Black hole, artwork. Surrounding gas and dust are spiralling into the black hole, dragged by immense gravitational forces. This forms an accretion disc (purple/yellow). The extreme conditions cause the emission of high-energy jets. 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 stars of a sufficient mass will form a black hole. In the background is the bright band of the stars of the Milky Way galaxy.