QUASAR

any of a class of rare cosmic objects of high luminosity as well as strong radio emission observed at extremely great distances. The term is also often applied to closely related objects that have the same optical appearance but that are radio quietthe so-called QSOs, which stands for quasi-stellar objects. Quasars are no more than a light-year or two in size, but they are up to 1,000 times more luminous than giant galaxies that have a diameter of about 100,000 light-years. The tremendous brilliance of quasars allows them to be observed at distances of more than 10,000,000,000 light-years. This enormous amount of radiation is released from a small area at the centre of a quasar. Many investigators attribute such energy generation to gas spiraling at high velocity into a massive black hole (q.v.). An outer atmosphere of tenuous gas produces emission lines that are observed in the spectrum of quasars. These emission lines are always shifted toward the red, corresponding to large Doppler velocities of recession. According to Hubble's law of the expansion of the universe, these large velocities correspond to large distancesa conclusion about quasar distances disputed by some astronomers. Quasars were more numerous and more luminous about 10 billion years ago than they are at present. The brightest quasar is 3C 273, which lies at a distance of 2,000,000,000 light-years from the Earth. Radio interferometry shows that the radio nucleus of 3C 273 is expanding at nearly the velocity of light.