Black hole, computer artwork. Planets, moons, stars, radiation and other matter are all 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.