DescriptionA quasar 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 accretion onto 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 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 quasar 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. About a million quasars have been identified with reliable spectroscopic redshifts and between 2-3 million identified in photometric catalogs. The nearest known quasar is about 600 million light-years from Earth, while the record for the most distant known AGN is at a redshift of 10.1, corresponding to a comoving distance of 31.6 billion light-years, or a look-back time of 13.2 billion years. Quasar discovery surveys have shown that quasar activity was more common in the distant past; the peak epoch was approximately 10 billion years ago. Concentrations of multiple quasars are known as large quasar groups and may constitute some of the largest known structures in the universe if the observed groups are good tracers of mass distribution.

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