How do black holes form?

A star is held up by the outward pressure from the nuclear reactions in its core. When a very massive star exhausts its fuel, this pressure fails and the core collapses catastrophically under gravity, often triggering a supernova explosion. If the remaining core is heavy enough, nothing can stop the collapse, and it shrinks to a point of extreme density, forming a black hole whose gravity is so strong that not even light can escape. The boundary around it, from which nothing can return, is called the event horizon. Supermassive black holes at the centres of galaxies formed through additional processes over billions of years.