Question: why do black holes happen ??

Vicky Fawcett answered on 23 Mar 2023:

When a massive star (about 25 times larger than our sun) runs out of fuel, it doesn’t have enough energy to overcome it’s own gravity so it collapses and then explodes in a “supernova”. The material that doesn’t get blown away is incredibly dense and has massive amount of gravity, which we call a black hole.
A black hole that is ten times as massive as the sun is only 30 km big – which is smaller than London! So imagine 10 suns crammed into London… that’s a lot of mass!