Quasar, or Quasi-stellar Object (QSO), a starlike object displaying an unusually large red shift. Red shift is a phenomenon in which the lines found in the spectra of the elements of a light source appear shifted from their normal positions toward the red end of the visible spectrum. Quasars were first discovered in the early 1960's and were originally called quasistellar radio sources; many have since been discovered that do not emit radio waves.

Most astronomers believe that the red shift of a quasar is explained by the Doppler effect that the shift is caused by the relative motion of the earth and the quasar away from one another. The greater this motion, the larger the red shift. The large red shifts of quasars would also seem to indicate that these objects are very distant: according to the theory of an expanding universe, the farther an object is from the Milky Way, the faster it appears to be moving away. Some quasars have been calculated to be about 13 billion light years away, farther than any other object known.

For an object to appear as bright as most quasars do at their calculated distances, it must emit more energy than several dozen galaxies put together. There is strong evidence, however, that quasars are much smaller than even a single galaxy. Astronomers have attempted to account for this discrepancy by developing a number of theories that describe violent processes that could produce great amounts of energy.

One theory is that a quasar is a dense cluster of stars in which large amounts of energy are released by the collision of stars within the cluster. Another is that a quasar is a rapidly rotating star that is more than a million times as massive as the sun and emits clouds of high-energy electrons. A third theory holds that a quasar is a disk of gases heated to extremely high temperatures while being drawn into an unusually large black hole.

Some astronomers argue that there is evidence that quasars are actually relatively close to the earth and therefore are not nearly as bright as most astronomers believe. They argue that the red shift in the case of quasars is unrelated to the Doppler effect, but is, instead, the result of some physical process that is not yet understood.