The famous space telescope has been in orbit 25 years. This is how it ushered in a new age of astronomy
Ask any person on the street to name one telescope, and there’s little doubt they would say the Hubble Space Telescope. Thanks to the Hubble, the excitement of cosmic discoveries has become part of the general culture, rather than being only the province of professional astronomers.
When considering Hubble’s major scientific achievements, we should recognize that because it studies an extremely wide range of astronomical phenomena, discoveries rarely belong exclusively to Hubble. More often, it was the combination of observations from several telescopes that led to the breakthrough findings. Hubble did play an absolutely crucial role, however, in all of the science highlighted here.
Philosophically, Hubble has changed our views on the physical place of humans in the cosmic landscape by strengthening our understanding of the “Copernican principle”—the idea that Earth and humans do not occupy any special place in the universe. And yet, Hubble has also shown us that we do live at a rather special point in time.
Through a series of deep-field observations, Hubble has expanded our horizons to unprecedented distances, showing us that in the observable universe, there are hundreds of billions of galaxies. A combination of observations from ground-based telescopes, along with those from the Kepler, Spitzer, and Hubble telescopes, has convinced even skeptics that planets are also very common. The Milky Way galaxy alone is, in fact, teeming with billions of extrasolar planets, many of them in the “habitable zone”—meaning they could have liquid water—around their parent stars. For the first time, Hubble has provided information about the atmospheric composition of a few extrasolar planets, and has even imaged some planets in orbit around their host stars. But the unremarkable nature of our physical location doesn’t end there. We have learned that the center of our home galaxy contains a black hole with the mass of about four million suns, but Hubble has found that nearly every galaxy with a central stellar bulge contains a supermassive black hole. The universe is teeming with them. Finally, Hubble-based research into the universe’s expansion rate combined with data from other projects has shown that ordinary matter—the stuff that stars, planets, and humans are made of—constitutes only 4.9 percent of the cosmos. Copernicus was right: In this universe, we are not at center stage.
The situation is very different, however, when it comes to our place in time. Hubble has played a key role in determining the age of the universe—about 13.8 billion years, with a possible error of no more than about three percent. Through its deep-field observations, Hubble has mapped the history of the birth rate of stars, which reached its peak about nine to 10 billion years ago and has been declining ever since. Our universe is definitely past its prime.
Hubble has also determined that the expansion of the universe started to accelerate around six billion years ago, apparently propelled by the repulsive force of “dark energy.” While the precise nature of dark energy is not known, observations to date tell us that it is energy associated with empty space. If dark energy represents the energy of the physical vacuum, the expansion of the universe will continue to accelerate forever. The physical and philosophical implications of such a universe are far-reaching. About one or two trillion years from now, the light from all the galaxies (outside those in our own Local Group) will be moving away to the point that observers in the Milky Way galaxy (if they still exist) will not be able to see any other galaxy. Such observers will find it essentially impossible to reconstruct the true history of the universe. While a trillion years may sound like an incredibly long time, in the cosmic chronicles of an ever-accelerating universe, it is really just a fleeting moment. In other words, we are fortunate to live at a special time—one during which telescopes such as Hubble can make progress in deciphering nature’s secrets. We should take full advantage of this finite opportunity.
In addition to its scientific achievements, discoveries by Hubble have formed the basis for what is arguably the most successful science outreach program in history. Art critics have even compared the telescope’s images to the best works of art of our time. Hubble’s longevity has turned it into an icon of science for an entire generation. I have spent a significant part of my career as an astrophysicist interpreting Hubble data, including four years heading the office that conducts the outreach and education activities.
After five servicing missions by space shuttle astronauts, the last in 2009, Hubble began to perform almost like a new observatory, even though it was approaching 20 years in orbit. NASA plans to continue Hubble operations until at least 2018. With the execution of six “frontier” deep-field observations due in the near future, we can be sure that new discoveries wait just around the corner. These will pave the way for Hubble’s successor: the James Webb Space Telescope.
The Beginning of Everything
In December 1995, Hubble spent 10 days peering into a tiny patch of the sky in the constellation Ursa Major. The result was the Hubble Deep Field, revealing nearly 3,000 galaxies in what was the longest look back to the beginning of the universe.
Since then, astronomers have continued to push Hubble’s view further and further back in time. The latest iteration is the Ultra Deep Field, taken over 200 hours in August and September 2012 with the Wide Field Camera 3, installed in 2009 during Hubble’s last servicing mission. The image was made by a team at the California Institute of Technology in Pasadena, led by Richard Ellis. The 1995 version, made with the Wide Field and Planetary Camera 2, was a look back at the 13.8-billion-year-old universe when it was a youthful one billion years. Some of the galaxies in the Ultra Deep Field date back to just 350 million years after the Big Bang, or about three percent of the universe’s age, explains Ellis.
“The results of this remarkable image are consistent with the picture that galaxies ‘switched on’ when the universe was about 200 million years old, and gradually developed their forms,” says Ellis. “Cosmic Dawn [the era when matter began to form stars and galaxies] was not likely a dramatic event narrowly confined in time.” Indeed, the series of deep fields from Hubble has shown that the farther back astronomers look, the more smoothly the number of galaxies and other objects declines.
Astronomers’ knowledge of the early universe has benefited greatly from Hubble’s observations, but the picture is not complete. Ellis believes even earlier galaxies will be found. Hubble won’t be the one to find them, though, since astronomers have now taken images with all the filters in WFC3’s near-infrared channel. Deep field exploration will be one of the many tasks handed off to Hubble’s successor, the James Webb Space Telescope.
What Happens When A Star Dies
“I was shocked,” recalls Arsen Hajian when he first saw this image of planetary nebula IC 418 in 1999. The astronomer, who now manages a company that develops imaging and spectroscopy equipment, picked the object for his postdoctoral research on measuring the distance to nebulae, because ground observations showed it to be round and featureless. “We took an image expecting it to be simple and straightforward; what we discovered was anything but,” he says. Hajian’s reaction was a common one among astronomers who were seeing the early images sent to Earth by the Hubble Space Telescope, especially in 1999, “the first time I could download Hubble data directly from the web page, as opposed to waiting for a tape to show up in the mail.”
A planetary nebula forms when a mid-size star, much like our sun, starts to die. The balance between the inward pull of gravity and the outward pressure from nuclear fusion shifts as the star’s hydrogen fuel runs out. When the star begins to collapse, the heat from this increased pressure ignites the helium, which “explodes like crazy compared to hydrogen,” Hajian explains. The force causes the star to “throw off its outer coat. Its atmosphere turns into a big puff of gas that rushes out into space,” reaching an area hundreds of times the size of our solar system. Meanwhile, with the new fuel expended, the core shrinks to about the size of Earth.
Astronomers used to believe most nebulae were simple, spherical objects, because they were imaging them with radio and ground-based optical telescopes, which could resolve only one or two pixels across. Now research is filled with the study of microstructures—the “little filaments and strands and blobs” that make up the wild nebulae we now know exist. “Hubble was really the driver behind that,” says Hajian.
The astronomer was mesmerized by the pattern that Hubble revealed on the surface of IC 418, which lies about 2,000 light-years from Earth. Hajian began to call it the Spirograph Nebula, “from when I was a kid, that little thing you put a pen in and twirl the little gear around and it makes a pretty picture.” Though Hajian suspects the pattern is created by some kind of magnetic effect, astronomers still haven’t pinned down the cause. If anything, Hubble has given astronomers as many new mysteries to solve as it has delivered answers.
Exoplanets in Detail
In the hunt for exoplanets, Hubble isn’t the first space telescope that comes to mind. NASA’s Kepler spacecraft, launched in 2009, has found thousands of exoplanet candidates. “The strength of Kepler is in the statistics—you learn a lot when you can detect thousands of planets,” says Paul Kalas of the University of California at Berkeley. “The strength of Hubble is in being able to study a few systems in exquisite detail.”
Kalas and his team began using Hubble to look for planets around the star Fomalhaut in 2004, and found an enormous dust belt. The detailed images, however, showed that the belt was an unusual shape, likely disturbed by the gravity of at least one planet, so in 2006 they went back to look again. Comparing the images, they found “a tiny dot of light, 10 billion times fainter than the star.” Fomalhaut b is the first exoplanet to have its picture taken.
But even a picture isn’t definitive proof for everyone. Kalas extrapolated an orbit, but it didn’t explain the debris ring shape, as the planet’s gravity would push and pull on it. What’s more, Kalas says some scientists were concerned that the planet didn’t appear brighter in the infrared, as a planet orbiting a youthful 440-million-year-old star should.
Kalas went back to Hubble and found that Fomalhaut b’s orbit is extremely elliptical, similar to those of comets in our solar system, which follow paths that swing inside close to the sun and then far out beyond Pluto into the Kuiper Belt. At the farthest point from its star, Fomalhaut b is 350 times farther than Earth is from the sun. In about 20 years, the planet should travel far enough out to cross the debris ring. “Fomalhaut b would then become extremely interesting to monitor because there are no other examples that we know of where a planet would be penetrating its Kuiper Belt,” says Kalas, who plans to look for a more massive planet in the system that nudged Fomalhaut b into its eccentric orbit.
Spying on the Neighbors
Years from now, Hubble will probably be most remembered for its breathtaking deep field images, showing thousands of galaxies billions of light-years away. But Hubble has done plenty of impressive work in our own neighborhood too. In 1994, it took pictures as Comet Shoemaker-Levy 9 approached Jupiter and was ripped to pieces by the massive planet’s gravity, making enormous waves as it plunged into the gaseous atmosphere. It was a rare, spectacular event, and Hubble’s observations gave astronomers new insights into Jupiter’s composition.
Though the planets in our solar system may not seem far compared to distant galaxies, ground telescopes looking through the atmosphere can still struggle to resolve small details. This photo of Uranus and its rings was part of a series that studied how the rings’ appearance changed from 2003 to 2007, when they were edge-on toward Earth. With Hubble, SETI Institute astronomer Mark Showalter and his team discovered two additional outer rings, plus two tiny moons, Mab and Cupid.
Astronomers can employ tricks to see such faint detail. In this case, Showalter overexposed the planet by a factor of 50, making the planet a big white ball but allowing the nearly invisible detail of the rings to appear. The image above is an overlay of a true-color image of the planet (showing a polar cap and a bright storm) on a black-and-white image of the ring structure.
“Hubble remains far and away the most powerful instrument we have for studying the families of small moons orbiting the outer planets,” Showalter says. “I find it remarkable that Mab and Cupid were too small to be noticed by the Voyager spacecraft during its 1986 flyby of Uranus, but we could see them using Hubble years later.” Showalter used the same “observing trick” on Pluto and discovered two tiny moons in 2011 and 2012. His Hubble observations will assist NASA’s New Horizons spacecraft during its 2015 flyby of Pluto.
How Do You ‘See’ A Black Hole?
The idea of black holes was first considered more than two centuries ago. Albert Einstein’s theory of general relativity started to define them in the early 1900s, and by the 1970s, astronomers were looking for signs of something that, by definition, they could not see. Instead of looking for the black hole, they looked for high-energy radiation given off by the matter being sucked into it.
In 1990, soon after launch, Hubble began to search for the signature emission spectra of supermassive black holes in active galactic nuclei, where astronomers theorized black holes might exist. Using its infrared camera, Hubble was able to see through these dusty regions and measure the subtle movement of stars near where black holes were thought to be. A sharp rise in the velocity of stars close to the nuclei revealed a powerful gravitational field that astronomers could attribute to a black hole.
In 1997, a team of astronomers pointed Hubble at the galaxy Centaurus A, about 12 million light-years away. The team, led by Ethan Schreier, now president of Associated Universities, Inc. (a company that manages large-scale astronomy facilities), imaged the X-ray and radio jets streaming from the center of Centaurus A. They further found evidence of a black hole with the mass of 55 million suns and discovered that it is feeding on a small galaxy that collided with its own. Hubble was able to peer through the galactic dust to see these turbulent interactions. “Needless to say, we do not see the black hole itself in this picture,” explains Schreier. “However, its location is not far from the forefront star in the dark region right near the center of the image.”
Thanks to Hubble’s high-resolution probing, we know today that most, if not all, galaxies have supermassive black holes in their centers. Schreier also puts into perspective Hubble’s contributions: “These observations marked the end of more than 25 years of my observing Centaurus A with different instruments at different wavelengths—from the time I helped confirm the location of an X-ray source associated with this active galaxy in 1971, to my discovery of its jet in X-rays with the Einstein Observatory in 1979, to the detection of the jet in radio [waves] with the Very Large Array, and then onto these optical and infrared observations with Hubble.” Hubble’s infrared capabilities enabled Schreier to see a “warped disk” of material around the black hole for the first time, providing information about how stars form in these active galactic nuclei. The famed space observatory has pushed our knowledge of the universe to new heights, but its discoveries often come from the combined efforts of many telescopes.
The Making of a Solar System
Since the first confirmed planet orbiting another star was found in 1994, thousands of candidates have been identified. While astronomers continue to search for solar systems that look similar to ours, other scientists are focused on the debris that came beforehand. These proto-planetary disks—the dust and gas and rocks chaotically orbiting a young star—are the key to understanding how planets form. An early and still popular target for Hubble was the Orion Nebula, the closest massive star-forming region to Earth and an excellent target for proto-planetary disks.
Not long before Hubble launched, the common belief was that solar systems formed in relative isolation—that orbiting dust and debris slowly coalesced into planets solely due to their own and their stars’ gravity. This image, taken in 1998, was among the first to show direct evidence that these disks are forming around stars in the nebula, in surprisingly close proximity to other stars. John Bally, professor at University of Colorado at Boulder and one of the astronomers who acquired the image, says, “Our early work demonstrated that there were large particles in the translucent outer portions of this disk, indicating that the first stages of planet formation were already occurring here.”
Until the advent of adaptive optics—computers that compensate for atmospheric light distortion—ground telescopes had trouble resolving the tiny disks. Hubble, though, has discovered hundreds of them in the Orion Nebula alone.
“Several decades ago, astronomers thought that most planets formed around stars, which in turn formed in relative isolation from the gravitational condensation of interstellar molecular clouds,” says Bally. Although the process of star formation is still a matter of debate, astronomers now believe that these stars “form in clusters, with hundreds or thousands of sibling stars, some of which are massive.” Gravity and radiation from nearby stars can affect the formation of the proto-planetary disk, or do even worse damage if a massive star explodes in a supernova. Combining Hubble’s data with research on our local meteors, we now know that some stellar activity just a few light-years away, either an exploding supernova or a massive star pushing off its gassy layers, affected our solar system when it was just a proto-planetary disk.
Among Hubble’s significant contributions is its ability to confirm theories astronomers develop as they try to understand the universe. This image of “The Mice,” NGC 4676, is one example of an observation that proved the model.
Astronomers study galaxy collisions both to learn how galaxies form today and to look back in time to see how the very first galaxies may have been created in the early universe. We can’t watch the collisions, as they take hundreds of millions of years to unfold, but looking out across the sky, we can find snapshots of galaxies in every stage of collision. Scientists use the data about the behavior of colliding galaxies at each stage to create computer models. Then, educated guesses about the initial state of a particular pair of galaxies are entered into the program, and fingers are crossed in the hope the result looks like what they observe.
The first model of The Mice was published in 1972, says Joshua Barnes of the Institute for Astronomy at Manoa in Hawaii, who became interested in the research a few years later as an undergraduate. By the early 1990s, computers became faster and more powerful, and observations from telescopes like the Very Large Array in New Mexico provided crucial data about how fast different sections of the galaxies were moving. In 1995, with relatively high-power computers crunching the new data, Barnes and colleague John Hibbard greatly improved on the 1972 model.
Coincidentally, in 2002 NASA picked The Mice as an early target to show off Hubble’s new Advanced Camera for Surveys, giving Barnes the opportunity to compare his model with the best image ever taken of the galaxies. The image supported his theories about the initial orbit and angles of the two galaxies, along with their incorporation of the effects of dark matter, “which was essentially unknown in 1972 and which has a huge effect on the dynamics of galaxy collisions,” says Barnes.
Once they had a new detailed image to work from, Barnes says, it was back to the drawing board. “One thing this image drove home for me was the widespread star formation triggered by galaxy interactions. We knew about this in general, but the superb resolution of [Hubble] revealed young star clusters and star-forming regions distributed throughout the tails of the two galaxies.” By the next year, Barnes was able to use the model to study the circumstances that trigger star formation
In 2002, astronomers noticed a star at the edge of the Milky Way that suddenly gave off such a brilliant burst of light that it was for a time one of the brightest stars in the galaxy. Hubble began to watch as the light bounced off nearby interstellar dust, “like sound from a yodeler echoing off of mountains,” says Howard Bond of the Space Telescope Science Institute in Baltimore.
The star, V838 Monocerotis, is 20,000 light-years away. Astronomers were able to calculate its distance from Earth by measuring how its light moves through the dust. V838 Mon is in a class of objects that change relatively quickly. For some, a pattern appears that can provide a way to determine distances in the universe. Type 1a supernovae, for example, are called “standard candles” because their violent explosions produce a predictable brightness. Astronomers have turned Hubble toward them to reliably measure distances; the detection of their movement through space confirmed that the universe’s expansion is accelerating, providing evidence for the existence of dark energy. “Time-dependent phenomena are very important in astronomy,” says Bond, who notes that several ground-based telescopes are now being devoted to hunting down these objects.
But observation doesn’t always provide easy answers. V838 Mon was first thought to be a nova—an explosion of a white dwarf, ejecting its outer layers, that occurs when hydrogen from a companion star piles up on the dwarf’s surface, causing a runaway nuclear reaction. Soon astronomers realized that V838 Mon had become significantly brighter not because it violently blew off material but because the entire star had ballooned in size. Bond says that mystery and the revelation of previously hidden beauty are what Hubble provides: “I think we were all surprised by the complex structures revealed in the dust surrounding the star…. On top of the scientific results, the images are just so aesthetically pleasing, making this kind of work doubly fun to do.”
In regions of active star formation, astronomers sometimes find mysteriously swirling, rapidly moving plumes. This “cosmic skyrocket,” as NASA put it in a 2012 press release, is Herbig-Haro (HH) 110, one of about 1,000 such objects. Astronomers believe they form when gas and dust from an accretion disk fall into a young star, which results in narrow twin jets shooting in opposite directions from the axis. The jets then interact with the interstellar medium, forming the Herbig-Haro object.
Bo Reipurth of the Institute for Astronomy Hilo in Hawaii says that HH 110 is located nearby, in the Orion Nebula, and is moving very fast. “This implies that we can actually see the motion of the object in images separated by only a few years, when imaged with Hubble’s amazingly sharp vision.” This composite, made from Hubble images taken between 2004 and 2011, shows “a powerful wind” colliding with the dark brown cloud seen in the lower right, which “deflects the gas and creates the impressive pyrotechnics we are seeing here,” says Reipurth.
The plumes in the jet are about a light-year across and have a particularly mysterious origin: Astronomers can’t find the source star. Scientists can use calculations created by studying the changes in the series of images to turn back time and estimate where the plumes began. Now astronomers believe HH 110 is actually is a diverted jet from nearby HH 270, part of which is colliding with a dense cloud. A Herbig-Haro display like this one is rare, says Reipurth, and “reminds us that young stars can spew gases across huge distances.” Hubble’s continued observations of these unusual phenomena and astronomers’ use of the “rewind” method will give even more insight into the source star’s formation as it accretes mass.
The Other Ninety-Five Percent
How do you study what you can’t see? When it comes to dark matter and dark energy, which we now know make up about 95 percent of our universe, astronomers have started to use an old technique: gravitational lensing. Thanks to Albert Einstein’s theory of general relativity, we know that matter curves space, and light will follow those curves. This allows observers on Earth to “see” dark objects by measuring how light from a distant source bends around it.
In 2010, Eric Jullo, now at the Aix-Marseille University in France, and his team at NASA’s Jet Propulsion Laboratory used Hubble to demonstrate the use of gravitational lensing to study dark matter and dark energy. They looked at Abell 1689, a massive galaxy cluster about 2.2 billion light-years away, “for the stunning number of lensed images it contains,” explains Jullo. By measuring how much each galaxy’s light is distorted, the astronomers used the relativity theory to calculate the mass in the cluster they couldn’t see.
The team eventually published this image, taken with Hubble’s Advanced Camera for Surveys, with the galaxies in false color, while “in purple we overlaid a reconstruction of the matter distribution.” With the information from Hubble, Jullo says, “we showed that the two groups of galaxies visible in the image are actually associated with two distinct halos of dark matter” that affect their orientation, shape, and density. Scientists still know very little about the nature of dark matter and dark energy, but reconstructions like the one Jullo’s team made “can tell us about the growth of the universe.”
Hubble’s ability to take detailed lensing images is just one contribution to the study of this exotic matter. After ground observations of distant supernovas in 1998 revealed that the universe’s expansion was speeding up—thus revealing the existence of dark energy, a discovery that in 2011 garnered a Nobel Prize in physics—astronomers turned to Hubble to confirm these astounding findings. The space telescope’s images were able to eliminate other explanations: This mysterious repulsive energy is causing the universe to expand at ever-faster rates. The continuing search for dark matter and dark energy is now one of the biggest inquiries in science, and Hubble will surely play a part in answering it.