A quasar is an astronomical source of electromagnetic energy, including light, which shows a very high redshift. The scientific consensus is that this high redshift is the result of Hubble's law. This implies that quasars are very distant. To be observable at that distance, the energy output of quasars must dwarf that of almost every known astrophysical phenomenon with the exception of comparatively short-lived supernovae and gamma-ray bursts. They may readily release energy in levels equal to the output of hundreds of average galaxies combined. The output of light is equivalent to one trillion suns. Quasars are believed to be 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 currently known mechanism appears able to explain the vast energy output and rapid variability.