quasar

Quasar, short for quasi-stellar radio source, is an extremely luminous active galactic nucleus powered by accretion of matter onto a supermassive black hole at the center of a distant galaxy. The energy output spans radio to X-ray wavelengths and originates as gas in the accretion disk and surrounding regions heats to extreme temperatures.

Quasars are among the most luminous persistent sources in the universe, with bolometric luminosities up to about 10^40 watts (roughly 10^47 erg/s). They appear point-like in optical images due to their distance, show strong broad emission lines from fast-moving gas, and have high redshifts, indicating great look-back times. They commonly vary on short timescales.

Quasars were identified in the 1960s from radio surveys as quasi-stellar objects; it is now known that many are radio-quiet and that the quasar phenomenon is part of the broader class of active galactic nuclei. The energy source is accretion onto a supermassive black hole; some quasars launch relativistic jets producing strong radio emission.

Host galaxies are typically massive and may be elliptical or irregular, with the bright nucleus often outshining the host at optical wavelengths. Quasars serve as cosmological probes, helping study the intergalactic medium, galaxy evolution, and the expansion history of the universe via absorption features in spectra and quasar abundance at different epochs.

Classification includes radio-loud versus radio-quiet quasars; when jets align with Earth, the sources are often called blazars.