A quasar (contraction of QUASi-stellAR radio source) is an astronomical source of electromagnetic energy, including light, that dwarfs the energy output of the brightest stars. A quasar may readily release energy in levels equal to the output of hundreds of average galaxies combined. In optical telescopes, a quasar looks like a very faint star (i.e. it is a point source), and has a very high redshift. The general consensus is that this high redshift is cosmological, the result of Hubble's law, which implies that quasars must be very distant and hence very luminous.

Some quasars display rapid changes in luminosity, which implies that they are small (an object cannot change faster than the time it takes light to travel from one end to the other; but see quasar J1819+3845 for another explanation). The highest redshift currently known for a quasar is 6.4. [1]

The scientific consensus is that quasars are powered by accretion of material onto supermassive black holes in the nuclei of distant galaxies, making these luminous versions of the general class of objects known as active galaxies. No other mechanism appears able to explain the vast energy output and rapid variability.