Black hole formation. Computer artwork showing the formation of a black hole in four steps (from left to right). A black hole (far right) is formed when the core of a star (far left) collapses (centre right) under its own weight. This increases its gravitational field to the point where, beyond a boundary known as the event horizon, nothing, not even light, can escape. This leads to its black appearance. Only sufficiently massive stars form black holes, doing so when they have used up the fuel that sustained their nuclear fires. This results in a loss of outwards heat pressure, allowing the total collapse that forms the singularity that is a black hole.