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{{Short description|Active galactic nucleus (AGN) containing a supermassive black hole}}
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{{redirect-distinguish|Quasi-stellar object|Quasi-star|Quaoar}}
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[[File:Artist's rendering ULAS J1120+0641.jpg|thumb|upright=1.35|Artist's rendering of the accretion disc in [[ULAS J1120+0641]], a very distant quasar containing a supermassive black hole with a mass two billion times that of the Sun<ref>{{cite news |title=Most Distant Quasar Found |url=http://www.eso.org/public/news/eso1122/ |access-date=4 July 2011 |newspaper=ESO Science Release}}</ref>]]
[[File:PKS 1127-145 X-rays.jpg|thumb|The [[Chandra X-ray Observatory|Chandra]] X-ray image is of the quasar [[PKS 1127-145]], a highly luminous source of X-rays and visible light about 10 billion light-years from Earth. An enormous X-ray jet extends at least a million light-years from the quasar. Image is 60 arcseconds on a side. [[Right ascension|RA]] 11h 30m 7.10s [[Declination|Dec]] −14° 49' 27" in [[Crater (constellation)|Crater]]. Observation date: May 28, 2000. Instrument: ACIS]]

A '''quasar''' ({{IPAc-en|ˈ|k|w|eɪ|z|ɑːr}} {{Respell|KWAY|zar}}) is an extremely [[Luminosity|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 mass]]es, surrounded by a gaseous [[Accretion disk|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 [[luminosity|luminosities]] thousands of times greater than that of a [[galaxy]] such as the [[Milky Way]].<ref>{{cite journal |last1=Wu |first1=Xue-Bing |display-authors=etal |title=An ultraluminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30 |journal=Nature |date=2015 |volume=518 |issue=7540 |pages=512–515 |doi=10.1038/nature14241 |pmid=25719667 |arxiv = 1502.07418 |bibcode = 2015Natur.518..512W |s2cid=4455954 }}</ref><ref>{{Cite book |last1=Frank |first1=Juhan |last2=King |first2=Andrew |last3=Raine |first3=Derek J. |title=Accretion Power in Astrophysics |edition=Third |bibcode=2002apa..book.....F |isbn=0521620538 |location=Cambridge, UK |publisher=Cambridge University Press |date=February 2002}}</ref> Quasars are usually categorized as a subclass of the more general category of AGN. The [[redshifts]] of quasars are of [[Expansion of the universe|cosmological origin]].<ref>{{Cite web |title=Quasars and Active Galactic Nuclei |url=https://ned.ipac.caltech.edu/level5/Kembhavi/Kem1_5.html |access-date=2020-08-31 |website=ned.ipac.caltech.edu}}</ref>

The term {{em|quasar}} originated as a [[Contraction (grammar)|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 [[Central massive object|centers of galaxies]], and that some host galaxies are strongly [[Interacting galaxy|interacting]] or [[Galaxy merger|merging]] galaxies.<ref>{{cite journal |last1=Bahcall |first1=J. N. |display-authors=etal |title=Hubble Space Telescope Images of a Sample of 20 Nearby Luminous Quasars |journal=The Astrophysical Journal |date=1997 |volume=479 |issue=2 |pages=642–658 |doi=10.1086/303926 |arxiv = astro-ph/9611163 |bibcode = 1997ApJ...479..642B|s2cid=15318893 }}</ref> 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 [[Astrophysical jet|jet]], and the degree of [[Extinction (astronomy)|obscuration]] by gas and [[Cosmic dust|dust]] within the host galaxy.

About a million quasars have been identified with reliable [[Astronomical spectroscopy|spectroscopic]] redshifts,<ref name="MILLIQUAS">{{cite web | title=Million Quasars Catalog, Version 8 (2 August 2023) | website=MILLIQUAS | date=2023-08-02 | url=https://heasarc.gsfc.nasa.gov/w3browse/all/milliquas.html | access-date=2023-11-20}}</ref> and between 2-3 million identified in [[Photometric_redshift|photometric]] catalogs.<ref name="Shu-2019">{{cite journal | last1=Shu | first1=Yiping | last2=Koposov | first2=Sergey E | last3=Evans | first3=N Wyn | last4=Belokurov | first4=Vasily | last5=McMahon | first5=Richard G | last6=Auger | first6=Matthew W | last7=Lemon | first7=Cameron A | title=Catalogues of active galactic nuclei from Gaia and unWISE data | journal=Monthly Notices of the Royal Astronomical Society | publisher=Oxford University Press (OUP) | volume=489 | issue=4 | date=2019-09-05 | issn=0035-8711 | doi=10.1093/mnras/stz2487 | pages=4741–4759| doi-access=free | arxiv=1909.02010 }}</ref><ref name="Quaia-2023">{{cite journal | last1=Storey-Fisher | first1=Kate | last2=Hogg | first2=David W. | last3=Rix | first3=Hans-Walter | last4=Eilers | first4=Anna-Christina | last5=Fabbian | first5=Giulio | last6=Blanton | first6=Michael | last7=Alonso | first7=David | year=2024 | title=Quaia, the Gaia-unWISE Quasar Catalog: An All-Sky Spectroscopic Quasar Sample | journal=AAS Journals | volume=964 | issue=1 | page=69 | doi=10.3847/1538-4357/ad1328 | doi-access=free | arxiv=2306.17749 | bibcode=2024ApJ...964...69S }}</ref> The [[Markarian 231|nearest known]] quasar is about 600 million [[light-year]]s from Earth, while the record for the [[UHZ1|most distant known AGN]] is at a redshift of 10.1, corresponding to a [[Comoving and proper distances|comoving distance]] of 31.6 billion light-years, or a look-back time of 13.2 billion years.<ref>{{Cite web |title=APOD: 2023 November 10 - UHZ1: Distant Galaxy and Black Hole |url=https://apod.nasa.gov/apod/ap231110.html |access-date=2023-11-10 |website=apod.nasa.gov}}</ref><ref name=":0">{{Cite journal |last1=Whalen |first1=Daniel J. |last2=Latif |first2=Muhammad A. |last3=Mezcua |first3=Mar |date=2023-10-01 |title=Radio Emission From a z = 10.1 Black Hole in UHZ1 |journal=The Astrophysical Journal |volume=956 |issue=2 |pages=133 |doi=10.3847/1538-4357/acf92c |issn=0004-637X|doi-access=free |arxiv=2308.03837 |bibcode=2023ApJ...956..133W }}</ref>

Quasar discovery surveys have shown that quasar activity was more common in the distant past; the peak epoch was approximately 10&nbsp;billion years ago.<ref>{{cite journal |last1=Schmidt |first1=Maarten |author-link=Maarten Schmidt |last2=Schneider |first2=Donald |last3=Gunn |first3=James |date=1995 |title=Spectroscopic CCD Surveys for Quasars at Large Redshift. IV.&nbsp;Evolution of the Luminosity Function from Quasars Detected by Their Lyman-Alpha Emission |journal=The Astronomical Journal |volume=110 |page=68 |bibcode=1995AJ....110...68S |doi=10.1086/117497}}</ref> Concentrations of multiple quasars are known as [[large quasar group]]s and may constitute some of the [[galaxy filament|largest known structures]] in the universe if the observed groups are good tracers of mass distribution.

==Naming==
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:<ref>{{cite journal |last1=Chiu |first1=Hong-Yee |title=Gravitational collapse |journal=Physics Today |volume=17 |issue=5 |page=21 |doi=10.1063/1.3051610 |year=1964 |bibcode=1964PhT....17e..21C |doi-access=free }}</ref>

{{Blockquote|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.}}

==History of observation and interpretation ==
<!-- This section is linked from [[Redshift]] -->
[[File:SDSS image of quasar 3C273.jpg|thumb|200px|[[Sloan Digital Sky Survey]] image of quasar [[3C 273]], illustrating the object's star-like appearance. The quasar's jet can be seen extending downward and to the right from the quasar.]]
[[File:Quasar viewed from Hubble.jpg|thumb|200px|[[Hubble Space Telescope|Hubble]] images of quasar [[3C 273]]. At right, a [[coronagraph]] is used to block the quasar's light, making it easier to detect the surrounding host galaxy.]]

=== Background ===
{{Main|Galaxy#Distinction from other nebulae}}

Between 1917 and 1922, it became clear from work by [[Heber Doust Curtis]], [[Ernst Öpik]] and others that some objects ("[[nebula]]e") seen by astronomers were in fact distant [[galaxies]] like the Milky Way. But when [[radio astronomy]] began in the 1950s, astronomers detected, among the galaxies, a small number of anomalous objects with properties that 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]]. The [[spectral line]]s of these objects, which identify the [[chemical element]]s of which the object is composed, were also extremely strange and defied explanation. 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]].<ref>{{cite web|url=https://hubblesite.org/contents/news-releases/1996/news-1996-35.html |title=Hubble Surveys the "Homes" of Quasars |publisher=HubbleSite |date=1996-11-19 |access-date=2011-07-01}}</ref> This implies an extremely high [[power density]].<ref>{{cite web |url=http://neutrino.aquaphoenix.com/un-esa/astrophysics/astro-chapter7.html |title=7. HIGH-ENERGY ASTROPHYSICS ELECTROMAGNETIC RADIATION |publisher=Neutrino.aquaphoenix.com |access-date=2011-07-01 |archive-url=https://web.archive.org/web/20110707154332/http://neutrino.aquaphoenix.com/un-esa/astrophysics/astro-chapter7.html |archive-date=2011-07-07 |url-status=dead }}</ref> Considerable discussion took place over what these objects might be. They were described as ''"quasi-stellar'' [meaning: star-like] ''radio sources"'', or ''"quasi-stellar objects"'' (QSOs), a name which reflected their unknown nature, and this became shortened to "quasar".

=== Early observations (1960s and earlier) ===
The first quasars ([[3C 48]] and [[3C 273]]) were discovered in the late 1950s, as radio sources in all-sky radio surveys.<ref name="Shields">{{cite journal |last1=Shields |first1=Gregory A. |title=A Brief History of Active Galactic Nuclei |journal=The Publications of the Astronomical Society of the Pacific |date=1999 |volume=111 |issue=760 |pages=661–678 |access-date=3 October 2014 |url=http://ned.ipac.caltech.edu/level5/Sept04/Shields/Shields3.html |doi=10.1086/316378 |arxiv=astro-ph/9903401 |bibcode=1999PASP..111..661S |s2cid=18953602 }}</ref><ref>{{cite web |title=Our Activities |url=http://www.esa.int/Our_Activities/Space_Science/18_June/(print) |publisher=[[European Space Agency]] |access-date=3 October 2014}}</ref><ref>{{Cite journal |last1=Matthews |first1=Thomas A. |author-link=Thomas A. Matthews |last2=Sandage |first2=Allan R. |author-link2=Allan Sandage |date=July 1963 |title=Optical Identification of 3c 48, 3c 196, and 3c 286 with Stellar Objects. |journal=The Astrophysical Journal |language=en |volume=138 |pages=30 |bibcode=1963ApJ...138...30M |doi=10.1086/147615 |issn=0004-637X |doi-access=free}}</ref><ref>{{cite book |url=https://books.google.com/books?id=W-cbw-QdcHUC&pg=PA237 |title=Physics: Imagination and Reality |isbn=9789971509293 |last1=Wallace |first1=Philip Russell |year=1991|publisher=World Scientific }}</ref> They were first noted as radio sources with no corresponding visible object. Using small telescopes and the [[Lovell Telescope]] as an [[Interferometry|interferometer]], they were shown to have a very small angular size.<ref name="jbo">{{cite web |url=http://www.jb.man.ac.uk/public/story/mk1quasars.html |title=The MKI and the discovery of Quasars |publisher=[[Jodrell Bank Observatory]] |access-date=2006-11-23}}</ref> By 1960, hundreds of these objects had been recorded and published in the [[Third Cambridge Catalogue]] while astronomers scanned the skies for their optical counterparts. In 1963, a definite identification of the radio source [[3C 48]] with an optical object was published by [[Allan Sandage]] and [[Thomas A. Matthews]]. Astronomers had detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum, which contained many unknown broad emission lines. The anomalous spectrum defied interpretation.

British-Australian astronomer [[John Gatenby Bolton|John Bolton]] made many early observations of quasars, including a breakthrough in 1962. Another radio source, [[3C 273]], was predicted to undergo five [[occultation]]s by the [[Moon]]. Measurements taken by [[Cyril Hazard]] and John Bolton during one of the occultations using the [[Parkes Radio Telescope]] allowed [[Maarten Schmidt]] to find a visible counterpart to the radio source and obtain an [[optical spectrum]] using the {{convert|200|in|m|adj=on}} [[Hale Telescope]] on [[Palomar Mountain|Mount Palomar]]. This spectrum revealed the same strange emission lines. Schmidt was able to demonstrate that these were likely to be the ordinary [[spectral line]]s of hydrogen redshifted by 15.8%—at the time, a high redshift (with only a handful of much fainter galaxies known with higher redshift). If this was due to the physical motion of the "star", then 3C 273 was receding at an enormous velocity, around {{val|47,000|fmt=commas|u=km/s}}, far beyond the speed of any known star and defying any obvious explanation.<ref name="schmidt1963" /> Nor would an extreme velocity help to explain 3C 273's huge radio emissions. If the redshift was cosmological (now known to be correct), the large distance implied that 3C 273 was far more luminous than any galaxy, but much more compact. Also, 3C 273 was bright enough to detect on archival photographs dating back to the 1900s; it was found to be variable on yearly timescales, implying that a substantial fraction of the light was emitted from a region less than 1&nbsp;light-year in size, tiny compared to a galaxy.

Although it raised many questions, Schmidt's discovery quickly revolutionized quasar observation. The strange spectrum of [[3C 48]] was quickly identified by Schmidt, Greenstein and Oke as [[hydrogen]] and [[magnesium]] redshifted by 37%. Shortly afterwards, two more quasar spectra in 1964 and five more in 1965 were also confirmed as ordinary light that had been redshifted to an extreme degree.<ref name="Caltech_p3">{{Cite web |last=Shields |first=Gregory A. |date=1999 |title=A Brief History of AGN. 3.&nbsp;The Discovery Of Quasars |url=http://ned.ipac.caltech.edu/level5/Sept04/Shields/Shields3.html |website=[[California Institute of Technology]]}}</ref> 

=== Development of physical understanding (1960s) ===
{{main|Redshift|Universe|Expansion of the universe}}

An extreme redshift could imply great distance and velocity but could also be due to extreme mass or perhaps some other unknown laws of nature. Extreme velocity and distance would also imply immense power output, which lacked explanation. The small sizes were confirmed by [[interferometry]] and by observing the speed with which the quasar as a whole varied in output, and by their inability to be seen in even the most powerful visible-light telescopes as anything more than faint starlike points of light. But if they were small and far away in space, their power output would have to be immense and difficult to explain. Equally, if they were very small and much closer to this galaxy, it would be easy to explain their apparent power output, but less easy to explain their redshifts and lack of detectable movement against the background of the universe.

Schmidt noted that redshift is also associated with the expansion of the universe, as codified in [[Hubble's law]]. If the measured redshift was due to expansion, then this would support an interpretation of very distant objects with extraordinarily high [[luminosity]] and power output, far beyond any object seen to date. This extreme luminosity would also explain the large radio signal. Schmidt concluded that 3C 273 could either be an individual star around 10&nbsp;km wide within (or near to) this galaxy, or a distant active galactic nucleus. He stated that a distant and extremely powerful object seemed more likely to be correct.<ref name="schmidt1963">{{Cite journal |last=Schmidt |first=M. |author-link=Maarten Schmidt |date=March 1963 |title=3C 273 : A Star-Like Object with Large Red-Shift |journal=Nature |language=en |volume=197 |issue=4872 |pages=1040 |bibcode=1963Natur.197.1040S |doi=10.1038/1971040a0 |issn=0028-0836 |s2cid=4186361 |doi-access=free}}</ref>

Schmidt's explanation for the high redshift was not widely accepted at the time. A major concern was the enormous amount of energy these objects would have to be radiating, if they were distant. In the 1960s no commonly accepted mechanism could account for this. The currently accepted explanation, that it is due to [[matter]] in an [[accretion disc]] falling into a [[supermassive black hole]], was only suggested in 1964 by [[Edwin E. Salpeter]] and [[Yakov Zeldovich]],<ref>{{cite journal |last1=Shields |first1=G. A. |title=A Brief History of Active Galactic Nuclei |journal=Publications of the Astronomical Society of the Pacific |date=1999 |volume=111 |issue=760 |page=661 |doi=10.1086/316378 |bibcode=1999PASP..111..661S |arxiv = astro-ph/9903401|s2cid=18953602 }}</ref> and even then it was rejected by many astronomers, as at this time the existence of [[black holes]] at all was widely seen as theoretical.

Various explanations were proposed during the 1960s and 1970s, each with their own problems. It was suggested that quasars were nearby objects, and that their redshift was not due to the [[Redshift#Expansion of space|expansion of space]] but rather to [[gravitational redshift|light escaping a deep gravitational well]]. This would require a massive object, which would also explain the high luminosities. However, a star of sufficient mass to produce the measured redshift would be unstable and in excess of the [[Hayashi limit]].<ref>{{Cite journal |last=Chandrasekhar |first=S. |author-link=Subrahmanyan Chandrasekhar |date=August 1964 |title=The Dynamical Instability of Gaseous Masses Approaching the Schwarzschild Limit in General Relativity. |journal=The Astrophysical Journal |language=en |volume=140 |issue=2 |pages=417 |bibcode=1964ApJ...140..417C |doi=10.1086/147938 |issn=0004-637X |s2cid=120526651 |doi-access=free}}</ref> Quasars also show [[forbidden lines|forbidden]] spectral emission lines, previously only seen in hot gaseous nebulae of low density, which would be too diffuse to both generate the observed power and fit within a deep gravitational well.<ref>{{Cite journal |last1=Greenstein |first1=Jesse L. |author-link=Jesse L. Greenstein |last2=Schmidt |first2=Maarten |date=July 1964 |title=The Quasi-Stellar Radio Sources 3c 48 and 3c 273. |journal=The Astrophysical Journal |language=en |volume=140 |issue=1 |pages=1 |bibcode=1964ApJ...140....1G |doi=10.1086/147889 |issn=0004-637X |s2cid=123147304 |doi-access=free}}</ref> There were also serious concerns regarding the idea of cosmologically distant quasars. One strong argument against them was that they implied energies that were far in excess of known energy conversion processes, including [[nuclear fusion]]. There were suggestions that quasars were made of some hitherto unknown stable form of [[antimatter]] in similarly unknown types of region of space, and that this might account for their brightness.<ref>{{Cite journal |last=Gray |first=G. K. |year=1965 |title=Quasars and Antimatter |journal=Nature |language=en |volume=206 |issue=4980 |page=175 |bibcode=1965Natur.206..175G |doi=10.1038/206175a0 |issn=0028-0836 |s2cid=4171869 |doi-access=free}}</ref> Others speculated that quasars were a [[white hole]] end of a [[wormhole]],<ref>{{cite book |last1=Haven |first1=Kendall F. |date=2001 |title=That's weird!: awesome science mysteries |others=Illustrated by Jason Lynch |publisher=Fulcrum Resources |location=Golden, Colo. |isbn=9781555919993|pages=39–41 |url=https://books.google.com/books?id=2zVo1lirhi4C&pg=PA39}}</ref><ref>{{cite book |last1=Santilli |first1=Ruggero Maria |title=Isodual theory of antimatter: with applications to antigravity, grand unification and cosmology |date=2006 |publisher=Springer |location=Dordrecht |isbn=978-1-4020-4517-2 |page=304 |url=https://books.google.com/books?id=xJwUB--qflEC&pg=PA304 |bibcode=2006itaa.book.....S }}</ref> or a [[chain reaction]] of numerous [[supernova]]e.<ref name="Caltech_ipac_4">{{Cite web |last=Shields |first=Gregory A. |date=1999 |title=A Brief History of AGN. 4.2.&nbsp;Energy Source |url=http://ned.ipac.caltech.edu/level5/Sept04/Shields/Shields4_2.html |website=[[California Institute of Technology]]}}</ref>

Eventually, starting from about the 1970s, many lines of evidence (including [[Uhuru (satellite)|the first]] [[X-ray]] [[space observatory|space observatories]], knowledge of [[black hole]]s and modern models of [[cosmology]]) gradually demonstrated that the quasar redshifts are genuine and due to the [[expansion of space]], that quasars are in fact as powerful and as distant as Schmidt and some other astronomers had suggested, and that their energy source is matter from an accretion disc falling onto a supermassive black hole.<ref name="keel2009">{{cite web |first=William C. |last=Keel |date=October 2009 |title=Alternate Approaches and the Redshift Controversy |publisher=The University of Alabama |url=http://www.astr.ua.edu/keel/galaxies/arp.html |access-date=2010-09-27}}</ref> This included crucial evidence from optical and X-ray viewing of quasar host galaxies, finding of "intervening" absorption lines, which explained various spectral anomalies, observations from [[gravitational lensing]], [[James Gunn (astronomer)|Gunn]]'s 1971 finding that galaxies containing quasars showed the same redshift as the quasars,<ref>{{cite journal | last1 = Gunn | first1 = James E. | title = On the Distances of the Quasi-Stellar Objects | journal = The Astrophysical Journal | date = March 1971 | volume = 164 | page = L113 | doi = 10.1086/180702 | bibcode = 1971ApJ...164L.113G | doi-access = free }}</ref> and [[Jerome Kristian|Kristian]]'s 1973 finding that the "fuzzy" surrounding of many quasars was consistent with a less luminous host galaxy.<ref>{{cite journal | last1 = Kristian | first1 = Jerome | title = Quasars as Events in the Nuclei of Galaxies: the Evidence from Direct Photographs | journal = The Astrophysical Journal | date = January 1973 | volume = 179 | page = L61 | doi = 10.1086/181117| bibcode = 1973ApJ...179L..61K }}</ref>

This model also fits well with other observations suggesting that many or even most galaxies have a massive central black hole. It would also explain why quasars are more common in the early universe: as a quasar draws matter from its accretion disc, there comes a point when there is less matter nearby, and energy production falls off or ceases, as the quasar becomes a more ordinary type of galaxy.

The accretion-disc energy-production mechanism was finally modeled in the 1970s, and black holes were also directly detected (including evidence showing that supermassive black holes could be found at the centers of this and many other galaxies), which resolved the concern that quasars were too luminous to be a result of very distant objects or that a suitable mechanism could not be confirmed to exist in nature. By 1987 it was "well accepted" that this was the correct explanation for quasars,<ref name="thomsen_1987" /> and the cosmological distance and energy output of quasars was accepted by almost all researchers.

===Modern observations (1970s and onward)===
[[File:MUSE spies accreting giant structure around a quasar.tif|thumb|Cloud of gas around the distant quasar SDSS J102009.99+104002.7, taken by [[Multi-unit spectroscopic explorer|MUSE]]<ref>{{cite web |title=MUSE spies accreting giant structure around a quasar |url=http://www.eso.org/public/images/potw1747a/ |access-date=20 November 2017 |website=eso.org}}</ref>]]

Later it was found that not all quasars have strong radio emission; in fact only about 10% are "radio-loud". Hence the name "QSO" (quasi-stellar object) is used (in addition to "quasar") to refer to these objects, further categorized into the "radio-loud" and the "radio-quiet" classes. The discovery of the quasar had large implications for the field of astronomy in the 1960s, including drawing physics and astronomy closer together.<ref>{{cite journal |last1=de Swart |first1=J. G. |last2=Bertone |first2=G. |last3=van Dongen |first3=J. |title=How dark matter came to matter |journal=Nature Astronomy |date=2017 |volume=1 |issue=59 |pages=0059 |arxiv=1703.00013 |doi=10.1038/s41550-017-0059 |bibcode = 2017NatAs...1E..59D|s2cid=119092226 }}</ref>

In 1979, the [[gravitational lens]] effect predicted by [[Albert Einstein]]'s [[general theory of relativity]] was confirmed observationally for the first time with images of the [[Twin Quasar|double quasar]] 0957+561.<ref>{{cite web |url=http://www.astr.ua.edu/keel/agn/q0957.html |title=Active Galaxies and Quasars – Double Quasar 0957+561 |publisher=Astr.ua.edu |access-date=2011-07-01}}</ref>

[[File:UZC J224030.2+032131.jpg|thumb|A cosmic mirage known as the [[Einstein Cross]]. Four apparent images are actually from the same quasar.]]

A study published in February 2021 showed that there are more quasars in one direction (towards [[Hydra (constellation)|Hydra]]) than in the opposite direction, seemingly indicating that the Earth is moving in that direction. But the direction of this dipole is about 28° away from the direction of the Earth's motion relative to the [[cosmic microwave background]] radiation.<ref>{{cite journal |last=Secrest |first=Nathan |display-authors=etal |date=25 February 2021 |title=A Test of the Cosmological Principle with Quasars |journal=The Astrophysical Journal Letters |volume=908 |issue=2 |pages=L51 |arxiv=2009.14826 |bibcode=2021ApJ...908L..51S |doi=10.3847/2041-8213/abdd40 |doi-access=free}}</ref>

In March 2021, a collaboration of scientists, related to the [[Event Horizon Telescope]], presented, for the first time, a [[Polarization (waves)|polarized-based image]] of a [[black hole]], specifically the black hole at the center of [[Messier 87]], an [[elliptical galaxy]] approximately 55 million light-years away in the [[constellation]] [[Virgo (constellation)|Virgo]], revealing the forces giving rise to quasars.<ref name="NYT-20210324">{{cite news |last=Overbye |first=Dennis |authorlink=Dennis Overbye |title=The Most Intimate Portrait Yet of a Black Hole - Two years of analyzing the polarized light from a galaxy's giant black hole has given scientists a glimpse at how quasars might arise. |url=https://www.nytimes.com/2021/03/24/science/astronomy-messier-87-black-hole.html |archive-url=https://ghostarchive.org/archive/20211228/https://www.nytimes.com/2021/03/24/science/astronomy-messier-87-black-hole.html |archive-date=2021-12-28 |url-access=limited |date=24 March 2021 |work=[[The New York Times]] |accessdate=25 March 2021 }}{{cbignore}}</ref>

== Current understanding==
{{Nature timeline}}

It is now known that quasars are distant but extremely luminous objects, so any light that reaches the [[Earth]] is redshifted due to the [[expansion of the universe]].<ref name="grupen_cowan2005">{{Cite book |last1=Grupen |first1=Claus |url=https://archive.org/details/astroparticlephy00grup |title=Astroparticle physics |last2=Cowan, Glen |date=2005 |publisher=Springer |isbn=978-3-540-25312-9 |location=Berlin ; New York |pages=[https://archive.org/details/astroparticlephy00grup/page/n25 11]–12 |oclc=ocm60561678 |url-access=limited}}</ref>

Quasars inhabit the centers of active galaxies and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the [[Milky Way]], which contains 200–400&nbsp;billion stars. This radiation is emitted across the electromagnetic spectrum almost uniformly, from X-rays to the far infrared with a peak in the ultraviolet optical bands, with some quasars also being strong sources of radio emission and of gamma-rays. With high-resolution imaging from ground-based telescopes and the [[Hubble Space Telescope]], the "host galaxies" surrounding the quasars have been detected in some cases.<ref>[http://hubblesite.org/newscenter/archive/releases/1996/35/image/a/ Hubble Surveys the "Homes" of Quasars]. Hubblesite News Archive, Release ID 1996–35.</ref> These galaxies are normally too dim to be seen against the glare of the quasar, except with special techniques. Most quasars, with the exception of [[3C 273]], whose average [[apparent magnitude]] is 12.9, cannot be seen with small telescopes.

Quasars are believed—and in many cases confirmed—to be powered by [[accretion (astrophysics)|accretion]] of material into supermassive black holes in the nuclei of distant galaxies, as suggested in 1964 by [[Edwin Salpeter]] and [[Yakov Zeldovich]].<ref name="Shields" /> Light and other radiation cannot escape from within the [[event horizon]] of a black hole. The energy produced by a quasar is generated {{em|outside}} the black hole, by gravitational stresses and immense [[friction]] within the material nearest to the black hole, as it orbits and falls inward.<ref name="thomsen_1987">{{cite journal |title = End of the World: You Won't Feel a Thing |first = D. E. |last = Thomsen |journal = Science News |volume = 131 |issue = 25 |pages = 391 |date = Jun 20, 1987 |jstor = 3971408 |doi = 10.2307/3971408}}</ref> The huge luminosity of quasars results from the accretion discs of central supermassive black holes, which can convert between 5.7% and 32% of the [[mass]] of an object into [[energy]],<ref name="Lambourne">{{Cite book |last=Lambourne |first=Robert J. |url=https://books.google.com/books?id=GUySYQaDM1cC&pg=PA222 |title=Relativity, gravitation and cosmology |date=2010 |publisher=Cambridge University Press [u.a.] |isbn=978-0-521-13138-4 |edition=Illustrated |location=Cambridge New York Melbourne |page=222}}</ref> compared to just 0.7% for the [[Proton–proton chain|p–p chain]] [[nuclear fusion]] process that dominates the energy production in Sun-like stars. Central masses of 10<sup>5</sup> to 10<sup>9</sup> [[solar mass]]es have been measured in quasars by using [[reverberation mapping]]. Several dozen nearby large galaxies, including the [[Milky Way]] galaxy, that do not have an active center and do not show any activity similar to a quasar, are confirmed to contain a similar supermassive black hole in their [[Active galactic nucleus|nuclei (galactic center)]]. Thus it is now thought that all large galaxies have a black hole of this kind, but only a small fraction have sufficient matter in the right kind of orbit at their center to become active and power radiation in such a way as to be seen as quasars.<ref name="DiMatteo2005">{{cite journal |first1=Tiziana|last1= Di Matteo|author1-link=Tiziana Di Matteo (astrophysicist) |display-authors=etal |title=Energy input from quasars regulates the growth and activity of black holes and their host galaxies |journal=Nature |date=10 February 2005 |volume=433 |issue=7026 |pages=604–607 |bibcode=2005Natur.433..604D |arxiv=astro-ph/0502199 |doi=10.1038/nature03335 |pmid=15703739|s2cid=3007350 }}</ref>

This also explains why quasars were more common in the early universe, as this energy production ends when the supermassive black hole consumes all of the gas and dust near it. This means that it is possible that most galaxies, including the Milky Way, have gone through an active stage, appearing as a quasar or some other class of active galaxy that depended on the black-hole mass and the accretion rate, and are now quiescent because they lack a supply of matter to feed into their central black holes to generate radiation.<ref name="DiMatteo2005" />

[[File:Quasars in interacting galaxies.jpg|thumb|left|Quasars in interacting galaxies<ref>{{cite web |title=Quasars in interacting galaxies |url=http://www.spacetelescope.org/images/opo1520a/ |website=ESA/Hubble |access-date=19 June 2015}}</ref>]]

The matter accreting onto the black hole is unlikely to fall directly in, but will have some angular momentum around the black hole, which will cause the matter to collect into an [[accretion disc]]. Quasars may also be ignited or re-ignited when normal galaxies merge and the black hole is infused with a fresh source of matter.<ref>{{Cite journal |last=Pierce |first=J C S |display-authors=etal |date=13 February 2023 |title=Galaxy interactions are the dominant trigger for local type 2 quasars |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=522 |issue=2 |pages=1736–1751 |arxiv=2303.15506 |doi=10.1093/mnras/stad455 |issn=0035-8711 |doi-access=free}}</ref> In fact, it has been suggested that a quasar could form when the [[Andromeda Galaxy]] collides with the [[Milky Way]] galaxy in approximately 3–5&nbsp;billion years.<ref name="thomsen_1987" /><ref>{{cite web |url=http://www.galaxydynamics.org/papers/GreatMilkyWayAndromedaCollision.pdf |title=Galaxy für Dehnungsstreifen |access-date=December 30, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20081217134733/http://www.galaxydynamics.org/papers/GreatMilkyWayAndromedaCollision.pdf |archive-date=December 17, 2008 }}</ref><ref>{{cite web |url=http://www.cfa.harvard.edu/~tcox/localgroup/lg.pdf |title=Archived copy |access-date=July 1, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20100202184123/http://www.cfa.harvard.edu/~tcox/localgroup/lg.pdf |archive-date=February 2, 2010}}</ref><ref>{{cite press release |author=<!--Not stated--> |title=Astronomers solve the 60-year mystery of quasars – the most powerful objects in the Universe |url=https://www.sheffield.ac.uk/news/astronomers-solve-60-year-mystery-quasars-most-powerful-objects-universe |publisher=University of Sheffield |date=2023-04-26 |access-date=2023-09-10 }}</ref>

In the 1980s, unified models were developed in which quasars were classified as a particular kind of [[active galaxy]], and a consensus emerged that in many cases it is simply the viewing angle that distinguishes them from other active galaxies, such as [[blazar]]s and [[radio galaxy|radio galaxies]].<ref>{{Cite journal |last=Barthel |first=Peter D. |year=1989 |title=Is every quasar beamed? |journal=The Astrophysical Journal |language=en |volume=336 |pages=606 |bibcode=1989ApJ...336..606B |doi=10.1086/167038 |issn=0004-637X}}</ref>

The highest-redshift quasar known ({{As of|2024|8|lc=on}}) is [[UHZ1]], with a redshift of approximately 10.1,<ref name="NAT-20171206">{{Cite journal |last=Bañados |first=Eduardo |display-authors=etal |date=2018 |title=An 800-million-solar-mass black hole in a significantly neutral Universe at a redshift of 7.5 |journal=Nature |language=en |volume=553 |issue=7689 |pages=473–476 |arxiv=1712.01860 |bibcode=2018Natur.553..473B |doi=10.1038/nature25180 |issn=0028-0836 |pmid=29211709 |s2cid=205263326}}</ref> which corresponds to a [[comoving distance]] of approximately 31.7 billion [[light-year]]s from Earth (these distances are much larger than the distance light could travel in the universe's 13.8-billion-year history because the universe is expanding).

== Properties ==
More than {{val|900000|fmt=commas}} quasars have been found (as of July 2023),<ref name="MILLIQUAS"/> most from the [[Sloan Digital Sky Survey]]. All observed quasar spectra have redshifts between 0.056 and 10.1 (as of 2024), which means they range between 600 million and 30&nbsp;billion [[comoving distance|light-years away from Earth]]. Because of the great distances to the farthest quasars and the finite velocity of light, they and their surrounding space appear as they existed in the very early universe.

The power of quasars originates from supermassive black holes that are believed to exist at the core of most galaxies. The Doppler shifts of stars near the cores of galaxies indicate that they are revolving around tremendous masses with very steep gravity gradients, suggesting black holes.

Although quasars appear faint when viewed from Earth, they are visible from extreme distances, being the most luminous objects in the known universe. The brightest quasar in the sky is [[3C 273]] in the [[constellation]] of [[Virgo (constellation)|Virgo]]. It has an average [[apparent magnitude]] of 12.8 (bright enough to be seen through a medium-size amateur [[telescope]]), but it has an [[absolute magnitude]] of −26.7.<ref name="3C 273 article">{{cite journal |title=The Quasi-Stellar Radio Sources 3C 48 and 3C 273 |bibcode=1964ApJ...140....1G |journal=The Astrophysical Journal |doi=10.1086/147889 |volume=140 |pages=1 |year=1964 |last1=Greenstein |first1=Jesse L. |last2=Schmidt |first2=Maarten|s2cid=123147304 |doi-access=free }}<!--|access-date=25 April 2014--></ref> From a distance of about 33&nbsp;light-years, this object would shine in the sky about as brightly as the [[Sun]]. This quasar's [[luminosity]] is, therefore, about 4&nbsp;trillion (4{{e|12}}) times that of the Sun, or about 100 times that of the total light of giant galaxies like the [[Milky Way]].<ref name="3C 273 article"/> This assumes that the quasar is radiating energy in all directions, but the active galactic nucleus is believed to be radiating preferentially in the direction of its jet. In a universe containing hundreds of billions of galaxies, most of which had active nuclei billions of years ago but only seen today, it is statistically certain that thousands of energy jets should be pointed toward the Earth, some more directly than others. In many cases it is likely that the brighter the quasar, the more directly its jet is aimed at the Earth. Such quasars are called [[blazars]].

The hyperluminous quasar [[APM&nbsp;08279+5255]] was, when discovered in 1998, given an [[absolute magnitude]] of −32.2. High-resolution imaging with the [[Hubble Space Telescope]] and the 10&nbsp;m [[Keck Telescope]] revealed that this system is [[gravitational lensing|gravitationally lensed]]. A study of the gravitational lensing of this system suggests that the light emitted has been magnified by a factor of ~10. It is still substantially more luminous than nearby quasars such as 3C 273.

Quasars were much more common in the early universe than they are today. This discovery by [[Maarten Schmidt]] in 1967 was early strong evidence against [[Steady-state theory|steady-state cosmology]] and in favor of the [[Big Bang]] cosmology. Quasars show the locations where supermassive black holes are growing rapidly (by [[Accretion (astrophysics)|accretion]]). Detailed simulations reported in 2021 showed that galaxy structures, such as spiral arms, use gravitational forces to 'put the brakes on' gas that would otherwise orbit galaxy centers forever; instead the braking mechanism enabled the gas to fall into the supermassive black holes, releasing enormous radiant energies.<ref>{{cite web |title=New simulation shows how galaxies feed their supermassive black holes |url=https://www.sciencedaily.com/releases/2021/08/210817131435.htm |website=sciencedaily.com |access-date=31 August 2021 |date=17 August 2021 |quote=First model to show how gas flows across universe into a supermassive black hole’s center.}}</ref><ref>{{Cite journal |last=Anglés-Alcázar |first=Daniel |display-authors=etal |date=August 2021 |title=Cosmological Simulations of Quasar Fueling to Subparsec Scales Using Lagrangian Hyper-refinement |journal=The Astrophysical Journal |volume=917 |issue=2 |page=53 |arxiv=2008.12303 |bibcode=2021ApJ...917...53A |doi=10.3847/1538-4357/ac09e8 |issn=0004-637X |s2cid=221370537 |doi-access=free}}</ref> These black holes co-evolve with the mass of stars in their host galaxy in a way not fully understood at present. One idea is that jets, radiation and [[Disk wind|winds]] created by the quasars shut down the formation of new stars in the host galaxy, a process called "feedback". The jets that produce strong radio emission in some quasars at the centers of [[clusters of galaxies]] are known to have enough power to prevent the hot gas in those clusters from cooling and falling on to the central galaxy.

Quasars' luminosities are variable, with time scales that range from months to hours. This means that quasars generate and emit their energy from a very small region, since each part of the quasar would have to be in contact with other parts on such a time scale as to allow the coordination of the luminosity variations. This would mean that a quasar varying on a time scale of a few weeks cannot be larger than a few light-weeks across. The emission of large amounts of power from a small region requires a power source far more efficient than the nuclear fusion that powers stars. The conversion of [[gravitational potential energy]] to radiation by infalling to a black hole converts between 6% and 32% of the mass to energy, compared to 0.7% for the conversion of mass to energy in a star like the Sun.<ref name="Lambourne" /> It is the only process known that can produce such high power over a very long term. (Stellar explosions such as [[supernova]]s and [[gamma-ray burst]]s, and direct [[matter]]–[[antimatter]] annihilation, can also produce very high power output, but supernovae only last for days, and the universe does not appear to have had large amounts of antimatter at the relevant times.)

Since quasars exhibit all the properties common to other [[active galaxy|active galaxies]] such as [[Seyfert galaxy|Seyfert galaxies]], the emission from quasars can be readily compared to those of smaller active galaxies powered by smaller supermassive black holes. To create a luminosity of 10<sup>40</sup>&nbsp;[[watt]]s (the typical brightness of a quasar), a supermassive black hole would have to consume the material equivalent of 10 solar masses per year. The brightest known quasars devour 1,000 solar masses of material every year (equivalent to 10 Earths per second). Quasar luminosities can vary considerably over time, depending on their surroundings. Since it is difficult to fuel quasars for many billions of years, after a quasar finishes accreting the surrounding gas and dust, it becomes an ordinary galaxy.

Radiation from quasars is partially "nonthermal" (i.e., not due to [[black-body radiation]]), and approximately 10% are observed to also have jets and lobes like those of [[radio galaxy|radio galaxies]] that also carry significant (but poorly understood) amounts of energy in the form of particles moving at [[relativistic speed]]s. Extremely high energies might be explained by several mechanisms (see [[Fermi acceleration]] and [[Centrifugal mechanism of acceleration]]). Quasars can be detected over the entire observable [[electromagnetic spectrum]], including [[radio waves|radio]], [[infrared]], [[visible light]], [[ultraviolet]], [[X-ray]] and even [[gamma ray]]s. Most quasars are brightest in their rest-frame ultraviolet [[wavelength]] of 121.6&nbsp;[[nanometer|nm]] [[Lyman series|Lyman-alpha]] emission line of hydrogen, but due to the tremendous redshifts of these sources, that peak luminosity has been observed as far to the red as 900.0&nbsp;nm, in the near infrared. A minority of quasars show strong radio emission, which is generated by jets of matter moving close to the speed of light. When viewed downward, these appear as [[blazar]]s and often have regions that seem to move away from the center faster than the speed of light ([[superluminal]] expansion).This is an optical illusion due to the properties of [[special relativity]].

Quasar redshifts are measured from the strong [[spectral line]]s that dominate their visible and ultraviolet emission spectra. These lines are brighter than the continuous spectrum. They exhibit [[Doppler broadening]] corresponding to a mean speed of several percent of the speed of light. Fast motions strongly indicate a large mass. Emission lines of hydrogen (mainly of the [[Lyman series]] and [[Balmer series]]), helium, carbon, magnesium, iron and oxygen are the brightest lines. The atoms emitting these lines range from neutral to highly ionized, leaving it highly charged. This wide range of ionization shows that the gas is highly irradiated by the quasar, not merely hot, and not by stars, which cannot produce such a wide range of ionization.

Like all (unobscured) active galaxies, quasars can be strong X-ray sources. Radio-loud quasars can also produce X-rays and gamma rays by [[inverse Compton scattering]] of lower-energy photons by the radio-emitting electrons in the jet.<ref name="Dooling">{{Cite web |last=Dooling |first=Dave |date=18 November 1999 |title=BATSE finds most distant quasar yet seen in soft gamma rays Discovery will provide insight on formation of galaxies |url=https://science.nasa.gov/NEWHOME/HEADLINES/ast24nov99_1.htm |url-status=dead |archive-url=https://web.archive.org/web/20090723104615/http://science.nasa.gov/newhome/headlines/ast24nov99_1.htm |archive-date=2009-07-23 |website=NASA Science}}</ref>

{{anchor|iron quasar}}''Iron quasars'' show strong emission lines resulting from low-ionization [[iron]] (Fe&nbsp;{{abbr|II|singly ionized}}), such as IRAS&nbsp;18508-7815.

<gallery widths="200" heights="200">
File:Bright halos around distant quasars.jpg|Bright halos around 18 distant quasars<ref>{{cite web |title=Bright halos around distant quasars |url=http://www.eso.org/public/images/eso1638a/ |access-date=26 October 2016 |website=eso.org}}</ref>
File:PKS 1127-145 X-rays.jpg|The [[Chandra X-ray Observatory|Chandra]] X-ray image is of the quasar PKS 1127-145, a highly luminous source of X-rays and visible light about 10 billion light-years from Earth. An enormous X-ray jet extends at least a million light-years from the quasar. Image is 60 arcseconds on a side. [[Right ascension|RA]] 11h 30m 7.10s [[Declination|Dec]] −14° 49' 27" in Crater. Observation date: May 28, 2000. Instrument: ACIS
File:Quasar HE 1104-1805.jpg|Gravitationally lensed quasar HE 1104-1805<ref>{{cite news |title=Gravitationally lensed quasar HE 1104-1805 |url=http://www.spacetelescope.org/images/heic1116a/ |access-date=4 November 2011 |newspaper=ESA/Hubble Press Release}}</ref>
File:Artist's impression of mysterious alignment of quasar rotation axes.ogv|Animation shows the alignments between the spin axes of quasars and the large-scale structures that they inhabit.
</gallery>

=== Spectral lines, reionization, and the early universe===
{{main|Reionization|Chronology of the Universe}}
[[File:Fingerprint of the early Universe.jpg|thumb|Spectrum from quasar HE&nbsp;0940-1050 after it has travelled through [[intergalactic medium]]]]
[[File:QuasarStarburst.jpg|thumb|right|This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous [[starburst (astronomy)|starburst]] ever seen in such a combination.]]

Quasars also provide some clues as to the end of the [[Big Bang]]'s [[reionization]]. The oldest known quasars ([[redshift|''z'']]&nbsp;=&nbsp;6){{needs update|date=January 2021}} display a [[Gunn–Peterson trough]] and have absorption regions in front of them indicating that the [[intergalactic medium]] at that time was [[neutral gas]]. More recent quasars show no absorption region, but rather their spectra contain a spiky area known as the [[Lyman-alpha forest]]; this indicates that the intergalactic medium has undergone reionization into [[plasma (physics)|plasma]], and that neutral gas exists only in small clouds.

The intense production of [[ionization|ionizing]] [[ultraviolet]] radiation is also significant, as it would provide a mechanism for reionization to occur as galaxies form. Despite this, current theories suggest that quasars were not the primary source of reionization; the primary causes of reionization were probably the earliest generations of [[star]]s, known as [[population III]] stars (possibly 70%), and [[dwarf galaxies]] (very early small high-energy galaxies) (possibly 30%).<ref name="popIII_sim">{{Cite journal |last1=Gnedin |first1=Nickolay Y. |last2=Ostriker |first2=Jeremiah P. |date=1997 |title=Reionization of the Universe and the Early Production of Metals |journal=The Astrophysical Journal |language=en |volume=486 |issue=2 |pages=581–598 |arxiv=astro-ph/9612127 |bibcode=1997ApJ...486..581G |doi=10.1086/304548 |issn=0004-637X |s2cid=5758398}}</ref><ref name="qso_z">{{cite arXiv |first=Limin |last=Lu |date=1998 |title=The Metal Contents of Very Low Column Density Lyman-alpha Clouds: Implications for the Origin of Heavy Elements in the Intergalactic Medium |eprint=astro-ph/9802189 |display-authors=etal }}</ref><ref name="Bouwens_LLG">{{Cite journal |last=Bouwens |first=R. J. |display-authors=etal |date=2012 |title=Lower-luminosity Galaxies Could Reionize the Universe: Very Steep Faint-end Slopes to the UV Luminosity Functions at ''z'' ⩾ 5–8 from the HUDF09 WFC3/IR Observations |journal=The Astrophysical Journal |volume=752 |issue=1 |pages=L5 |arxiv=1105.2038 |bibcode=2012ApJ...752L...5B |doi=10.1088/2041-8205/752/1/L5 |issn=2041-8205 |s2cid=118856513}}</ref><ref name="qso_source1">{{Cite journal |last=Madau |first=Piero |display-authors=etal |date=April 1999 |title=Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Sources |journal=The Astrophysical Journal |language=en |volume=514 |issue=2 |pages=648–659 |arxiv=astro-ph/9809058 |bibcode=1999ApJ...514..648M |doi=10.1086/306975 |issn=0004-637X |s2cid=17932350}}</ref><ref name="qso_source0">{{Cite journal |last1=Shapiro |first1=Paul R. |author-link=Paul R. Shapiro |last2=Giroux |first2=Mark L. |date=October 1987 |title=Cosmological H II regions and the photoionization of the intergalactic medium |journal=The Astrophysical Journal |language=en |volume=321 |pages=L107 |bibcode=1987ApJ...321L.107S |doi=10.1086/185015 |issn=0004-637X |doi-access=free}}</ref><ref name="qso_source2">{{Cite journal |last=Fan |first=Xiaohui |display-authors=etal |date=December 2001 |title=A Survey of ''z'' > 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at ''z'' ~ 6 |journal=The Astronomical Journal |volume=122 |issue=6 |pages=2833–2849 |arxiv=astro-ph/0108063 |bibcode=2001AJ....122.2833F |doi=10.1086/324111 |s2cid=119339804}}</ref>

Quasars show evidence of elements heavier than [[helium]], indicating that galaxies underwent a massive phase of [[star formation]], creating [[population III stars]] between the time of the [[Big Bang]] and the first observed quasars. Light from these stars may have been observed in 2005 using [[NASA]]'s [[Spitzer Space Telescope]],<ref>{{cite web |url=http://www.nasa.gov/centers/goddard/news/topstory/2005/universe_objects.html |title=NASA Goddard Space Flight Center: News of light that may be from population III stars |publisher=Nasa.gov |access-date=2011-07-01 |archive-date=2011-04-16 |archive-url=https://web.archive.org/web/20110416191522/http://www.nasa.gov/centers/goddard/news/topstory/2005/universe_objects.html |url-status=dead }}</ref> although this observation remains to be confirmed.

== Subtypes ==
The [[Taxonomy (general)|taxonomy]] of quasars includes various subtypes representing subsets of the quasar population having distinct properties.

* '''Radio-loud quasars''' are quasars with powerful [[Astrophysical jet|jets]] that are strong sources of radio-wavelength emission. These make up about 10% of the overall quasar population.<ref name="peterson1997">{{cite book |last1=Peterson |first1=Bradley |title=Active Galactic Nuclei |date=1997 |publisher=Cambridge University Press |isbn=0-521-47911-8}}</ref>
* '''Radio-quiet quasars''' are those quasars lacking powerful jets, with relatively weaker radio emission than the radio-loud population. The majority of quasars (about 90%) are radio-quiet.<ref name="peterson1997"/>
* '''Broad absorption-line (BAL) quasars''' are quasars whose spectra exhibit broad absorption lines that are blue-shifted relative to the quasar's rest frame, resulting from gas flowing outward from the active nucleus in the direction toward the observer. Broad absorption lines are found in about 10% of quasars, and BAL quasars are usually radio-quiet.<ref name="peterson1997"/> In the rest-frame ultraviolet spectra of BAL quasars, broad absorption lines can be detected from ionized carbon, magnesium, silicon, nitrogen, and other elements.
* '''Type 2 (or Type II) quasars''' are quasars in which the accretion disc and broad emission lines are highly obscured by dense gas and [[Cosmic dust|dust]]. They are higher-luminosity counterparts of Type 2 Seyfert galaxies.<ref>{{cite journal |last1=Zakamska |first1=Nadia |display-authors=etal|title=Candidate Type II Quasars from the Sloan Digital Sky Survey. I. Selection and Optical Properties of a Sample at 0.3 < ''Z'' < 0.83 |journal=The Astronomical Journal |year=2003 |volume=126 |issue=5 |page=2125 |doi=10.1086/378610 |bibcode=2003AJ....126.2125Z |arxiv=astro-ph/0309551 |s2cid=13477694 }}</ref> 
* '''Red quasars''' are quasars with optical colors that are redder than normal quasars, thought to be the result of moderate levels of dust [[Extinction (astronomy)|extinction]] within the quasar host galaxy. Infrared surveys have demonstrated that red quasars make up a substantial fraction of the total quasar population.<ref>{{cite journal |last1=Glikman |first1=Eilat |display-authors=et al |title=The FIRST-2MASS Red Quasar Survey |journal=The Astrophysical Journal |date=2007 |volume=667 |issue=2 |page=673 |doi=10.1086/521073 |bibcode=2007ApJ...667..673G |arxiv=0706.3222 |s2cid=16578760 }}</ref>
* '''[[OVV quasar|Optically violent variable]] (OVV) quasars''' are radio-loud quasars in which the jet is directed toward the observer. Relativistic beaming of the jet emission results in strong and rapid variability of the quasar brightness. OVV quasars are also considered to be a type of [[blazar]].
* '''Weak emission line quasars''' are quasars having unusually faint emission lines in the ultraviolet/visible spectrum.<ref>{{cite journal |last1=Diamond-Stanic |first1=Aleksandar |display-authors=etal|title=High-redshift SDSS Quasars with Weak Emission Lines |journal=The Astrophysical Journal |date=2009 |volume=699 |issue=1 |pages=782–799 |doi=10.1088/0004-637X/699/1/782 |bibcode=2009ApJ...699..782D |arxiv=0904.2181 |s2cid=6735531 }}</ref>

==Role in celestial reference systems==
[[File:Quasar HE 0109-3518.tiff|thumb|The energetic radiation of the quasar makes [[dark galaxies]] glow, helping astronomers to understand the obscure early stages of galaxy formation.<ref>{{cite press release |title=Dark Galaxies of the Early Universe Spotted for the First Time |url=http://www.eso.org/public/news/eso1228/|access-date=13 July 2012|publisher=ESO}}</ref> ]]

Because quasars are extremely distant, bright, and small in apparent size, they are useful reference points in establishing a measurement grid on the sky.<ref>{{cite web |url=http://aa.usno.navy.mil/faq/docs/ICRS_doc.php |title=ICRS Narrative |access-date=2012-06-07 |publisher=<!---http://aa.usno.navy.mil/index.php--->U.S. Naval Observatory Astronomical Applications |archive-date=2011-07-09 |archive-url=https://web.archive.org/web/20110709100504/http://aa.usno.navy.mil/faq/docs/ICRS_doc.php |url-status=dead }}</ref>
The [[International Celestial Reference System]] (ICRS) is based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky. Because they are so distant, they are apparently stationary to current technology, yet their positions can be measured with the utmost accuracy by [[very-long-baseline interferometry]] (VLBI). The positions of most are known to 0.001 [[arcsecond]] or better, which is orders of magnitude more precise than the best optical measurements.

==Multiple quasars==
A grouping of two or more quasars on the sky can result from a chance alignment, where the quasars are not physically associated, from actual physical proximity, or from the effects of gravity bending the light of a single quasar into two or more images by [[gravitational lensing]]. Discerning between these cases can be difficult.

===Lensed multiples===
A multiple-image quasar is a quasar whose light undergoes [[gravitational lensing]], resulting in double, triple or quadruple images of the same quasar. The first such gravitational lens to be discovered was the double-imaged quasar [[Q0957+561]] (or Twin Quasar) in 1979.<ref>{{Cite journal |last1=Blandford |first1=R. D. |author-link=Roger Blandford |last2=Narayan |first2=R. |date=September 1992 |title=Cosmological Applications of Gravitational Lensing |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=30 |issue=1 |pages=311–358 |bibcode=1992ARA&A..30..311B |doi=10.1146/annurev.aa.30.090192.001523 |issn=0066-4146}}</ref>
An example of a triply lensed quasar is PG1115+08.<ref>{{Cite journal |last1=Henry |first1=J. Patrick |last2=Heasley |first2=J. N. |date=May 1986 |title=High-resolution imaging from Mauna Kea: the triple quasar in 0.3-arc s seeing |journal=Nature |language=en |volume=321 |issue=6066 |pages=139–142 |bibcode=1986Natur.321..139H |doi=10.1038/321139a0 |issn=0028-0836 |s2cid=4244246}}</ref>
Several quadruple-image quasars are known, including the [[Einstein Cross]] and the [[Cloverleaf Quasar]], with the first such discoveries happening in the mid-1980s.

The [[Gaia (spacecraft)|Gaia]] astrometric all sky survey has already discovered a dozen or so Einstein crosses, and continued work with its data releases has potential to identify many more still.<ref name=GaiaHunt/><ref>{{cite web |title=IoW_20150409 - Gaia - Cosmos |url=https://www.cosmos.esa.int/web/gaia/iow_20150409}}</ref>

A relatively recent topic of research called "time delay cosmology" uses multiple-lens quasars to probe the [[Hubble tension]].<ref name=TreuMarshall2016>{{cite journal |bibcode=2016A&ARv..24...11T}}</ref> When differing images of a single source have differing path lengths, that difference can be measured via the time delay between flickers in the images. The path length difference plus modelling the deflecting mass can be used to estimate the absolute distance of the source; that enables a cosmic distance measurement independent of the standard [[cosmic distance ladder]]. Some results from 2020, claiming precision of 2-3%, were later determined to suffer from systematic errors due to overly strong assumptions; in 2025, the TDCOSMO collaboration significantly weakened the assumptions, publishing a result of {{val|71.6|3.9|3.3|u=km/s|up=Mpc}} (around 5% precision).<ref name=TDCOSMO2025>{{cite arXiv |last1=Collaboration |first1=TDCOSMO |last2=Birrer |first2=Simon |last3=Buckley-Geer |first3=Elizabeth J. |last4=Cappellari |first4=Michele |last5=Courbin |first5=Frédéric |last6=Dux |first6=Frédéric |last7=Fassnacht |first7=Christopher D. |last8=Frieman |first8=Joshua A. |last9=Galan |first9=Aymeric |last10=Gilman |first10=Daniel |last11=Huang |first11=Xiang-Yu |last12=Knabel |first12=Shawn |last13=Langeroodi |first13=Danial |last14=Lin |first14=Huan |last15=Millon |first15=Martin |last16=Morishita |first16=Takahiro |last17=Motta |first17=Veronica |last18=Mozumdar |first18=Pritom |last19=Paic |first19=Eric |last20=Shajib |first20=Anowar J. |last21=Sheu |first21=William |last22=Sluse |first22=Dominique |last23=Sonnenfeld |first23=Alessandro |last24=Spiniello |first24=Chiara |last25=Stiavelli |first25=Massimo |last26=Suyu |first26=Sherry H. |author27=Chin Yi Tan |last28=Treu |first28=Tommaso |author29=Lyne Van de Vyvere |last30=Wang |first30=Han |display-authors=1 |title=TDCOSMO 2025: Cosmological constraints from strong lensing time delays |date=2025 |class=astro-ph.CO |eprint=2506.03023}}</ref>

===Physical multiples===
When two quasars appear to be very close to each other as seen from Earth (separated by a few [[arcsecond]]s or less), they are commonly referred to as a "double quasar". If a visual pair are shown to be also close together in space (i.e. observed to have similar redshifts), they are termed a "quasar pair", or as a "binary quasar" or "dual quasar"<ref name=GaiaDualQuasar>{{cite web |title=Gaia Image of the Week |url=https://www.cosmos.esa.int/web/gaia/iow_20230405}}</ref><ref name=Chen+2023>{{cite journal |last1=Chen |first1=Yu-Ching |last2=Liu |first2=Xin |last3=Foord |first3=Adi |last4=Shen |first4=Yue |last5=Oguri |first5=Masamune |last6=Chen |first6=Nianyi |last7=Di Matteo |first7=Tiziana |last8=Holgado |first8=Miguel |last9=Hwang |first9=Hsiang-Chih |last10=Zakamska |first10=Nadia |title=A close quasar pair in a disk–disk galaxy merger at z = 2.17 |url=https://www.nature.com/articles/s41586-023-05766-6 |journal=Nature |date=2023 |volume=616 |issue=7955 |pages=45–49 |doi=10.1038/s41586-023-05766-6 |pmid=37020007 |arxiv=2209.11249 |bibcode=2023Natur.616...45C }}</ref> if they are close enough that their host galaxies are likely to be physically interacting.<ref>{{Cite journal |last=Myers |first=Adam D. |display-authors=et al |year=2008 |title=Quasar Clustering at 25 ''h''<sup>−1</sup> kpc from a Complete Sample of Binaries |journal=The Astrophysical Journal |language=en |volume=678 |issue=2 |pages=635–646 |arxiv=0709.3474 |bibcode=2008ApJ...678..635M |doi=10.1086/533491 |issn=0004-637X |s2cid=15747141}}</ref>

As quasars are overall rare objects in the universe, the probability of three or more separate quasars being found near the same physical location is very low, and determining whether the system is closely separated physically requires significant observational effort, which most quasar-like sources never receive; as a result, not many physical multiple quasars are known.

Physical multiple quasars can be important mechanisms to study [[Interacting galaxy|galactic interactions]] and evolution, especially in the early universe, such as when star formation and galactic activity were highest (redshift z=2).<ref>{{cite journal |last1=Shen |first1=Yue |last2=Chen |first2=Yu-Ching |last3=Hwang |first3=Hsiang-Chih |last4=Liu |first4=Xin |last5=Zakamska |first5=Nadia |last6=Oguri |first6=Masamune |last7=Li |first7=Jennifer I-Hsiu |last8=Lazio |first8=Joseph |last9=Breiding |first9=Peter |title=A hidden population of high-redshift double quasars unveiled by astrometry |journal=Nature Astronomy |date=2021 |volume=5 |issue=6 |pages=569–574 |doi=10.1038/s41550-021-01323-1 |arxiv=2105.03298 |bibcode=2021NatAs...5..569S |url=https://www.nature.com/articles/s41550-021-01323-1}}</ref>

The [[Gaia (spacecraft)|Gaia]] astrometric all sky survey, in its first foray into multiple quasars, suggested that the [[Sloan Digital Sky Survey]]-discovered quasar J0749+2255 was in fact a dual quasar. Followup observations by Hubble, Keck, Gemini, Chandra and VLA confirmed the true nature of this source, an interacting galaxy pair at redshift z=2.17.<ref name=GaiaDualQuasar/><ref name=Chen+2023/> Future work with Gaia data releases can be expected to significantly expand the catalog of both lensed and physical multiple quasars, and associated galactic research.<ref name=GaiaHunt>{{cite web |url=https://www.cosmos.esa.int/web/gaia/iow_20220926 |title=Gaia on the hunt for dual quasars and gravitational lenses}}</ref><ref name=Mannucci+2022>{{cite journal | last1=Mannucci | first1=F. | last2=Pancino | first2=E. | last3=Belfiore | first3=F. | last4=Cicone | first4=C. | last5=Ciurlo | first5=A. | last6=Cresci | first6=G. | last7=Lusso | first7=E. | last8=Marasco | first8=A. | last9=Marconi | first9=A. | last10=Nardini | first10=E. | last11=Pinna | first11=E. | last12=Severgnini | first12=P. | last13=Saracco | first13=P. | last14=Tozzi | first14=G. | last15=Yeh | first15=S. | title=Unveiling the population of dual and lensed active galactic nuclei at sub-arcsec separations | journal=Nature Astronomy | date=2022 | volume=6 | issue=10 | pages=1185–1192 | doi=10.1038/s41550-022-01761-5 | arxiv=2203.11234 | bibcode=2022NatAs...6.1185M | url=https://www.nature.com/articles/s41550-022-01761-5 }}</ref>

The first true triple quasar was found in 2007 by observations at the [[W. M. Keck Observatory]] in [[Mauna Kea]], [[Hawaii]].<ref>{{cite news |first=Paul |last=Rincon |author-link=Paul Rincon |date=2007-01-09 |title=Astronomers see first quasar trio |work=BBC News |url=http://news.bbc.co.uk/2/hi/science/nature/6243361.stm }}</ref> [[LBQS 1429-008]] (or QQQ J1432-0106) was first observed in 1989 and at the time was found to be a double quasar. When [[astronomer]]s discovered the third member, they confirmed that the sources were separate and not the result of gravitational lensing. This triple quasar has a redshift of ''z'' = 2.076.<ref>{{cite web |title=Triple quasar QQQ 1429-008 |publisher=ESO |access-date=2009-04-23 |url=http://www.eso.org/gallery/v/ESOPIA/Quasars/phot-02-07.jpg.html |url-status=dead |archive-url=https://web.archive.org/web/20090208213256/http://eso.org/gallery/v/ESOPIA/Quasars/phot-02-07.jpg.html |archive-date=2009-02-08}}</ref> The components are separated by an estimated 30–50&nbsp;[[kiloparsecs]] (roughly 97,000–160,000 light-years), which is typical for interacting galaxies.<ref>{{Cite journal |last=Djorgovski |first=S. G. |author-link=Stanislav George Djorgovski |display-authors=etal |date=2007 |title=Discovery of a Probable Physical Triple Quasar |journal=The Astrophysical Journal |language=en |volume=662 |issue=1 |pages=L1–L5 |arxiv=astro-ph/0701155 |bibcode=2007ApJ...662L...1D |doi=10.1086/519162 |issn=0004-637X |s2cid=22705420}}</ref> In 2013, the second true triplet of quasars, QQQ&nbsp;J1519+0627, was found with a redshift ''z'' = 1.51, the whole system fitting within a physical separation of 25&nbsp;kpc (about 80,000 light-years).<ref>{{cite web |date=12 March 2013 |title=Extremely rare triple quasar found |url=https://phys.org/news/2013-03-extremely-rare-triple-quasar.html |access-date=1 May 2025 |website=phys.org}}</ref><ref>{{Cite journal |last=Farina |first=E. P. |display-authors=et al |year=2013 |title=Caught in the act: discovery of a physical quasar triplet |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=431 |issue=2 |pages=1019–1025 |arxiv=1302.0849 |bibcode=2013MNRAS.431.1019F |doi=10.1093/mnras/stt209 |issn=1365-2966 |s2cid=54606964 |doi-access=free}}</ref>

The first true quadruple quasar system was discovered in 2015 at a redshift ''z''&nbsp;=&nbsp;2.0412 and has an overall physical scale of about 200&nbsp;kpc (roughly 650,000 light-years).<ref>{{Cite journal |last=Hennawi |first=Joseph F. |display-authors=et al |year=2015 |title=Quasar quartet embedded in giant nebula reveals rare massive structure in distant universe |journal=Science |language=en |volume=348 |issue=6236 |pages=779–783 |arxiv=1505.03786 |bibcode=2015Sci...348..779H |doi=10.1126/science.aaa5397 |issn=0036-8075 |pmid=25977547 |s2cid=35281881}}</ref>

== Gallery ==
<gallery mode="packed" heights="160">
File:Hubble_Resolves_Two_Pairs_of_Quasars.jpg|These two NASA/ESA Hubble Space Telescope images reveal two pairs of quasars that existed 10 billion years ago and reside at the hearts of merging galaxies.<ref>{{cite web |title=Hubble Resolves Two Pairs of Quasars |url=https://esahubble.org/images/opo2114a/ |access-date=April 13, 2021 |website=esahubble.org}}</ref>
File:Webb identifies the earliest strands of the cosmic web (annotated) (aspire1).jpg|This image from the NASA/ESA/CSA [[James Webb Space Telescope]] shows an arrangement of ten galaxies. The 3-million-light-year-long filament is anchored by a quasar.
File:Webb's View Around the Extremely Red Quasar SDSS J165202 64+172852 3 (weic2217a).jpeg|The quasar SDSS J165202.64+172852.3 images in the center and on the right present new observations from the JWST in multiple wavelengths to demonstrate the distribution of gas around the object.<ref>{{cite news |date=October 19, 2023 |title=Webb's View Around the Extremely Red Quasar SDSS J165202.64+172852.3. |url=https://esawebb.org/images/weic2217a/ |work=esawebb.org}}</ref> 
File:Quasar HE0450-2958.jpg|Quasar HE0450-2958 as imaged by the Hubble Space Telescope with advanced image processing known as MCS-deconvolution.
File:Quasar HE 1013-2136 with Tidal Tails.jpg|This photograph shows an image of the Quasar HE 1013-2136 (center) and its surroundings.
File:Quasar 3C 273.jpg| Quasar 3C 273 as imaged by the Hubble Space Telescope's Advanced Camera for Surveys.
File:Radio map of quasar 3C48.jpg| Radio map of the quasar 3C48 made from VLBA 1.4-GHz observations, made by uploader. Cross indicates core, arrow shows 0.5 arcsec.
File:Best image of bright quasar 3C 273 (10953173335).jpg| This image from Hubble’s Wide Field and Planetary Camera 2 (WFPC2) is likely the best of ancient and brilliant quasar 3C 273, which resides in a giant elliptical galaxy in the constellation of Virgo (The Virgin).
</gallery>

== See also ==
{{Portal|Astronomy|Stars|Outer space|Physics}}
* [[Galaxy formation and evolution]]
* [[Large quasar group]]
* [[List of microquasars]]
* [[List of quasars]]
* [[Microquasar]]
* [[Quasi-star]]

== References ==
{{Reflist|30em}}

== External links ==
{{Commons category|Quasars}}
{{wikiquote}}
* [http://www.aavso.org/vstar/vsots/ 3C 273: Variable Star Of The Season]
* [http://www.sky-map.org/?object=3C%20273&zoom=12&img_source=SDSS SKY-MAP.ORG SDSS image of quasar 3C 273]
* [http://www.perseus.gr/Astro-DSO-Quasars.htm Expanding Gallery of Hires Quasar Images]
* <!---[http://www.sdss.org/gallery/gal_zqso.html--->[https://web.archive.org/web/20170208212333/http://classic.sdss.org/gallery/ Gallery of Quasar Spectra from SDSS]
* [http://cas.sdss.org/dr6/en/proj/advanced/quasars/default.asp SDSS Advanced Student Projects: Quasars]
* [https://web.archive.org/web/20080517091731/http://hubblesite.org/go/blackholes Black Holes: Gravity's Relentless Pull] Award-winning interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
* [http://www.astronomycast.com/astronomy/galaxies/ep-98-quasars/ Audio: Fraser Cain/Pamela L. Gay – Astronomy Cast. Quasars – July 2008]
* {{Cite web |last1=Merrifield |first1=Michael |last2=Copland |first2=Ed |title=z~1.3 – An implausibly large structure [in the Universe] |url=http://www.sixtysymbols.com/videos/largest_structure_universe.htm |website=Sixty Symbols |publisher=[[Brady Haran]] for the [[University of Nottingham]]}}

{{Galaxy}}
{{black holes}}
{{Stellar core collapse}}
{{Authority control}}

[[Category:Quasars| ]]
[[Category:Active galaxy types]]
[[Category:Articles containing video clips]]
[[Category:Radio astronomy]]
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