Black hole, conceptual computer artwork. Matter is spiralling into the black hole, dragged by immense gravitational forces. This will cause the material to give off high-energy X-rays. A black hole is formed when the core of a star collapses under its own weight, increasing 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 black hole.