Question: what gas is suked up by black holes

Sam Connolly answered on 13 Jun 2014:

Depends on the type of black hole!

Small black holes, which there are quite a lot of in our galaxy and are formed from massive stars collapsing, often don’t suck up any gas, so we can’t see them (well we think so, but we can’t see them if they aren’t sucking up gas, so we don’t know!). The only time they suck up enough gas to see is when they’re in a ‘binary’, with another normal star orbiting around them (we see the gas as it heats up whilst falling onto the black hole). Because they have very strong gravity compared to the normal star, they suck gas off the other star, which swirls around the black hole as it falls into it, giving off a lot of light which we can see.

Supermassive black holes are really big ones at the centres of galaxies, and also don’t always suck up gas – the one at the centre of our galaxy (called Sagittarius A*) isn’t sucking up much gas at the moment, so the only reason we know it’s there is because we can see lots of stars orbiting around an empty bit of space! What I look at is ‘active’ black holes, which have enough gas nearby that they’re constantly sucking up a lot of it and giving of a lot of light, which we can study. The gas could be left over from when the galaxy formed, or could be blown towards it by supernovae (which is stars exploding at the end of their lives). We’re not entirely sure how exactly the gas gets to these supermassive black holes though, that’s one of the things we’re still trying to figure out!