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==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 Γpik]] 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=661β678 |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 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. The Discovery Of Quasars |url=http://ned.ipac.caltech.edu/level5/Sept04/Shields/Shields3.html |website=[[California Institute of Technology]]}}</ref> While the observations and redshifts themselves were not doubted, their correct interpretation was heavily debated, and Bolton's suggestion that the radiation detected from quasars were ordinary [[spectral line]]s from distant highly redshifted sources with extreme velocity was not widely accepted at the time. === 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 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=39β41 |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. 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 β 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 28Β° 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>
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