Astronomers have spotted a gas cloud on a collision course with the supermassive black hole at the centre of the Milky Way. The black hole, known as Sagittarius A* (Sgr A*) is a hungry beast that it is 4.3 million times more massive than the Sun. Relatively little material has been engulfed by the black hole in recent years, but that is about to change.  

The path of the doomed gas cloud is shown here in red, with nearby stars’ orbits shown in blue. (ESO)

It is suspected that all galaxies are hiding a central supermassive black hole, but the one in our own Galaxy is the only one close enough for astronomers to study in sufficient detail. So when they realised that a cool gas cloud was headed straight for Sgr A*, it caused quite a lot of excitement.

“We have not expected to find such an event. We never actually observed with the specific purpose to detect such a cloud,” says Stefan Gillessen, lead author of the paper announcing the discovery and fate of the cloud. This will be the first time that any object has ever been observed as it gets swallowed by the central black hole.

The gas cloud was discovered by taking advantage of the adaptive optics on the European Southern Observatory’s Very Large Telescope in Chile. While the gas cloud has yet to cross the point of no return, known as the event horizon, it is already showing signs of being swept in by the immense gravitational force of the monstrous black hole. The speed of the cloud has doubled in the last seven years, and as it whizzes ever closer to its doom it will start to become compressed and stretched out in a process known as spaghettification.

The gas cloud most likely originated from the stellar winds in a nearby binary colliding, and as the cloud approaches the black hole it will start to heat up. This heat will result in X-ray emission, and NASA’s Chandra X-ray Observatory in space will have its eyes peeled in anticipation.

The X-ray display will begin shortly before the gas cloud reaches the pericentre of its orbit in mid 2013, i.e. the closest point to the black hole. It will not necessarily by immediately gobbled by the black hole – this process could take up to 10 years.

“The total amount of energy available is rather large,” Gillessen tells Skymania News. “Even if the material would rain down onto the black hole over the next 10 years, the resulting luminosity from that might still be pretty spectacular.” The light emitted from the area around the black hole could increase 1000-fold as the gas cloud nears the black hole.

The supermassive black hole’s existence, hidden behind clouds of dust, was confirmed by ESO telescopes quite recently. But Sgr A* has been monitored since 1992 and since then only two other contenders came within dangerous distances of the black hole. The two stars actually passed by the black hole at a distance closer than the gas cloud, but they continued safely on their way. “A star is a compact, self-gravitating object that can survive the tidal forces and the flight through the ambient gas without feeling it really,” explains Gillessen. “The cloud on the other hand is a much thinner object and has no self-gravity – such that the external forces can destroy it.”

Gillessen is confident that much can be learned about the physics of black holes as the gas cloud is wolfed down by Sgr A*. “We can watch how the material ultimately makes it to the event horizon. How does the matter get rid of its angular momentum? In other words: How does the black hole get fed? Another aspect is that the cloud is a well-defined (in astronomical terms) probe that starts to dive into the innermost accretion flow zone. This region is still unprobed, since there was no way to characterize the properties of the gas there. Now, this will become possible because of the cloud falling through that.”