Quasar | Wikipedia audio article
Channel: wikipedia tts
Date: 2018-12-08

This is an audio version of the Wikipedia Article:
Quasar


00:01:54 1 Naming
00:02:13 2 History of observation and interpretation
00:02:58 2.1 Background
00:03:09 2.2 Early observations (1960s and earlier)
00:04:34 2.3 Development of physical understanding (1960s)
00:07:28 2.4 Modern observations (1970s onward)
00:13:52 3 Current understanding
00:14:45 4 Properties
00:18:56 4.1 Spectral lines, reionization, and the early universe
00:26:49 5 Quasar subtypes
00:28:21 6 Role in celestial reference systems
00:30:31 7 Multiple quasars
00:31:22 8 See also
00:34:39 9 References



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SUMMARY
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A quasar () (also known as a QSO or quasi-stellar object) is an extremely luminous active galactic nucleus (AGN). It has been theorized that most large galaxies contain a supermassive central black hole with mass ranging from millions to billions of times the mass of our Sun. In quasars and other types of AGN, the black hole is surrounded by a gaseous accretion disk. As gas falls toward the black hole, energy is released in the form of electromagnetic radiation, which can be observed across the electromagnetic spectrum. The power radiated by quasars is enormous: the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way.The term "quasar" originated as a contraction of quasi-stellar [star-like] 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 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, and quasar discovery surveys have demonstrated that quasar activity was more common in the distant past. The peak epoch of quasar activity was 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, estimated at 800 million solar masses, is the most distant black hole identified to date.