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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

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(Continued from page 2)

Mark Morris thinks that "sinkhole" makes an apt metaphor for a black hole. If you were in space near the black hole," he says, "you would see things disappear into it from all directions."

Both Ghez and Morris like to imagine looking out from a black hole. "This is the thriving city center of the galaxy, compared to the suburbs where we are," says Ghez. "Stars are moving at tremendous speeds. You'd see things change on a time scale of tens of minutes." Morris picks up on this theme. "If you look at the night sky from a beautiful mountaintop, it takes your breath away how many stars there are," he says. "Now, multiply that by a million. That's what the sky at the galactic center would look like. It would be like a sky full of Jupiters, and a few stars as bright as the full Moon."

In such a magnificent setting, the laws of physics are wonderfully twisted. Ghez and Morris hope to gather the first evidence that stars do indeed travel along the weird orbital paths predicted by Einstein's relativity theory. If so, each star would trace something like a pattern from a Spirograph drawing toy: a series of loops that gradually shift in position relative to the black hole. Ghez thinks she and her colleagues are several years away from spotting that shift.

With each new finding, the Milky Way's core becomes more perplexing and fascinating. Both Ghez's and Genzel's teams were startled to discover many massive young stars in the black hole's neighborhood. There are scores of them, all just five to ten million years old—infants, in cosmic terms—and they are roughly ten times as massive as our sun. No one is entirely sure how they got so close to the black hole or how they came to be. Elsewhere in the galaxy, gestating stars require a cold, calm womb within a large cloud of dust and gas. The galactic core is anything but calm: intense radiation floods the area, and the black hole's gravity should shred gaseous nurseries before anything incubates there. As Reinhard Genzel put it at a conference several years ago, those young stars "have no damn right to be there." It's possible some of them were born farther out and migrated inward, but most theorists think they're too young for that scenario. Morris thinks the intense gravity compresses spiraling gas into a disk around the black hole, creating the new suns in a type of star birth not seen in any other galactic environment.

These young stars will self-destruct a few million years from now. And when they do, the most massive ones will leave behind small black holes. Morris theorizes that hundreds of thousands of these stellar-mass black holes, accumulated from past generations of stars, swarm around the central, supermassive black hole. The stellar-mass black holes are only about 20 miles wide, so collisions between them would be rare. Instead, Morris says, "You'll have black holes swinging past each other in the night, and stars moving through this destruction derby. A near miss between one of the black holes and a star could scatter the star into the supermassive black hole or out of the galactic center entirely." Theorists think the supermassive black hole may gobble a star once every tens of thousands of years—an event that would flood the center of the galaxy with radiation. "It would be a spectacular event," Morris says.

Astronomers see signs of such gobbling when they examine the Milky Way's interior with X-ray and radio telescopes, which detect the shock waves of past explosions. Giant black holes in other galaxies are too far away for astronomers to study in such depth, says Avi Loeb, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. That's why he hangs on every announcement from the Ghez and Genzel teams. "The advances made by the observers in such a short time have been truly remarkable," he says. "We theorists are all cheerleaders for them."

Loeb and others are painting a new picture of how the universe and its 100 billion galaxies evolved since the Big Bang 13.7 billion years ago. They believe that all galaxies started with as-yet-unexplained "seed" black holes—tens to thousands of times the mass of our sun—that grew exponentially during violent feeding cycles when galaxies collided, which they did more frequently when the universe was younger and galaxies were closer together. In a collision, some stars catapult into deep space and other stars and gases plummet into the newly combined black hole at the galaxies' center. As the black hole grows, Loeb says, it turns into a raging quasar with gas heated to billions of degrees. The quasar then blasts the rest of the gas out of the galaxy entirely. After the gas is depleted, Loeb says, "the supermassive black hole sits at the center of the galaxy, dormant and starved."

It appears that our Milky Way, with its modest-sized black hole, has absorbed only a few smaller galaxies and has never fueled a quasar. However, a fearsome collision looms. The closest large galaxy, called Andromeda, is on a collision course with the Milky Way. The two will start to merge about two billion years from now, gradually forming a massive galaxy that Loeb and his former Harvard-Smithsonian colleague T. J. Cox call "Milkomeda." The galaxies' supermassive central black holes will collide, devouring torrents of gas and igniting a new quasar for a short time in this sedate part of the universe. "We are late bloomers in that regard," Loeb notes. "It happened to most other galaxies early on." (Earth won't get thrown out of the Sun's orbit by the collision and it shouldn't be whacked by anything during the merger. But there will be a lot more stars in the sky.)

Our galaxy's disturbing future aside, Loeb hopes that soon—perhaps within a decade—we'll have the first image of the Milky Way's supermassive black hole, thanks to an emerging global network of "millimeter wave" telescopes. Named for the wavelength of the radio waves they detect, the instruments won't actually see the black hole itself. Rather, in concert they'll map the shadow it casts on a curtain of hot gas behind it. If all goes well, the shadow will have a distinctive shape. Some theorists expect the black hole to be spinning. If so, according to the counterintuitive dragging of space predicted by Einstein, our view of the shadow will be distorted into something like a lopsided and squashed teardrop. "It would be the most remarkable picture we could have," says Loeb.

On the fourth and final night of Ghez's planned observations, wind and fog at the Mauna Kea summit keep the telescope domes closed. So the astronomers review their data from previous nights. Images from the first two nights ranged from good to excellent, says Ghez; the third night was "respectable." She's says she's content: her students have enough to keep them busy, and Tuan Do from the University of California at Irvine identified a few big, young stars to add to the team's analysis. "I feel incredibly privileged to work at something I have this much fun at," Ghez says. "It's hard to believe that black holes really exist, because it's such an exotic state of the universe. We've been able to demonstrate it, and I find that really profound."


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