Question: What is a black hole, how is it formed and what does it do?

Christina Pagel answered on 28 Jun 2013:

A black hole is the left over bits of a massive star…

You need to start off with a huge star (5 or more times as heavy as our sun) – such a star is always trying to shrink under its own weight (as gravity pulls stuff towards the middle), but as this happens the hydrogen atoms in the star (stars are mostly hydrogen) get so close together they fuse together to create helium and release loads of energy (this is fusion). That energy makes the star want to get bigger again and so for billions of years you have this push (fusion)/pull (gravity) thing going on that keeps stars at about the same size… eventually, the star can;t make enough energy any more to keep pushing out (all its hydrogen is gone) and then it will shrink under gravity – if it’s big enough the gravity is so strong that eventually you get something so big in a tiny tiny space that even light can’t escape from it… and that’s a black hole!!

to think how amazing this is think about neutron stars – these are what’s left over if the star isn’t quite big enough to make a black hole (so about 3 times the size of the sun). A neutron star is so tightly packed that it weighs 3 times as much as the sun but is only 24 kilometres wide!!! That’s less than the size of London!