In 2017, the quasar ULAS J1342+0928 was detected at redshift z = 7.54. The record for the most distant known quasar continues to change. More than a million quasars have been found, with the nearest known being about 600 million light-years away from Earth. As with other categories of AGN, the observed properties of a quasar depend on many factors, including the mass of the black hole, the rate of gas accretion, the orientation of the accretion disc relative to the observer, the presence or absence of a jet, and the degree of obscuration by gas and dust within the host galaxy. High-resolution images of quasars, particularly from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, and that some host galaxies are strongly interacting or merging galaxies. The term quasar originated as a contraction of 'quasi-stellar radio source'-because quasars were first identified during the 1950s as sources of radio-wave emission of unknown physical origin-and when identified in photographic images at visible wavelengths, they resembled faint, star-like points of light.

The redshifts of quasars are of cosmological origin. Usually, quasars are categorized as a subclass of the more general category of AGN. The radiant energy of quasars is enormous the most powerful quasars have luminosities thousands of times greater than that of a galaxy such as the Milky Way. Gas in the disc falling towards the black hole heats up because of friction and releases energy in the form of electromagnetic radiation. This emission from an AGN is powered by a supermassive black hole with a mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. It is sometimes known as a quasi-stellar object, abbreviated QSO. Artist's rendering of the accretion disc in ULAS J1120+0641, a very distant quasar powered by a supermassive black hole with a mass two billion times that of the Sun Ī quasar ( / ˈ k w eɪ z ɑːr/ KWAY-zar) is an extremely luminous active galactic nucleus (AGN).

Quasar microwave