Milky Way's Black Hole Prepares for Massive Feast, A doomed gas cloud accelerating towards the black hole at the centre of the Milky Way has been observed for the first time.

The gas cloud, moving at more than eight million kilometres an hour, has nearly doubled in speed over the last seven years, and is predicted to pass within 40 billion km of the black hole's event horizon in mid 2013. This event will be the first time that an object approaching a supermassive black hole has been observed.

Formally known as Sgr A* (pronounced Sagittarius A star), the black hole at the centre of the Milky Way is the closest black hole known to astronomers, "about 100 times closer to Earth than the nearest supermassive black hole in another galactic nucleus, that of the Andromeda galaxy", said Mark Morris from the University of California, Los Angeles Department of Physics and Astronomy, author of the 'News & Views' article accompanying the study in Nature today.


Watching the black hole

Sgr A* is 26,000 light years from Earth, and the cloud will pass it at less than 10 times the distance from the Sun to Neptune. A light year is calculated as the distance that light travels in one hour - roughly the distance between the Sun and Jupiter.

Relatively speaking, the Earth's proximity to this monumental event will allow observers unprecedented access to such an enormous astronomical event, and could provide valuable information as to how gaseous matter behaves once it is within the force field of a black hole.

Stefan Gillessen from the Max Planck Institute for Extraterrestrial Physics in Germany and the paper's lead author has been researching Sgr A*'s activity since 1992. "This is an extremely exciting event because it shows how objects assemble their mass. This is a new kind of object, and we've observed that the gas cloud is being elongated and stretched as it moves towards the black hole," he said.

Gillessen added, "The idea of an astronaut close to a black hole being stretched out like spaghetti is a familiar idea, thanks to science fiction. But we can now see this happening for real. The newly discovered cloud is not going to survive the experience."

Waking a sleeping giant

Acknowledging that the events of 2013 are sure to "wake a sleeping giant", Morris has reservations about the estimated mass of the cloud. Gillessen and his team estimate the cloud to be the size of the Solar System, with a mass equivalent to three times that of Earth.

As the density of the cloud will directly impact the resulting observation, the researchers asserting that the mass began as a "spheroidal, thick blob" and that its movement towards Sgr A* has retained the properties expected of the black hole's tidal forces, shearing the cloud along the direction of its motion. A thin, dense sheet, they purport, "would by now already have fragmented and disintegrated".

Morris maintains that the scale of the incoming cloud is modest, and while there certainly is a mass moving towards the black hole that will undoubtedly make a close pass, "I am not entirely convinced it is a three-Earth mass. It's hard for a small piece of gas to thrive in that hostile environment for long, and therefore it is possible that it may be held together by something more massive, like a star that hasn't been seen because it's too dim, or another enveloping magnetic field."

The inevitable doom

Morris describes his doubt that the cloud's ability to retain its own gravity as it approaches the black hole is that the "inevitable doom of such a blob of gas is its inexorable tendency towards fragmentation, such as the Rayleigh-Taylor and Kelvin-Helmholtz instabilities, cleave and pare the cloud into a cluster of smaller and more ill-defined fragments, like a disintegrating satellite entering Earth's atmosphere."

Whether or not the gas cloud is able to retain an independent gravitational field on its approach to the black hole, it is a near-certainty that some of the gases will fuel an accretion-induced brightening of the black hole. Light, be it a giant radiation flare or a faint illumination, will inevitably shed light on the mysterious workings of galactic black holes.