A quasar (/ˈkwzɑːr/) (also known as a QSO or quasi-stellar object) is an active galactic nucleus (AGN) of very high luminosity. Most large galaxies contain a supermassive central black hole with mass ranging from millions to billions of Solar masses. In quasars and other types of AGN, the black hole is surrounded by a gaseous accretion disk. As gas in the accretion disk falls toward the black hole, energy is released in the form of electromagnetic radiation. Quasars emit energy across the electromagnetic spectrum and can be observed at radio, infrared, visible, ultraviolet, and X-ray wavelengths. The most powerful quasars have luminosities exceeding 1041 W, thousands of times greater than an ordinary large galaxy such as the Milky Way.

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. High-resolution images of quasars, particularly from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, and that some quasar 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 disk relative to the observer, the presence or absence of a jet, and the degree of obscuration by gas and dust within the host galaxy.

Quasars are found over a very broad range of distances (corresponding to redshifts ranging from z < 0.1 for the nearest quasars to z > 7 for the most distant known quasars), and quasar discovery surveys have demonstrated that quasar activity was more common in the distant past. The peak epoch of quasar activity in the Universe corresponds to redshifts around 2, or approximately 10 billion years ago. As of 2017, the most distant known quasar is ULAS J1342+0928 at redshift z=7.54; light observed from this quasar was emitted when the Universe was only 690 million years old. The supermassive black hole in this quasar is the most distant black hole identified to date, and is estimated to have a mass that is 800 million times the mass of our Sun.

The term "quasar" was coined by Chinese-born U.S. astrophysicist Hong-Yee Chiu in May 1964, in Physics Today, to describe certain astronomically-puzzling objects:

So far, the clumsily long name 'quasi-stellar radio sources' is used to describe these objects. 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. For convenience, the abbreviated form 'quasar' will be used throughout this paper.

Between 1917 and 1922, 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 our own. But when radio astronomy commenced in the 1950s, astronomers detected, among the galaxies, a small number of anomalous objects with properties that defied explanation.

This page was last edited on 26 May 2018, at 20:58.
Reference: https://en.wikipedia.org/wiki/Quasar under CC BY-SA license.

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