Supermassive black hole. Artwork of an accretion disc of infalling material (orange) and a high-energy particle jet (blue) associated with a supermassive black hole at the centre of a galaxy. Black holes are formed when matter collapses to densities where gravity prevents light escaping. Supermassive ones can contain the mass of billions of stars. X-rays from the base of the jet illuminates the accretion disc, allowing studies such as those carried out by NASA's NuSTAR mission (Nuclear Spectroscopic Telescope Array). This allows calculations of the rate of rotation within the accretion disc, and the rate of spin of the black hole. Image published in 2013.