What is a black hole?

Dr Steve Curran, from the School of Chemical and Physical Sciences, answers:

A black hole is what happens to a star once it runs out of nuclear fuel and collapses.

The sun won’t become a black hole because it’s not big enough—instead it’ll become a white dwarf. But if it was three times as heavy it would collapse to the stage it formed an event horizon—that’s the boundary point at which the gravitational pull becomes so great it makes it impossible for anything, even light, to escape. This is why black holes are completely dark. Also, time slows down the closer you get to a black hole. So a black hole’s essentially a star frozen in time with an event horizon where you just cannot get any information in or out of it.