Question: why is there a black hole?

Sammie Buzzard answered on 13 Mar 2018:

Some black holes can be created by dying stars but they need to be much bigger than our Sun, about 20 times bigger.

Normally a star produces energy through nuclear reactions inside its core and this balances gravity trying to pull the star in. But when the star runs out of fuel for these reactions, the star dies. Small stars can cool down and during into white dwarfs. But for big stars gravity wins and the core collapses in on itself and can sometimes form a black hole. The rest of the star explodes and a supernova is formed.

The black hole has a boundary called an ‘event horizon’, anything that passes that boundary is trapped in the black hole, including light!