EVEN gluttons can’t eat forever. When black holes at the hearts of galaxies swell to 50 billion times the mass of the sun, they may lose the discs of gas they use as cosmic feedlots.

Most galaxies host a supermassive black hole at their centre. Around it is a region of space where gas settles into an orbiting disc. This gas can lose energy and fall inwards, feeding the black hole. But these discs are known to be unstable and prone to crumbling into stars.

Theoretically, a black hole could grow so big that it swallows up the stable part of the disc and destroys it. However, no one thought that black holes would grow so big that they could do this. “It didn’t occur to us to worry about it, because the mass required was so large,” says Andrew King of the University of Leicester, UK.

But that’s changed in recent years. The heaviest black holes we’ve now seen have a mass up to 40 billion times that of our sun. That fact prompted King to calculate how big a black hole would have to be for its outer edge to keep a disc from forming. He came up with the figure of 50 billion solar masses (arxiv.org/abs/1511.08502).

Without a disc, the black hole would stop growing, meaning 50 billion suns would be about the upper limit. The only way it could get larger is if a star happened to fall straight in or another black hole merged with it. Neither process fattens them up as well as a gas disc. “Unless you merge with another monster, you’ll make almost no difference to the black hole mass,” King says.