quasar, one of a class of blue celestial objects having the appearance of stars when viewed through a telescope and currently believed to be the most distant and most luminous objects in the universe; the name is shortened from quasi-stellar radio source (QSR). Quasars were discovered as the visible counterparts of certain discrete celestial sources of radio waves (see radio astronomy). Similar starlike objects that do not emit radio waves were subsequently discovered and named quasi-stellar objects (QSOs). Although their visible light is faint, the quasars are optically brighter than the galaxies with which radio sources had been identified before 1963. Before their spectra were studied carefully, it was believed that the quasars were stars in our galaxy. However, the lines in their spectra have enormous red shifts that seem to imply that they are receding from the Milky Way with speeds as great as 95% of the speed of light. Only shifts toward the red end of the spectrum have been observed for quasars; blue-shifted ones that would indicate a quasar approaching our galaxy have not yet been found. If quasars were simply objects being ejected from nearby galaxies at high speeds, and not the distant objects they appear to be, then some would have blue shifts. If Hubble's law for the expansion of the universe is extrapolated to include the quasars, they would be many billion light-years away and consequently as luminous intrinsically as 1,000 galaxies combined. To account for such brilliant light, astronomers believe that quasars are supermassive black holes in galactic nuclei, releasing energy by the accretion of matter through a rotating viscous disk (see cosmology).

in full quasi-stellar radio source

Any of a class of enigmatic cosmic objects of high luminosity and strong radio emission observed at extremely great distances; also, a closely related object that has the same optical appearance but does not emit radio waves, i.e., a so-called quasi-stellar object (QSO). Most quasars exhibit very large redshifts, suggesting that they are moving away from Earth at tremendous speeds (approaching the speed of light); they thus are some of the most distant known objects in the universe. Quasars are no more than a light-year or two across but as much as 1,000 times more luminous than a giant galaxy having a diameter of 100,000 light-years; their extreme brightness allows them to be observed at distances of more than 10 billion light-years. Many investigators attribute such energy generation to matter spiraling at high velocity into a supermassive black hole (millions or billions of times as much mass as the Sun) at the centre of a distant galaxy. Seealso active galactic nucleus.

Learn more about quasar with a free trial on Britannica.com.