How massive can a black hole get?

Black holes are common throughout the Universe, and it is highly likely that there is one at the heart of most galaxies, including our own. A black hole is the result of a supergiant star coming to the end of its life. After exploding as a supernova, the core collapses under its own gravity into an object that is so dense that nothing can escape from it, not even light. That means that the object cannot be seen and it has, in effect, opted out of the Universe and the laws of physics. The black hole may only be a few kilometres in diameter in terms of physical size.

It is thought that most black holes are relatively small, at around three to six stellar masses, but there is also evidence to suggest that others could be much more massive, and these are the ones that may lie at the centres of galaxies, including our own.

In terms of mass, there is an upper size limit for black holes. However, that limit is almost unimaginably large at 50 billion times that of the sun!

Calculations have been done by Andrew King, an astronomer at the University of Leicester in the United Kingdom, based on the fact that the mass of a black hole continues to grow by absorbing gas at its margin. This gas, which may have come from a star that formed a binary pair with the original collapsed supergiant, forms an “accretion disc” that orbits the black hole at considerable speed. As it loses energy the gas falls inwards and feeds the black hole, thus adding to its mass. It will emit x-rays as it does so, thus enabling the black hole to be detected from Earth.

In theory, a black hole could grow so big that it absorbed all the gas in the orbiting disc, but the mass required to do this would be incredibly large. As it is, it has been calculated that there are black holes in the Universe that have masses of around 40 billion suns.

But how big would be a black hole have to get before it prevented a disc of gas from forming around it? Andrew King’s calculation sets this at 50 billion suns. At this mass it would not be possible for any gas to remain in its vicinity and it could not get any larger.

However, it is possible to speculate that a black hole could grow by swallowing whole stars. Andrew King contends that this would make almost no difference to the overall mass of the black hole. For any real difference to be noticed, two supermassive black holes would have to merge together.

© John Welford