Question: How does a black hole form and what exactly is it?

Susan Cartwright answered on 23 Jun 2015:

A black hole is any object sufficiently dense that its escape velocity – i.e. the speed you would need to be going to escape its gravitational field – exceeds the speed of light. Escape velocity decreases as you get further from the source of the gravity, because gravity is an inverse square law; the surface at which the escape velocity is exactly the speed of light is called the “event horizon”, and nothing can escape from inside the event horizon. As a result of the lack of information from inside the event horizon, black holes have only three measurable properties: their mass, their rate of spin, and their electric charge (which, in any natural black hole, will be essentially zero – but if it were possible to create a black hole in the lab, I think you’d make very sure it was charged, so that you could use magnetic fields to control it).

Black holes are formed by the deaths of massive stars. When a massive star has used nuclear fusion to create the elements up to iron, it is doomed, because whereas fusing elements lighter than iron releases energy, fusing iron actually consumes energy. So when the star has made a core of iron which is too massive to support its own weight, the core will collapse under gravity, and nothing can stop it.

What usually happens in this collapse is that the core forms a neutron star: the electrons and protons combine to form neutrons (releasing a lot of neutrinos), and the resulting object, which is essentially a single atomic nucleus with a mass about 1.5 times the mass of the Sun (but only about 20 km across!!) is stable, because of the Pauli Exclusion Principle (two neutrons cannot occupy the same space at the same time). However, if the core has a mass more than about 3 times the Sun, theory predicts that the neutron star will become dense enough to collapse into a black hole. Thus, black holes result from the deaths of very massive stars.

Black holes can combine to form larger black holes. In the centre of our Galaxy, and in fact of nearly all large galaxies, there is a “supermassive black hole” – our Galaxy’s is about 4 million times the mass of the Sun, and it’s a small one as these things go. These are formed because stars in the centres of galaxies are quite tightly packed together, so any black hole that forms can easily acquire additional mass, either by colliding with other black holes or by attracting surrounding gas and stars.

Note that black holes don’t “suck”: unless you are really close, their gravity is just like anything else with the same mass. Our Sun is no more in danger of being “sucked into the Galactic black hole” than it would be if the centre of the Galaxy was just tightly packed stars, as we used to think.