quasar (quasi-stellar object, QSO)

A small extragalactic object that is exceedingly luminous for its angular size and has a high redshift. The word originated as a contraction of "quasi-stellar" radio source, the term given in 1963 to a class of apparently star-like objects, emitting radio waves and having high redshifts. Quasars are now generally believed to be the most luminous type of active galactic nuclei. Around a small number, the faint nebulous light of a surrounding galaxy has been detected. Many thousands of quasars have been catalogued. In general, quasars have a spectrum that shows emission lines, high redshifts (typically from 0.5 to 4, although values higher and lower than these have been recorded), and they are so compact that they appear as sharp as stars on photographs. Although the quasars discovered in the 1960s were all radio sources, most of those now known are not strong radio sources. Quasars have the largest redshifts found, and their importance in astronomy stems primarily from this feature. If the redshifts follow from the expansion of the universe, Hubble's law can be applied. It follows that they are the most distant objects observable, some of them more than 10 billion light years from our Galaxy. The light from quasars has reached us from long ago, so in principle it can tell us about the state of the universe billions of years in the past. The light from distant quasars shows the Lyman-alpha forest of numerous absorption lines of hydrogen at a variety of lower redshifts. This absorption occurs in hydrogen clouds near the quasar. If the light from a distant quasar passes through an intervening galaxy, a gravitational lens effect may be seen. The fact that we see such remote objects means that they are intrinsically very luminous, from a few to a hundred times the luminosity of a normal galaxy. The presence of emission lines implies that energy is generated through non-thermal mechanisms. Very-long-baseline interferometry shows that the central energy source in quasars is confined to a volume of space similar to the size of the solar system. This implies that the energy source is probably the infall of matter to a supermassive black hole.