Cloud suicide will wake black hole sleeping giant
18:00 14 December 2011
Stephen Battersby
Translation by Autumnson Blog
自殺的氣體雲在2013年夠時候分離（圖片提供：ESO / MPE/馬克 Schartmann）
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 supermassive 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.
在銀河系中心的沉睡巨人差不多要醒了。一把自殺的氣體雲正朝向星系的超大型黑洞，它可能會吞下該雲，產生巨大的輻射耀斑，可有助於解釋為什麼黑洞通常是如此平和的。
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.
注定了的雲對天文學家是一驚訝， “我們一直在看著銀河系中心凡20年，但主要是觀察恆星的動靜，：”德國嘉興馬克斯普朗克地外物理研究所的萊因哈德 Genzel說。
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.
Genzel的同事斯特凡Gillessen從智利的非常大望遠鏡陣列拍於今年3月的圖像中發現該雲。它是一頗不尋常地密集的雲，比我們太陽系的並沒有大些和攜帶約三倍於地球的質量。
撕碎的雲
Shredded cloud
The team realised 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.
不幸的時機
Unlucky timing
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.
"By dumping more material in there, the cloud could drive the system into a higher efficiency regime," says Ginzel. There may be one huge flare of radiation or several over the coming decades.
There's no danger of the active black hole harming Earth. And though sadly not visible to the naked eye, this radiation will give astronomers clues as to why our black hole is normally so different from quasars.
"There is evidence that the galactic centre was more luminous within the last few thousand years, and we are unlucky in living at a time when it appears to be unusually dormant," says astrophysicist Martin Rees at the University of Cambridge, who was not part of the study.
http://www.newscientist.com/article/dn21288-cloud-suicide-will-wake-black-hole-sleeping-giant.html
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