A quasar is the highly energetic core of a remote active galaxy; quasars are the most luminous objects in the universe, capable of radiating over a trillion times as much energy as the Sun from a region little larger than the Solar System. The first quasars were found because of their radio emission and were called quasistellar radio sources. But these represent only about 1% of the quasar population. When others turned up that were radio quiet, the name was changed to quasistellar object (QSO). In either case, quasar is a contraction of "quasistellar," in reference to the fact that the visual appearance is star-like. Following Maarten Schmidt's discovery that quasars have very high redshifts, it became clear that they lie at remote cosmological distances and, therefore, to appear as bright as they do, must be fantastically luminous. Moreover, some quasars show marked variability over a period of just a few days, pointing to an incredibly compact source. Their radio structures often include jets and lobes similar to what we see from radio galaxies. Indeed, quasars and radio galaxies are simply different aspects of the same active galactic nucleus phenomenon, all powered ultimately by the same engines – supermassive black holes. The nearest quasar is 3C273, at a distance of about 2.5 billion light-years in Virgo (R.A. 12h 29.1m, Dec. +20° 3.1'), the first of its type to be identified (1963) and the brightest quasar in apparent magnitude (12.8); it is visible with a good 25-cm telescope under dark skies and therefore qualifies as the most remote object normally accessible to serious amateurs. 3C273 is unusually luminous for being, in quasar terms, relatively nearby: most of its brilliant brethren are found populating the early universe at distances of around 10 billion light-years. 3C273 also has by far the brightest optical jet known among quasars.

Several quasars have been discovered with redshifts greater than 6, which places them at distances of around 13 billion light-years and a time of less one billion years after the Big Bang. Observations of these remote objects thus shed valuable light on conditions in the early universe.