A Front-Row Seat to a Black Hole in Action
A gas cloud is nearing a black hole  -  and the celestial fireworks show is going to be impressive By Michael Lemonick Feb. 04, 2013 Share
ESO / MPE / Marc SchartmannA simulation of how the gas cloud may break apart as it approaches the black hole Email
If you want to feel a shiver of cosmic menace, just ponder black holes. Venture a bit too close to one of these voracious monsters, and you'll never get out  -  although it hardly matters, since you'll be torn to shreds and flash-heated to millions of degrees along the way. Star-size black holes are bad enough, but the supermassive holes that lurk at the centers of most galaxies are millions of times more powerful. When they swallow a star or a giant gas cloud, we call the resulting flare of energy a quasar, which can be visible halfway across the universe.
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Now it's about to happen  -  albeit with less spectacular fireworks  -  right in our backyard. Back in 2011, astronomers spotted an interstellar gas cloud plunging more or less toward the Milky Way's own supermassive black hole, which is about the mass of 4 million suns. And by the scientists' calculations, the cloud will meet its doom this coming September or October. "The impact will be deeper and more exciting than we thought," says Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and the lead author of the Nature report that first announced the cloud's existence.
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Ordinarily, astronomers would expect an interstellar cloud like this (as well as any stars in the vicinity) to be orbiting the central black hole at an angle, spiraling in only gradually. Not this one, though. "It's remarkable how directly it's moving toward the black hole," says Reinhard Genzel of the University of California, Berkeley, one of Gillessen's co-authors. "Someone really aimed it very well."
Astronomers have already seen changes in the cloud's structure since it was first discovered. "There are clear signs that it's being stretched," says Gillessen. That's a result of tidal forces: the cloud's leading edge feels the black hole's gravity much more strongly than the trailing edge. The difference in speed between front and rear is about 360 miles (580 km) per second, and by April, says Gillessen, "we're pretty sure the cloud should be starting to shred apart." It is reminiscent, albeit on a much larger scale, of the fragmentation of Comet Shoemaker-Levy 9, which was tidally broken apart by Jupiter's gravity before plunging to its death in July 1994.
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Even at its closest approach later in the year, the cloud fragments will not have reached the black hole's Schwarzschild radius  -  the point of no return, where a final plunge into infinite density and pressure is inescapable. But it should soon be slamming into the black hole's "atmosphere"  -  the thin haze of gas that whirls around it at a safe distance. "That could create shock waves, which could be visible in X-ray wavelengths," says Gillessen.
The bits of cloud may eventually funnel into the black hole itself, orbiting faster and faster, like water spiraling down a drain as the cloud's own internal friction heats it to millions of degrees, giving off bursts of energy as it goes. Nobody knows quite how long it might take for that to happen. "I don't necessarily expect fireworks next fall," says Genzel, "but there could be. It might be that bits and pieces might shoot directly in."
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Astronomers around the world will be tuned in just in case. "People will be looking with telescopes in all wave bands, from radio to gamma rays," says Gillessen. "There's a list of proposals from those who want to observe it. We'll certainly see the cloud shredded this year ... Whether we see something more, well, that's the fun part."
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