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See Inside Scientific American Volume 308, Issue 6

Summer Blockbuster: A Black Hole Swallows a Cloud

This summer's best fireworks may take place at the center of our galaxy
black hole, black hole swallows cloud



Don Dixon

Astronomers have seen it coming. Starting this summer—possibly this month—a large cloud of gas and dust and perhaps a star will begin to ricochet through the dead center of the Milky Way galaxy, the home of a supermassive black hole. The ensuing celestial fireworks should reveal much about the mysterious central core of the galaxy, a region kept shrouded in darkness by dust and distance.

Scientists have long wondered why the black hole at the center of the Milky Way, unlike the black holes at the center of other large galaxies, is perplexingly quiet. It doesn't seem to be gobbling up matter at nearly the rate we would expect.

Unfortunately, the interesting region around the black hole is just too small for our telescopes to resolve from so far away. (Think of painting the Mona Lisa on a thumbtack, launching it to the moon, then trying to make out her smile.) Our blurry view makes it hard to understand why it is not flaring with energy as it sucks in gas with the gravitational force of four million suns. This observational frustration is why the incoming cloud is so exciting. “If you watch a meteor go through the atmosphere of Earth, it burns up by friction,” says Eliot Quataert, an astrophysicist at the University of California, Berkeley. “We're hoping to see something similar—how the cloud interacts with all the other gas spiraling into the black hole.” The cloud will act like a probe, and astronomers will spend years interpreting its readout: a giant cosmic flare.

Researchers are also hoping to sort out exactly where the cloud came from. Some have suggested that two clumps of gas may have collided near the galaxy's center, draining away the momentum that kept them in orbit. Others think the cloud might be a dim, young solar system, the dust not yet petrified into planetary form, the star obscured by gas.

No matter where the unlucky cloud originated from, its fate is sealed: within a few years it will be sucked past the event horizon of the black hole, its existence obliterated. But astronomers will be studying its long farewell, using every kind of telescope available to them. Perhaps the cloud isn't so unlucky after all, suggests Stefan Gillessen, a member of the team that first discovered the cloud in 2011. “It's unlucky in the sense that it will be destroyed,” he quips, “but lucky in the sense that it becomes famous.”

This article was originally published with the title "Cloud to Black Hole: Eat My Dust."

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