Black hole. Artist's impression of a black hole with a glowing accretion disk. The matter in the disk may have been pulled from the outer layers of a companion star. A disk is formed because matter has angular momentum transferred to it by gravita- tional effects as it falls towards the rapidly- spinning hole. The matter in the disk is compress- ed & heated so that it begins to glow. Parts of the disk can reach millions of degrees celsius, hot enough to emit X-rays. Black holes are incred- ibly dense objects that are formed from the cores of huge stars which exploded as supernovae when they exhausted their fuel supply. Their gravity is so strong that light cannot escape from them.