Black hole. Artwork of the Cygnus X-1 system, thought to contain a black hole. The system comprises a blue giant star (right) and a small dense object that accretes matter from the star. This object is a powerful emitter of X-rays, due to being heated to extremely high temperatures in the accretion disc. Its mass and X-ray emission both suggest it is a black hole, but no definitive answer has been found. A black hole is a region of space in which the gravitational field is so strong that even light cannot escape. They are thought to form in supernovae, the explosive deaths of massive stars. Cygnus X-1 is around 82,000 light years away.