There's a limit to how big black holes can get, but it's a pretty big limit – a mass around 50 billion times that of our Sun, researchers say.

The black hole stops growing and will cease to support itself, when it runs out of the surrounding disk of gas it consumes as food, says Professor Andre King of the University of Leicester in Britain.

As the gas gives up energy and falls inward to feed the growing black hole, the disk can become unstable and eventually collapse as stars begin to form in it.

After calculating how large black holes might need to grow so that their outer edges prevent a gas disc from forming, the researchers came up with a figure of around 50 billion solar masses.

"The significance of this discovery is that astronomers have found black holes of almost the maximum mass, by observing the huge amount of radiation given off by the gas disc as it falls in," King says, who is also the study's leader. "The mass limit means that this procedure should not turn up any masses much bigger than those we know, because there would not be a luminous disc."

For a black hole to be larger that the calculated limit, it would need a star to fall into it as food, or it would need to merge with a second black hole, the researchers say in their study set to appear in Monthly Notices of the Royal Astronomical Society.

Bigger black holes resulting from such a merging would be difficult to detect, King explains, since it would not possess a surrounding disk of gas creating visible light as it was consumed.

King suggested that one may detect it in other ways, for example, as it bent light rays passing nearby, which is a phenomenon known as gravitational lensing.

Also, in the future, we might be able to detect gravitational waves that would be emitted upon merging, as predicted in Einstein's General Theory of Relativity.