An X-ray image of half the universe

The 900,000 sources include around 710,000 supermassive black holes in distant galaxies (active galactic nuclei), 180.000 X-ray emitting stars in our own Milky Way, 12,000 clusters of galaxies, plus a small number of other exotic classes of sources like X-ray emitting binary stars, supernova remnants, pulsars, and other objects.