There’s a giant black hole with the mass of 4 million Suns at the centre of the Milky Way. But that’s still tiny compared to its counterparts in other galaxies
Black holes are the most extreme astrophysical phenomena in the universe. And there’s a good chance they’re the best known, too. Not everyone has heard of neutron stars, planetary nebulae or brown dwarfs, but most people have come across the idea of a black hole, even if it was only in a sciencefiction film. Part of the fascination lies in the way black holes seem to break – or at least twist beyond recognition – all the commonsense laws of physics. Looked at more closely, though, they’re really just an inevitable consequence of these laws.
As a theoretical concept, black holes first made their appearance when scientists were thinking about gravity, particularly in the context of large objects such as stars.
Gravity is a force that acts between particles of matter, tending to pull them closer and closer together. In everyday objects in the world around us, the force of gravity is counterbalanced by other physical effects such as electrostatic and nuclear forces – with the result that things maintain a finite size rather than collapsing down to a point. The same is true of stars like the Sun, where gravity is counteracted by fluid pressure. Even in an ultra-compact star like a white dwarf, where a mass similar to that of the Sun is squashed down to a planet-sized volume, there are other more exotic forces that come into play to stop them collapsing forever.
By the middle of the 20th century, however, scientists knew there was an upper limit to the amount of gravity that these exotic forces could resist. If a star was sufficiently massive to start with, it would eventually collapse all the way down to an infinitesimally small point. All the original mass would still be there, but compressed down to a state of infinitely high density. This sounded so impossible – particularly to mathematicians, who dislike having ‘infinity’ in their equations – that many thought some new kind of physics would come to the rescue to prevent the infinite collapse. But it didn’t, and by the 1960s it was generally accepted that these mind-bending phenomena really were a possibility. It was around this time that they acquired the name by which we know them now: black holes.
It may seem like a rather big leap to go from the idea of matter being compressed down to an infinitesimally small point to talking about a ‘hole’ in space – one that it’s possible to fall into, but never climb out of. Yet this really is a logical step to take if we th
