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Friday, November 14, 2014

Super Black Hole: Sagittarius A and G2

"In 1974, an unusually compact source of radio waves at the exact center of the Milky Way, some 26,000 light-years away. Named Sagittarius A, after the constellation where it appears in the sky, it hints of something strange: a trickle of X-rays from the same spot, tendrils of agitated gas surrounding it and, most telling, a small group of stars racing around madly for no obvious reason.
Tracking the motions of those stars enabled astronomers to estimate the mass of the unseen object directing the action. From there they built a convincing case that Sagittarius A* was in fact a black hole — the biggest one in the galaxy, with a mass 4.3 million times that of the sun and a diameter of about 25 million kilometers.... when Sagittarius A* was acting up, it was at least a million times as energetic as it is today. Put another way, it briefly shone as brilliantly as a million suns.
"In fact, other researchers may have already caught Sagittarius A* in the act of such a feeding frenzy, albeit on a much smaller scale. Every day or so, Chandra captures modest flare-ups during which the black hole brightens up to a factor of 160 for a few hours.... those events are burps caused when an asteroid or comet, at least 10 kilometers across, passes within about 100 million kilometers of the black hole, quickly getting shredded and consumed."
And now a large gas cloud known as G2 is whipping past the black hole at 10 million kph. It is the first time scientists have ever seen such a thing.... the black hole’s potent gravity has warped G2 into a long, snaking blob, with the leading part already coiled all the way around Sagittarius A*. At its closest, G2 will pass about 25 billion kilometers from the event horizon, far enough that it is unlikely to get entirely sucked in. But the cloud is so elongated that it will take a full year for all of it to clear the black hole, and there is plenty of room for surprise.
G2 might make fireworks another way, because Sagittarius A* is probably surrounded by smaller black holes. Much as hair collects around a bathtub drain, the most massive objects in the Milky Way tend to spiral downward toward the center. As a result, an estimated 20,000 black holes, each about the size of a city and containing a few times the mass of the sun, are thought to be circling Sagittarius A*. As G2 blows past that dark swarm, the smaller black holes may scoop up bits of loose gas and light up with bursts of X-rays that could be visible to Chandra or NASA’s NuStar space telescope.