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Beast in the MIDDLE, The

Muse, May/Jun 2004 by Irion, Robert

The night sky seems so peaceful. Stars appear in the old familiar patterns night after night and year after year. And when the sky is dark, we can see our Milky Way galaxy stretching far above the horizon like a faint cloud, drifting peacefully through the black universe.

But deep within that quiet cloud, at the center of our galaxy, lurks a ravenous monster: a gigantic black hole that sucks gas, dust, and sometimes entire stars down its throat, crushing them beyond recognition. And once something disappears into the hole, it's gone forever, vanished from our universe, never to be seen again.

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Astronomers now think almost every galaxy hosts one of these celestial beasts. But what are black holes? What are they made of, and where do they come from? No one knows for sure, but the more we learn, the more deceptive the peaceful vista of the night sky begins to seem.

Black holes are so weird it took astronomers a long time to believe they exist, much less to start looking for them. Albert Einstein's theories about light and gravity allowed for such things, but everyone thought that was just an odd side effect of the math-even Einstein, who hated the whole idea of black holes.

In the 1930s, astronomers calculated that giant stars might collapse into black holes at the end of their lives. Giant stars are like raging furnaces in space. They burn fuel deep inside their cores. Heat and light streaming out of the star keep the entire ball of gas inflated like a hot balloon. But when the star runs out of fuel, it can no longer support the vast weight of the gas around it. As the star collapses inward, all hell breaks loose. The outer layers rip apart, throwing off a glowing, expanding shell of gas called a supernova. Deep within the supernova, gravity crushes the core of the star until it fits within a tiny region. If Earth experienced the same kind of squeeze, its entire mass would fit within a space no bigger than a gumball.

In the 1970s and 1980s astronomers finally began to find black holes. They've now located about 20 objects in the disk of the Milky Way, mostly toward the center, that have the right signs. These compact objects, which typically weigh about 10 times more than our sun, were probably created by the collapse of single stars that had 20 or 30 times the sun's mass.

Exciting as these discoveries were, other astronomers suspected there were even stranger beasts to come. They imagined that gigantic black holes-weighing millions or billions of times more than our sun-might squat at the centers of galaxies. Gradually they began to collect evidence that this was really true. Shooting from the cores of some galaxies, they saw brilliant beacons of light and x-rays. These beacons, which were powerful enough to shine across the entire universe, might be coming from blistering hot gas, dust, and stars spiraling into the maw of a giant black hole that flared up one last time before being forever extinguished. And there were other clues as well.

As evidence built for giant black holes in other galaxies, astronomers itched to study the core of our own galaxy, the Milky Way. Since it's so much closer, you'd think it would be easier to see. Paradoxically, telescopes could see the centers of other galaxies better than the center of our own. There is too much dust in the way, so to see what was going on in the center astronomers had to wait for telescopes that could see through dust. (To find out how this works, see "How Do You See through a Veil?" on page 18.)

But by the mid-1990s, two teams of astronomers were pointing the world's biggest telescopes at our galaxy's heart. The telescopes, located in Hawaii and in Chile, South America, have mirrors about 30 feet wide. That's as wide as a tennis court. Just as an owl's big eyes collect enough light to let it hunt at night, these huge eyes gathered enough light to let the astronomers spy on the Milky Way's innermost stars.

What did they see? Both teams were excited to discover that the stars near the center of our galaxy could be seen to move from year to year-unlike the stars near our sun, which are so nearly fixed they might as well be pinned to the heavens. This might not seem all that exciting, but it meant the stars were zipping about like angry bees rather than sailing sedately through the heavens as most stars do. The researchers mapped the stars' positions to trace their orbits and to calculate their orbital speeds.

The results were startling. One star passed within 6 billion miles of the suspected black hole, nearly as close as Pluto is to the sun. For a star diving near a black hole, that's a close call. As the star approached the suspect spot, scientists calculated its speed to be nearly 6000 miles per second, or 20 million miles per hour. That's more than 300 times faster than the speed at which Earth orbits the sun. No one knew a star could move that fast.

Once they had measured the speeds and distances of several stars, the teams turned to the laws of gravity to "weigh" the black hole. A light object orbiting a heavy one is a bit like an ice skater whose friend, feet firmly planted, is pulling him around in circles. If the friend is really heavy, he can whip the skater around quite fast. But if he's light, he can't twirl the skater around as quickly. In the same way, if Earth were heavier, the moon would revolve around it in less than the month it currently takes.

Beast in the MIDDLE, The

Muse, May/Jun 2004 by Irion, Robert

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