Illustration of Cygnus X-1 an X-ray emitting black hole in the constellation Cygnus, around 7,000 light years from Earth. This was the first object identified as a black hole. It is in a binary system with the blue supergiant star HDE226868 (not shown). As matter from the star falls into the black hole it is heated and some is emitted as X-rays. A black hole is formed when the core of a star collapses 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. Only sufficiently massive stars form black holes, doing so when they have used up the fuel that sustained their nuclear explosions.