch more than a million light-years away from the AGN. Centaurus A is a starburst galaxy (see page 150) producing huge numbers of hot young stars and bluish star clusters, most of it happening near its central core, which is believed to harbor a supermassive black hole more than 50 million times the mass of the Sun. Ground-based photos of the full extent of Centaurus A reveal its equatorial disk to be not perfectly flat but warped. This suggests that the galaxy may have interacted, or even collided and merged, with another galaxy long ago. Its past interaction may have even been the trigger for the current burst of star-formation activity in Centaurus A, from the shock waves and

tidal forces between merging molecular clouds of gas and dust spurring the eventual collapse of those clouds into the spinning protostellar disks from which new stars form.

Hubble’s WFC3 false-color photo of the giant elliptical galaxy known as Centaurus A. At a distance of only 11 million lightyears, Centaurus A contains the nearest supermassive black hole to our own galaxy. The colors are a combination of individual measurements made from ultraviolet, visible, and near-infrared filters.

OPPOSITE: Hubble’s WFC3 false-color photo of the “peculiar” galaxies known as Arp 273, a pair of spiral galaxies whose gravitational interaction is causing warping and twisting of their spiral arms. Arp 273 is located around 300 million lightyears away in the constellation Andromeda. The photo is a composite of ultraviolet, blue, and red filters.

MOST PECULIAR GALAXIES December 2010 In the mid-1960s, American astronomer Halton Arp published an atlas of 338 galaxy photographs he

dubbed “peculiar” objects because of their unusual appearances compared to “normal” galaxies. Dozens of galactic peculiarities were cataloged by Arp, including spirals with split or detached arms; ellipticals connected to spirals; galaxies with filaments, tails, fragments, and other detached parts; as well as multiple galaxies that appear to be interacting with each other. Arp’s catalog is a delightful sampling of oddball members of the cosmic zoo of galaxies. Among the more photogenic members of Arp’s catalog subsequently photographed by the HST is a pair of galaxies about 300 million light-years away from Earth known as Arp 273, located in the constellation Andromeda. This photo shows a pair of highly misshapen spiral galaxies distorted by their gravitational interaction. Computer simulations of the distorted upper galaxy, particularly how the outer spiral arm is stretched into a wide ring, are consistent with the lower (also distorted) galaxy having plunged right through the upper galaxy’s outer arms. Like a rock thrown into a lake, the gravity ripples from the lower galaxy’s collision produce a characteristic warping, tilting, and distortion of the upper galaxy. The extreme stretching of the lower galaxy’s spiral arms and the development of faint tails of stars trailing away from them are also likely a result of the collision. Hubble’s images reveal that the gravitational interaction between the two galaxies has also set off a new wave of star formation in both of them. In the upper galaxy, new stars appear as a spray of bright blue clusters across the top of the photo, along the galaxy’s outermost spiral arm. In the lower galaxy, most of the new star formation appears to be focused in the center core of the spiral. The births of these new stars appear to have been triggered by the “collisionally” induced gravitational compression of dust and gas clouds within the pre-collision galaxies. Interactions like this represent one way that galaxies can become peculiar, and another way that new generations of stars can be formed.

Hubble’s WFC3 false-color photo of the beautiful spiral galaxy known as Messier 83, or the Southern Pinwheel Galaxy, located 15 million light-years away in the southern-hemisphere constellation Hydra (the Serpent). The colors are a combination of individual measurements made from ultraviolet, visible, and infrared filters.

THE SOUTHERN PINWHEEL September 2012 Spiral galaxies are among the most beautiful and mathematically elegant astronomical objects in the Universe. Their photogenic nature, particularly spirals oriented “face-on” to us so we can see their intricate structures, makes them popular targets for amateur astronomers. A small number of “face-on” spirals that are relatively close to us (in the grand scale of the Universe) provide opportunities for professional astronomers to tease out subtle details of structure, composition, origin, and evolution by way of HST and other space-based observatories. A good example is Messier 83, known as the “Southern Pinwheel Galaxy”—a relatively close and stunningly beautiful spiral that’s a favorite target of amateurs and professionals. The Pinwheel is some 50,000 light-years across (about half the size of our spiral Milky Way galaxy) and

“only” about 15 million light-years away. It spans an angular size about half as large as the full Moon in the sky. Because of its size and proximity, Hubble can discern finer details in its structure than in more distant spiral galaxies. Images and mosaics from Hubble reveal abundant evidence of stellar evolution, the “circle of life” represented by thousands of young-star clusters, hundreds of thousands of individual stars from young to old, and hundreds of nebular “bubbles” associated with relatively recent supernova explosions. Particularly stunning are the numerous bright-blue patches that arc along the galaxy’s dusty and loosely wound spiral arms, tracing the locations of hot young stars that have recently emerged from their nebular cocoons. Many of those stars are less than a few million years old and emit enormous amounts of ultraviolet energy that ionizes residual hydrogen gas collected in its spiral arms. Energy from those stars, as well as other newborns still embedded in those clouds of gas and dust, makes them glow bright pink in this image. The older stars in the Pinwheel are more focused toward the galaxy’s central bar, and they bulge and glow with a more yellowish hue. Deep within the central core, Hubble images reveal more of the brightblue colors typical of highly active star formation regions. One hypothesis for this is the Pinwheel’s central bar of stars funneling new materials into the core region, helping to fuel accelerated star birth.

THE LONG COLLISION OF THE ANTENNAE GALAXIES July 2013 Even though most of space is empty space, every so often things run into each other. Asteroids and comets crash into planets. Stars collide and merge into more massive stars and black holes. Even entire galaxies can collide with each other, although “collide” isn’t quite the right description. What actually happens is these giant astronomical objects passing through each other with few or even no individual stars actually colliding. Still, the gravitational effects can be devastating. Among the most dramatic examples of the gravitational violence wrought by colliding galaxies is seen in Hubble’s photographs of the galaxies known as NGC 4038 and NGC 4039. These two giant collections of stars started out as normal spiral galaxies, but about a billion years ago their paths through space started drawing them closer together, and then eventually onto a collision course. Over hundreds of millions of years, the two galaxies passed through each other, their immense gravities tearing apart their elegant spiral structures, ripping stars and huge clouds of gas and dust away from each other, and forming long streamers of gas stretching far beyond their bright cores. The streams of far-flung gas are reminiscent of an insect’s antennae, thus their nickname: the Antennae Galaxies. The massive galactic collision created a stellar baby boom among the survivors. Huge clouds of gas and dust likely slammed into each other, intermingled, compressed, and collapsed into enormous numbers of bright blue clusters of newly formed hot young stars that dominate the remains of the outer spiral arms of the original galaxies. New stars are being formed at rates comparable to those in so-called starburst galaxies (see page 150). Clouds of bright pink gas, partially obscured by dark dust, are also sprinkled through the

wreckage and serve as raw materials for the birth of yet more new stars. The Antennae are now locked in a slow-motion celestial embrace as a result of the collision, appearing to have captured each other with their mutual gravity. Some computer simulations predict that the two galaxies will continue to orbit each other in ever-tightening circles until they eventually merge into a single giant elliptical galaxy. A similar violent fate might await the graceful spirals of the Milky Way and Andromeda galaxies, which are on course to collide with each other in approximately three to five billion years.

OPPOSITE: A Hubble merged WFC3 and ACS instrument montage of the Antennae Galaxies, a pair of distorted spiral galaxies locked in a cosmic dance as they have gone through a slow-motion cosmic collision over the past billion years. The Antennae are 45 million light-years away, in the southern-hemisphere constellation Corvus (the Raven).

COSMIC ZOOM LENS June 2002 A lens is an object (usually made of clear glass or plastic) that can bend and focus light onto a desired

point, usually a piece of film or a digital detector in a camera. Lenses can zoom in on—or magnify— images as well. It turns out that light can also be bent by the gravitational force of “lenses” made of extremely massive astronomical objects, like black holes or enormous clusters of galaxies. Hubble has taken images and captured other data of such “gravitational lenses,” including the massive cluster of galaxies known as Abell 1689. Abell 1689 is a cluster of hundreds of galaxies and at least 160,000 globular clusters that is about 2 million light-years across and is located about 2.2 billion light-years away from us in the constellation Virgo. It is one of the largest and most massive galaxy clusters known, with a total mass of many trillions of Suns. The cluster contains so much mass that it acts as a gravitational lens that bends and zooms in on the light of many hundreds more galaxies located much farther away, but which are behind Abell 1689 when viewed from our part of the Universe. The bending of light like this by enormous amounts of mass was one of the key predictions of Albert Einstein’s early–twentieth-century theory of relativity. Astronomers have found examples of such “lensed” galaxies that appear intermingled with those of the Abell 1689 cluster from much farther away, including one lensed galaxy that could be as old as 13 billion years (dating back to the very earliest epoch in the history of star and galaxy formation). Gravitational lenses allow instruments like those on Hubble to see deeper into space, and thus farther back in time, than they normally could. The more distant galaxies behind Abell 1689 appear in the Hubble photo as blue and red arcs of light, as their original appearance has been smeared out by the massive lens of the cluster. Hubble’s exceptional resolution and imaging sensitivity, and the ability to collect the faint light from such targets over exposure times of dozens of hours, has provided a rare window into deep time. Abell 1689 is just one of dozens of known sources of such strong gravitational lensing, and astronomers have exploited all the known occurrences of this type to learn as much as possible about the previously out-of-reach extremely early Universe, including clues to how the first generation of stars formed.

OPPOSITE: Hubble’s ACS photo of the distant cluster of galaxies known as Abell 1689, which shows the effect of gravitational lensing, whereby massive foreground objects, in this case the galactic cluster that is some 2.2 billion light-years away, amplifies the appearance of even more distant objects behind it along the line of sight. This color composite is a combination of individual green, red, and infrared filter images. PAGES 166 and 167: In this merged Hubble ACS and WFC3 view of the galaxy cluster known as Abell 370, a number of smeared-out, arc-shaped images of more distant galaxies (amplified by the gravitational lensing effect of the cluster) mingle with other more narrow, curved streaks caused by asteroids in our solar system (see page 48) passing through the field of view of the long-exposure galaxy images.

QUASARS April 2003 After the discovery in the 1930s that the center of the Milky Way galaxy is a strong emitter of radio waves, astronomers began systematically surveying the skies for other objects that could also be strong radio sources, to try to help determine how and why such objects exist. In the late 1950s, a group in the UK published the results of their search in the Third Cambridge Catalog of Radio Sources, also known as “3C.” The 273rd object identified in their catalog, which was later found to match the position of a previously identified star, turned out to be extremely interesting. Astronomers had been puzzled by the star’s location because its spectrum—the way it emits light of different colors and energies—was unlike that of any other known star. The radio signals provided the answer: it wasn’t a star at all, but a phenomenally luminous and energetic core of a massive elliptical galaxy that is more than 2.5 billion light-years away. 3C 273 is an example of a “quasar,” or quasi-stellar object, one of the most energetic classes of astronomical objects in the known Universe.

INSET: Hubble’s WFPC2 photo of the quasi-stellar radio source, or quasar, known as 3C 273, located about 2.5 billion lightyears away in the constellation Virgo. Even though they look like stars in the sky, quasars are actually the intensely bright and active centers of entire galaxies. This color composite was made from individual blue and red filter images. OPPOSITE: An artist’s impression of what an ancient, distant quasar like 3C273 and its bi-polar jets of high-energy particles might look like up close.

Some galaxies, deep in their star-packed central cores, have massive black holes that spew enormous amounts of energy into space. Such active galactic nuclei (AGNs; see page 158) are fueled by collapsed stars with a mass of millions to billions times that of our Sun. 3C 273 is the first, and brightest, example of such AGNs discovered at what astronomers call “cosmological distances,” or objects so far away that their light has taken a significant fraction of the age of the Universe to reach us. 3C 273 was the first quasar discovered because it is one of the closest to us, even at a staggering 2.5 billion light-years away. This also means that 3C 273 is the most luminous known quasar, shining with the light of more than 4 trillion times the Sun. 3C 273 is only visible in telescopes, but it is so intrinsically bright that if it instead happened to be located only about 30 light-years from us it would appear as bright to us as is the Sun in our sky. Like many AGNs much closer to us, 3C 273 is shooting enormous jets of high-energy particles into space, as gas and dust are accelerated by a central black hole 900 million times as massive as the Sun. The longest of those jets, photographed here in great detail by HST, is nearly 200,000 light-years long.

EINSTEIN’S CROSS August 2003 The ability of the most massive objects in the Universe, like giant galaxies or clusters of galaxies, to bend light in the space around them is known as gravitational lensing (see Cosmic Zoom Lens, page 168). Most examples of lensing involve the amplification and focusing of more distant “normal” galaxies. However, a

small number of examples have been discovered of a foreground galaxy acting as a lens for more distant quasars, the ultra-high energy cores of active galaxies formed early in the history of the Universe (see Quasars, page 170). The most elegant and picturesque of these kinds of gravitational lenses occur when the light from a single background quasar is split into smeared-out arcs or multiple separate re-focused images. In a small number of cases, the original quasar is split into a cross-shaped pattern of four lensed images. To honor the famous physicist’s prediction of the optical illusion created by this kind of light-bending in his general theory of relativity, the pattern is called an “Einstein Cross.” OPPOSITE: A merged Hubble ACS and WFC3 false-color photo of the gravitationally lensed quasar known as HE04351223. The gravity of the yellowish foreground galaxy splits and magnifies the light of the much more distant whitish quasar. This RGB composite was generated from green and infrared ACS images and an infrared WFC3 image. BELOW: Detail from opposite page.

Hubble has taken spectacular photos of a number of Einstein Crosses, allowing astronomers to study the gravitational lensing effect in great detail and provide a window into astronomical objects from very early in the Universe’s 13.8-billion-year history that would otherwise be too faint for Hubble to study. One particularly spectacular example is from combined ACS and WFC3 instrument photos of the gravitationally lensed quasar known as HE0435-1223, located in the constellation Eridanus. Spectroscopy of the lensed system from ground-based telescopes reveals that the yellowish elliptical galaxy in the foreground is about 4 billion light-years away, and that the lensed background quasar is perhaps nearly 10 billion light-years away—a staggering distance that implies we are seeing an active galactic nucleus as it appeared only about 3 to 4 billion years after the Big Bang. Einstein Crosses like this are more than just a beautiful curiosity for astronomers. Quasars vary over time, and the spectra of all four re-focused images of the same background quasar vary identically but not at the same time, because the split-up light has traveled different distances before re-focusing. Using the resulting time delays from multiple Einstein Crosses, astronomers have been able to constrain the rate of expansion of the Universe (the so-called “Hubble constant”) to better than 5% accuracy.

DARK MATTER? November 2004 One of the fundamental revolutions in cosmology (the study of the origin and evolution of the Universe) over the past few decades is the hypothesis that everything we can see and directly study in the Cosmos is only a small fraction of everything that really is. Specifically, astronomers have theorized that a mysterious, unseen form of matter permeates the Universe and can only be detected indirectly, for example from the gravitational influence that it exerts on “normal” matter. This putatively unseen component of our Universe is called Dark Matter—not because it appears as a dark color, but rather because it cannot be seen at all. How can we detect something that is, by definition, undetectable? The answer is that even though Dark Matter does not reflect or emit electromagnetic radiation like normal matter does, it does have mass and can thus influence the movement of normal matter. In fact, one of the reasons Dark Matter was first hypothesized is that it could explain the otherwise puzzling movements of globular clusters or spiral arms in massive galaxies—the idea being that these otherwise unexplainable movements are influenced by enormous amounts of unseen mass surrounding those galaxies. Gravitational lensing by clusters of distant galaxies (see Cosmic Zoom Lens, page 168) also provides a way to indirectly study Dark Matter. For example, astronomers have analyzed Hubble images of the galaxy cluster Cl 0024+17, located about 4 billion light-years away in the constellation Pisces, and discovered that the cluster is amplifying light from numerous more-distant galaxies. By mapping the strength of the lightbending or lensing by the foreground cluster, astronomers have been able to map out the gravity field of the

cluster and have discovered that there is simply not enough visible mass among the seen galaxies to explain the amount of mass needed to produce that level of lensing. According to some astronomers, computer modeling of the gravity field of Cl 0024+17 shows that the cluster is surrounded by an enormous “ring” of Dark Matter. One hypothesis is that this ring is the result of the collision of this cluster with another giant galaxy cluster long ago, causing their halos of Dark Matter to interact and form a ring. However, other astronomers are skeptical of this hypothesis, so research is ongoing. In some cosmological models, Dark Matter could represent 85% or more of all matter in the Universe. If so, it might represent the ultimate demotion, relegating us and all the planets, stars, and galaxies that we can directly observe to just a minor component of the Cosmos.

OPPOSITE: False-color view of a Hubble ACS Wide Field Camera image of the galaxy cluster known as Cl 0024+17,

located about 4 billion light-years away in the constellation Pisces. Superimposed in cyan on the RGB composite (made from blue, red, and infrared filter images) is a computer-modeled map of the gravity of the cluster. Some astronomers interpret the ring-like nature of the gravity map as evidence of Dark Matter.

Hubble’s ACS photo of the cluster of more than a thousand elliptical and spiral galaxies known as the Coma cluster, because of its location in the constellation Coma Berenices. The Coma cluster is located some 320 million light-years away. This color composite was made from individual blue and red filter images.

THE BOUNDARIES OF THE UNIVERSE January 2007 Superclusters, among the largest known structures in the Cosmos, are collections of hundreds to thousands of galaxies bound to each other gravitationally. The interconnected network, or web, of smaller galaxy clusters and giant superclusters discovered so far by astronomers outlines the overall large-scale structure of walls, filaments, and voids that define the boundaries of our Universe. The outstanding resolution of the Hubble Space Telescope makes it possible to identify not only individual galaxies within clusters and superclusters, but also individual globular clusters (spherical collections of millions of stars bound together by gravity) within many of those galaxies in relatively nearby collections of galaxies. A wonderful example is from Hubble’s ACS images of the distant Coma cluster, a collection of more than 1,000 individual gravitationally bound galaxies located about 320 million

light-years away in the constellation Coma Berenices (“Berenice’s Hair”). Despite its vast distance, the Coma cluster is actually one of the closest galaxy clusters to us (besides the Virgo cluster, of which the Milky Way is a part). It’s also part of a much larger supercluster of galaxies called (imaginatively) the Coma supercluster. Hubble’s images of the Coma cluster reveal more than 22,000 point-like globular clusters, some of which orbit galaxies within the cluster, but others of which are apparently free-floating in orbits between the galaxies. Collisions or near-collisions between galaxies within the cluster are thought to have ripped many of these globular clusters from their original host galaxies. For example, a string of these bright blue orphaned globular clusters appears to form a sort of bridge between the two largest diffuse elliptical galaxies in Hubble’s photo of the central region of the Coma cluster. These wandering spherical collections of stars are known as “intracluster globular clusters” because they are bound to the overall gravity of the cluster, not to that of any individual galaxy within the cluster. Rogue globular clusters like those wandering through the Coma cluster provide additional ways to assess the gravitational fields of such large-scale structures. Indeed, “anomalies” in the gravity and motions of both the intracluster globular clusters as well as many individual galaxies in the Coma cluster constituted one of the first pieces of evidence found (back in the 1930s) to support the hypothesis that the Universe is dominated by Dark Matter, an unseen form of matter than can only be detected indirectly because of its gravitational influence on normal matter (see page 174).

STEPHAN’S QUINTET August 2009 While many hundreds of thousands of galaxies have been identified in clusters or superclusters across the Universe, fewer exist in small groups close to each other in our general intergalactic neighborhood. Such collections are referred to as “compact groups,” and only about a hundred or so have been cataloged. The first one ever identified is also the most famous—Stephan’s Quintet. Discovered in 1877 by French astronomer Édouard Stephan, this compact group is composed of five different galaxies located in the constellation Pegasus. As seen in great detail in Hubble WFC3 photos, two of the galaxies are classical spiral galaxies, two are spiral galaxies interacting with each other, and one is a classical elliptical galaxy. Four of the five galaxies are dominated by older, yellowish stars, while the fifth is dominated by younger, blue-white stars. The quintet is something of an illusion, though, because only four of the five members of the group are actually close to each other. The four yellowest galaxies in the photo are part of a closely associated compact group some 290 million light-years away. The fifth galaxy (the bluer one at the upper left) is actually about seven times closer to us (some 40 million light-years away) and is not traveling with the others. In truth, the name of the group really should be “Stephan’s Quartet, Plus One in the Foreground.” Regardless, three of the four closely associated and more-distant galaxies show evidence that they once had—or continue to have—collisions or close gravitational interactions that significantly modified their structures. The two galaxies in the center of the photo appear to be in mid-collision, and the upper right galaxy has had its spiral arms significantly modified (extending to a full 180° around its central bar),

probably by its close pass with the interacting pair below. Because of the gravitational stirring of the gas and dust in their spiral arms, the interacting pair, as well as the close-passing barred spiral, show abundant evidence of starburst activity. This can be seen in the recently initiated formation of hot, bluish, young stars along the disrupted spiral arms, as well as in the pockets of reddish-pink glowing hydrogen gas cocoons where new stars are being formed but have yet to emerge. Coincidentally, similar starburst activity is also seen in the spiral arms of the much closer, presumably younger galaxy at upper left.

OPPOSITE: Hubble’s WFC3 false-color composite of the five closely spaced galaxies known as Stephan’s Quintet, in the constellation Pegasus. Only the three yellowish objects (which make up four galaxies total) are actually close to each other; the bluer galaxy at upper left is actually seven times closer to us than the other four. This RGB color composite was made from separate blue, green, and infrared filter images.

THE FIRST GALAXIES? March 2011 One of the reasons that gravitationally lensing clusters of galaxies (see Cosmic Zoom Lens, page 168) are so interesting to—and coveted by—astronomers is that they can bring into focus much more distant objects that could not be seen otherwise. At the cosmological distances already involved, that means that lensing clusters create the opportunity to see specific astronomical objects as they existed much farther back in time than any other observational methods. The Big Bang theory for the origin and evolution of the early Universe hypothesizes that the first stars and galaxies formed relatively early, perhaps within the first billion years or less. However, technology does not yet exist for telescopes—even highly capable space telescopes—to see the 10 to 12 billion lightyears or more needed to go that far back in time. And even if the technology did exist, dimming of the light from such vast distances by intervening gas and dust might make most objects that far away simply undetectable. However, this is where gravitational lensing comes to the rescue, and one of the most stunning examples comes from Hubble images of the giant cluster of elliptical galaxies known as Abell 383, located about 2.5 billion light-years away in the constellation Eridanus. This cluster of thousands of individual galaxies and their associated halo of Dark Matter (as inferred from the gravity structure of the visible galaxies) produces a significant amount of lensing and focusing of galaxies “behind” the cluster, along our line of sight. Many of those background galaxies are smeared out into arcs by the cluster’s strong gravity. But at least one distant background galaxy has been brought into sharp focus by Abell 383’s lensing effect. This background galaxy has been split into two separately focused images by the foreground cluster’s gravity (see Einstein’s Cross, page 172), making it appear as two dim, insignificant specks just above and to the lower left of the cluster’s bright central core. Astronomers have discovered that that particular galaxy appears to be around 12.75 billion years old: that is, it formed only about 950 million years after the Big Bang. Other similar examples of ancient galaxies brought into view by lensing clusters have revealed even older ones, some going back to just a mere 200 million years after the Big Bang. Apparently, the first stars—and the first galaxies in which they gathered—formed quickly from the raw materials of the early Universe.

OPPOSITE: This merged Hubble ACS and WFC3 false-color photo shows the giant galactic cluster Abell 383, located about 2.5 billion light-years away. Among the smeared-out and zoomed-in views of lensed galaxies from behind the cluster is a pair of faint specks that correspond to a background galaxy located some 12.75 billion light-years away—among the oldest galaxies ever found. This RGB composite was generated from green and infrared ACS images and infrared WFC3 images.

ULTRA DEEP FIELD September 2012 One advantage of using the Hubble Space Telescope is that observing time is never clouded out by the Earth’s atmosphere. Every moment spent staring into deep space is perfectly clear; and if the instruments can remain pointed in stable fashion while the telescope orbits Earth, the view will remain the same. Astronomers have exploited this stability to take super-long exposure photographs, holding Hubble’s shutter open for multiple photographs adding up to days or even weeks of exposure time pointed at a few specific places in the sky, collecting light from the faintest, most distant galaxies ever seen. Astronomers call such long-exposure imaging “deep-field” photography, because the longer the exposure, the resulting light comes from deeper and deeper back in time. One of the more recent examples of this point-and-stare approach was deep-field imaging using the full range of Hubble’s color capabilities, from the ultraviolet to the infrared. Called the Ultra Deep Field, the photo was compiled from more than two million seconds (more than three weeks) of total exposure time using the ACS and WFC3 instruments

between 2002 and 2012. The region to stare at was a tiny, relatively “blank” and essentially random area of the sky in the constellation Fornax, chosen mostly because it happens to be free of most nearby stars over the area of Hubble’s field of view. Deep-field imaging like this has gone deeper and deeper during Hubble’s lifetime, providing tangible proof that, in some ways, the telescope is something of a time machine. Some of the oldest galaxies that can be identified in the Ultra Deep Field appear to have formed around 13.3 billion years ago, or only about 400 million years after the Big Bang. The photo is like a core sample, reaching from nearby space all the way back into deep time. The Hubble Ultra Deep Field photo is among the most stunning views of astronomical color and form ever acquired over HST’s lifetime. More than 10,000 objects can be seen in the photo, and the vast majority of them—even ones that look like mere dots—are entire galaxies. This random field of view is only one half of 1% of the width of the full Moon in the sky—an equivalent sliver of sky to what you would see if looking up through an 8-foot-long soda straw. Imagine extrapolating the amount and diversity of galaxies seen in this photo across the entire sky to appreciate the truly astronomical number of galaxies out there. OPPOSITE TOP: Hubble’s Ultra Deep Field photo, acquired by combining hundreds of images of the same tiny patch of sky taken between 2002 and 2012. This color composite was generated from 13 different ACS and WFC3 filters covering wavelengths from the ultraviolet to the infrared. OPPOSITE BOTTOM: Each successive deep field photographed by Hubble has reached farther back in time, now to within less than half a billion years after the Big Bang.

GAMMA RAY BURSTS July 2013 In the 1960s, the U.S. military deployed a series of Earth-orbiting satellites designed to detect high-energy radiation emitted from covert nuclear weapons tests by the Soviet Union or others. Surprisingly, the satellites detected intense bursts of high-energy gamma rays coming from deep space instead. After a few decades of observational follow-up, astronomers discovered that these gamma ray bursts (GRBs) are enormous explosions caused when high-mass stars collapse into compact objects like neutron stars or black holes. Gamma ray bursts are the single brightest electromagnetic event in the Universe, creating as much energy in a few seconds as our Sun will over its entire lifetime. They are also extremely rare events, predicted to occur only a few times every million years in each galaxy. Because these events provide a direct way to study the violent and energetic end of stellar evolution, astronomers have launched their own satellites to detect GRBs and have established a special network to alert other astronomers when one goes off.

HST is part of this network, as it’s one of the observatories that can follow up the detection of a GRB with imaging and spectroscopy in the UV, visible, and infrared. Details of the way the energy from the explosions tails off, over hours to days later, provide details about the type of event that has occurred, as well as insights on the physics of the creation of compact objects (see A Monster Black Hole Gazes Back, page 136). An example is the GRB detected by NASA’s Swift GRB satellite on June 3, 2013, in a previously uncharted galaxy in the constellation Leo later known as GRB 130603B. Within 10 days, ground controllers were able to point Hubble at that galaxy and obtain visible and infrared images that showed a faint afterglow from the enormous explosion, helping to pinpoint the exact location of the original event. Analysis of the Hubble data and other observations of the event have interpreted it as the decaying afterglow from a new kind of supernova explosion called a “kilonova,” or the hypothesized collapse of a relatively low-mass white dwarf star. Astronomers have discovered that GRBs vary in duration, from short (up to a few seconds), to long (up to a few hours), to ultra-long (more than a few hours), with each duration providing unique insights on a specific kind of hypothesized stellar explosion or compact-object merger.

Hubble’s ACS and WFC3 false-color photo of the galaxy called GRB 130603B, where a gamma ray burst was detected on June 3, 2013. Hubble imaging 10 days later showed an infrared afterglow of the explosion, which faded away after ten more days (lower inset). This RGB composite was generated from a red-filter ACS image and an infrared-filter WFC3 image.

COLLIDING CLUSTERS August 2014 Collisions occur in the Universe on every possible scale, from tiny impacts between dust grains to the massive interactions of entire galaxy clusters. Hubble and other cutting-edge astronomical observatories provide a front-row seat to numerous examples of the latter, enabling unique insights into gravitational interactions among the largest-scale features in the Universe, as well as the role of Dark Matter (see Dark Matter?, page 174) in shaping the large-scale structure of the Cosmos. A spectacular example comes from a joint observational campaign to study the impending collision and merging of two enormous clusters of galaxies located about 4.3 billion light-years away in the constellation Eridanus. In Hubble ACS and WFC3 images, the two parts of the merging cluster, which is known as MACS J0416, each shows evidence of gravitational lensing of background galaxies (see Cosmic Zoom Lens, page 168), smearing out the light from those more distant objects into distorted shapes and arcs because of the enormous amount of mass involved (estimated to be more than 160 trillion times the mass of

the Sun). The nature of that gravitational distortion, plus other clues among the many thousands of galaxies involved in the merger, allows astronomers to map out the distribution of Dark Matter associated with the clusters. Hubble photos have been combined with a map of X-ray emission from the clusters from NASA’s Chandra X-ray Observatory, which reveals the temperatures of the gas associated with these massive collections of galaxies, as well as a map of the radio emission from the clusters from the National Science Foundation’s Very Large Array ground-based radio telescope facility, which is sensitive to the presence of turbulence and shock waves generated by the gravitational forces among individual and groups of interacting galaxies. The combined data sets show that the collision of these particular enormous structures has only just begun, because the hot gases and inferred Dark Matter distribution within each of the two merging clusters have not yet been disrupted in the ways that would be expected if they had already met. Rather, each cluster’s distribution of galaxies, gas, and Dark Matter appears largely self-contained. However, the entire system is on the brink of chaos, as the collision is expected to unfold over the next millions to billions of years, leading to an eventual merger into a single supercluster.

A false-color photo image of galaxy cluster MACS J0416, located about 4.3 billion light-years away in the constellation Eridanus. This composite uses data from three different telescopes: Hubble ACS and WFC3 visible and infrared filter data in red, green, and blue; Chandra X-ray satellite data in diffuse blue; and Very Large Array radio wavelength data in diffuse pink.

JAMES WEBB SPACE TELESCOPE 2020 What comes next for space-based telescopes after the expected demise of the Hubble Space Telescope

sometime in the 2020s? NASA’s answer is the James Webb Space Telescope, or JWST, a larger and more complex observatory that is optimized to observe the Universe in the infrared. JWST is the successor to Hubble and is named after former NASA Administrator James Webb, who led the agency from 1961 to 1968 in the lead-up to the Apollo Moon landings. Like Hubble, JWST took a long time to go from concept to mission reality. Planning for an infrared space observatory with an 8-meter–diameter primary mirror, initially called the Next Generation Space Telescope, began in the mid-1990s. By the early 2000s, a plan was approved by NASA and Congress to scale the observatory back to a 6.5-meter mirror, to achieve a total mission cost of just under $1 billion and an expected launch in 2010. However, technical and management problems have plagued the design, as well as the fabrication and testing of JWST, and its price tag has ballooned to over $10 billion, with a launch possible no earlier than May 2020. The European Space Agency and Canadian Space Agency are also contributing to the project. PAGES 188 and 189: This artist’s impression of the breakup and re-entry of the European Space Agency’s Automated Transfer Vehicle satellite is a good analogy for the fate awaiting the Hubble Space Telescope, once its altitude control and stabilization systems stop working sometime in the 2020s. BELOW: A December 2017 photo of the primary mirror and support structure of the James Webb Space Telescope in front of a giant thermal vacuum chamber located at NASA’s Johnson Space Center in Houston, Texas.

JWST’s primary science goals are to search for light from the first stars and galaxies, to study the formation and evolution of galaxies, to understand the formation of stars and planetary systems, and to study planetary systems and the origins of life. Despite its rocky start, persistent budget troubles, and continuing launch delays, by all assessments JWST should be worth the cost and wait. The telescope’s primary mirror has about five times the light-collecting area of Hubble, so it will be able to detect much fainter objects. JWST uses an “open tube” design and graphite composite materials that allow it to be half the mass of Hubble despite the much larger mirror. The open design and needed infrared performance require the observatory to use a large reflective sunshield to keep the mirror and instruments out of direct sunlight and stay as cold as possible. Further, JWST will be

deployed about 930,000 miles (1.5 million km) beyond Earth’s orbit, in a gravitational balance point called L2 that will allow it to remain close enough to Earth for good radio communication, but far enough to avoid any infrared “contamination” from the Earth or Moon. This distant location also means that it is unlikely—or will at least be quite difficult—for JWST to be serviced by astronauts for maintenance or repair.

A comparison of the Hubble Space Telescope and its 2.4-meter–diameter primary mirror with JWST and its 6.5-meter– diameter primary mirror.

WFIRST Mid to Late 2020s? After the pointing systems or other critical components of the Hubble Space Telescope have reached the end of their expected lifetime sometime in the early to mid-2020s, astronomers won’t be able to continue to observe the Universe with a large space-based observatory optimized for visible-light wavelengths, as the JWST will be optimized for infrared wavelengths (see James Webb Space Telescope, page 190). So planning is in the works for a replacement space telescope that can pick up the part of the electromagnetic spectrum where Hubble left off. The replacement is to be called the Wide Field Infrared Survey Telescope, or WFIRST. Using a 2.4meter–diameter primary mirror donated to NASA from the National Reconnaissance Office and originally planned for a spy satellite, WFIRST would have the same high-resolution imaging capability as Hubble, but with a field of view 100 times larger (comparable in angular size to the full Moon). The very-wide field of view would allow WFIRST to scan the sky more efficiently for Hubble-like deep surveys in visible to short-

wave–infrared wavelengths. The main science goals of the telescope are to study Dark Matter and Dark Energy (the mysterious force that could be leading to the acceleration of the expansion of the Universe) via observations of distant supernovae and gravitational lensing, to survey the properties of all known exoplanets, and to search for and characterize new exoplanets by separating their light from the “contaminating” light of their parent star. WFIRST would use two main instruments: an infrared camera and spectrometer for wide-field survey imaging and compositional studies; and a device called a coronagraph that would enable direct imaging of exoplanets by blocking the starlight and allowing detection of planets very close to the star, but up to a billion times dimmer. Like JWST, WFIRST would be launched into a stable orbit some distance away from the Earth to allow it to observe more frequently than Hubble (which is blocked by Earth half the time from its low orbit) and without the influence of scattered light from the Earth or Moon. WFIRST was initially funded for detailed study by NASA in 2014, and has steadily progressed in Congressional support and funding since then. With a planned maximum price tag of $3.2 billion, however, and given NASA’s difficulties in keeping the development of JWST on schedule and on budget, WFIRST still faces some political opposition as well as a number of technical hurdles that designers need to overcome before it is formally approved as a new NASA space observatory. If that approval occurs in 2020, then WFIRST potentially could be launched before the end of that decade.

An artist’s rendering of the Wide Field Infrared Survey Telescope, or WFIRST, a 2.4-meter–diameter space telescope designed to conduct surveys of large regions of the sky at comparable resolution to Hubble.

LUVOIR Mid-2030s Once the mission of the Hubble Space Telescope is over sometime in the 2020s, and assuming the successful launch and operation of follow-on telescopes like JWST and WFIRST, astronomers will still not be able to observe the Universe in the ultraviolet (UV) like they could with Hubble. This is because, first, the Earth’s atmosphere absorbs ultraviolet light, preventing it from being detected by ground-based

telescopes, and because, second, the follow-on telescopes after Hubble are being optimized to work only at visible or infrared wavelengths.

An artist’s concept of the possible Large Ultraviolet Optical Infrared Surveyor space telescope, or LUVOIR. Designed as a possible successor to JWST, LUVOIR would use a primary mirror between 8 and 15 meters in diameter, and a JWST-like sunshield to help keep the telescope and instruments cool.

As a result, the astronomical community is looking for ways to re-establish and enhance space-based ultraviolet observing capabilities after Hubble. A possible mission that has emerged as a leading candidate is called the Large UV Optical Infrared Surveyor telescope, or LUVOIR. Nominally, LUVOIR would look superficially like JWST, with an open telescope design thermally protected by a sunshield and operating far enough away from the Earth and Moon to avoid contaminating scattered light but close enough to allow high-bandwidth radio communications. However, the intention is for LUVOIR to be a much larger telescope than JWST, with up to a 15-meter–diameter primary mirror and five and forty times, respectively, more collecting area than JWST and Hubble. Launch would hypothetically be sometime in the early 2030s. LUVOIR would leverage its multi-wavelength capabilities to achieve a wide range of science goals spanning cosmology; stellar and galactic astrophysics; and exoplanet and Solar System studies. Specifically, for astrophysics and cosmology, LUVOIR instruments and observations would focus on expanding Hubble’s exploration of the birth of stars and planetary systems; the formation and evolution of galaxies; and the mapping of cosmic structure in the early Universe. For Solar System and exoplanet research, LUVOIR instruments and observations would be able to analyze the structure and composition of exoplanet atmospheres and surfaces, with particular focus on the search for “biosignatures” (like excess oxygen or methane) that could indicate the presence of life on those worlds. Within our Solar System, LUVOIR’s UV and visible wavelength capabilities would enable more detailed studies than Hubble on the composition and time-varying dynamics of giant planet atmospheres, magnetospheres, and aurorae; the composition of geysers and other clues to the nature of putative subsurface oceans on icy moons like Europa and Enceladus; and the compositions of comets, asteroids, and distant Kuiper Belt Objects. The ultimate fate of LUVOIR currently rests with the astronomical community, which could or could not recommend its development based on detailed studies to be conducted in the 2020s.

HABEX/STARSHADE Mid to Late 2030s?

The past few decades have seen a revolution in the discovery and initial identification of exoplanets, or planets around other stars. More than four thousand exoplanets are currently known to be orbiting around nearby Sunlike stars, and several dozen of these are suspected of being comparable in size to the Earth and potentially similar in terms of environmental conditions. The next big leap in exoplanet studies will come when astronomers are able to characterize the surfaces and atmospheres of these potentially Earthlike worlds in enough detail to truly assess their habitability. One such space-telescope concept that is being studied for possible launch in the mid to late 2030s is a mission called the Habitable Exoplanet Imaging Mission, or HabEx. HabEx would focus on finding and characterizing planets within the so-called “habitable zone” around stars that are not so different from the Sun—that is, stars with long, stable, hydrogen-burning lifetimes and levels of ultraviolet or other highenergy radiation that are not lethal to organic molecules. Such stars are common in our celestial neighborhood and in our galaxy (and others) in general, and have been the focus of exoplanet search and characterization missions to date, like the NASA Kepler Space Telescope mission. Kepler and other search programs have found that nearly all Sunlike stars have planetary systems, with those systems ranging from just one planet to upwards of ten or more (like our Solar System). The prospects for deeper planet-hunting and for discovering potential “Earth 2.0’s” out there are promising. In the current concept design (which is sure to change over time), HabEx would deploy a telescope of approximately 4 meters in diameter equipped with imaging and spectroscopy capabilities focused on ultraviolet and visible wavelengths, but also with some limited infrared capability. Deployed in tandem would be a large (tens of meters in diameter when unfurled) “starshade,” a shield of lightweight materials that can “occult” or block the light from a host star while HabEx obtains images and spectra of any planets close in to the star that are not blocked by the starshade. The use of a starshade could allow the faint light from a planet just one ten-billionth as bright as its star to be detected and measured. Like LUVOIR, the HabEx concept (and, independently, the concept of using a starshade with any spacebased telescope) is being judged on its worthiness by the space science community as part of the next National Academy of Sciences Decadal Survey of Astronomy and Astrophysics, slated for public release in 2020.

An artist’s concept of a possible design of the Habitable Exoplanet Imaging Mission (HabEx) space telescope, along with a deployed starshade that is designed to block starlight so that small, close-in planets can be more easily detected.

ORIGINS Mid to Late 2030s? Just as planning for Hubble started many decades before its launch, as did planning for Hubble’s successor the James Webb Space Telescope (JWST), planning for JWST’s successor is beginning now. Specifically, NASA and the space-science community are considering four large-scale space telescopes for possible deployment in the 2030s: Lynx, an X-ray observatory to enhance the discoveries of the Chandra X-ray telescope; the UV-Optical-IR capable space telescopes known as LUVOIR and HabEx (see pages 193 and 194, respectively), and a fourth possible concept known as the Origins Space Telescope, or OST.

Artist’s concept of one of the early possible designs for NASA’s Origins space telescope, a 6-to-10-meter–class observatory designed as an infrared-optimized follow-on to the James Webb Space Telescope.

OST would, as its name states, focus on origins—of planets and the protoplanetary disks from which they form; of stars and the interstellar molecular clouds from which they form; of galaxies and the supermassive black holes that lie at their hearts; and of the Universe in general, dating back to the first moments after the Big Bang. Like LUVOIR and HabEx, OST would also place special emphasis on characterizing the surfaces and atmospheres of small, rocky, potentially Earthlike exoplanets orbiting within the habitable zones around their host stars. What sets OST apart from other large space telescopes under consideration for the 2030s is that it would be optimized for observations deep in the infrared part of the electromagnetic spectrum. The infrared is where planetary bodies give off most of their heat energy and where many of the dark dusty lanes seen in nebulae and galaxies are relatively transparent, allowing objects and processes in their interiors to be studied in detail. Current concept studies are focusing on a primary mirror in the 6-to-10-meter class, to improve upon JWST’s resolution and sensitivity. New-technology detectors and sunshields would be designed to give OST something like 100 to 1,000 times greater sensitivity than JWST or any previous infrared space telescope. In a sense, Lynx, LUVOIR, HabEx, and OST are all competing against each other to vie for what will

likely be just a single multi-billion-dollar–class space-telescope mission that NASA and other interested space-agency partners might be able to afford in the 2030s. Like Hubble and other spectacular space telescopes that have flown (or soon will), the winning concept for the 2030s will have to pass the tough scientific standards and reviews from the global-space-science community, as well as political scrutiny of its predicted costs.

NOTES AND FURTHER READING

This Hubble ACS false-color photo shows part of the Whale galaxy (also known as NGC 4631), about 19 million light years away in the constellation Canes Venatici. The bright core of this edge-on spiral galaxy (on the left side of the photo) is a region of intense star-forming activity, viewed through the dark, dusty lanes of the galaxy’s spiral arms.

I consulted many different resources in doing the research for this book, including a variety of general historical and encyclopedic sources and peer-reviewed publications, to verify the factual information (or the range of accepted consensus), as well as a variety of websites for additional details and follow-up. Here I’ve listed some of those resources that I hope you will consult if your interest and curiosity are sparked by any of the spectacular images, mysteries, and discoveries that astronomers and planetary scientists have used Hubble to explore. The Hubble Space Telescope has taken hundreds of thousands of images of tens of thousands of different astronomical objects over its 30-year lifetime so far. Thus, I’ve obviously taken some liberties and introduced my own personal and professional biases as a Hubble user on what images to include here, given the limitations of the number of pages in this book. Someone else writing this book would surely have picked a different set of greatest hits photos, although I like to believe that there would be significant overlap with the ones I have chosen. Still, I would be delighted to consider suggestions for other images to swap in for possible future editions of this book, and of course would also welcome any corrections or suggestions in general about the content. Please feel free to write to me by email at [email protected].

HISTORY AND HIGHLIGHTS OF THE HUBBLE SPACE TELESCOPE The Hubble Cosmos: 25 Years of New Vistas in Space, D. H. Devorkin, R. W. Smith, and R. P. Kirshner, National Geographic, 2015. “Hubblesite,” http://hubblesite.org “Hubble Space Telescope,” NASA Web page: https://www.nasa.gov/mission_pages/hubble/main/index.html “Hubble Space Telescope,” Wikipedia entry: https://en.wikipedia.org/wiki/Hubble_Space_Telescope

The Hubble Space Telescope: From Concept to Success, Springer Praxis, 2015. Hubble’s Universe: Greatest Discoveries and Latest Images, T. Dickinson, Firefly Books, 2017. NASA Hubble Space Telescope: Haynes Users’ Guide, Haynes, 2015. Spitzer, Lyman Jr., “Report to Project Rand: Astronomical Advantages of an Extra-Terrestrial Observatory” (1946), reprinted in NASA SP-2001–4407: Exploring the Unknown, Chapter 3, Doc. III-1, p. 546. Spitzer, Lyman S. (March 1979). “History of the Space Telescope”. Quarterly Journal of the Royal Astronomical Society. 20: 29–36. Bibcode:1979QJRAS.20. . . .29S.

GENERAL ASTRONOMY BOOKS A Brief History of Time, S. Hawking, Bantam Books, 1998. Astronomy: A Self-Teaching Guide, D. L. Moché, Wiley, 2014. Astronomy for Kids: How to Explore Outer Space with Binoculars, a Telescope, or Just Your Eyes!, B. Betts and E. Colón, Rockridge Press, 2018. The Astronomy Book: Big Ideas, Simply Explained, DK Books, 2017. The Cosmic Perspective (textbook), by J. O. Bennett, M. O. Donahue, N. Schneider, and M. Voit, Pearson Education Inc., 2019. The Space Book, Jim Bell, Sterling, 2018. Turn Left at Orion, G. Consolmagno and D. M. Davis, Cambridge Univ. Press, 2019.

GENERAL ASTRONOMY WEBSITES Astronomy Magazine: http://www.astronomy.com Astronomy Picture of the Day: https://apod.nasa.gov/apod Astronomical Society of the Pacific: https://www.astrosociety.org Bad Astronomy by Phil Plait: https://www.syfy.com/tags/bad-astronomy Curious About Astronomy?: http://curious.astro.cornell.edu European Space Agency: http://www.esa.int/ESA NASA: http://nasa.gov Sky and Telescope Magazine: https://www.skyandtelescope.com Space.com: http://space.com

SOLAR SYSTEM The Great Comet Crash: The Collision of Comet Shoemaker-Levy 9 and Jupiter, edited by J. R. Spencer

and J. Mitton, Cambridge, 1995. The Extrasolar Planets Encyclopaedia, http://exoplanet.eu “HST Studies of Mars,” J. F. Bell, in A Decade of Hubble Space Telescope Science, eds. M. Livio, K. Noll, & M. Stiavelli, Cambridge, 2003. Hubble Outer Planet Legacy Program, https://archive.stsci.edu/prepds/opal Hubblesite: Solar System Highlights: http://hubblesite.org/images/news/82-solar-system Hubblesite: Exoplanets Highlights: http://hubblesite.org/images/news/51-exoplanets NASA’s Planetary Science Division, https://science.nasa.gov/solar-system The Planetary Society, http://www.planetary.org The Ultimate Interplanetary Travel Guide, Jim Bell, Sterling, 2018.

STARS Black Holes & Time Warps: Einstein’s Outrageous Legacy, K. Thorne and S. Hawking, W. W. Norton, 2014. The Complex Lives of Star Clusters, D. Stevenson, Springer, 2015. Extreme Explosions: Supernovae, Hypernovae, Magnetars, and Other Unusual Cosmic Blasts, D. Stevenson, Springer, 2014. “How Stars Work,” How Stuff Works web site, https://science.howstuffworks.com/star5.htm Hubblesite: Stars: http://hubblesite.org/images/news/2-stars Star Clusters, https://en.wikipedia.org/wiki/Star_cluster Stars, J.B. Kaler, W. H. Freeman, 1998.

NEBULAE An Introduction to Planetary Nebulae, J. J. Nishiyama, IOP Concise Physics, 2018. Dark Nebulae: http://abyss.uoregon.edu/~js/glossary/dark_nebula.html H II Regions (Emission Nebulae): https://en.wikipedia.org/wiki/H_II_region “‘Hubble Goes High-Definition to Revisit Iconic ‘Pillars of Creation,’” NASA Web Page, Jan. 5, 2015: https://www.nasa.gov/content/goddard/hubble-goes-high-definition-to-revisit-iconic-pillars-of-creation Hubblesite: Nebulae: http://hubblesite.org/images/news/3-nebulae Planetary Nebulae, https://en.wikipedia.org/wiki/Planetary_nebula Reflection Nebula: https://www.nasa.gov/multimedia/imagegallery/image_feature_701.html

GALAXIES

Edwin Hubble: Mariner of the Nebulae, G. E. Christianson, Taylor & Francis, 2019. “The first picture of a black hole opens a new era of astrophysics,” L. Grossman and E. Conover, Science News, April 10, 2019: https://www.sciencenews.org/article/black-hole-first-picture-event-horizontelescope Galaxies, T. Ferris, Random House, 1988. Galaxy: https://en.wikipedia.org/wiki/Galaxy Galaxy: Mapping the Cosmos, J. Geach, Reaktion Books, 2014. Galaxy Classification: https://lco.global/spacebook/galaxies/galaxy-classification/ Galaxy Mergers: http://astronomyonline.org/Cosmology/GalaxyMergers.asp Hubblesite: Galaxies: http://hubblesite.org/images/news/4-galaxies

THE DISTANT UNIVERSE Clusters of Galaxies: https://www.astronomynotes.com/galaxy/s9.htm Cosmology: https://en.wikipedia.org/wiki/Cosmology “Dark Matter and Dark Energy,” National Geographic Web Site, https://www.nationalgeographic.com/science/space/dark-matter Hubblesite: Cosmology, http://hubblesite.org/images/news/12-cosmology Hubblesite: Deep Fields, http://hubblesite.org/images/news/14-deep-fields Hubblesite: Galaxy Clusters, https://hubblesite.org/images/news/15-galaxy-clusters Hubblesite: Gravitational Lensing, http://hubblesite.org/images/news/18-gravitational-lensing “What is the Big Bang Theory?” E. Howell, Nov. 7, 2017, https://www.space.com/25126-big-bangtheory.html

BEYOND HUBBLE “Astro 2020: Decadal Survey on Astronomy and Astrophysics,” National Academy of Sciences Web Site, https://sites.nationalacademies.org/SSB/CurrentProjects/SSB_185159 Cain, F., “What Comes After James Webb and WFIRST? Four Amazing Future Space Telescopes,” Universe Today, June 13, 2018, https://www.universetoday.com/139461/what-comes-after-james-webband-wfirst-four-amazing-future-space-telescopes “James Webb Space Telescope,” NASA Web site, https://www.jwst.nasa.gov List of Proposed Space Observatories: https://en.wikipedia.org/wiki/List_of_proposed_space_observatories

Astronauts on the first servicing mission took this dramatic photo from the cabin of the space shuttle Endeavour while slowly creeping up on Hubble high above Western Australia. During their December 1993 mission, the astronauts captured the telescope and installed corrective optics that restored Hubble to full perfect vision (see page 21).

This Hubble ACS photomosaic shows part of the Andromeda galaxy (also known as M31), the nearest spiral galaxy to our Milky Way, around 2 million light years away. More than 100 million stars and thousands of separate star clusters produce the scene we see here, which stretches across more than 40,000 light years from the galaxy’s bright yellowish central core of older stars (at left) out to its dusty spiral arms and their blue-ish pockets of new star forming regions (at right).

ACKNOWLEDGMENTS The incredible images and discoveries of the Hubble Space Telescope would never have been possible without the tireless decades of advocacy, design, construction, operation, maintenance, and data analysis by the tens of thousands of people that it takes to make any such Big Science project like this happen. I feel a particularly deep sense of gratitude to the astronauts who, during five space shuttle missions between 1990 and 2009, literally risked their lives to deploy and service Hubble. These heroic explorers fixed the telescope’s focus problem and upgraded the cameras and other instruments and systems during the observatory’s first two decades so that it could make discoveries for at least another ten years, and hopefully many more. As an HST observer myself, I am also grateful to the staff at STScI for their skill and dedicated work to help plan complex observations that take advantage of every possible minute of observing time on the facility. I am also grateful to John Meils, my editor at Sterling, for believing in this book project, and to the rest of the team at Sterling, including Kevin Ullrich, Michael Cea, and Scott Russo. Also, a big thank you to Michael Bourret, my literary agent at Dystel, Goderich, & Bourret, for his many years of support of my writing in general. Finally, special thanks and love go to my prime focus and guiding star Jordana Blacksberg, for her unwavering patience, support, and wisdom during my many photographic trips to space. Jim Bell Mesa, Arizona May, 2019

Hubble photographed this giant, billowing tower of cool gas and dust called the Eagle Nebula (also known as M16) in November 2004 using the ACS instrument. Hot young stars that are forming within the nebula are eroding this huge cloud of hydrogen gas into intricate and lovely structures. The section here is nearly ten light years long, or more than twice the distance from the Sun to our next nearest star.

IMAGE CREDITS ALMA/Hyosun Kim: 78 Ken Crawford Rancho Del Sol Observatory: 131 (inset) EHT Collaboration: 136 ESA: D. Ducros 188—189, Hubble 10—11, M. Kornmesser 191 Getty Images: Corbis 5 (top) Lockheed Missiles and Space Company: 12—13 (bottom) NASA: i, ix, xii, xv, xvi, 3, 15, 16 (inset), 18—19, 20, 24—25, 28, 31, 35, 182 (bottom), Dana Berry 47, ESA 6—7, 14, 17, 21, 22, 40—41, 84—85, 101, 102—103, 126, 196—197, ESA (ASU) vxiii, ESA, and J. Anderson and R. van der Marel 92—93, ESA, S. Beckwith and the Hubble Heritage Team 148—149, ESA. N. Benitez/JHU, T. Broadhurst/The Hebrew U, H. Ford/JHU, M. Clampin, G. Hartig, G. Illingworth /UCO, Lick Observatory, the ACS Science Team and ESA 168—169, ESA and M. Buie/SW Research Institute 61, ESA, Chris Burrow, Space Telescope Science Institue, J. Krist and IDT Team 46, ESA, J. Clarke/Boston U and Z. Levay 55, ESA and John T. Clarke/U of Michigan 50, ESA, CXC, and JPL Caltech iix—ix, ESA, CXC, NRAO/AUI/NSF, and G. Ogrean/Stanford U 187, SA, CXC and the University of Potsdam, JPL-Caltech 144, ESA, J. Dalcanton/U of Washington, B. F. Williams U of Washington, L. C. Johnson U of Washington, the PHAT team, and R. Gendle 200—201, ESA, D. Ehrenreich/Institut de Planétologie et d’Astrophysique de Grenoble/CNRS/Université Joseph Fourier 63, ESA/Hubble and H. Olofsson/Onsala Space Observatory 83, ESA H. Ford/JHU, G. Illingworth/UCSC/LO, M. Clampin, G. Hartig and the ACS Science Team 139, ESA/Hubble, Karl Stapelfeldt, B. Stecklum and A. Choudhary/ Thüringer Landessternwarte Tautenburg, Germany 91, ESA and Hubble 98—99, 125, 151, 165, 170 (inset), 172, 173, ESA and The Hubble Heritage Team ii—iii, 36—37, 77, 94—95, 96—97, 100 —101, 105, 108, 112, 123—124, 129, 130—131, 132—133, 137, 141, 143, 146—147, 155, 160—161, 162 —163, 204, ESA, the Hubble Heritage Team and R. Gendler 134—135, ESA/Hubble, M. Kornmesser 170 —171, ESA and The Hubble 20th Anniversary Team 34, ESA, and the Hubble Heritage/Hubble Collaboration 80—81, 156—157, 158—159, ESA/Hubble, HST Frontier Fields 166—167, ESA, The Hubble Heritage Team, A. Nota and The Westerlund 2 Science Team 72—73, ESA/Hubble, A. Filippenko, R. Jansen vi—vii, ESA, and the Hubble SM4 ERO Team 179, ESA, M.J. Jee and H. Ford/Johns Hopkin 175, ESA, D. Jewitt/UC LA, J. Agarwal/Max Planck Institute for Solar System Research, H. Weaver/Johns Hopkins U, M. Mutchler, and S. Larson/U of Arizona 67, ESA, J. Mack, and J. Madrid 176—177, ESA and A. Nota 206, ESA and U of MA/JPL, and Spitzer Science Center/Caltech 26—27, ESA and R. Sahai 79,

ESA, R Sahai and John Trauger/JPL, the WFPC2 science team, 107, ESA and P. Kalas/UC Berkeley 65, ESA, and R. Kirshner/Harvard-Smithsonian Center for Astrophysics/ 86—87, ESA and Jesús Maíz Apellániz/Instituto de Astrofísica de Andalucía, Spain 88—89, ESA, C.R. O’Dell/Vanderbilt U, M. Meixner and P. McCullough 115, ESA, J. Richard (Center for Astronomical Research/Observatory of Lyon, France), and J.-P. Kneib (Astrophysical Lab of Marseille, France) 181, ESA, M. Robberto/Space Telescope Science Institute, and the Hubble Space Telescope Orion Treasury Project Team 116, ESA, A. Simon, M. Wong/UC Berkeley, and G. Orton/JPL-Caltech 68, ESA, A. Simon/Goddard Space Flight Center and M.H. Wong and A. Hsu Berkeley 71, ESA, N. Smith/UC Berkeley and The Hubble Heritage Team (Hubble Image) an d N. Smith/UC Berkely and NOAO/AURA/NSF (CTIO Image) 119, 120—121, ESA, H. Teplitz, M. Rafelsk/(IPAC/Caltech, A. Koekemoer R. Windhorst/Arizona State U and Z. Levay 182 (top), ESA, N. Smith/U of Arizona, and J. Morse/BoldlyGo Institute, NY 76, ESA, N. Tanvir U of Leicester, A. Fruchter, and A. Levan U of Warwick 184, 185, ESA, U of Colorado, Cornell, Space Science Institute 52, ESA, H. Weaver, M. Mutchler and Z. Levay 58, ESA, H. Weaver, A. Stern and the HST Pluto Companion Search Team 56, H. Hammel and MIT 43, Andrew Fruchter and the ERO Team111, Goddard Space Flight Center/CI Lab 192, 194, Goddard Space Flight Center/CI Lab/Adriana Manrique 193, The Hubble Heritage Team 152—153, Johns Hopkins U Applied Physics Lab/SW Research Institute/Alex Park 60, JPL 45, 49, JPL/Caltech viii-ix, 194, Johnson Space Center iv—v, 199, Marshall Space Flight Center Collection 5 (bottom), Jack Pfaller xvii, Desiree Stover 190, H. Weaver, HST Comet Hyakutake Observing Team 38—39 © Chris Schur: 44 The U.S. National Archives: 32—33 Courtesy Wikimedia Commons: Andrew Buck 13 (top)

The stellar nursery known as NGC 346 was captured in this spectacular Hubble ACS false-color photo from 2005. This nebula is located about 210,000 light years away in the Small Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. Embedded within the nebula’s glowing clouds of gas and dark lanes of dust is a bright cluster of hot, energetic, newly formed stars that are ionizing and sculpting their surroundings.