X-ray sourcesLuminous sources of X-ray emission lying well beyond the Solar System. More than 120 000 are now known, the majority from the ROSAT all-sky survey and deep pointing observations. Optical counterparts include a wide range of astronomical objects, from high-redshift quasars to nearby stars, and even comets. The brightest sources lie in our Galaxy and are identified principally as X-ray binaries (e.g. Scorpius X-1, Cygnus X-1, Cygnus X-3, Centaurus X-3, Hercules X-1), many having an X-ray power 100 to 1000 times the power of the Sun. A smaller subset of bright galactic X-ray sources are identified with young supernova remnants, such as the Crab nebula and the remnant of Tycho's star. Fainter but intrinsically much more powerful X-ray emission is detected from many extragalactic objects, especially rich clusters of galaxies and active galaxies – such as Seyfert galaxies, quasars, and powerful radio galaxies, including Virgo A (M87) and Centaurus A.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006