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Concept
Quasar
Summary
A quasar (ˈkweɪzɑːr ) is an extremely luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The 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. Gas in the disc falling towards the black hole heats up because of friction and releases energy in the form of electromagnetic radiation. 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. Quasars are usually categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.
The term originated as a contraction of "quasi-stellar [star-like] radio source"—because they 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. High-resolution images of quasars, particularly from the Hubble Space Telescope, have shown that quasars occur in the centers of galaxies, and that some host galaxies are strongly interacting or merging galaxies. 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.
More than a million quasars have been found, with the nearest known being about 600 million light-years from Earth. The record for the most distant known quasar continues to change. In 2017, quasar ULAS J1342+0928 was detected at redshift z = 7.54. Light observed from this 800-million-solar-mass quasar was emitted when the universe was only 690 million years old. In 2020, quasar Pōniuāʻena was detected from a time only 700 million years after the Big Bang, and with an estimated mass of 1.
Official source
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