Quasars

Quasars are quasi-stellar objects that represent the most luminous active galactic nuclei. They are powered by accretion of matter onto supermassive black holes at the centers of distant galaxies. Gas in the surrounding region forms an accretion disk around the black hole, heating to extreme temperatures and emitting across the electromagnetic spectrum from radio to X-ray. In many cases the nuclear emission outshines the host galaxy by a large margin.

Quasars were discovered in the 1960s as star-like radio sources with unusually strong and broad emission lines, revealing their extragalactic and very distant nature. The term stands for quasi-stellar radio source, though most quasars are radio-quiet and later recognized as a broad class of active galactic nuclei rather than a single type of radio source.

Quasars host supermassive black holes with masses from about a million to tens of billions of solar masses. Their bolometric luminosities can reach up to roughly 10^47 erg/s, corresponding to trillions of solar luminosities. The energy output can approach the Eddington limit, and some quasars exhibit relativistic jets that are detectable in radio wavelengths.

Spectroscopically, quasars show strong, broad emission lines and a blue continuum produced by the accretion disk. They are observed across a wide redshift range, with some found at redshifts beyond z = 7, making them valuable probes of the early universe and the intergalactic medium.

Quasars play a key role in studies of galaxy evolution and cosmology. They illuminate the intergalactic medium, contribute to the cosmic X-ray background, and help map large-scale structure. The class includes both radio-loud and radio-quiet objects; orientation relative to the observer explains much of the diversity, with blazars representing a jet-aligned subset.