Astronomers see gas cloud ripped apart by black hole

Astronomers around the world are taking advantage of a unique opportunity to watch as a cloud of gas is ripped apart by the supermassive black hole at the centre of our Milky Way galaxy.

Simulation of gas cloud meeting black hole
A simulation of the gas cloud passing close to the supermassive black hole at the centre of the galaxy. Credit: ESO/S. Gillessen/MPE/Marc Schartmann

An array of telescopes on the ground and in space have been observing the first encounter ever witnessed by the hungry monster, which is up to five million times the mass of the Sun, and the swirling gas and dust.

Latest results from the European Southern Observatory’s Very Large Telescope in Chile show that the gas cloud, dubbed G2, has become hugely stretched by the black hole like strands of spaghetti.

Its front part has zipped passed the powerful giant, labelled Sgr A* (pronounced Sagittarius A star), getting about five times as close to it as Neptune is to the Sun. But the gas in its trailing tail is still falling towards this cosmic plughole.

Moving cloud
VLT observations of the cloud in 2006, 2010 and 2013 are shown in false colour of blue, green and red respectively. Credit: ESO/S. Gillessen

Nothing can escape a black hole. So anything that crosses its event horizon and gets sucked inside will leave our Universe never to return. By implication, we can never see one directly, but they give themselves away from their gravitational effects on stars orbiting them and radiation emitted from gas and dust that has collected in what is called an accretion disk around them.

The changing shape of the cloud has been monitored by a spectroscopic instrument called SINFONI on the VLT with its 8.2-metre wide light-gathering mirror.

A black hole’s scientific name is a singularity. Supermassive black holes exist at the hearts of many galaxies and are often considerably more massive than that in the Milky Way. So observing our own local version can tell us a lot about the evolution of stars and galaxies.

The gas cloud, which has been simply labelled G2, was only discovered in 2011 and completely by chance as astronomers examined images taken nine years earlier using Europe’s Very Large Telescope in Chile.

Stefan Gillessen, of the Max Planck Institute for Extraterrestrial Physics in Germany, led the team that discovered the gas cloud. He said: “The gas at the head of the cloud is now stretched over more than 160 billion kilometres around the closest point of the orbit to the black hole.

“And the closest approach is only a bit more than 25 billion kilometres from the black hole itself — barely escaping falling right in. The cloud is so stretched that the close approach is not a single event but rather a process that extends over a period of at least one year.”

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Gillesen told Skymania News that his team first spotted the cloud in 2011 then found that they had recorded images of it as far back as 2002. He said: “As is typical in science, it was a bit of a random find. We were looking for something else about the black hole. It struck us that there was something rather faint but visible moving over the years towards the position of the black hole.

“We decided to look back at our previous observations and found that it had been sitting there quietly in the data since 2002. The first spectroscopy was achieved in 2004. We found we had recorded spectroscopic data which showed that the object was larger in 2011 than it was in 2004. It had stretched along an elliptical orbit around the black hole.”

Gillessen says: “Watching what happens to this gas cloud will help tell us how atmospheres around black holes are structured. From the amount of friction the cloud experiences, we can estimate how much gas there is and so learn how such an atmosphere is made. It will also be useful to see how the gas circles the black hole to help tell us how it is being fed. Nobody has ever seen that.”

Reinhard Genzel, leader of the research group that has been studied this region for nearly 20 years, said: “The most exciting thing we now see in the new observations is the head of the cloud coming back towards us at more than 10 million km/h along the orbit — about one per cent of the speed of light. This means that the front end of the cloud has already made its closest approach to the black hole.”

Astronomers are uncertain about the gas cloud’s origin. Gillessen told Skymania News: “It could be a result of the collisions between winds from massive stars flying around the galactic centre.”

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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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