When Galaxies Collide
Gazing at the night sky, at the lights that seem eternally glued onto the black of space, a starwatcher would never guess at the violence that lurks within the starry seas of tranquillity. But out beyond our solar system, beyond even our Milky Way, worlds collide. Billions of stars, swirling gas clouds and dusty nebulae sometimes pass through each other virtually unscathed, like ghosts in a movie; sometimes they collide and ignite the fires of millions of thermonuclear infernos. And sometimes they bring sweet vindication to astronomers bold enough to think they can fathom the mysteries of the cosmos. Francois Schweizer, now at the Carnegie Institution in Washington, D.C., thought he had done that. At a telescope in Chile in the late 1970s, he observed that when galaxies collide, they change shape and trigger spectacular starbursts. He concluded that, contrary to the textbooks, newborn ""globular clusters'' of glowing stars are not nearly as old as creation. Formed much later, from the collision of giant clouds of hydrogen in smashing galaxies, they are instead the road kill of the cosmos. Few believed him.
Then the Hubble Space Telescope (HST) was launched, floating out of the cargo bay of the space shuttle Discovery in April 1990. Ever since, whenever they could win a few hours of observing time, Schweizer and colleague Brad Whitmore of the Space Telescope Science Institute in Baltimore have used HST to spy on colliding galaxies. Their persistence paid off. Last week they announced that Hubble, photographing the two Antennae galaxies 63 million light-years away, spied more than 1,000 brilliant clusters of stars exploding into being. With these images the space telescope showed that galaxies--the 50 billion collections of stars, gas and dust that speckle the universe like beacons in a dark sea--are not the isolated, static structures they were once thought to be. Instead, they collide and merge, cannibalize each other, fade, flare and change shape like Flubber. ""This is a huge revolution in thought,'' says astrophysicist Alan Dressler of the Carnegie Observatories in Pasadena, Calif. ""We all believed that galaxies looked the same for all time. But they don't. HST, by telling us about how galaxies formed and evolved, is telling us something about how we got here. It's amazing that humans could learn that.''
But they can, because the space telescope, orbiting some 370 miles above Earth, has provided Earthlings with front-row seats for some of the grandest shows the universe has staged. It has brought us spectaculars in our own backyard--like Comet Shoemaker-Levy pummeling Jupiter in 1994--as well as fireworks far, far away, like the black hole at the center of galaxy M87 slurping up surrounding matter. It has found exploding stars--supernovas--blowing celestial smoke rings, and ""pillars of creation'' trillions of miles high where stars are being born. ""The thing that is so amazing,'' says astronomer Mario Livio of the telescope institute, ""is that, literally every place HST has looked, it has found something fantastic.''
It wasn't always so. Soon after the telescope was launched, technicians looking at the raw images had the ultimate ""oops'' moment. Photos from the $1.5 billion instrument showed galaxies and stars as smeared-out cotton balls, not the sharp objects astronomers expected (and promised Congress). The main mirror, 94 inches across, had a ""spherical aberration''--in human terms, astigmatism. But in December 1993, two spacewalking astronauts dropped what amounts to a contact lens over the warped mirror. And ever since, says Andrew Fruchter of the institute, ""the space telescope has seen farther and more clearly than any of us dared hope.''
What it has seen has answered some of the thorniest questions in astronomy. Some of those questions are of interest mainly to scientists who have made it their life's work to learn, for instance, how some stars spray out jets of matter like a cosmic fire hose. Other discoveries reek of science fiction, like the massive black holes that gobble down anything that approaches. Still other finds speak to questions so basic that they began in theology and only in this century became the province of science: when was the universe created, and how? Some of the space telescope's greatest hits:
Are we alone? Looking at young stars in Orion, the ""hunter'' constellation, the telescope has spied platters of swirling dust called protoplanetary discs. Their name reflects their destiny: these discs, astronomers believe, will eventually break into distinct clumps of matter orbiting the star. We call these clumps of matter planets. Our sun, then, is probably not alone in having a retinue. To the contrary: Hubble suggests that it is a rare star that does not eventually have planets. Which makes the odds that we are not alone in the universe even shorter.
Young and old: To astronomers' surprise, HST discovered hundreds of blue stars at the center of globular clusters. Globular clusters are compact blobs containing anywhere from 100,000 to several million stars, the whole array sparkling like a cloud of diamonds. Our own Milky Way has about 150 globular clusters. ""These were among the first stars to appear in the universe,'' says one leading astronomy book, ""formed from the primeval gases.'' That's why the blue stars were a shock: blue means hot (think of the flame on a gas range), and hot means young (stars, like people, burn hottest in their youth). ""Seeing these hot young stars in clusters 12 billion years old was like finding a teenager in an old-age home,'' says the institute's Michael Shara. ""What were they doing there?'' Hubble answered: the driving force was stellar collisions. In the ultimate heavenly recycling, old stars in globular clusters merge, and out of their ashes (well, gas) a new star is born. Contrary to the textbooks, globular clusters do not contain only old stars.
Black holes: Hubble has found that black holes grow like weeds in the cosmic garden. ""In the center of every galaxy where the space telescope looked,'' says Livio, ""there is a massive black hole.'' A black hole is a region of such immense mass, and hence such powerful gravity, that nothing, not even light, can escape its clutches. One does not ""see'' black holes, therefore; one infers them. The Hubble did it by measuring, with one of its four instruments, the velocity at which a cloud of gas swirled around a galaxy's center. From that number, astronomers can calculate how much mass is in the center, keeping the orbiting gas from flying away like a discus after an Olympian's windup. The answer: an object with the mass of 3 billion to 5 billion suns. Hubble also found that these black holes were voracious. They swallow burning gas equivalent to 1 million suns every year. The resulting burp of radio waves shows up as a quasar, a once mysterious beacon that shines with the light of 1,000 entire galaxies, which is equivalent to trillions of stars.
Takeover artists: The space telescope has shown that galaxies do not evolve ""in splendid isolation,'' as conventional astronomic wisdom held. Instead, they shape each other. The astronomer after whom the telescope is named--Edwin Hubble--discovered in the 1920s that the universe is expanding. But the common-sense idea that it is expanding into empty space is wrong. The universe is all there is; there is no space beyond it to expand into. (If there were extra space, it would be, by definition, part of the universe.) Instead, the universe expands by actually creating, out of nothing, new space between groups of galaxies. The universe is therefore getting bigger, and galaxies farther apart. But there are exceptions. Galaxies close enough together to feel each other's gravity undergo a coy, slow courtship. When they meet, they might slide by, passing in the night like cosmic strangers. But they might, instead,collide. Our own Milky Way is now swallowing a dwarf galaxy called Sagittarius. ""This galaxy probably started with an almost spherical shape,'' says astrophysicist David Weinberg of Ohio State University. ""But now that the Milky Way is stretching it out, it is five times longer than it is wide.'' In another 200 million years the Milky Way will have captured Sagittarius, adding its 10 million stars to our 50 billion.
OF ALL THE SECRETS THAT the Hubble has unlocked, its images of galaxies that formed when the universe was in its infancy ""would be at the top of my list of its achievements,'' says astrophysicist John Bahcall of the Institute for Advanced Study in Princeton, N.J. ""HST speaks eloquently to how galaxies started and got together, something we argue about endlessly.'' The most important clue is an image called the Hubble Deep Field, which was taken over the course of 10 days in December 1995. It shows, more dramatically than any other, how the Hubble Space Telescope is, equally, the Hubble Time Machine. Images form when light hits a telescope (or the retina of an eye). Light travels at a finite speed: 186,000 miles per second. So light falling on the space telescope today left its source at some time in the past. Light from the sun takes 8.3 minutes to reach Earth; we are thus seeing the sun as it was 8.3 minutes ago. Light from an object billions of miles away began its journey years ago; astronomical distances can thus be measured in ""light-years,'' the distance light travels in a year. Thus the image we see is the object as it appeared in the distant past. (The writers of ""Star Wars'' apparently knew what they were doing when they paired ""a long time ago'' with ""far, far away.'') The images in the Hubble Deep Field appear as they were roughly 11.7 billion to 10.4 billion years ago. What do these objects look like today? Only an astronomer living 10 billion years in the future will know.
The first surprise from the Deep Field is how many more galaxies there were in the past. ""We're all trying to figure out if this is real,'' says Bahcall, or whether they are misinterpreting the image. ""What happened to all these galaxies? Did they fade? Or are the galaxies we see today formed of many pieces''--that is, did many primordial galaxies merge to form today's?
The second surprise from the Deep Field is that the universe's lights, contrary to astronomers' hunch, turned on in one great burst. It was as if every chandelier in a mansion were flicked on simultaneously on a moonless night. Today, only about two stars turn on in our Milky Way every year; back then, tens of thousands turned on every year. How could the lights go on practically at once? The universe was smaller 11 billion years ago. Perhaps, suggests Claus Leitherer of the telescope institute, protogalaxies with lots of gas but few shining stars collided more often in the young, crowded universe. When they did, the gas within them compressed and ignited into stars.
THE THIRD SURPRISE FROM the Deep Field is that its 2,500 galaxies together form a Miro-esque panorama of balls and eggs, arcs and lines. Among the multitudes of oddly shaped galaxies are hints that nearly one third are ""interacting,'' which is a genteel way of saying that they are ripping the #@*! out of one another. Some are colliding like the Antennae. Others are cannibalizing each other. In still others the gravity of one is distorting the other, as a passing breeze makes an unholy mess of a neat pile of leaves. If humans manage to stick it out for another 5 billion years, we may see up close what happens when worlds collide. (But we won't be seeing it from Earth, which will have been fried as our dying sun expands.) The Milky Way is closing in on the Andromeda galaxy at 300,000 miles per hour. Since Andromeda is 2.2 million light-years (13 billion billion miles) away, nothing will happen for 5 billion years, at least. And since galaxies are mostly dust and gas, the collision will not be like ricocheting billiard balls. Instead, when the two galaxies are upon one another, the cold clouds of gas in the Milky Way will be compressed, like those in the Antennae, and millions of new stars will ignite. The Milky Way's flat disc, on the outer fringes of which our sun and nine planets orbit, will be ripped apart. Eventually Andromeda will plunge into the heart of the Milky Way, triggering a Fourth of July finale of starbursts. And then the two galaxies, originally shaped like pinwheels, will be one, shaped like an egg.
A theologian in the 1650s fearlessly dated Genesis to 4004 B.C. Hubble finds that an underestimate. It has peered 10 times farther than ground-based telescopes for clues to the age of the universe. These clues are called cepheids. These are special, ""variable,'' stars that grow brighter and fainter, as if a fidgety child were fooling with its dimmer switch. The time it takes a star to go from bright to faint and back again, it turns out, depends on how intrinsically bright they are. Comparing the intrinsic brightness of a distant object to its apparent brightness in a telescope reveals how far away it is. Astronomers then measure how fast the object is receding. With these two numbers, they can calculate how long it took for the universe to reach its current size. (If the cosmos is expanding really, really fast, it takes less time to grow up than if it is expanding slowly.) The answer: the universe is 13 billion to 14 billion years old.
Paradoxically, the greatest measure of the Hubble's success may be that it is uncovering as many mysteries as it is solving. By the end of the next decade, therefore, NASA plans to launch an armada of three dozen new space telescopes to, primarily, glimpse the first and last days of the universe. Clues to the beginning reside in what astronomers call the Dark Zone. On the far side of this region, a telescope called the Cosmic Background Explorer (COBE) has already found, are lumps of energy beginning to coalesce into matter, 300,000 years after the big bang that started the universe. On the near side of the Dark Zone, the Hubble Deep Field shows newborn galaxies, created 1 billion to 2 billion years after the big bang. ""The universe somehow went from amorphous to well structured,'' says Ed Wiler, director of NASA's Origins program, ""and a lot of people are interested in how. But Hubble can't look that far back.'' The $500 million Next Generation Space Telescope, scheduled for launch in 2007, will, and it should see the first galaxies forming. The Space Infrared Telescope Facility, to be launched in 2001, will detect heat rather than visible light and so will peer through dust at stars being born. ""It'll also study the [dusty]central regions of our own galaxy, where there might be a black hole,'' says Michael Werner, SIRTF project scientist.
If everything breaks right, the new instruments should also reveal whether the universe will keep expanding, come to a permanent halt or reverse itself and implode in a Big Crunch. Cosmic fate is in the hands of cosmic weight. If the universe contains enough matter, it will implode, because one piece of matter (Earth) attracts others (the moon) and so acts as a force against expansion. But if the universe contains less than this magic amount of matter, then expansion will continue. Instruments including the Microwave Anisotropy Probe, X-ray telescopes and infrared telescopes will help weigh the universe. They will look for hidden pockets of matter, invisible, superheated gas and massive but faint ""brown dwarf'' stars. If the cosmic weigh-in doesn't provide an answer, still another new telescope will look at whether our fate is already visible in the stars. It will compare the recession of galaxies billions of years ago to the recession of galaxies today, to figure out if the universal expansion is already slowing down.
The space telescope's ""nominal mission,'' as it's called in NASA-speak, runs until 2005. In 1999 it is scheduled for another service call; astronauts will install an advanced camera. In 2002 spacewalkers will install a new spectrograph. Astronomers hope that, with the fixes, Hubble can operate until 2010. No one wants to see it become just another hunk of space debris. As Space Telescope Science Institute director Robert Williams, paraphrasing a turn-of-the-century astronomer, says, ""The HST has shown us not only that the universe is stranger than we supposed, but that it is stranger than we can suppose.'' The Hubble has extended humankind's eyes to the farthest reaches of time and space. Even more, it has extended our imaginations.
Solar system 0.13 light-year across
Nearest galaxy 2.2 million light-years away
Mature clusters Up to 5 billion light years
Baby galaxies Up to 11 billion light-years
Dark Zone 13-14 billion light-years
Big bang About 14 billion light-years