Question: what causes black holes

Sam Connolly answered on 13 Jun 2014:

There are two types of black hole we can see in space, called ‘stellar’ black holes, which means they come from stars (the same as interstellar travel is travelling between stars), and ‘supermassive’ black holes, which means they have a lot of mass, and are much heavier than the stellar ones.

Stellar black holes are made when really big stars run out fuel to burn and collapse under their own gravity. Supermassive black holes are made at the centres of galaxies, just from the fact that there’s a lot of gas there when the galaxy forms, which is all pulled together by gravity until there’s enough stuff to make a black hole.

The basic way you get a black hole is always the same though – if you get enough stuff in a small enough space (a high enough density), you get a black hole. A black hole is just anything that has strong enough gravity that nothing, including light, can go fast enough to escape. The strength of the gravity of something depends on how heavy something is and how small a space the object is squashed into to.

So any way to squash a lot of matter into a small space is a way to make a black hole! In fact we think that you can have black holes of all sizes, from tiny ones smaller than a person all the way up to the supermassive ones which are as big as the solar system! But we’ve only seen stellar and supermassive ones so far, and perhaps a few in between those two sizes. There’s still a lot to find out still, which is my job!