Homing In On Black Holes

To gain insight into the most mysterious objects in the universe, astronomers shine a light at the chaotic core of our Milky Way

(Laurie Hatch)
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Editor's Note, Sept. 23, 2008: Smithsonian magazine profiled astrophysicist Andrea Ghez in April, 2008. Today, Ghez was one of 28 recipients of a prestigious MacArthur genius grant, acknowledging her contributions to the study of black holes in the evolution of galaxies.

From the summit of Mauna Kea, nearly 14,000 feet above the Pacific Ocean, the Milky Way tilts luminously across the night sky, an edge-on view of our galaxy. Parts of the great disk are obscured by dust, and beyond one of those dusty blots, near the teapot of the constellation Sagittarius, lies the center of the Milky Way. Hidden there is a deeply mysterious structure around which more than 200 billion stars revolve.

Behind me atop the craggy rocks of this dormant volcano on the island of Hawaii are the twin domes of the W. M. Keck Observatory. Each dome houses a telescope with a giant mirror almost 33 feet wide and, like a fly's eye, made of interlocking segments. The mirrors are among the world's largest for gathering starlight, and one of the telescopes has been equipped with a dazzling new tool that greatly increases its power. I gaze at the nearest of the Milky Way's graceful spiral arms as I wait for technicians to flip the switch.

Then, suddenly and with the faint click of a shutter sliding open, a golden-orange laser beam shoots into the sky from the open dome. The ray of light, 18 inches wide, appears to end inside one of the blackest spots in the Milky Way. It actually ends 55 miles above the surface of Earth. The signal it makes there allows the telescope to compensate for the blur of Earth's atmosphere. Instead of jittery pictures smeared by the constantly shifting rivers of air over our heads, the telescope produces images as clear as any obtained by satellites in space. Keck was one of the first observatories to be outfitted with a laser guide; now half a dozen others are beginning to use them. The technology provides astronomers with a sharp view of the galaxy's core, where stars are packed as tightly as a summer swarm of gnats and swirl around the darkest place of all: a giant black hole.

The Milky Way's black hole is undoubtedly the strangest thing in our galaxy—a three-dimensional cavity in space ten times the physical size of our sun and four million times the mass, a virtual bottomless pit from which nothing escapes. Every major galaxy, it's now believed, has a black hole at its core. And for the first time, scientists will be able to study the havoc these mind-boggling entities wreak. Throughout this decade, Keck astronomers will track thousands of stars caught in the gravity of the Milky Way's black hole. They will try to figure out how stars are born in its proximity and how it distorts the fabric of space itself. "I find it amazing that we can see stars whipping around our galaxy's black hole," says Taft Armandroff, director of the Keck Observatory. "If you had told me as a graduate student that I'd see that during my career, I'd have said it was science fiction."

To be sure, the evidence for black holes is entirely indirect; astronomers have never actually seen one. Albert Einstein's general theory of relativity predicted that the gravity of an extremely dense body could bend a ray of light so severely that it could not escape. For example, if something with the mass of our sun were shrunk into a ball a mile and a half in diameter, it would be dense enough to trap light. (For Earth to become a black hole, its mass would have to be compressed to the size of a pea.)

In 1939, J. Robert Oppenheimer, the man credited with developing the atom bomb, calculated that such drastic compression could happen to the biggest stars after they ran out of hydrogen and other fuel. Once the stars sputtered out, Oppenheimer and a colleague posited, the remaining gas would collapse due to its own gravity into an infinitely dense point. Telescope observations in the 1960s and 1970s backed up the theory. A few researchers suggested the only possible power source for something so luminous as quasars—extremely bright beacons billions of light-years away—would be a concentration of millions of suns pulled together by what scientists later dubbed a supermassive black hole. Astronomers then found stars that seemed to whip around invisible entities in our Milky Way, and they concluded that only the pull of gravity from small black holes—containing several times the mass of our sun and known as stellar-mass holes—could keep the stars in such tight orbits.

The Hubble Space Telescope added to the evidence for black holes in the 1990s by measuring how quickly the innermost parts of other galaxies rotate—up to 1.1 million miles per hour in big galaxies. The startling speeds pointed to cores containing up to a billion times the mass of the Sun. The discovery that supermassive black holes are at the core of most, if not all, galaxies was one of Hubble's greatest achievements. "At the beginning of the Hubble survey, I would have said black holes are rare, maybe one galaxy in 10 or 100, and that something went wrong in the history of that galaxy," says Hubble scientist Douglas Richstone of the University of Michigan. "Now we've shown they are standard equipment. It's the most remarkable thing."

Even from Hubble, though, the Milky Way's core remained elusive. If our galaxy harbored a supermassive black hole, it was quiet, lacking the belches of energy seen from others. Hubble, which was serviced and upgraded for the final time in 2009, can track groups of stars near the centers of distant galaxies, but because of its narrow angle of view and our galaxy's thick dust clouds, it can't take the same kind of pictures in our galaxy. Another approach would be to track individual stars in the black hole's vicinity using infrared light, which travels through dust, but the stars were too faint and too crowded for most ground-based telescopes to resolve. Still, some astronomers in the 1990s ventured that observations of the Milky Way's core might be possible. A number of tantalizing questions could then be addressed: How do stars live and die in that wild setting? What does a black hole consume? And can we witness, at the heart of the Milky Way, the warped space and time predicted by Einstein nearly a century ago?

The Keck control room is 20 miles from the telescope, in the ranching town of Waimea. To the researchers there, the spectacular laser is visible only as a wan beam on a computer monitor. The astronomers check their notebooks and watch screens full of data from the telescope, weather readings and the latest picture of the stars they're targeting. They use a video link to talk to the telescope operator, who will spend all night at the summit. Things are going so smoothly that there isn't much to do. The telescope will stay locked on the same spot in the sky for four hours; the laser's working fine, and a camera attached to the telescope takes one 15-minute exposure after another in an automated sequence. "This is just about the dullest kind of observing there is," University of California at Los Angeles astronomer Mark Morris says to me apologetically.


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