X-ray map of the whole sky obtained by the ROSAT satellite. It shows about 50,000 X-ray sources whose intensity runs from red for the less intense sources through yellow and blue for the brightest ones. More than half of these sources are thought to be active galactic nuclei such as Seyfert galaxies and quasars. The X-ray emission from these objects is probably powered by a supermassive black hole, weighing as much as one billion Suns, located in their centre. Other X-ray sources seen here include X-ray binary star systems which are formed by a normal star orbiting around a black hole or a neutron star.