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{{Short description|Astronomical structure}}

{{About|the astronomical structure|our galaxy|Milky Way|other uses}}

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{{Use mdy dates|date=February 2015}}

{{Multiple image |direction=vertical |align=right |width=310|image1=NGC 4414 (NASA-med).jpg|caption1=[[NGC 4414]], a typical [[spiral galaxy]] in the [[constellation]] [[Coma Berenices]], is about 55,000 [[light-year]]s in diameter and approximately 60 million light-years from Earth.}}

A '''galaxy''' is a [[gravitation]]ally bound system of [[star]]s, [[stellar remnant]]s, [[interstellar medium|interstellar gas]], [[cosmic dust|dust]], and [[dark matter]].<ref name="sparkegallagher2000" /><ref name=nasa060812 /> The word is derived from the [[Ancient Greek|Greek]] ''{{transl|grc|galaxias}}'' ({{lang|grc|γαλαξίας}}), literally 'milky', a reference to the [[Milky Way]] galaxy that contains the [[Solar System]]. Galaxies range in size from [[dwarf galaxy|dwarfs]] with just a few hundred million ({{10^|8}}) stars to [[IC 1101|giants]] with one hundred [[Orders of magnitude (numbers)#1012|trillion]] ({{10^|14}}) stars,<ref name=science250_4980_539 /> each orbiting its galaxy's [[center of mass]].

Galaxies are categorized according to their visual [[morphology (astronomy)|morphology]] as [[elliptical galaxy|elliptical]],<ref name=uf030616 /> [[Spiral galaxy|spiral]], or [[irregular galaxy|irregular]].<ref name="IRatlas" /> Many are thought to have [[supermassive black hole]]s at their centers. The Milky Way's central black hole, known as [[Sagittarius A*]], has a mass four million times greater than the [[Sun]].<ref name="smbh" /> As of March 2016, [[GN-z11]] is the oldest and most distant galaxy observed. It has a [[comoving distance]] of 32 billion [[light-years]] from [[Earth]], and is seen as it existed just 400 million years after the [[Big Bang]].

In 2021, data from NASA's [[New Horizons]] space probe was used to revise the previous estimate to roughly 200 billion galaxies ({{val|2e11}}),<ref>{{Cite web|title=Astronomers were wrong about the number of galaxies in universe|url=https://www.jpost.com/health-science/astronomers-were-wrong-about-the-number-of-galaxies-in-universe-655425|access-date=2021-01-14|website=The Jerusalem Post {{!}} JPost.com|language=en-US|archive-date=January 14, 2021|archive-url=https://web.archive.org/web/20210114153938/https://www.jpost.com/health-science/astronomers-were-wrong-about-the-number-of-galaxies-in-universe-655425|url-status=live}}</ref> which followed a 2016 estimate that there were two trillion ({{val|2e12}}) or more<ref name="Conselice" /><ref name="NYT-20161017">{{cite news |last=Fountain |first=Henry |title=Two Trillion Galaxies, at the Very Least |url=https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html |date=17 October 2016 |work=[[The New York Times]] |access-date=17 October 2016 |archive-date=December 31, 2019 |archive-url=https://web.archive.org/web/20191231233343/https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html |url-status=live }}</ref> galaxies in the [[observable universe]], overall, and as many as an estimated {{val|1e24}} stars<ref name="ESA-2019">{{cite web |author=Staff |title=How Many Stars Are There In The Universe? |url=https://www.esa.int/Our_Activities/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe |date=2019 |work=[[European Space Agency]] |access-date=21 September 2019 |archive-date=September 23, 2019 |archive-url=https://web.archive.org/web/20190923134902/http://www.esa.int/Our_Activities/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe |url-status=live }}</ref><ref>{{Cite book|chapter=The Structure of the Universe|doi=10.1007/978-1-4614-8730-2_10|title=The Fundamentals of Modern Astrophysics|pages=279–294|year=2015|last1=Marov|first1=Mikhail Ya.|isbn=978-1-4614-8729-6}}</ref> (more stars than all the [[Sand|grains of sand]] on all beaches of the planet [[Earth]]).<ref name="SU-20020201">{{cite web |last=Mackie |first=Glen |title=To see the Universe in a Grain of Taranaki Sand |url=http://astronomy.swin.edu.au/~gmackie/billions.html |date=1 February 2002 |work=[[Centre for Astrophysics and Supercomputing]] |access-date=28 January 2017 |archive-date=January 7, 2019 |archive-url=https://web.archive.org/web/20190107010855/http://astronomy.swin.edu.au/~gmackie/billions.html%0A%20 |url-status=live }}</ref> Most of the galaxies are 1,000 to 100,000 [[parsec]]s in diameter (approximately 3,000 to 300,000 [[light year]]s) and are separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 ly) and is separated from the [[Andromeda Galaxy]] (with diameter of about 220,000 ly), its nearest large neighbor, by 780,000 parsecs (2.5 million ly.)

The [[intergalactic space|space]] between galaxies is filled with a tenuous gas (the [[Outer space#Intergalactic space|intergalactic medium]]) with an average density of less than one [[atom]] per cubic meter. Most galaxies are gravitationally organized into [[galaxy group|groups]], [[galaxy cluster|clusters]] and [[supercluster]]s. The [[Milky Way]] is part of the [[Local Group]], which it dominates along with [[Andromeda Galaxy]]. The group is part of the [[Virgo Supercluster]]. At the [[Large-scale structure of the Cosmos|largest scale]], these associations are generally arranged into [[galaxy filament|sheets and filaments]] surrounded by immense [[void (astronomy)|voids]].<ref name=camb_lss /> Both the Local Group and the [[Virgo Supercluster]] are contained in a much larger cosmic structure named [[Laniakea Supercluster|Laniakea]].<ref>{{cite journal | last1 = Gibney | first1 = Elizabeth | s2cid = 124323774 | year = 2014 | title = Earth's new address: 'Solar System, Milky Way, Laniakea' | journal = Nature | doi = 10.1038/nature.2014.15819 }}</ref>

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== Etymology ==

The word ''galaxy'' was borrowed via [[French language|French]] and [[Medieval Latin]] from the [[Greek language|Greek]] term for the Milky Way, ''{{transl|grc|galaxías (kúklos)}}'' {{lang|grc|{{linktext|γαλαξίας}}}} ({{lang|grc|{{linktext|κύκλος}}}})<ref>C. T. Onions et al., ''The Oxford Dictionary of English Etymology'', Oxford, 1966, p. 385.</ref><ref name=oed /> 'milky (circle)', named after its appearance as a milky band of light in the sky. In [[Greek mythology]], [[Zeus]] places his son born by a mortal woman, the infant [[Heracles]], on [[Hera]]'s breast while she is asleep so the baby will drink her divine milk and thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way.<ref name=waller_hodge2003 /><ref name=konecny2006 />

In the astronomical literature, the capitalized word "Galaxy" is often used to refer to our galaxy, the [[Milky Way]], to distinguish it from the other galaxies in our [[universe]]. The English term ''Milky Way'' can be traced back to a story by [[Chaucer]] {{circa|1380}}:

{{Quote|See yonder, lo, the Galaxyë<br />&nbsp;Which men {{linktext|clepe}}th ''the Milky Wey'',<br />&nbsp;For hit is whyt.|Geoffrey Chaucer|''[[The House of Fame]]''<ref name=oed />}}

Galaxies were initially discovered telescopically and were known as ''[[spiral nebula]]e''. Most 18th to 19th century astronomers considered them as either unresolved [[star cluster]]s or anagalactic [[nebula]]e, and were just thought of as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the [[Andromeda Galaxy]], began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called ''island universes'', but this term quickly fell into disuse, as the word ''universe'' implied the entirety of existence. Instead, they became known simply as galaxies.<ref name=rao2005 />

== Nomenclature ==

[[File:Probing the distant past SDSS J1152+3313.tif|thumb|[[Galaxy cluster]] [[SDSS J1152+3313]]. SDSS stands for [[Sloan Digital Sky Survey]], J for [[Julian epoch]], and 1152+3313 for [[right ascension]] and [[declination]] respectively.]]

Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the [[Andromeda Galaxy]], the [[Magellanic Clouds]], the [[Whirlpool Galaxy]], and the [[Sombrero Galaxy]]. Astronomers work with numbers from certain catalogues, such as the [[Messier catalogue]], the NGC ([[New General Catalogue]]), the IC ([[Index Catalogue]]), the CGCG ([[Catalogue of Galaxies and of Clusters of Galaxies]]), the MCG ([[Morphological Catalogue of Galaxies]]), the UGC ([[Uppsala General Catalogue]] of Galaxies), and the PGC ([[Catalogue of Principal Galaxies]], also known as LEDA). All the well-known galaxies appear in one or more of these catalogs but each time under a different number.

For example, [[Messier 109]] (or "M109") is a spiral galaxy having the number 109 in the catalog of Messier. It also has the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, and PGC 37617 (or LEDA 37617). Millions of fainter galaxies are known by their identifiers in [[sky surveys]] such as the [[Sloan Digital Sky Survey]], in which M109 is cataloged as SDSS J115735.97+532228.9.

== Observation history ==

The realization that ''we live in a galaxy that is one among many'' parallels major discoveries about the [[Milky Way]] and other [[nebula]]e.

=== Milky Way ===

{{Main|Milky Way}}

[[Greek philosophy|Greek]] philosopher [[Democritus]] (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.<ref name="Plutarch">{{cite book | title=The Complete Works Volume 3: Essays and Miscellanies | publisher=Echo Library | author=Plutarch | author-link=Plutarch | date=2006 | page=66 | isbn=978-1-4068-3224-2 | url=https://books.google.com/books?id=I34rSPrX1tQC | access-date=July 25, 2018 | archive-date=March 24, 2021 | archive-url=https://web.archive.org/web/20210324071205/https://books.google.com/books?id=I34rSPrX1tQC | url-status=live }}</ref>

[[Aristotle]] (384–322 BCE), however, believed the Milky Way was caused by "the ignition of the fiery exhalation of some stars that were large, numerous and close together" and that the "ignition takes place in the upper part of the [[atmosphere]], in the [[Sublunary sphere|region of the World that is continuous with the heavenly motions]]."<ref name=Montada>{{cite encyclopedia

| last1=Montada

| first1=J. P.

| date=September 28, 2007

| title=Ibn Bâjja

| encyclopedia=[[Stanford Encyclopedia of Philosophy]]

| url=http://plato.stanford.edu/entries/ibn-bajja

| access-date=July 11, 2008

| archive-date=March 16, 2020

| archive-url=https://web.archive.org/web/20200316085852/https://plato.stanford.edu/entries/ibn-bajja/

| url-status=live

}}</ref> [[Neoplatonism|Neoplatonist]] philosopher [[Olympiodorus the Younger]] ({{circa|495}}–570 CE) was critical of this view, arguing that if the Milky Way was [[sublunary]] (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have [[parallax]], which it did not. In his view, the Milky Way was celestial.<ref name=heidarzadeh23 />

According to Mohani Mohamed, [[Islamic astronomy|Arabian]] astronomer [[Alhazen]] (965–1037) made the first attempt at observing and measuring the Milky Way's parallax,<ref name=mohamed /> and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere."<ref>{{cite web

| last1=Bouali

| first1=H.-E.

| last2=Zghal

| first2=M.

| last3=Lakhdar

| first3=Z. B.

| date=2005

| title=Popularisation of Optical Phenomena: Establishing the First Ibn Al-Haytham Workshop on Photography

| publisher=The Education and Training in Optics and Photonics Conference

| url=http://spie.org/etop/ETOP2005_080.pdf

| access-date=July 8, 2008

| archive-date=May 24, 2011

| archive-url=https://web.archive.org/web/20110524041243/http://spie.org/etop/ETOP2005_080.pdf

| url-status=live

}}</ref> [[Persian people|Persian]] astronomer [[al-Bīrūnī]] (973–1048) proposed the Milky Way galaxy was "a collection of countless fragments of the nature of nebulous stars."<ref>{{MacTutor Biography|id=Al-Biruni|title=Abu Arrayhan Muhammad ibn Ahmad al-Biruni}}</ref> [[Al-Andalus|Andalusian]] astronomer [[Ibn Bâjjah]] ("Avempace", {{abbr|d.|died}} 1138) proposed that it was composed of many stars that almost touched one another, and appeared to be a continuous image due to the effect of [[refraction]] from sublunary material,<ref name=Montada /><ref name="heidarzadeh25" /> citing his observation of the [[Conjunction (astronomy and astrology)|conjunction]] of Jupiter and Mars as evidence of this occurring when two objects were near.<ref name=Montada /> In the 14th century, Syrian-born [[Ibn Qayyim]] proposed the Milky Way galaxy was "a myriad of tiny stars packed together in the sphere of the fixed stars."<ref name=Livingston>{{cite journal

|last1=Livingston |first1=J. W.

|date=1971

|title=Ibn Qayyim al-Jawziyyah: A Fourteenth Century Defense against Astrological Divination and Alchemical Transmutation

|journal=[[Journal of the American Oriental Society]]

|volume=91 |issue=1 |pages=96–103 [99]

|doi=10.2307/600445

|jstor=600445

}}</ref>

[[File:Herschel-Galaxy.png|thumb|The shape of the Milky Way as estimated from star counts by [[William Herschel]] in 1785; the Solar System was assumed to be near the center.]]

Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer [[Galileo Galilei]] used a [[optical telescope|telescope]] to study it and discovered it was composed of a huge number of faint stars.<ref>Galileo Galilei, ''Sidereus Nuncius'' (Venice, (Italy): Thomas Baglioni, 1610), [https://archive.org/stream/Sidereusnuncius00Gali#page/n37/mode/2up pages 15 and 16.]<br />

English translation: Galileo Galilei with Edward Stafford Carlos, trans., ''The Sidereal Messenger'' (London, England: Rivingtons, 1880), [https://archive.org/stream/siderealmessenge80gali#page/42/mode/2up/ pages 42 and 43.]</ref><ref>{{cite web

|last1=O'Connor

|first1=J. J.

|last2=Robertson

|first2=E. F.

|date=November 2002

|title=Galileo Galilei

|url=http://www-gap.dcs.st-and.ac.uk/~history/Biographies/Galileo.html

|publisher=[[University of St. Andrews]]

|access-date=January 8, 2007

|archive-date=May 30, 2012

|archive-url=https://archive.today/20120530/http://www-gap.dcs.st-and.ac.uk/~history/Biographies/Galileo.html

|url-status=live

}}</ref>

In 1750, English astronomer [[Thomas Wright (astronomer)|Thomas Wright]], in his ''An Original Theory or New Hypothesis of the Universe'', correctly speculated that it might be a rotating body of a huge number of stars held together by [[gravitation]]al forces, akin to the [[Solar System]] but on a much larger scale, and that the resulting disk of stars could be seen as a band on the sky from our perspective inside it.<ref>Thomas Wright, ''An Original Theory or New Hypothesis of the Universe''{{nbsp}}... (London, England: H. Chapelle, 1750). [https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA48 From p.48:] {{Webarchive|url=https://web.archive.org/web/20161120194825/https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA48 |date=November 20, 2016 }} "...{{nbsp}}the stars are not infinitely dispersed and distributed in a promiscuous manner throughout all the mundane space, without order or design,{{nbsp}}... this phænomenon [is] no other than a certain effect arising from the observer's situation,{{nbsp}}... To a spectator placed in an indefinite space,{{nbsp}}... it [i.e., the Milky Way (''Via Lactea'')] [is] a vast ring of stars{{nbsp}}..."<br />

[https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA73 On page 73] {{Webarchive|url=https://web.archive.org/web/20161120194830/https://books.google.com/books?id=80VZAAAAcAAJ&pg=PA73 |date=November 20, 2016 }}, Wright called the Milky Way the ''Vortex Magnus'' (the great whirlpool) and estimated its diameter at 8.64×10¹² miles (13.9×10¹² km).</ref><ref name="our_galaxy" /> In his 1755 treatise, [[Immanuel Kant]] elaborated on Wright's idea about the Milky Way's structure.<ref>Immanuel Kant, [https://books.google.com/books?id=nCcaAQAAMAAJ&pg=PP9 {{Webarchive|url=https://web.archive.org/web/20161120195036/https://books.google.com/books?id=nCcaAQAAMAAJ&pg=PP9 |date=November 20, 2016 }} ''Allgemeine Naturgeschichte und Theorie des Himmels''{{nbsp}}...] [Universal Natural History and Theory of the Heavens{{nbsp}}...], (Königsberg and Leipzig, (Germany): Johann Friederich Petersen, 1755).<br />Available in English translation by Ian Johnston at: [http://records.viu.ca/~johnstoi/kant/kant2e.htm Vancouver Island University, British Columbia, Canada] {{Webarchive|url=https://web.archive.org/web/20140829071546/http://records.viu.ca/~johnstoi/kant/kant2e.htm |date=August 29, 2014 }}</ref>

The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by [[William Herschel]] in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with [[Galactocentrism|the Solar System close to the center]].<ref>{{cite book |author=William Herschel |s2cid=186213203 |journal=Philosophical Transactions of the Royal Society of London |title=Giving Some Accounts of the Present Undertakings, Studies, and Labours, of the Ingenious, in Many Considerable Parts of the World |url=https://books.google.com/books?id=IU9FAAAAcAAJ&pg=PA213 |chapter=XII. On the construction of the heavens |chapter-url=http://rstl.royalsocietypublishing.org/content/75/213.full.pdf+html |volume=75 |year=1785 |location=London |pages=213–266 |doi=10.1098/rstl.1785.0012 |issn=0261-0523 |access-date=January 27, 2016 |archive-date=November 20, 2016 |archive-url=https://web.archive.org/web/20161120170623/https://books.google.com/books?id=IU9FAAAAcAAJ&pg=PA213 |url-status=live }} Herschel's diagram of the galaxy appears immediately after the article's last page.</ref><ref name=paul1993 /> Using a refined approach, [[Jacobus Kapteyn|Kapteyn]] in 1920 arrived at the picture of a small (diameter about 15&nbsp;kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by [[Harlow Shapley]] based on the cataloguing of [[globular cluster]]s led to a radically different picture: a flat disk with diameter approximately 70&nbsp;kiloparsecs and the Sun far from the center.<ref name="our_galaxy" /> Both analyses failed to take into account the [[extinction (astronomy)|absorption of light]] by [[cosmic dust|interstellar dust]] present in the [[galactic plane]]; but after [[Robert Julius Trumpler]] quantified this effect in 1930 by studying [[open cluster]]s, the present picture of our host galaxy emerged.<ref>{{cite journal

|last1=Trimble |first1=V.

|date=1999

|title=Robert Trumpler and the (Non)transparency of Space

|journal=[[Bulletin of the American Astronomical Society]]

|volume=31 |issue=31 |page=1479

|bibcode=1999AAS...195.7409T

}}</ref>

=== Distinction from other nebulae ===

A few galaxies outside the Milky Way are visible on a dark night to the [[naked eye|unaided eye]], including the [[Andromeda Galaxy]], [[Large Magellanic Cloud]], the [[Small Magellanic Cloud]], and the [[Triangulum Galaxy]]. In the 10th century, Persian astronomer [[Al-Sufi]] made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud".<ref name="NSOG" /> In 964, he probably mentioned the Large Magellanic Cloud in his ''[[Book of Fixed Stars]]'' (referring to "Al Bakr of the southern Arabs",<ref name="obspm2"/> since at a [[declination]] of about 70° south it was not visible where he lived); it was not well known to Europeans until [[Ferdinand Magellan|Magellan]]'s voyage in the 16th century.<ref name="obspm">{{cite web

|title=Abd-al-Rahman Al Sufi (December 7, 903&nbsp;– May 25, 986 A.D.)

|url=http://messier.obspm.fr/xtra/Bios/alsufi.html

|publisher=[[Observatoire de Paris]]

|access-date=April 19, 2007

|archive-date=April 16, 2007

|archive-url=https://web.archive.org/web/20070416144810/http://messier.obspm.fr/xtra/Bios/alsufi.html

|url-status=live

}}</ref><ref name="obspm2">{{cite web

|title=The Large Magellanic Cloud, LMC

|url=http://messier.obspm.fr/xtra/ngc/lmc.html|publisher=Observatoire de Paris

|archive-url=https://web.archive.org/web/20170622160536/http://messier.obspm.fr/xtra/ngc/lmc.html

|archive-date=June 22, 2017|url-status=live

|date=Mar 11, 2004

}}</ref> The Andromeda Galaxy was later independently noted by [[Simon Marius]] in 1612.<ref name="NSOG" />

In 1734, philosopher [[Emanuel Swedenborg]] in his ''Principia'' speculated that there might be galaxies outside our own that were formed into galactic clusters that were minuscule parts of the universe that extended far beyond what we could see. These views "are remarkably close to the present-day views of the cosmos."<ref name="Gordon2002">{{cite web

|last1=Gordon

|first1=Kurtiss J.

|title=History of our Understanding of a Spiral Galaxy: Messier 33

|url=https://ned.ipac.caltech.edu/level5/March02/Gordon/Gordon2.html

|website=Caltech.edu

|access-date=11 June 2018

|archive-date=January 25, 2021

|archive-url=https://web.archive.org/web/20210125092657/http://ned.ipac.caltech.edu/level5/March02/Gordon/Gordon2.html

|url-status=live

}}</ref>

In 1745, [[Pierre Louis Maupertuis]] conjectured that some [[nebula]]-like objects were collections of stars with unique properties, including a [[Relativistic jets|glow exceeding the light]] its stars produced on their own, and repeated [[Johannes Hevelius]]'s view that the bright spots were massive and flattened due to their rotation.<ref>Kant, Immanuel, ''[[Universal Natural History and Theory of the Heavens]]'' (1755)</ref>

In 1750, [[Thomas Wright (astronomer)|Thomas Wright]] correctly speculated that the Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways.<ref name="our_galaxy">{{cite web

|last1=Evans |first1=J. C.

|date=November 24, 1998

|title=Our Galaxy

|url=http://physics.gmu.edu/~jevans/astr103/CourseNotes/ECText/ch20_txt.htm

|publisher=[[George Mason University]]

|access-date=January 4, 2007

|url-status=dead

|archive-url=https://archive.today/20120630/http://physics.gmu.edu/~jevans/astr103/CourseNotes/ECText/ch20_txt.htm

|archive-date=June 30, 2012

|df=mdy-all}}</ref><ref>See text quoted from Wright's ''An original theory or new hypothesis of the Universe'' in {{Cite book

|last1=Dyson

|first1=F.

|date=1979

|title=Disturbing the Universe

|page=245

|publisher=[[Pan Books]]

|isbn=978-0-330-26324-5

|url=https://books.google.com/books?id=uOlOPgAACAAJ

|access-date=July 25, 2018

|archive-date=March 24, 2021

|archive-url=https://web.archive.org/web/20210324071314/https://books.google.com/books?id=uOlOPgAACAAJ

|url-status=live

}}</ref>

[[File:Pic iroberts1.jpg|thumb|right|Photograph of the "Great Andromeda Nebula" by [[Isaac Roberts]], 1899, later identified as the [[Andromeda Galaxy]]]]

Toward the end of the 18th century, [[Charles Messier]] compiled a [[Messier object|catalog]] containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae.<ref name="our_galaxy" /> In 1845, [[William Parsons, 3rd Earl of Rosse|Lord Rosse]] constructed a new telescope and was able to distinguish between elliptical and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.<ref>[http://parsonstown.info/people/william-rosse "Parsonstown | The genius of the Parsons family | William Rosse"] {{Webarchive|url=https://web.archive.org/web/20210324071322/https://parsonstown.info/people/william-rosse |date=March 24, 2021 }}. ''parsonstown.info''.</ref>

In 1912, [[Vesto Slipher]] made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high [[Doppler shift]]s, indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us.<ref>{{cite journal

|last1=Slipher |first1=V. M.

|date=1913

|title=The radial velocity of the Andromeda Nebula

|journal=Lowell Observatory Bulletin

|volume=1 |pages=56–57

|bibcode=1913LowOB...2...56S

}}</ref><ref>{{cite magazine

|last1=Slipher |first1=V. M.

|date=1915

|title=Spectrographic Observations of Nebulae

|magazine=[[Popular Astronomy (US magazine)|Popular Astronomy]]

|volume=23 |pages=21–24

|bibcode=1915PA.....23...21S

}}</ref>

In 1917, [[Heber Curtis]] observed nova [[S Andromedae]] within the "Great [[Andromeda (constellation)|Andromeda]] Nebula" (as the Andromeda Galaxy, [[Messier object]] [[Andromeda Galaxy|M31]], was then known). Searching the photographic record, he found 11 more [[nova]]e. Curtis noticed that these novae were, on average, 10 [[magnitude (astronomy)|magnitudes]] fainter than those that occurred within our galaxy. As a result, he was able to come up with a distance estimate of 150,000&nbsp;[[parsec]]s. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.<ref>{{cite journal

|last1=Curtis |first1=H. D.

|date=1988

|title=Novae in Spiral Nebulae and the Island Universe Theory

|journal=[[Publications of the Astronomical Society of the Pacific]]

|volume=100 |page=6

|bibcode=1988PASP..100....6C

|doi=10.1086/132128

|doi-access=free

}}</ref>

In 1920 a debate took place between [[Harlow Shapley]] and [[Heber Curtis]] (the [[Great Debate (astronomy)|Great Debate]]), concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.<ref>{{cite web

|last1=Weaver

|first1=H. F.

|title=Robert Julius Trumpler

|url=http://www.nap.edu/readingroom/books/biomems/rtrumpler.html

|publisher=[[United States National Academy of Sciences|US National Academy of Sciences]]

|access-date=January 5, 2007

|archive-date=December 24, 2013

|archive-url=https://web.archive.org/web/20131224112329/http://www.nap.edu/readingroom/books/biomems/rtrumpler.html

|url-status=live

}}</ref>

In 1922, the [[Estonia]]n astronomer [[Ernst Öpik]] gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object.<ref>{{cite journal

|last1=Öpik |first1=E.

|date=1922

|title=An estimate of the distance of the Andromeda Nebula

|journal=[[The Astrophysical Journal]]

|volume=55 |page=406

|bibcode=1922ApJ....55..406O

|doi=10.1086/142680

}}</ref> Using the new 100&nbsp;inch [[Mount Wilson Observatory|Mt. Wilson]] telescope, [[Edwin Hubble]] was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some [[Cepheid variable]]s, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.<ref>{{cite journal

|last1=Hubble |first1=E. P.

|date=1929

|title=A spiral nebula as a stellar system, Messier 31

|journal=[[The Astrophysical Journal]]

|volume=69 |pages=103–158

|bibcode=1929ApJ....69..103H

|doi=10.1086/143167

}}</ref> In 1936 Hubble produced a classification of [[Galaxy morphological classification|galactic morphology]] that is used to this day.<ref>{{cite journal

|last1=Sandage

|first1=A.

|date=1989

|title=Edwin Hubble, 1889–1953

|journal=[[Journal of the Royal Astronomical Society of Canada]]

|volume=83

|issue=6

|pages=351–362

|url=http://antwrp.gsfc.nasa.gov/diamond_jubilee/1996/sandage_hubble.html

|access-date=January 8, 2007

|bibcode=1989JRASC..83..351S

|archive-date=May 30, 2012

|archive-url=https://archive.today/20120530/http://antwrp.gsfc.nasa.gov/diamond_jubilee/1996/sandage_hubble.html

|url-status=live

}}</ref>

=== Modern research ===

[[File:GalacticRotation2.svg|thumb|right|200px|[[Galaxy rotation curve|Rotation curve]] of a typical spiral galaxy: predicted based on the visible matter (A) and observed (B). The distance is from the [[Bulge (astronomy)|galactic core]].]]

In 1944, [[Hendrik C. van de Hulst|Hendrik van de Hulst]] predicted that [[microwave]] radiation with [[hydrogen line|wavelength of 21 cm]] would be detectable from interstellar atomic [[hydrogen]] gas;<ref>{{cite web

|last1=Tenn

|first1=J.

|title=Hendrik Christoffel van de Hulst

|url=http://www.phys-astro.sonoma.edu/BruceMedalists/vandeHulst/

|publisher=[[Sonoma State University]]

|access-date=January 5, 2007

|archive-date=May 29, 2012

|archive-url=https://archive.today/20120529/http://www.phys-astro.sonoma.edu/BruceMedalists/vandeHulst/

|url-status=dead

}}</ref> and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in our galaxy. These observations led to the hypothesis of a rotating [[barred spiral galaxy|bar structure]] in the center of our galaxy.<ref>{{cite journal

|last1=López-Corredoira |first1=M.

|s2cid=18399375

|display-authors=etal

|date=2001

|title=Searching for the in-plane Galactic bar and ring in DENIS

|journal=[[Astronomy and Astrophysics]]

|volume=373

|issue=1 |pages=139–152

|bibcode=2001A&A...373..139L

|doi=10.1051/0004-6361:20010560

|arxiv = astro-ph/0104307 }}</ref> With improved [[radio telescope]]s, hydrogen gas could also be traced in other galaxies.

In the 1970s, [[Vera Rubin]] uncovered a discrepancy between observed galactic [[galaxy rotation curve|rotation speed]] and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen [[dark matter]].<ref>{{cite magazine

|last1=Rubin |first1=V. C.

|date=1983

|title=Dark matter in spiral galaxies

|magazine=[[Scientific American]]

|volume=248

|issue=6 |pages=96–106

|bibcode=1983SciAm.248f..96R

|doi=10.1038/scientificamerican0683-96

}}</ref><ref>{{cite journal

|last1=Rubin |first1=V. C.

|date=2000

|title=One Hundred Years of Rotating Galaxies

|journal=[[Publications of the Astronomical Society of the Pacific]]

|volume=112 |issue=772 |pages=747–750

|bibcode=2000PASP..112..747R

|doi=10.1086/316573

}}</ref>

[[File:GOODS South field.jpg|left|thumb|Scientists used the galaxies visible in the [[Great Observatories Origins Deep Survey|GOODS]] survey to recalculate the total number of galaxies.<ref>{{cite web|title=Observable Universe contains ten times more galaxies than previously thought|url=https://www.spacetelescope.org/news/heic1620/|website=www.spacetelescope.org|access-date=17 October 2016|archive-date=December 23, 2020|archive-url=https://web.archive.org/web/20201223155303/https://www.spacetelescope.org/news/heic1620/|url-status=live}}</ref>]]

Beginning in the 1990s, the [[Hubble Space Telescope]] yielded improved observations. Among other things, its data helped establish that the missing dark matter in our galaxy could not consist solely of inherently faint and small stars.<ref>{{cite news

|title=Hubble Rules Out a Leading Explanation for Dark Matter

|publisher=Hubble News Desk

|date=October 17, 1994

|url=http://hubblesite.org/newscenter/archive/releases/1994/41/text/

|access-date=January 8, 2007

|archive-date=August 1, 2012

|archive-url=https://archive.today/20120801/http://hubblesite.org/newscenter/archive/releases/1994/41/text/

|url-status=live

}}</ref> The [[Hubble Deep Field]], an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion ({{val|1.25|e=11}}) galaxies in the observable universe.<ref>{{cite web

|date=November 27, 2002

|title=How many galaxies are there?

|url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/021127a.html

|publisher=NASA

|access-date=January 8, 2007

|archive-date=July 11, 2012

|archive-url=https://archive.today/20120711/http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/021127a.html

|url-status=live

}}</ref> Improved technology in detecting the [[electromagnetic spectrum|spectra]] invisible to humans (radio telescopes, infrared cameras, and [[x-ray astronomy|x-ray telescopes]]) allows detection of other galaxies that are not detected by Hubble. Particularly, surveys in the [[Zone of Avoidance]] (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.<ref>{{cite journal

|last1=Kraan-Korteweg |first1=R. C.

|last2=Juraszek |first2=S.

|s2cid=17900483

|date=2000

|title=Mapping the hidden Universe: The galaxy distribution in the Zone of Avoidance

|journal=[[Publications of the Astronomical Society of Australia]]

|volume=17 |issue=1 |pages=6–12

|bibcode=2000PASA...17....6K

|arxiv = astro-ph/9910572

|doi=10.1071/AS00006 }}</ref>

A 2016 study published in ''[[The Astrophysical Journal]],'' led by [[Christopher Conselice]] of the [[University of Nottingham]], used 20 years of Hubble images to estimate that the observable universe contained at least two trillion ({{val|2|e=12}}) galaxies.<ref name="Conselice">{{cite journal|title=The Evolution of Galaxy Number Density at z <{{nbsp}}8 and its Implications|author=Christopher J. Conselice|s2cid=17424588|display-authors=etal|journal=The Astrophysical Journal|volume=830|issue=2|year=2016|arxiv=1607.03909|bibcode= 2016ApJ...830...83C|doi=10.3847/0004-637X/830/2/83|page=83}}</ref><ref name="NYT-20161017" /> However, later observations with the [[New Horizons]] space probe from outside the [[zodiacal light]] reduced this to roughly 200 billion ({{val|2|e=11}}).<ref name="Lauer">{{cite journal |last1=Lauer |first1=Tod R. |last2=Postman |first2=Marc |last3=Weaver |first3=Harold A. |last4=Spencer |first4=John R. |last5=Stern |first5=S. Alan |last6=Buie |first6=Marc W. |last7=Durda |first7=Daniel D. |last8=Lisse |first8=Carey M. |last9=Poppe |first9=A. R. |last10=Binzel |first10=Richard P. |last11=Britt |first11=Daniel T. |last12=Buratti |first12=Bonnie J. |last13=Cheng |first13=Andrew F. |last14=Grundy |first14=W. M. |last15=Horányi |first15=Mihaly |last16=Kavelaars |first16=J. J. |last17=Linscott |first17=Ivan R. |last18=McKinnon |first18=William B. |last19=Moore |first19=Jeffrey M. |last20=Núñez |first20=J. I. |last21=Olkin |first21=Catherine B. |last22=Parker |first22=Joel W. |last23=Porter |first23=Simon B. |last24=Reuter |first24=Dennis C. |last25=Robbins |first25=Stuart J. |last26=Schenk |first26=Paul |last27=Showalter |first27=Mark R. |last28=Singer |first28=Kelsi N. |last29=Verbiscer |first29=Anne J. |last30=Young |first30=Leslie A. |title=New Horizons Observations of the Cosmic Optical Background |journal=The Astrophysical Journal |date=11 January 2021 |volume=906 |issue=2 |pages=77 |doi=10.3847/1538-4357/abc881 |url=https://iopscience.iop.org/article/10.3847/1538-4357/abc881 |access-date=15 January 2021 |language=en |issn=1538-4357|arxiv=2011.03052 |bibcode=2021ApJ...906...77L |hdl=1721.1/133770 |s2cid=226277978 }}</ref><ref>{{cite journal |title=New Horizons spacecraft answers the question: How dark is space? |website=phys.org |url=https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |access-date=15 January 2021 |language=en |archive-date=January 15, 2021 |archive-url=https://web.archive.org/web/20210115110710/https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |url-status=live }}</ref>

== Types and morphology ==

{{Main|Galaxy morphological classification}}

[[File:Hubble sequence photo.png|thumb|360px|Types of galaxies according to the [[Hubble Space Telescope|Hubble]] classification scheme: an ''E'' indicates a type of [[elliptical galaxy]]; an ''S'' is a [[Spiral galaxy|spiral]]; and ''SB'' is a [[barred spiral galaxy]].<ref group=note>Galaxies to the left side of the Hubble classification scheme are sometimes referred to as "early-type", while those to the right are "late-type".</ref>]]

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the [[Hubble sequence]]. Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as [[star formation]] rate in [[Starburst galaxy|starburst galaxies]] and activity in the cores of [[active galaxy|active galaxies]].<ref name="IRatlas" />

=== Ellipticals ===

{{Main|Elliptical galaxy}}

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an [[ellipsoid]]al profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little [[interstellar medium|interstellar matter]]. Consequently, these galaxies also have a low portion of [[open cluster]]s and a reduced rate of new star formation. Instead, they are dominated by generally older, more [[stellar evolution|evolved stars]] that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller [[globular cluster]]s.<ref name="elliptical">{{cite web

|last1=Barstow |first1=M. A.

|date=2005

|title=Elliptical Galaxies

|url=http://www.star.le.ac.uk/edu/Elliptical.shtml

|archive-url=https://web.archive.org/web/20120729081504/http://www.star.le.ac.uk/edu/Elliptical.shtml

|archive-date=2012-07-29

|publisher=[[Leicester University]] Physics Department

|access-date=June 8, 2006

}}</ref>

The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the [[interacting galaxy|interaction of galaxies]], resulting in a collision and merger. They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy clusters.<ref>{{cite web

|date=October 20, 2005

|title=Galaxies

|url=http://curious.astro.cornell.edu/galaxies.php

|archive-url=https://web.archive.org/web/20140629115612/http://curious.astro.cornell.edu/galaxies.php

|archive-date=2014-06-29

|publisher=[[Cornell University]]

|access-date=August 10, 2006

}}</ref>

==== Shell galaxy ====

[[File:NGC 3923 Elliptical Shell Galaxy.jpg|thumb|[[NGC 3923]] Elliptical Shell Galaxy (Hubble photograph)]]

A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy [[NGC 3923]] has over 20 shells.<ref>{{cite web|title = Galactic onion|url = http://www.spacetelescope.org/images/potw1519a/|website = www.spacetelescope.org|access-date = 2015-05-11|archive-date = August 6, 2020|archive-url = https://web.archive.org/web/20200806221639/https://www.spacetelescope.org/images/potw1519a/|url-status = live}}</ref>

=== Spirals ===

{{Main|Spiral galaxy|Barred spiral galaxy}}

[[File:M101 hires STScI-PRC2006-10a.jpg|thumb|right|The [[Pinwheel Galaxy]], NGC 5457]]

Spiral galaxies resemble spiraling [[pinwheel (toy)|pinwheels]]. Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of [[dark matter]] which extends beyond the visible component, as demonstrated by the universal rotation curve concept.<ref name="Williams2009">{{Cite journal | last1 = Williams | first1 = M. J. | last2 = Bureau | first2 = M. | last3 = Cappellari | first3 = M. | s2cid = 17940107 | doi = 10.1111/j.1365-2966.2009.15582.x | title = Kinematic constraints on the stellar and dark matter content of spiral and S0 galaxies | journal = Monthly Notices of the Royal Astronomical Society | volume = 400 | issue = 4 | pages = 1665–1689 | year = 2010 |arxiv = 0909.0680 |bibcode = 2009MNRAS.400.1665W }}</ref>

Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the [[bulge (astronomy)|bulge]] are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type ''S'', followed by a letter (''a'', ''b'', or ''c'') which indicates the degree of tightness of the spiral arms and the size of the central bulge. An ''Sa'' galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an ''Sc'' galaxy has open, well-defined arms and a small core region.<ref>{{cite web

|last1 = Smith

|first1 = G.

|date = March 6, 2000

|url = http://casswww.ucsd.edu/public/tutorial/Galaxies.html

|title = Galaxies&nbsp;— The Spiral Nebulae

|publisher = [[University of California]], San Diego Center for Astrophysics & Space Sciences

|access-date = November 30, 2006

|url-status = dead

|archive-url = https://archive.today/20120710/http://casswww.ucsd.edu/public/tutorial/Galaxies.html

|archive-date = July 10, 2012

|df = mdy-all

}}</ref> A galaxy with poorly defined arms is sometimes referred to as a [[flocculent spiral galaxy]]; in contrast to the [[grand design spiral galaxy]] that has prominent and well-defined spiral arms.<ref name=bergh1998 /> The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense.<ref>[http://phys.org/news/2014-02-fat-flat-galaxies.html "Fat or flat: Getting galaxies into shape"] {{Webarchive|url=https://web.archive.org/web/20210324072603/https://phys.org/news/2014-02-fat-flat-galaxies.html |date=March 24, 2021 }}. ''phys.org''. February 2014</ref>

[[File:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg|right|thumb|300px|[[NGC 1300]], an example of a [[barred spiral galaxy]]]]

In spiral galaxies, the spiral arms do have the shape of approximate [[logarithmic spiral]]s, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant [[angular velocity]]. The spiral arms are thought to be areas of high-density matter, or "[[Density wave theory|density waves]]".<ref name=bertin_lin1996 /> As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.<ref name=belkora355 />

[[File:Hoag's object.jpg|thumb|right|[[Hoag's Object]], an example of a [[ring galaxy]]]]

==== Barred spiral galaxy ====

A majority of spiral galaxies, including our own [[Milky Way]] galaxy, have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.<ref>{{cite journal

|last1=Eskridge |first1=P. B.

|last2=Frogel |first2=J. A.

|s2cid=189840251

|date=1999

|title=What is the True Fraction of Barred Spiral Galaxies?

|journal=[[Astrophysics and Space Science]]

|volume=269/270 |pages=427–430

|bibcode=1999Ap&SS.269..427E

|doi=10.1023/A:1017025820201

}}</ref> In the Hubble classification scheme, these are designated by an ''SB'', followed by a lower-case letter (''a'', ''b'' or ''c'') which indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a [[Galactic tide|tidal interaction]] with another galaxy.<ref>{{cite journal

|last1=Bournaud |first1=F.

|last2=Combes |first2=F.

|s2cid=17562844

|date=2002

|title=Gas accretion on spiral galaxies: Bar formation and renewal

|journal=[[Astronomy and Astrophysics]]

|volume=392

|issue=1 |pages=83–102

|bibcode=2002A&A...392...83B

|doi=10.1051/0004-6361:20020920

|arxiv = astro-ph/0206273 }}</ref> Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.<ref>{{cite journal

|last1=Knapen |first1=J. H.

|last2=Perez-Ramirez |first2=D.

|last3=Laine |first3=S.

|s2cid=10845683

|date=2002

|title=Circumnuclear regions in barred spiral galaxies&nbsp;— II. Relations to host galaxies

|journal=[[Monthly Notices of the Royal Astronomical Society]]

|volume=337 |issue=3 |pages=808–828

|bibcode=2002MNRAS.337..808K

|doi=10.1046/j.1365-8711.2002.05840.x

|arxiv = astro-ph/0207258 }}</ref>

Our own galaxy, the [[Milky Way]], is a large disk-shaped barred-spiral galaxy<ref>{{cite journal

|last1=Alard |first1=C.

|s2cid=18018228

|date=2001

|title=Another bar in the Bulge

|journal=[[Astronomy and Astrophysics Letters]]

|volume=379 |issue=2 |pages=L44–L47

|bibcode=2001A&A...379L..44A

|doi=10.1051/0004-6361:20011487

|arxiv = astro-ph/0110491 }}</ref> about 30&nbsp;kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×10¹¹)<ref>{{cite news

|last1=Sanders

|first1=R.

|date=January 9, 2006

|title=Milky Way galaxy is warped and vibrating like a drum

|publisher=[[UC Berkeley|UCBerkeley News]]

|url=http://www.berkeley.edu/news/media/releases/2006/01/09_warp.shtml

|access-date=May 24, 2006

|archive-date=January 25, 2014

|archive-url=https://www.webcitation.org/6MtkdRN6G?url=http://www.berkeley.edu/news/media/releases/2006/01/09_warp.shtml

|url-status=live

}}</ref> stars and has a total mass of about six hundred billion (6×10¹¹) times the mass of the Sun.<ref>{{cite journal

|last1=Bell |first1=G. R.

|last2=Levine |first2=S. E.

|date=1997

|title=Mass of the Milky Way and Dwarf Spheroidal Stream Membership

|journal=[[Bulletin of the American Astronomical Society]]

|volume=29 |issue=2 |page=1384

|bibcode=1997AAS...19110806B

}}</ref>

==== Super-luminous spiral ====

Recently, researchers described galaxies called super-luminous spirals. They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way's 100,000 light-year diameter). With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-infrared light. They are thought to have an increased star formation rate around 30 times faster than the Milky Way.<ref>{{Cite web|url=http://futurism.com/just-discovered-new-type-colossal-galaxy/|title=We Just Discovered a New Type of Colossal Galaxy|website=Futurism|language=en-US|access-date=2016-03-21|date=2016-03-21|archive-date=March 24, 2021|archive-url=https://web.archive.org/web/20210324071443/https://futurism.com/just-discovered-new-type-colossal-galaxy|url-status=live}}</ref><ref>{{Cite journal|last1=Ogle|first1=Patrick M.|last2=Lanz|first2=Lauranne|last3=Nader|first3=Cyril|last4=Helou|first4=George|s2cid=35287348|date=2016-01-01|title=Superluminous Spiral Galaxies|journal=The Astrophysical Journal|language=en|volume=817|issue=2|pages=109|doi=10.3847/0004-637X/817/2/109|issn=0004-637X|arxiv = 1511.00659 |bibcode = 2016ApJ...817..109O }}</ref>

=== Other morphologies ===

* [[Peculiar galaxy|Peculiar galaxies]] are galactic formations that develop unusual properties due to tidal interactions with other galaxies.

** A [[ring galaxy]] has a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy.<ref>{{cite journal

|last1=Gerber |first1=R. A.

|last2=Lamb |first2=S. A.

|last3=Balsara |first3=D. S.

|date=1994

|title=Ring Galaxy Evolution as a Function of "Intruder" Mass

|journal=[[Bulletin of the American Astronomical Society]]

|volume=26 |page=911

|bibcode=1994AAS...184.3204G

}}</ref> Such an event may have affected the [[Andromeda Galaxy#Structure|Andromeda Galaxy]], as it displays a multi-ring-like structure when viewed in [[infrared]] radiation.<ref>{{cite press release

|publisher=[[European Space Agency]]

|date=October 14, 1998

|title=ISO unveils the hidden rings of Andromeda

|url=http://www.iso.vilspa.esa.es/outreach/esa_pr/andromed.htm

|access-date=May 24, 2006

|url-status=dead

|archive-url=https://archive.today/19990828194420/http://www.iso.vilspa.esa.es/outreach/esa_pr/andromed.htm

|archive-date=August 28, 1999

|df=mdy-all

}}</ref>

* A [[lenticular galaxy]] is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars<ref>{{cite web

|date=May 31, 2004

|title=Spitzer Reveals What Edwin Hubble Missed

|url=http://www.cfa.harvard.edu/press/pr0419.html

|archive-url=https://web.archive.org/web/20060907042809/http://www.cfa.harvard.edu/press/pr0419.html

|archive-date=2006-09-07

|publisher=[[Harvard-Smithsonian Center for Astrophysics]]

|access-date=December 6, 2006

}}</ref> ([[Barred lenticular galaxy|barred lenticular galaxies]] receive Hubble classification SB0.)

* [[Irregular galaxy|Irregular galaxies]] are galaxies that can not be readily classified into an elliptical or spiral morphology.

** An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme.

** Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted.<ref>{{cite web

|last1=Barstow |first1=M. A.

|date=2005

|title=Irregular Galaxies

|url=http://www.star.le.ac.uk/edu/Irregular.shtml

|archive-url=https://web.archive.org/web/20120227172628/http://www.star.le.ac.uk/edu/Irregular.shtml

|archive-date=2012-02-27

|publisher=[[University of Leicester]]

|access-date=December 5, 2006

}}</ref> Nearby examples of (dwarf) irregular galaxies include the [[Magellanic Clouds]].

* An [[ultra diffuse galaxy]] (UDG) is an extremely-low-density galaxy. It may be the same size as the Milky Way, but have a visible star count only one percent of the Milky Way's. Its lack of luminosity is due to a lack of star-forming gas, resulting in old stellar populations.

=== Dwarfs ===

{{Main|Dwarf galaxy}}

Despite the prominence of large elliptical and spiral galaxies, most galaxies are dwarf galaxies. They are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, with only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100&nbsp;parsecs across.<ref>{{cite journal

|last1=Phillipps |first1=S.

|last2=Drinkwater |first2=M. J.

|last3=Gregg |first3=M. D.

|last4=Jones |first4=J. B.

|s2cid=18297376

|date=2001

|title=Ultracompact Dwarf Galaxies in the Fornax Cluster

|journal=[[The Astrophysical Journal]]

|volume=560 |issue=1 |pages=201–206

|bibcode=2001ApJ...560..201P

|doi=10.1086/322517

|arxiv = astro-ph/0106377 }}</ref>

Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered.<ref>{{cite magazine

|last1=Groshong

|first1=K.

|date=April 24, 2006

|title=Strange satellite galaxies revealed around Milky Way

|magazine=[[New Scientist]]

|url=https://www.newscientist.com/article/dn9043-strange-satellite-galaxies-revealed-around-milky-way.html

|access-date=January 10, 2007

|archive-date=July 2, 2015

|archive-url=https://web.archive.org/web/20150702024442/http://www.newscientist.com/article/dn9043-strange-satellite-galaxies-revealed-around-milky-way.html

|url-status=live

}}</ref> Dwarf galaxies may also be classified as [[dwarf elliptical galaxy|elliptical]], [[dwarf spiral galaxy|spiral]], or [[irregular galaxy|irregular]]. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called [[dwarf spheroidal galaxy|dwarf spheroidal galaxies]] instead.

A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million [[solar mass]]es, regardless of whether it has thousands or millions of stars. This suggests that galaxies are largely formed by [[dark matter]], and that the minimum size may indicate a form of [[warm dark matter]] incapable of gravitational coalescence on a smaller scale.<ref>{{cite web

|last1=Schirber

|first1=M.

|date=August 27, 2008

|url=http://news.sciencemag.org/physics/2008/08/no-slimming-down-dwarf-galaxies

|title=No Slimming Down for Dwarf Galaxies

|publisher=[[ScienceNOW]]

|access-date=August 27, 2008

|archive-date=May 30, 2020

|archive-url=https://web.archive.org/web/20200530044532/https://www.sciencemag.org/news/2008/08/no-slimming-down-dwarf-galaxies

|url-status=live

}}</ref>

== Other types of galaxies ==

=== Interacting ===

{{Main|Interacting galaxy}}

[[File:Antennae galaxies xl.jpg|thumb|right|200px|The [[Antennae Galaxies]] are undergoing a collision that will result in their eventual merger.]]

Interactions between galaxies are relatively frequent, and they can play an important role in [[galaxy formation and evolution|galactic evolution]]. Near misses between galaxies result in warping distortions due to [[galactic tide|tidal interactions]], and may cause some exchange of gas and dust.<ref name="umda">{{cite web

|url=http://www.astro.umd.edu/education/astro/gal/interact.html

|title=Galaxy Interactions

|publisher=[[University of Maryland]] Department of Astronomy

|access-date=December 19, 2006

|archive-url=https://web.archive.org/web/20060509074300/http://www.astro.umd.edu/education/astro/gal/interact.html

|archive-date=May 9, 2006

}}</ref><ref name="suia">{{cite web

|title=Interacting Galaxies

|url=http://cosmos.swin.edu.au/entries/interactinggalaxies/interactinggalaxies.html?e=1

|publisher=[[Swinburne University]]

|access-date=December 19, 2006

|archive-date=July 7, 2012

|archive-url=https://archive.today/20120707/http://cosmos.swin.edu.au/entries/interactinggalaxies/interactinggalaxies.html?e=1

|url-status=live

}}</ref>

Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies usually do not collide, but the gas and dust within the two forms interacts, sometimes triggering star formation. A collision can severely distort the galaxies' shapes, forming bars, rings or tail-like structures.<ref name="umda" /><ref name="suia" />

At the extreme of interactions are galactic mergers, where the galaxies' relative momentums are insufficient to allow them to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to the galaxies' original morphology. If one of the galaxies is much more massive than the other, the result is known as [[Interacting galaxy#Galactic cannibalism|cannibalism]], where the more massive larger galaxy remains relatively undisturbed, and the smaller one is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the [[Sagittarius Dwarf Elliptical Galaxy]] and the [[Canis Major Dwarf Galaxy]].<ref name="umda" /><ref name="suia" />

=== Starburst ===

{{Main|Starburst galaxy}}

[[File:M82 HST ACS 2006-14-a-large web.jpg|thumb|right|200px|[[Messier 82|M82]], a starburst galaxy that has ten times the star formation of a "normal" galaxy<ref>{{cite web

|date=April 24, 2006

|url=http://hubblesite.org/newscenter/archive/releases/2006/14/image/a

|title=Happy Sweet Sixteen, Hubble Telescope!

|publisher=[[NASA]]

|access-date=August 10, 2006

|archive-date=July 14, 2012

|archive-url=https://archive.today/20120714/http://hubblesite.org/newscenter/archive/releases/2006/14/image/a

|url-status=live

}}</ref>]]

Stars are created within galaxies from a reserve of cold gas that forms giant [[molecular cloud]]s. Some galaxies have been observed to form stars at an exceptional rate, which is known as a ''starburst''. If they continue to do so, they would consume their reserve of gas in a time span less than the galaxy's lifespan. Hence starburst activity usually lasts only about ten million years, a relatively brief period in a galaxy's history. Starburst galaxies were more common during the universe's early history,<ref name="chandra">{{cite web

|date=August 29, 2006

|url=http://chandra.harvard.edu/xray_sources/starburst.html

|title=Starburst Galaxies

|publisher=[[Harvard-Smithsonian Center for Astrophysics]]

|access-date=August 10, 2006

|archive-date=March 16, 2019

|archive-url=https://web.archive.org/web/20190316081832/http://chandra.harvard.edu/xray_sources/starburst.html

|url-status=live

}}</ref> but still contribute an estimated 15% to total star production.<ref>{{cite conference

|last1=Kennicutt Jr. |first1=R. C.

|display-authors=etal

|date=2005

|title=Demographics and Host Galaxies of Starbursts

|work=Starbursts: From 30 Doradus to Lyman Break Galaxies

|page=187

|publisher=[[Springer (publisher)|Springer]]

|bibcode=2005ASSL..329..187K

|doi = 10.1007/1-4020-3539-X_33 }}</ref>

Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to create [[H II region]]s.<ref>{{cite web

|last1 = Smith

|first1 = G.

|date = July 13, 2006

|title = Starbursts & Colliding Galaxies

|url = http://casswww.ucsd.edu/public/tutorial/Starbursts.html

|publisher = [[University of California]], San Diego Center for Astrophysics & Space Sciences

|access-date = August 10, 2006

|url-status = dead

|archive-url = https://archive.today/20120707/http://casswww.ucsd.edu/public/tutorial/Starbursts.html

|archive-date = July 7, 2012

|df = mdy-all

}}</ref> These stars produce [[supernova]] explosions, creating expanding [[supernova remnant|remnants]] that interact powerfully with the surrounding gas. These outbursts trigger a chain reaction of star-building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the activity end.<ref name="chandra" />

Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is [[Messier 82|M82]], which experienced a close encounter with the larger [[Messier 81|M81]]. Irregular galaxies often exhibit spaced knots of starburst activity.<ref>{{cite web

|last1=Keel

|first1=B.

|date=September 2006

|title=Starburst Galaxies

|url=http://www.astr.ua.edu/keel/galaxies/starburst.html

|publisher=[[University of Alabama]]

|access-date=December 11, 2006

|archive-date=June 4, 2012

|archive-url=https://archive.today/20120604/http://www.astr.ua.edu/keel/galaxies/starburst.html

|url-status=live

}}</ref>

=== Active galaxy ===

{{Main|Active galactic nucleus}}

[[File:M87 jet.jpg|thumb|right|200px|A jet of particles is being emitted from the core of the elliptical radio galaxy [[Messier 87|M87]].]]

Some observable galaxies are classified as "active" if they contain an active galactic nucleus (AGN). A significant portion of the galaxy's total energy output is emitted by the active nucleus instead of its stars, dust and [[interstellar medium]]. There are multiple classification and naming schemes for AGNs, but those in the lower ranges of luminosity are called [[Seyfert galaxy|Seyfert galaxies]], while those with luminosities much greater than that of the host galaxy are known as quasi-stellar objects or [[quasar]]s. AGNs emit radiation throughout the [[electromagnetic spectrum]] from radio wavelengths to X-rays, though some of it may be absorbed by dust or gas associated with the AGN itself or with the host galaxy.

The standard model for an [[active galactic nucleus]] is based on an [[accretion disc]] that forms around a [[supermassive black hole]] (SMBH) at the galaxy's core region. The radiation from an active galactic nucleus results from the [[gravitational energy]] of matter as it falls toward the black hole from the disc.<ref name="keel">{{cite web

|last1=Keel

|first1=W. C.

|date=2000

|url=http://www.astr.ua.edu/keel/galaxies/agnintro.html

|title=Introducing Active Galactic Nuclei

|publisher=University of Alabama

|access-date=December 6, 2006

|archive-date=July 27, 2012

|archive-url=https://archive.today/20120727/http://www.astr.ua.edu/keel/galaxies/agnintro.html

|url-status=live

}}</ref> The AGN's luminosity depends on the SMBH's mass and the rate at which matter falls onto it.

In about 10% of these galaxies, a diametrically opposed pair of [[Astrophysical jet|energetic jets]] ejects particles from the galaxy core at velocities close to the [[speed of light]]. The mechanism for producing these jets is not well understood.<ref name="monster">{{cite web

|last1=Lochner

|first1=J.

|last2=Gibb

|first2=M.

|title=A Monster in the Middle

|url=http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html

|publisher=NASA

|access-date=December 20, 2006

|archive-date=July 10, 2012

|archive-url=https://archive.today/20120710/http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html

|url-status=live

}}</ref>

==== Blazars ====

{{Main|Blazar}}

[[Blazar]]s are believed to be active galaxies with a [[relativistic jet]] pointed in the direction of Earth. A [[radio galaxy]] emits radio frequencies from relativistic jets. A unified model of these types of active galaxies explains their differences based on the observer's position.<ref name="monster" />

==== LINERS ====

{{Main|Low-ionization nuclear emission-line region}}

Possibly related to active galactic nuclei (as well as [[starburst (astronomy)|starburst]] regions) are [[low-ionization nuclear emission-line region]]s (LINERs). The emission from LINER-type galaxies is dominated by weakly [[ion]]ized elements. The excitation sources for the weakly ionized lines include post-[[Asymptotic giant branch|AGB]] stars, AGN, and shocks.<ref name="heckman1980">{{cite journal

|last1=Heckman |first1=T. M.

|date=1980

|title=An optical and radio survey of the nuclei of bright galaxies&nbsp;— Activity in normal galactic nuclei

|journal=[[Astronomy and Astrophysics]]

|volume=87 |pages=152–164

|bibcode=1980A&A....87..152H

}}</ref> Approximately one-third of nearby galaxies are classified as containing LINER nuclei.<ref name="keel" /><ref name="heckman1980" /><ref name="hoetal1997b">{{cite journal

|last1=Ho |first1=L. C.

|last2=Filippenko |first2=A. V.

|last3=Sargent |first3=W. L. W.

|s2cid=16742031

|date=1997

|title=A Search for "Dwarf" Seyfert Nuclei. V. Demographics of Nuclear Activity in Nearby Galaxies

|journal=[[The Astrophysical Journal]]

|volume=487

|issue=2 |pages=568–578

|bibcode=1997ApJ...487..568H

|doi=10.1086/304638

|arxiv = astro-ph/9704108 }}</ref>

==== Seyfert galaxy ====

{{Main|Seyfert galaxy}}

Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses; but unlike quasars, their host galaxies are clearly detectable.<ref name=Peterson1997>{{cite book |url=http://ned.ipac.caltech.edu/level5/Cambridge/frames.html |title=An Introduction to Active Galactic Nuclei |publisher=[[Cambridge University Press]] |first=Bradley M. |last=Peterson |year=1997 |isbn=978-0-521-47911-0}}</ref> Seyfert galaxies account for about 10% of all galaxies. Seen in visible light, most look like normal spiral galaxies; but when studied under other wavelengths, their cores' luminosity is equivalent to the luminosity of whole galaxies the size of the Milky Way.

==== Quasar ====

{{Main|Quasar}}

Quasars (/ˈkweɪzɑr/) or quasi-stellar radio sources, are the most energetic and distant members of active galactic nuclei. Extremely luminous, they were first identified as high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared more similar to stars than to extended sources similar to galaxies. Their luminosity can be 100 times that of the Milky Way.

=== Luminous infrared galaxy ===

{{Main|Luminous infrared galaxy}}

Luminous infrared galaxies (LIRGs) are galaxies with luminosities—the measurement of electromagnetic power output—above 10¹¹ L☉ (solar luminosities). In most cases, most of their energy comes from large numbers of young stars which heat surrounding dust, which reradiates the energy in the infrared. Luminosity high enough to be a LIRG requires a star formation rate of at least 18&nbsp;M☉&nbsp;yr⁻¹. Ultra-luminous infrared galaxies (ULIRGs) are at least ten times more luminous still and form stars at rates >180&nbsp;M☉&nbsp;yr⁻¹. Many LIRGs also emit radiation from an AGN. Infrared galaxies emit more energy in the infrared than all other wavelengths combined, with peak emission typically at wavelengths of 60 to 100 microns. LIRGs are uncommon in the local universe but were much more common when the universe was younger.

== Properties ==

===Magnetic fields===

Galaxies have [[magnetic field]]s of their own.<ref name="galactic_magnetic_fields">{{Cite encyclopedia|title = Galactic magnetic fields|journal = Scholarpedia|volume = 2|issue = 8|pages = 2411|last = Beck|first = Rainer|doi = 10.4249/scholarpedia.2411|bibcode = 2007SchpJ...2.2411B |year = 2007|doi-access = free}}</ref> They are strong enough to be dynamically important, as they:

* Drive mass inflow into the centers of galaxies

* Modify the formation of spiral arms

* Can affect the rotation of gas in the galaxies' outer regions

* Provide the transport of angular momentum required for the collapse of gas clouds, and hence the formation of new stars

The typical average [[Equipartition theorem|equipartition]] strength for [[Spiral galaxy|spiral galaxies]] is about 10 μG ([[Gauss (unit)|microGauss]]) or 1{{nbsp}}nT ([[Tesla (unit)|nanoTesla]]). By comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss or 30 μT ([[Tesla (unit)|microTesla]]). Radio-faint galaxies like [[Andromeda Galaxy|M 31]] and [[Triangulum Galaxy|M33]], our [[Milky Way]]'s neighbors, have weaker fields (about 5{{nbsp}}μG), while gas-rich galaxies with high star-formation rates, like M 51, M 83 and NGC 6946, have 15 μG on average. In prominent spiral arms, the field strength can be up to 25 μG, in regions where cold gas and dust are also concentrated. The strongest total equipartition fields (50–100 μG) were found in [[Starburst galaxy|starburst galaxies]]—for example, in M 82 and the [[Antennae Galaxies|Antennae]]; and in nuclear starburst regions, such as the centers of NGC 1097 and other [[Barred spiral galaxy|barred galaxies]].<ref name="galactic_magnetic_fields"/>

== Formation and evolution ==

{{Main|Galaxy formation and evolution}}

Galactic formation and evolution is an active area of research in [[astrophysics]].

=== History ===

==== Formation ====

[[File:Artist's impression of a protocluster forming in the early Universe.jpg|right|thumb|Artist's impression of a protocluster forming in the early universe<ref>{{cite web|title=Construction Secrets of a Galactic Metropolis|url=http://www.eso.org/public/news/eso1431/|website=www.eso.org|publisher=ESO Press Release|access-date=October 15, 2014|archive-date=March 24, 2021|archive-url=https://web.archive.org/web/20210324071552/https://www.eso.org/public/news/eso1431/|url-status=live}}</ref>]]

Current cosmological models of the early universe are based on the [[Big Bang]] theory. About 300,000 years after this event, atoms of [[hydrogen]] and [[helium]] began to form, in an event called [[Recombination (cosmology)|recombination]]. Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. As a result, this period has been called the "[[Timeline of the Big Bang#Dark Ages|dark ages]]". It was from density fluctuations (or [[anisotropy|anisotropic]] irregularities) in this primordial matter that [[structure formation|larger structures]] began to appear. As a result, masses of [[baryon]]ic matter started to condense within [[cold dark matter]] halos.<ref name="hqrdvj">{{cite web

|date=November 18, 1999

|title=Protogalaxies

|url=http://cfa-www.harvard.edu/~aas/tenmeter/proto.htm

|archive-url=https://web.archive.org/web/20080325183740/http://cfa-www.harvard.edu/~aas/tenmeter/proto.htm

|archive-date=2008-03-25

|publisher=[[Harvard-Smithsonian Center for Astrophysics]]

|access-date=January 10, 2007

}}</ref><ref name=rmaa17_107 /> These primordial structures eventually became the galaxies we see today.

[[File:Young Galaxy Accreting Material.jpg|thumb|right|200px|Artist's impression of a young galaxy accreting material]]

===== Early galaxy formation =====

Evidence for the appearance of galaxies very early in the Universe's history was found in 2006, when it was discovered that the galaxy [[IOK-1]] has an unusually high [[redshift]] of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and earliest-to-form galaxy seen at that time.<ref>{{cite journal

|last1=McMahon |first1=R.

|s2cid=28977650

|date=2006

|title=Astronomy: Dawn after the dark age

|journal=[[Nature (journal)|Nature]]

|volume=443 |issue=7108 |pages=151–2

|doi=10.1038/443151a

|pmid=16971933

|bibcode = 2006Natur.443..151M }}</ref>

While some scientists have claimed other objects (such as [[Galaxy Abell 1835 IR1916|Abell 1835 IR1916]]) have higher redshifts (and therefore are seen in an earlier stage of the universe's evolution), IOK-1's age and composition have been more reliably established. In December 2012, astronomers reported that [[UDFj-39546284]] is the most distant object known and has a redshift value of 11.9. The object, estimated to have existed around 380 million years<ref name="Space-20121212">{{cite web |last=Wall |first=Mike |title=Ancient Galaxy May Be Most Distant Ever Seen |url=http://www.space.com/18879-hubble-most-distant-galaxy.html |date=December 12, 2012 |publisher=[[Space.com]] |access-date=December 12, 2012 |archive-date=May 4, 2019 |archive-url=https://web.archive.org/web/20190504165521/https://www.space.com/18879-hubble-most-distant-galaxy.html |url-status=live }}</ref> after the [[Big Bang]] (which was about 13.8 billion years ago),<ref name="Cosmic Detectives">{{cite web

|title = Cosmic Detectives

|url = http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives

|publisher = The European Space Agency (ESA)

|date = April 2, 2013

|access-date = April 15, 2013

|archive-date = February 11, 2019

|archive-url = https://web.archive.org/web/20190211204726/http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives

|url-status = live

}}</ref> is about 13.42 billion [[Distance measures (cosmology)|light travel distance years]] away. The existence of galaxies so soon after the Big Bang suggests that [[protogalaxy|protogalaxies]] must have grown in the so-called "dark ages".<ref name="hqrdvj" /> As of May 5, 2015, the galaxy [[EGS-zs8-1]] is the most distant and earliest galaxy measured, forming 670 million years after the [[Big Bang]]. The light from EGS-zs8-1 has taken 13 billion years to reach Earth, and is now 30 billion light-years away, because of the [[expansion of the universe]] during 13 billion years.<ref>{{cite web|title = HubbleSite – NewsCenter – Astronomers Set a New Galaxy Distance Record (05/05/2015) – Introduction|url = http://hubblesite.org/newscenter/archive/releases/2015/22/|website = hubblesite.org|access-date = 2015-05-07|archive-date = December 9, 2016|archive-url = https://web.archive.org/web/20161209080358/http://hubblesite.org/newscenter/archive/releases/2015/22/|url-status = live}}</ref><ref>{{cite web|title = This Galaxy Far, Far Away Is the Farthest One Yet Found|website = [[Space.com]]|date = May 5, 2015|url = http://www.space.com/29319-farthest-galaxy-ever-found.html|access-date = 2015-05-07|archive-date = October 2, 2015|archive-url = https://web.archive.org/web/20151002063401/http://www.space.com/29319-farthest-galaxy-ever-found.html|url-status = live}}</ref><ref name="phys.org">{{cite web|title = Astronomers unveil the farthest galaxy|url = http://phys.org/news/2015-05-astronomers-unveil-farthest-galaxy.html|access-date = 2015-05-07|archive-date = September 11, 2017|archive-url = https://web.archive.org/web/20170911142756/https://phys.org/news/2015-05-astronomers-unveil-farthest-galaxy.html|url-status = live}}</ref><ref>{{Cite news|title = Astronomers Measure Distance to Farthest Galaxy Yet|url = https://www.nytimes.com/2015/05/06/science/astronomers-measure-distance-to-farthest-galaxy-yet.html|newspaper = The New York Times|date = 2015-05-05|access-date = 2015-05-07|issn = 0362-4331|first = Dennis|last = Overbye|archive-date = April 13, 2019|archive-url = https://web.archive.org/web/20190413220851/https://www.nytimes.com/2015/05/06/science/astronomers-measure-distance-to-farthest-galaxy-yet.html|url-status = live}}</ref><ref>{{Cite journal|title = A Spectroscopic Redshift Measurement for a Luminous Lyman Break Galaxy at z=7.730 using Keck/MOSFIRE|doi = 10.1088/2041-8205/804/2/L30 | arxiv = 1502.05399 |date = 2015-02-18|first1 = P. A.|last1 = Oesch|first2 = P. G.|last2 = van Dokkum|first3 = G. D.|last3 = Illingworth|first4 = R. J.|last4 = Bouwens|first5 = I.|last5 = Momcheva|first6 = B.|last6 = Holden|first7 = G. W.|last7 = Roberts-Borsani|first8 = R.|last8 = Smit|first9 = M.|last9 = Franx|s2cid = 55115344 |bibcode = 2015ApJ...804L..30O|volume=804|issue = 2 |journal=The Astrophysical Journal|pages=L30}}</ref>

[[File:Signatures of the Earliest Galaxies.jpg|thumb|Different components of near-infrared background light detected by the [[Hubble Space Telescope]] in deep-sky surveys<ref>{{cite web|title=Signatures of the Earliest Galaxies|url=http://www.spacetelescope.org/images/opo1534a/|access-date=15 September 2015|archive-date=August 6, 2020|archive-url=https://web.archive.org/web/20200806191830/https://www.spacetelescope.org/images/opo1534a/|url-status=live}}</ref>]]

The detailed process by which the earliest galaxies formed is an open question in astrophysics. Theories can be divided into two categories: top-down and bottom-up. In top-down correlations (such as the Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years.<ref>{{cite journal

|last1=Eggen |first1=O. J.

|last2=Lynden-Bell |first2=D.

|last3=Sandage |first3=A. R.

|date=1962

|title=Evidence from the motions of old stars that the Galaxy collapsed

|journal=[[The Astrophysical Journal]]

|volume=136 |page=748

|bibcode=1962ApJ...136..748E

|doi=10.1086/147433

}}</ref> In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as [[globular cluster]]s form first, and then a number of such bodies accrete to form a larger galaxy.<ref>{{cite journal

|last1=Searle |first1=L.

|last2=Zinn |first2=R.

|date=1978

|title=Compositions of halo clusters and the formation of the galactic halo

|journal=[[The Astrophysical Journal]]

|volume=225 |issue=1 |pages=357–379

|bibcode=1978ApJ...225..357S

|doi=10.1086/156499

}}</ref>

Once protogalaxies began to form and contract, the first [[halo star]]s (called [[Population 3 stars|Population III stars]]) appeared within them. These were composed almost entirely of hydrogen and helium and may have been more massive than 100 times the Sun's mass. If so, these huge stars would have quickly consumed their supply of fuel and became [[supernova]]e, releasing heavy elements into the [[interstellar medium]].<ref>{{cite journal

|last1=Heger |first1=A.

|last2=Woosley |first2=S. E.

|s2cid=16050642

|date=2002

|title=The Nucleosynthetic Signature of Population III

|journal=[[The Astrophysical Journal]]

|volume=567 |issue=1 |pages=532–543

|bibcode=2002ApJ...567..532H

|doi=10.1086/338487

|arxiv = astro-ph/0107037 }}</ref> This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.<ref>{{cite journal

|last1=Barkana

|first1=R.

|last2=Loeb

|first2=A.

|s2cid=119094218

|year=2001

|title=In the beginning: the first sources of light and the reionization of the Universe

|journal=[[Physics Reports]]

|volume=349

|issue=2

|pages=125–238

|bibcode=2001PhR...349..125B

|arxiv=astro-ph/0010468

|doi=10.1016/S0370-1573(01)00019-9

|url=http://cds.cern.ch/record/471794/files/0010468.pdf

|type=Submitted manuscript

|access-date=July 25, 2018

|archive-date=March 14, 2021

|archive-url=https://web.archive.org/web/20210314114618/http://cds.cern.ch/record/471794/files/0010468.pdf

|url-status=live

}}</ref>

In June 2015, astronomers reported evidence for [[Population 3 stars|Population III stars]] in the [[Cosmos Redshift 7]] galaxy at {{math|''z'' {{=}} 6.60}}. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of [[chemical element]]s heavier than [[hydrogen]] that are needed for the later formation of planets and life as we know it.<ref name="AJ-20150604">{{cite journal |last1=Sobral |first1=David |last2=Matthee |first2=Jorryt |last3=Darvish |first3=Behnam |last4=Schaerer |first4=Daniel |last5=Mobasher |first5=Bahram |last6=Röttgering |first6=Huub J. A. |last7=Santos |first7=Sérgio |last8=Hemmati |first8=Shoubaneh |s2cid=18471887 |title=Evidence for POPIII-like Stellar Populations in the Most Luminous LYMAN-α Emitters at the Epoch of Re-ionisation: Spectroscopic Confirmation |date=4 June 2015 |journal=[[The Astrophysical Journal]] |doi=10.1088/0004-637x/808/2/139 |bibcode=2015ApJ...808..139S |volume=808 |issue=2 |page=139|arxiv = 1504.01734 }}</ref><ref name="NYT-20150617">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Traces of Earliest Stars That Enriched Cosmos Are Spied |url=https://www.nytimes.com/2015/06/18/science/space/astronomers-report-finding-earliest-stars-that-enriched-cosmos.html |date=17 June 2015 |work=[[The New York Times]] |access-date=17 June 2015 |archive-date=June 29, 2019 |archive-url=https://web.archive.org/web/20190629125022/https://www.nytimes.com/2015/06/18/science/space/astronomers-report-finding-earliest-stars-that-enriched-cosmos.html |url-status=live }}</ref>

=== Evolution ===

Within a billion years of a galaxy's formation, key structures begin to appear. [[Globular cluster]]s, the central supermassive black hole, and a [[bulge (astronomy)|galactic bulge]] of metal-poor [[metallicity|Population II stars]] form. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added.<ref>{{cite news

|date = February 9, 2005

|title = Simulations Show How Growing Black Holes Regulate Galaxy Formation

|url = http://www.cmu.edu/PR/releases05/050209_blackhole.html

|publisher = [[Carnegie Mellon University]]

|access-date = January 7, 2007

|url-status = dead

|archive-url = https://archive.today/20120604/http://www.cmu.edu/PR/releases05/050209_blackhole.html

|archive-date = June 4, 2012

|df = mdy-all

}}</ref> During this early epoch, galaxies undergo a major burst of star formation.<ref>{{cite news

|last1=Massey |first1=R.

|date=April 21, 2007

|title=Caught in the act; forming galaxies captured in the young Universe

|url=http://www.ras.org.uk/index.php?option=com_content&task=view&id=1190&Itemid=2

|archive-url=https://web.archive.org/web/20131115031412/http://www.ras.org.uk/index.php?option=com_content&task=view&id=1190&Itemid=2

|archive-date=2013-11-15

|publisher=[[Royal Astronomical Society]]

|access-date=April 20, 2007

}}</ref>

During the following two billion years, the accumulated matter settles into a [[disc (galaxy)|galactic disc]].<ref>{{cite journal

|last=Noguchi |first=M.

|s2cid=17963236

|date=1999

|title=Early Evolution of Disk Galaxies: Formation of Bulges in Clumpy Young Galactic Disks

|journal=[[The Astrophysical Journal]]

|volume=514 |issue=1 |pages=77–95

|bibcode=1999ApJ...514...77N

|doi=10.1086/306932

|arxiv = astro-ph/9806355 }}</ref> A galaxy will continue to absorb infalling material from [[high-velocity cloud]]s and [[dwarf galaxy|dwarf galaxies]] throughout its life.<ref>{{cite web

|last1=Baugh |first1=C.

|last2=Frenk |first2=C.

|date=May 1999

|url=http://physicsweb.org/articles/world/12/5/9

|archive-url=https://web.archive.org/web/20070426043157/http://physicsweb.org/articles/world/12/5/9

|archive-date=2007-04-26

|title=How are galaxies made?

|publisher=[[PhysicsWeb]]

|access-date=January 16, 2007

}}</ref> This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the [[planetary formation|formation]] of [[planet]]s.<ref>{{cite conference

|last1=Gonzalez |first1=G.

|date=1998

|title=The Stellar Metallicity&nbsp;— Planet Connection

|work=Brown dwarfs and extrasolar planets: Proceedings of a workshop ...

|pages=431

|bibcode=1998ASPC..134..431G

}}</ref>

{{Multiple image |direction=vertical |align=right |width=200 |image1=XDF-scale.jpg|image2=The Hubble eXtreme Deep Field.jpg |image3=XDF-separated.jpg |caption1=''[[Hubble Extreme Deep Field|XDF]]'' view field compared to the [[angular diameter|angular size]] of the [[Moon]]. Several thousand galaxies, each consisting of billions of [[star]]s, are in this small view. |caption2=''[[Hubble Extreme Deep Field|XDF]]'' (2012) view: Each light speck is a galaxy, some of which are as old as 13.2 billion years<ref name="Space-20120925">{{cite web |last=Moskowitz |first=Clara |title=Hubble Telescope Reveals Farthest View Into Universe Ever |url=http://www.space.com/17755-farthest-universe-view-hubble-space-telescope.html |date=September 25, 2012 |publisher=[[Space.com]] |access-date=September 26, 2012 |archive-date=May 5, 2020 |archive-url=https://web.archive.org/web/20200505111220/https://www.space.com/17755-farthest-universe-view-hubble-space-telescope.html |url-status=live }}</ref>&nbsp;– the [[observable universe]] is estimated to contain 200 billion to two trillion galaxies. |caption3=''[[Hubble Extreme Deep Field|XDF]]'' image shows (from left) fully mature galaxies, nearly mature galaxies (from five to nine billion years ago), and [[Protogalaxy|protogalaxies]], blazing with [[young star]]s (beyond nine billion years). |header=''[[Hubble Extreme Deep Field|Hubble eXtreme Deep Field (XDF)]]'' }}

The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology.<ref name="sa296">{{cite magazine

|last1=Conselice |first1=C. J.

|date=February 2007

|title=The Universe's Invisible Hand

|magazine=[[Scientific American]]

|volume=296 |issue=2 |pages=35–41

|doi=10.1038/scientificamerican0207-34

|bibcode = 2007SciAm.296b..34C }}</ref> Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails. Examples of these formations can be seen in [[NGC 4676]]<ref>{{cite news

|last1=Ford

|first1=H.

|display-authors=etal

|date=April 30, 2002

|title=The Mice (NGC 4676): Colliding Galaxies With Tails of Stars and Gas

|url=http://hubblesite.org/newscenter/archive/releases/2002/11/image/d

|publisher=Hubble News Desk

|access-date=May 8, 2007

|archive-date=September 7, 2016

|archive-url=https://web.archive.org/web/20160907062239/http://hubblesite.org/newscenter/archive/releases/2002/11/image/d/

|url-status=live

}}</ref> or the [[Antennae Galaxies]].<ref>{{cite journal

|last1=Struck |first1=C.

|s2cid=119369136

|date=1999

|title=Galaxy Collisions

|doi=10.1016/S0370-1573(99)00030-7

|journal=Physics Reports

|volume=321

|issue=1–3

|pages=1–137

|arxiv=astro-ph/9908269

|bibcode = 1999PhR...321....1S }}</ref>

The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130&nbsp;[[metre per second|km/s]], and—depending upon the lateral movements—the two might collide in about five to six billion years. Although the Milky Way has never collided with a galaxy as large as Andromeda before, evidence of past collisions of the Milky Way with smaller dwarf galaxies is increasing.<ref>{{cite news

|last1=Wong |first1=J.

|date=April 14, 2000

|title=Astrophysicist maps out our own galaxy's end

|url=http://www.news.utoronto.ca/bin/000414b.asp

|publisher=[[University of Toronto]]

|access-date=January 11, 2007

|archive-url=https://web.archive.org/web/20070108183824/http://www.news.utoronto.ca/bin/000414b.asp

|archive-date=January 8, 2007

}}</ref>

Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common. Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked about ten billion years ago.<ref>{{cite journal

|last1=Panter |first1=B.

|last2=Jimenez |first2=R.

|last3=Heavens |first3=A. F.

|last4=Charlot |first4=S.

|s2cid=15174718

|date=2007

|title=The star formation histories of galaxies in the Sloan Digital Sky Survey

|journal=[[Monthly Notices of the Royal Astronomical Society]]

|volume=378 |issue=4 |pages=1550–1564

|arxiv=astro-ph/0608531

|doi=10.1111/j.1365-2966.2007.11909.x |bibcode=2007MNRAS.378.1550P

}}</ref>

=== Future trends ===

{{Main|Future of an expanding universe}}

Spiral galaxies, like the Milky Way, produce new generations of stars as long as they have dense [[molecular cloud]]s of interstellar hydrogen in their spiral arms.<ref>{{cite journal

|last1=Kennicutt Jr. |first1=R. C.

|last2=Tamblyn |first2=P.

|last3=Congdon |first3=C. E.

|date=1994

|title=Past and future star formation in disk galaxies

|journal=[[The Astrophysical Journal]]

|volume=435 |issue=1 |pages=22–36

|bibcode=1994ApJ...435...22K

|doi=10.1086/174790

}}</ref> Elliptical galaxies are largely devoid of this gas, and so form few new stars.<ref>{{cite book

|last1=Knapp

|first1=G. R.

|date=1999

|title=Star Formation in Early Type Galaxies

|journal=Star Formation in Early Type Galaxies

|volume=163

|pages=119

|publisher=[[Astronomical Society of the Pacific]]

|bibcode=1999ASPC..163..119K

|oclc=41302839

|isbn=978-1-886733-84-8

|arxiv=astro-ph/9808266

|url=https://books.google.com/books?id=tpDvAAAAMAAJ

|access-date=July 25, 2018

|archive-date=March 24, 2021

|archive-url=https://web.archive.org/web/20210324071724/https://books.google.com/books?id=tpDvAAAAMAAJ

|url-status=live

}}</ref> The supply of star-forming material is finite; once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.<ref name="cosmic_battle">{{cite web

|last1=Adams

|first1=Fred

|last2=Laughlin

|first2=Greg

|date=July 13, 2006

|title=The Great Cosmic Battle

|url=http://www.astrosociety.org/pubs/mercury/0001/cosmic.html

|publisher=[[Astronomical Society of the Pacific]]

|access-date=January 16, 2007

|archive-date=July 31, 2012

|archive-url=https://archive.today/20120731/http://www.astrosociety.org/pubs/mercury/0001/cosmic.html

|url-status=live

}}</ref><ref>{{cite web|title = Cosmic 'Murder Mystery' Solved: Galaxies Are 'Strangled to Death'|website = [[Space.com]]|date = May 13, 2015|url = http://www.space.com/29398-galaxy-strangulation-death-mystery.html|access-date = 2015-05-14|archive-date = March 24, 2021|archive-url = https://web.archive.org/web/20210324071733/https://www.space.com/29398-galaxy-strangulation-death-mystery.html|url-status = live}}</ref>

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (10¹³–10¹⁴&nbsp;years), as the smallest, longest-lived stars in our universe, tiny [[red dwarf]]s, begin to fade. At the end of the stellar age, galaxies will be composed of [[compact star|compact objects]]: [[brown dwarf]]s, [[white dwarf]]s that are cooling or cold ("[[black dwarf]]s"), [[neutron star]]s, and [[black hole]]s. Eventually, as a result of [[Relaxation (physics)#Relaxation in astronomy|gravitational relaxation]], all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.<ref name="cosmic_battle" /><ref>{{cite web

|last1=Pobojewski

|first1=S.

|date=January 21, 1997

|title=Physics offers glimpse into the dark side of the Universe

|url=http://www.umich.edu/~urecord/9697/Jan21_97/artcl17.htm

|publisher=[[University of Michigan]]

|access-date=January 13, 2007

|archive-date=June 4, 2012

|archive-url=https://archive.today/20120604/http://www.umich.edu/~urecord/9697/Jan21_97/artcl17.htm

|url-status=live

}}</ref>

== Larger-scale structures ==

{{Main|Observable universe#Large-scale structure|Galaxy filament|Galaxy groups and clusters}}

{{multiple image

| align = left

| direction = vertical

| width = 230

| image1 = Seyfert Sextet full.jpg

| width1 =

| alt1 =

| caption1 = [[Seyfert's Sextet]] is an example of a compact galaxy group.

| image2 =

| width2 =

| alt2 =

| caption2 = [[Millennium Simulation]] showing large-scale structure of the Cosmos. The image spans about 400 million light years across.

}}

Deep-sky surveys show that galaxies are often found in groups and [[Clusters of galaxies|clusters]]. Solitary galaxies that have not significantly interacted with other galaxies of comparable mass in the past billion years are relatively scarce. Only about 5% of the galaxies surveyed have been found to be truly isolated; however, they may have interacted and even merged with other galaxies in the past, and may still be orbited by smaller satellite galaxies. Isolated galaxies<ref group=note>The term "field galaxy" is sometimes used to mean an isolated galaxy, although the same term is also used to describe galaxies that do not belong to a cluster but may be a member of a group of galaxies.</ref> can produce stars at a higher rate than normal, as their gas is not being stripped by other nearby galaxies.<ref>{{cite magazine

|last1=McKee

|first1=M.

|date=June 7, 2005

|title=Galactic loners produce more stars

|url=https://www.newscientist.com/article.ns?id=dn7478

|magazine=[[New Scientist]]

|access-date=January 15, 2007

|archive-date=August 11, 2011

|archive-url=https://www.webcitation.org/60r7bjRkM?url=http://www.newscientist.com/article/dn7478

|url-status=live

}}</ref>

On the largest scale, the universe is continually expanding, resulting in an average increase in the separation between individual galaxies (see [[Hubble's law]]). Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This ongoing merging process (as well as an influx of infalling gas) heats the intergalactic gas in a cluster to very high temperatures of 30–100 [[megakelvin]]s.<ref>{{cite web

|url=http://chandra.harvard.edu/xray_sources/galaxy_clusters.html

|title=Groups & Clusters of Galaxies

|publisher=[[NASA]]/[[Chandra]]

|access-date=January 15, 2007

|archive-date=July 7, 2012

|archive-url=https://archive.today/20120707/http://chandra.harvard.edu/xray_sources/galaxy_clusters.html

|url-status=live

}}</ref> About 70–80% of a cluster's mass is in the form of dark matter, with 10–30% consisting of this heated gas and the remaining few percent in the form of galaxies.<ref>{{cite web

|last1=Ricker

|first1=P.

|title=When Galaxy Clusters Collide

|url=http://www.sdsc.edu/pub/envision/v15.2/ricker.html

|publisher=[[San Diego Supercomputer Center]]

|access-date=August 27, 2008

|archive-date=August 5, 2012

|archive-url=https://archive.today/20120805/http://www.sdsc.edu/pub/envision/v15.2/ricker.html

|url-status=dead

}}</ref>

Most galaxies are gravitationally bound to a number of other galaxies. These form a [[fractal]]-like hierarchical distribution of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster; these formations contain the majority of galaxies (as well as most of the [[baryon]]ic mass) in the universe.<ref>{{cite web

|last1=Dahlem |first1=M.

|date=November 24, 2006

|title=Optical and radio survey of Southern Compact Groups of galaxies

|url=http://www.atnf.csiro.au/people/mdahlem/sci/SCGs.html

|publisher=[[University of Birmingham]] Astrophysics and Space Research Group

|access-date=January 15, 2007

|archive-url=https://web.archive.org/web/20070613151936/http://www.atnf.csiro.au/people/mdahlem/sci/SCGs.html

|archive-date=June 13, 2007

}}</ref><ref>{{cite web

|last1=Ponman |first1=T.

|date=February 25, 2005

|title=Galaxy Systems: Groups

|url=http://www.sr.bham.ac.uk/research/groups.html

|archive-url=https://web.archive.org/web/20090215023446/http://www.sr.bham.ac.uk/research/groups.html

|archive-date=2009-02-15

|publisher=University of Birmingham Astrophysics and Space Research Group

|access-date=January 15, 2007

}}</ref> To remain gravitationally bound to such a group, each member galaxy must have a sufficiently low velocity to prevent it from escaping (see [[Virial theorem]]). If there is insufficient [[kinetic energy]], however, the group may evolve into a smaller number of galaxies through mergers.<ref>{{cite journal

|last1=Girardi |first1=M.

|last2=Giuricin |first2=G.

|s2cid=14059401

|date=2000

|title=The Observational Mass Function of Loose Galaxy Groups

|journal=[[The Astrophysical Journal]]

|volume=540 |issue=1 |pages=45–56

|bibcode=2000ApJ...540...45G

|doi=10.1086/309314

|arxiv = astro-ph/0004149 }}</ref>

{{unsolved|physics|The [[List of largest cosmic structures|largest structures]] in the universe are larger than expected. Are these actual structures or random density fluctuations?}}

Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity.<ref name="Hubble protocluster">{{cite news|title=Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen|url=http://www.spacetelescope.org/news/heic1201/|access-date=January 22, 2015|newspaper=ESA/Hubble Press Release|archive-date=June 12, 2018|archive-url=https://web.archive.org/web/20180612140011/http://www.spacetelescope.org/news/heic1201/|url-status=live}}</ref> Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as the [[brightest cluster galaxy]], which, over time, [[tidal force|tidally]] destroys its satellite galaxies and adds their mass to its own.<ref>{{cite journal

|last = Dubinski

|first = J.

|s2cid = 3137328

|date = 1998

|title = The Origin of the Brightest Cluster Galaxies

|url = http://www.cita.utoronto.ca/~dubinski/bcg/

|journal = [[The Astrophysical Journal]]

|volume = 502

|issue = 2

|pages = 141–149

|doi = 10.1086/305901

|bibcode = 1998ApJ...502..141D

|arxiv = astro-ph/9709102

|access-date = January 16, 2007

|archive-url = https://web.archive.org/web/20110514155953/http://www.cita.utoronto.ca/~dubinski/bcg/

|archive-date = May 14, 2011

|url-status = dead

|df = mdy-all

}}</ref>

[[Supercluster]]s contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the [[large-scale structure of the Cosmos|supercluster scale]], galaxies are arranged into sheets and filaments surrounding vast empty voids.<ref>{{cite journal

|last1=Bahcall |first1=N. A.

|date=1988

|title=Large-scale structure in the Universe indicated by galaxy clusters

|journal=[[Annual Review of Astronomy and Astrophysics]]

|volume=26

|issue=1 |pages=631–686

|bibcode=1988ARA&A..26..631B

|doi=10.1146/annurev.aa.26.090188.003215

}}</ref> Above this scale, the universe appears to be the same in all directions ([[isotropy|isotropic]] and [[wikt:Homogeneity|homogeneous]]).,<ref>{{cite journal

|last1=Mandolesi |first1=N.

|s2cid=4349689

|display-authors=etal

|date=1986

|title=Large-scale homogeneity of the Universe measured by the microwave background

|journal=[[Letters to Nature]]

|volume=319

|issue=6056 |pages=751–753

|doi=10.1038/319751a0

|bibcode = 1986Natur.319..751M }}</ref> though this notion has been challenged in recent years by numerous findings of large-scale structures that appear to be exceeding this scale. The [[Hercules-Corona Borealis Great Wall]], currently the [[List of largest cosmic structures|largest structure]] in the universe found so far, is 10 billion [[light-year]]s (three gigaparsecs) in length.<ref name=HBHT2>{{cite journal|last1=Horváth|first1=István|last2=Bagoly|first2=Zsolt|last3=Hakkila|first3=Jon|last4=Tóth|first4=L. Viktor|s2cid=56073380|title=New data support the existence of the Hercules-Corona Borealis Great Wall|journal=Astronomy & Astrophysics|volume = 584|pages = A48|arxiv=1510.01933|year = 2015|doi = 10.1051/0004-6361/201424829|bibcode = 2015A&A...584A..48H }}</ref><ref name=HBHT>{{cite journal|last1=Horváth|first1=István|last2=Bagoly|first2=Zsolt|last3=Hakkila|first3=Jon|last4=Tóth|first4=L. Viktor|title=Anomalies in the GRB spatial distribution|journal=Proceedings of Science|pages=78|arxiv=1507.05528|bibcode = 2014styd.confE..78H |year=2014}}</ref><ref name=cookie>{{Cite journal|arxiv =1507.00675 |last1 = Balazs|first1 = L. G.|title = A giant ring-like structure at 0.78<z<0.86 displayed by GRBs|journal = Monthly Notices of the Royal Astronomical Society|volume = 452|issue = 3|pages = 2236|last2 = Bagoly|first2 = Z.|last3 = Hakkila|first3 = J. E.|last4 = Horváth|first4 = I.|last5 = Kobori|first5 = J.|last6 = Racz|first6 = I.|last7 = Tóth|first7 = L. V.|s2cid = 109936564|year = 2015|doi = 10.1093/mnras/stv1421|bibcode = 2015MNRAS.452.2236B }}</ref>

The Milky Way galaxy is a member of an association named the [[Local Group]], a relatively small group of galaxies that has a diameter of approximately one&nbsp;megaparsec. The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group; many of the other member galaxies are dwarf companions of these two.<ref>{{cite journal

|last1=van den Bergh |first1=S.

|s2cid=1805423

|date=2000

|title=Updated Information on the Local Group

|journal=Publications of the Astronomical Society of the Pacific

|volume=112 |issue=770 |pages=529–536

|bibcode=2000PASP..112..529V

|doi=10.1086/316548

|arxiv = astro-ph/0001040 }}</ref> The Local Group itself is a part of a cloud-like structure within the [[Virgo Supercluster]], a large, extended structure of groups and clusters of galaxies centered on the [[Virgo Cluster]].<ref name="tully1982">{{cite journal

|last1=Tully |first1=R. B.

|date=1982

|title=The Local Supercluster

|journal=[[The Astrophysical Journal]]

|volume=257 |pages=389–422

|bibcode=1982ApJ...257..389T

|doi=10.1086/159999

}}</ref> And the Virgo Supercluster itself is a part of the [[Pisces-Cetus Supercluster Complex]], a giant [[galaxy filament]].

[[File:NASA-HubbleLegacyFieldZoomOut-20190502.webm|thumb|225px|right|<div align="center">[[Hubble Legacy Field]] (50-second video)<ref name="EA-2019052">{{cite news |author=NASA |title=Hubble astronomers assemble wide view of the evolving universe |url=https://www.eurekalert.org/pub_releases/2019-05/nsfc-haa050219.php |date=May 2, 2019 |work=[[EurekAlert!]] |access-date=May 2, 2019 |author-link=NASA |archive-date=March 24, 2021 |archive-url=https://web.archive.org/web/20210324071832/https://www.eurekalert.org/pub_releases/2019-05/nsfc-haa050219.php |url-status=live }}</ref></div>]]

[[File:The southern plane of the Milky Way from the ATLASGAL survey.jpg|left|thumb|Southern plane of the Milky Way from submillimeter wavelengths<ref>{{cite web|title=ATLASGAL Survey of Milky Way Completed|url=http://www.eso.org/public/news/eso1606/|access-date=7 March 2016|archive-date=March 24, 2021|archive-url=https://web.archive.org/web/20210324074529/https://www.eso.org/public/news/eso1606/|url-status=live}}</ref>]]

== Multi-wavelength observation ==

{{See also|Observational astronomy}}

{{multiple image

| align = right

| direction = vertical

| width = 220

| image1 =

| caption1 = A visual light image of [[Andromeda Galaxy]] shows the emission of ordinary stars and the light reflected by dust.

| image2 = Andromeda galaxy.jpg

| caption2 = This ultraviolet image of [[Andromeda Galaxy|Andromeda]] shows blue regions containing young, massive stars.

}}

The peak radiation of most stars lies in the [[visible spectrum]], so the observation of the stars that form galaxies has been a major component of [[optical astronomy]]. It is also a favorable portion of the spectrum for observing ionized [[H II region]]s, and for examining the distribution of dusty arms.

The [[cosmic dust|dust]] present in the interstellar medium is opaque to visual light. It is more transparent to [[far infrared astronomy|far-infrared]], which can be used to observe the interior regions of giant molecular clouds and [[Bulge (astronomy)|galactic cores]] in great detail.<ref>{{cite web

|title=Near, Mid & Far Infrared

|url=http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html

|publisher=[[Infrared Processing and Analysis Center|IPAC]]/[[NASA]]

|access-date=January 2, 2007

|url-status=dead

|archive-url=https://web.archive.org/web/20061230203454/http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html

|archive-date=December 30, 2006

}}</ref> Infrared is also used to observe distant, [[redshift|red-shifted]] galaxies that were formed much earlier. Water vapor and [[carbon dioxide]] absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for [[infrared astronomy]].

The first non-visual study of galaxies, particularly active galaxies, was made using [[radio astronomy|radio frequencies]]. The Earth's atmosphere is nearly transparent to radio between 5&nbsp;[[Hertz|MHz]] and 30&nbsp;GHz. (The [[ionosphere]] blocks signals below this range.)<ref>{{cite web

|title=The Effects of Earth's Upper Atmosphere on Radio Signals

|url=http://radiojove.gsfc.nasa.gov/education/educ/radio/tran-rec/exerc/iono.htm

|publisher=[[NASA]]

|access-date=August 10, 2006

|archive-date=May 29, 2012

|archive-url=https://archive.today/20120529/http://radiojove.gsfc.nasa.gov/education/educ/radio/tran-rec/exerc/iono.htm

|url-status=live

}}</ref> Large radio [[interferometry|interferometers]] have been used to map the active jets emitted from active nuclei. [[Radio telescope]]s can also be used to observe neutral hydrogen (via [[hydrogen line|21&nbsp;cm radiation]]), including, potentially, the non-ionized matter in the early universe that later collapsed to form galaxies.<ref>{{cite web

|title=Giant Radio Telescope Imaging Could Make Dark Matter Visible

|url=https://www.sciencedaily.com/releases/2006/12/061214135537.htm

|website=[[ScienceDaily]]

|date=December 14, 2006

|access-date=January 2, 2007

|archive-date=July 3, 2017

|archive-url=https://web.archive.org/web/20170703211527/https://www.sciencedaily.com/releases/2006/12/061214135537.htm

|url-status=live

}}</ref>

[[UV astronomy|Ultraviolet]] and [[X-ray astronomy|X-ray telescopes]] can observe highly energetic galactic phenomena. Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole.<ref>{{cite news

|title=NASA Telescope Sees Black Hole Munch on a Star

|url=http://www.nasa.gov/mission_pages/galex/galex-20061205.html

|publisher=NASA

|date=December 5, 2006

|access-date=January 2, 2007

|archive-date=June 4, 2012

|archive-url=https://archive.today/20120604/http://www.nasa.gov/mission_pages/galex/galex-20061205.html

|url-status=live

}}</ref> The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of supermassive black holes at the cores of galaxies was confirmed through X-ray astronomy.<ref>{{cite web

|last1=Dunn

|first1=R.

|title=An Introduction to X-ray Astronomy

|url=http://www-xray.ast.cam.ac.uk/xray_introduction/

|publisher=[[Institute of Astronomy, Cambridge|Institute of Astronomy]] X-Ray Group

|access-date=January 2, 2007

|archive-date=July 17, 2012

|archive-url=https://archive.today/20120717/http://www-xray.ast.cam.ac.uk/xray_introduction/

|url-status=live

}}</ref>

== Gallery ==

<gallery mode="packed" heights="200">

File:Squabbling Galactic Siblings.jpg|Squabbling Galactic Siblings<ref>{{cite web|title=Squabbling Galactic Siblings|url=https://esahubble.org/images/potw2130a/|access-date=July 16, 2021|archive-date=July 26, 2021|archive-url=https://web.archive.org/web/20210726051958/https://esahubble.org/images/potw2130a/|url-status=live}}</ref>

File:Hubble Returns to Science Operations.jpg|LEFT: ARP-MADORE2115-273 is a rare example of an interacting galaxy pair in the southern hemisphere. RIGHT: ARP-MADORE0002-503 is a large [[spiral galaxy]] with unusual, extended spiral arms, at a distance of 490 million light-years.<ref>{{cite web|title=Hubble Returns to Science Operations|url=https://esahubble.org/images/opo2145a/|access-date=July 26, 2021|archive-date=July 19, 2021|archive-url=https://web.archive.org/web/20210719223517/https://esahubble.org/images/opo2145a/|url-status=live}}</ref>

</gallery>

== See also ==

{{div col|colwidth=30em}}

* [[Dark galaxy]]

* [[Galactic orientation]]

* [[Galaxy formation and evolution]]

* [[Illustris project]]

* [[List of galaxies]]

* [[List of nearest galaxies]]

* [[Luminous infrared galaxy]]

* [[Outline of galaxies]]

* [[Supermassive black hole]]

* [[Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure]]

* [[UniverseMachine]]

{{div col end}}

== Notes ==

{{reflist|group=note}}

== References ==

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<ref name="sparkegallagher2000">{{harvnb|Sparke|Gallagher|2000|p=i}}</ref>

<ref name="heidarzadeh23">{{harvnb|Heidarzadeh|2008|pp=23–25}}</ref>

<ref name="heidarzadeh25">{{harvnb|Heidarzadeh|2008|p=25, Table 2.1}}</ref>

<ref name=paul1993>{{harvnb|Paul|1993|pp=16–18}}</ref>

<ref name=mohamed>{{harvnb|Mohamed|2000|pp=49–50}}</ref>

<ref name="NSOG">{{harvnb|Kepple|Sanner|1998|p=18}}</ref>

<ref name=bergh1998>{{harvnb|Van den Bergh|1998|p=17}}</ref>

<ref name=waller_hodge2003>{{harvnb|Waller|Hodge|2003|p=91}}</ref>

<ref name=bertin_lin1996>{{harvnb|Bertin|Lin|1996|pp=65–85}}</ref>

<ref name=belkora355>{{harvnb|Belkora|2003|p=355}}</ref>

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* Based upon: {{Cite journal

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<ref name=camb_lss>{{cite web

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{{refend}}

== External links ==

{{Sister project links|auto=1|wikt=galaxy|n=y|b=High School Earth Science/Galaxies}}

* [http://ned.ipac.caltech.edu/ NASA/IPAC Extragalactic Database (NED)] ([http://ned.ipac.caltech.edu/Library/Distances/ NED-Distances])

* {{In Our Time|Galaxies|p003c1cn|Galaxies}}

* [https://web.archive.org/web/20150718054637/http://www.atlasoftheuniverse.com/ An Atlas of The Universe]

* [https://web.archive.org/web/20150912191650/http://www.nightskyinfo.com/galaxies/ Galaxies—Information and amateur observations]

* [https://web.archive.org/web/20060411094750/http://science.nasa.gov/headlines/y2002/08feb_gravlens.htm The Oldest Galaxy Yet Found]

* [http://www.galaxyzoo.org/ Galaxy classification project, harnessing the power of the internet and the human brain]

* [http://www.physics.org/facts/sand-galaxies.asp How many galaxies are in our universe?] {{Webarchive|url=https://web.archive.org/web/20150821071507/http://www.physics.org/facts/sand-galaxies.asp |date=August 21, 2015 }}

* [https://www.youtube.com/watch?v=08LBltePDZw 3-D Video (01:46)&nbsp;– Over a Million Galaxies of Billions of Stars each&nbsp;– BerkeleyLab/animated.]

{{Galaxy}}