Editing Quasar (section)

== 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).