Some of them changed their luminosity very rapidly in the optical range and even more rapidly in the X-ray range, suggesting an upper limit on their size, perhaps no larger than the Solar System. The spectral lines of these objects, which identify the chemical elements of which the object is composed, were also extremely strange and defied explanation. The objects emitted large amounts of radiation of many frequencies, but no source could be located optically, or in some cases only a faint and point-like object somewhat like a distant star. But when radio astronomy began in the 1950s, astronomers detected, among the galaxies, a small number of anomalous objects with properties that defied explanation. Main article: Galaxy § Distinction from other nebulaeīetween 19, it became clear from work by Heber Curtis, Ernst Öpik and others that some objects (" nebulae") seen by astronomers were in fact distant galaxies like the Milky Way. History of observation and interpretation For convenience, the abbreviated form "quasar" will be used throughout this paper. Because the nature of these objects is entirely unknown, it is hard to prepare a short, appropriate nomenclature for them so that their essential properties are obvious from their name. So far, the clumsily long name "quasi-stellar radio sources" is used to describe these objects. The term "quasar" was first used in an article by astrophysicist Hong-Yee Chiu in May 1964, in Physics Today, to describe certain astronomically puzzling objects: 4.1 Spectral lines, reionization, and the early universe.2.3 Development of physical understanding (1960s).
2.2 Early observations (1960s and earlier).2 History of observation and interpretation.Concentrations of multiple, gravitationally attracted quasars are known as large quasar groups and constitute some of the largest known structures in the universe. Quasar discovery surveys have demonstrated that quasar activity was more common in the distant past the peak epoch was approximately 10 billion years ago. In early 2021, the quasar J0313–1806, with a 1.6-billion-solar-mass black hole, was reported at z = 7.64, 670 million years after the Big Bang. In 2020, the quasar Pōniuāʻena was detected from a time only 700 million years after the Big Bang, and with an estimated mass of 1.5 billion times the mass of the Sun. Light observed from this 800-million- solar-mass quasar was emitted when the universe was only 690 million years old. 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. 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 also known as a quasi-stellar object, abbreviated QSO) is an extremely luminous active galactic nucleus (AGN), powered by a supermassive black hole, with mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc.
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