Question: how do black holes form?

Edward Bovill answered on 12 Mar 2013:

Black holes form when really, really huge stars die.

A star reaches the end of it’s life when all the fuel inside it runs out. The fuel for a star is hydrogen. Inside a star it is so hot that hydrogen atoms have enough energy to smash together and form helium. When this happens it produces lots of energy in the form of light and heat, which is why stars are so hot and bright.

Stars are so incredibly large that their own gravity wants to constantly make them collapse in on themselves. The heat and energy produced by smashing hydrogen atoms together is all that keeps them from collapsing.

When the hydrogen fuel runs out, the inside of the star stops producing heat and the star starts to collapse in on itself.

For smallish stars, like our Sun, first what happens is the star starts to smash helium together to form carbon and nitrogen. When this happens the star grows in size and forms a red giant star; for the Sun it will grow so big it will engulf the Earth, totally destroying any life left at that point (this won’t happen for billions of years so nothing to worry about). Eventually, the star will run out of helium and then it collapses down to a ball about the size of the Earth called a white dwarf star. These still shine a little bit, but not much compared to a normal star.

If the star is a lot bigger than our Sun, when it starts to collapse (after it has expanded to make a red giant or supergiant) it will form something called a neutron star. These are really tiny, about the size of a big city, but contain the mass of over 3 suns! A teaspoon full of a neutron star would weigh 900 times as much as the great pyramid of Egypt! Neutron stars do not shine at all.

If the star is really, really big (about 20 times as big as our Sun), then when the star collapses it is so dense and heavy that it will continue collapsing onto itself until nothing remains. All the mass from the star will be compressed into a point so tiny you cannot measure it. The effects of the gravity of this tiny object are so strong that once you get caught in its gravitational pull you cannot escape. Not even light escapes a black hole’s gravitational pull – which is why they are called ‘black holes’.