December 19, 2011 – SPACE – The sleeping giant at the centre of the Milky Way is about to wake up. A suicidal gas cloud is heading towards the galaxy’s super-massive black hole, which will probably swallow the cloud, generating enormous flares of radiation that could help explain why the black hole is normally so placid. The doomed cloud was a surprise to astronomers. “We have been looking at the galactic centre for 20 years, but mainly to observe the motion of stars,” says Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. Genzel’s colleague Stefan Gillessen spotted the cloud in images from the Very Large Telescope array in Chile, taken in March this year. It is an unusually dense cloud, not much bigger than our solar system and carrying about three times the mass of Earth. The team realized that the cloud also appears in earlier images, giving them a sequence that reveals its path. It is moving at almost 2500 kilometres per second towards our galaxy’s black hole, Sagittarius A*. At present Sagittarius A* is strangely quiet, unlike quasars, the hyperactive black holes that emit huge amounts of radiation, fuelled by inflowing gas. Our black hole gets much less gas, and for some reason this starvation state makes it much less efficient than a quasar, producing only a thousandth as much radiation per kilogram of fuel. While a star would just sail past our black hole unscathed, the loose mass of gas heading towards it is more vulnerable. It is already being stretched out by the black hole’s gravity, and when it gets closer in 2013 it will plough into the halo of hot gas around the hole. That should send shockwaves through the cloud to heat it to several million degrees, and according to the group’s simulations the gaseous collision will shred the cloud into filaments. This turmoil may mean that much of the cloud ends up swirling right down into the black hole. There’s no danger of the active black hole harming Earth. –New Scientist
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