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{{otheruses}}
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A '''galaxy''' is a very large group of stars.
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[[Image:NGC 4414 (NASA-med).jpg|right|thumb|280px|'''[[NGC 4414]]''', a typical spiral galaxy in the [[constellation]] [[Coma Berenices]], is about 17,000 [[parsec]]s in diameter and approximately 20 million parsecs distant. Credit:[[Hubble Space Telescope]][[NASA]]/[[ESA]].]]
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A '''galaxy''' is a massive, [[gravitation|gravitationally bound]] system consisting of [[star]]s, an [[interstellar medium]] of gas and [[cosmic dust|dust]], and [[dark matter]].<ref name="sparkegallagher2000">{{cite book
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[[Category:Cosmos]]
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  | author=L. S. Sparke, J. S. Gallagher III
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  | year=2000
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  | title=Galaxies in the Universe: An Introduction
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  | publisher=Cambridge University Press
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  | location=Cambridge
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  | id=ISBN 0-521-59704-4}}</ref><ref>{{cite web | author=Hupp, E.; Roy, S.; Watzke, M. | date = August 21, 2006 | url = http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html | title = NASA Finds Direct Proof of Dark Matter | publisher = NASA | accessdate = 2007-04-17 }}</ref> The name is from the Greek root ''galaxias'' [γαλαξίας], meaning "milky," a reference to the [[Milky Way]] galaxy. Typical galaxies range from [[dwarf galaxy|dwarfs]] with as few as ten million<ref>{{cite web |date= [[May 3]] [[2000]] | url = http://www.eso.org/outreach/press-rel/pr-2000/pr-12-00.html | title = Unveiling the Secret of a Virgo Dwarf Galaxy | publisher = ESO | accessdate = 2007-01-03 }}</ref> (10<sup>7</sup>) stars up to giants with one trillion<ref name="M101">{{cite web |date= [[February 28]] [[2006]] | url = http://www.nasa.gov/mission_pages/hubble/science/hst_spiral_m10.html | title = Hubble's Largest Galaxy Portrait Offers a New High-Definition View | publisher = NASA | accessdate = 2007-01-03 }}</ref> (10<sup>12</sup>) stars, all orbiting a common [[center of mass]]. Galaxies can also contain many [[Star system#Multiple star systems|multiple star systems]], [[star cluster]]s, and various [[interstellar cloud]]s.  The [[Sun]] is one of the stars in the [[Milky Way]] galaxy; the [[Solar System]] includes the Earth and all the other objects that orbit the Sun.
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Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the [[elliptical galaxy]],<ref>{{cite news | first=Aaron | last=Hoover | title=UF Astronomers: Universe Slightly Simpler Than Expected | publisher=Hubble News Desk | date=June 16, 2003 | url=http://www.napa.ufl.edu/2003news/galaxies.htm | accessdate=2007-02-05 }}</ref> which has an [[ellipse]]-shaped light profile. [[Spiral galaxy|Spiral galaxies]] are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as [[peculiar galaxy|peculiar galaxies]], and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased [[star formation]], producing what is called a [[starburst galaxy]]. Small galaxies that lack a coherent structure could also be referred to as [[irregular galaxy|irregular galaxies]].<ref name="IRatlas">{{cite web | last = Jarrett | first = T.H. | url = http://www.ipac.caltech.edu/2mass/gallery/galmorph/ | title = Near-Infrared Galaxy Morphology Atlas | publisher = California Institute of Technology | accessdate = 2007-01-09 }}</ref>
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There are probably more than 100 billion (10<sup>11</sup>) galaxies in the [[observable universe]].<ref>{{cite web | last = Mackie | first = Glen |date= [[February 1]] [[2002]] | url = http://astronomy.swin.edu.au/~gmackie/billions.html | title = To see the Universe in a Grain of Taranaki Sand | publisher = Swinburne University | accessdate = 2006-12-20 }}</ref> Most galaxies are 1,000 to 100,000<ref name="M101" /> [[parsec]]s in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs).<ref>{{cite web | author=D. Gilman | url = http://www.hq.nasa.gov/office/pao/History/EP-177/ch4-7.html | title = The Galaxies: Islands of Stars | publisher = NASA WMAP | accessdate = 2006-08-10 }}</ref> [[Intergalactic space]] (the space between galaxies) is filled with a tenuous gas of an average density less than one [[atom]] per [[cubic meter]]. The majority of galaxies are organized into a hierarchy of associations called [[galaxy groups and clusters|clusters]], which, in turn, can form larger groups called [[supercluster]]s. These larger structures are generally arranged into [[Great Wall (astronomy)|sheets]] and [[galaxy filament|filaments]], which surround immense [[void (astronomy)|voids]] in the [[universe]].<ref>{{cite web | url = http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html | title = Galaxy Clusters and Large-Scale Structure | publisher = University of Cambridge | accessdate = 2007-01-15 }}</ref>
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Although it is not yet well understood, [[dark matter]] appears to account for around 90% of the [[mass]] of most galaxies. Observational data suggests that [[supermassive black hole]]s may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of [[active galactic nucleus|active galactic nuclei]] found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.<ref name="smbh">{{cite web | author = D. Finley, D. Aguilar |date= [[November 2]] [[2005]] | url = http://www.nrao.edu/pr/2005/sagastar/ | title = Astronomers Get Closest Look Yet At Milky Way's Mysterious Core | publisher = National Radio Astronomy Observatory | accessdate = 2006-08-10 }}</ref>
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==Etymology==
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The word ''galaxy'' derives from the [[Greek language|Greek]] term for our own galaxy, ''galaxias'' (''γαλαξίας''), or ''kyklos galaktikos,'' meaning "milky circle" for its appearance 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 that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away and a jet of her milk sprays the night sky, producing the faint band of light known as the Milky Way.<ref>{{cite web | last = Koneãn˘ | first = Lubomír | url = http://www.udu.cas.cz/collegium/tintoretto.pdf | format=PDF | title = Emblematics, Agriculture, and Mythography in The Origin of the Milky Way | publisher=Academy of Sciences of the Czech Republic | accessdate = 2007-01-05 }}</ref>
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In the astronomical literature, the capitalized word 'Galaxy' is used to refer to our ([[Milky Way]]) galaxy, to distinguish it from the billions of other galaxies.
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The term ''Milky Way'' first appeared in the English language in a poem by [[Geoffrey Chaucer|Chaucer]].
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{{Quote|"See yonder, lo, the Galaxyë<br />&nbsp;Which men clepeth the Milky Wey,<br />&nbsp;For hit is whyt."|Geoffrey Chaucer|Geoffrey Chaucer ''[[The House of Fame]]'', ''c.'' 1380.<ref>{{cite web
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| url = http://www.etymonline.com/index.php?term=galaxy
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| title = Online Etymology Dictionary
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| accessdate = 2007-01-03
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}}</ref>}}
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When [[William Herschel]] constructed his catalog of deep sky objects, he used the name ''[[spiral nebula]]'' for certain objects such as [[Andromeda Galaxy|M31]]. These would later be recognized as immense conglomerations of stars, when the true distance to these objects began to be appreciated, and they would be termed ''island universes.'' However, the word ''universe'' was understood to mean the entirety of existence, so this expression fell into disuse and the
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objects instead became known as galaxies.<ref>{{cite web | last = Rao | first = Joe |date= [[September 2]] [[2005]] | url = http://www.space.com/spacewatch/050902_teapot.html | title = Explore the Archer's Realm | publisher = space.com | accessdate = 2007-01-03 }}</ref>
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==Observation history==
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The realization that we live in a galaxy, and that there were, in fact, many other galaxies, parallel discoveries that were made about the [[Milky Way]] and other [[nebula]]e in the night sky.
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===The Milky Way===
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The [[Greek philosophy|Greek philosopher]] [[Democritus]] (450&ndash;370 <small>B.C.</small>) proposed that the bright band on the night sky known as the [[Milky Way]] might consist of distant stars.<ref>{{cite news
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| last=Burns | first=Tom | date=[[July 31]], [[2007]]
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| title=Constellations reflect heroes, beasts, star-crossed lovers
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| url=http://www.dispatch.com/live/content/now/stories/2007/07/stars.html
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| publisher=The Dispatch | accessdate=2008-03-18 }}</ref> The [[Islamic astronomy|Persian astronomer]] [[Abū Rayhān al-Bīrūnī]] (973-1048 <small>A.D.</small>) likewise proposed the Milky Way galaxy to be a collection of countless [[Nebula|nebulous]] stars.<ref>{{MacTutor Biography|id=Al-Biruni|title=Abu Rayhan Muhammad ibn Ahmad al-Biruni}}</ref> Actual proof of this came in 1610 when [[Galileo Galilei]] used a [[optical telescope|telescope]] to study the Milky Way and discovered that it is composed of a huge number of faint stars.<ref>{{cite web
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| author=J. J. O'Connor, E. F. Robertson
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| date = November 2002
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| url = http://www-gap.dcs.st-and.ac.uk/~history/Biographies/Galileo.html
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| title = Galileo Galilei
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| publisher = University of St. Andrews
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| accessdate = 2007-01-08 }}</ref> In a treatise in 1755, [[Immanuel Kant]], drawing on earlier work by [[Thomas Wright (astronomer)|Thomas Wright]], speculated (correctly) that the Galaxy might be a rotating body of a huge number of stars held together by [[gravitation|gravitational forces]], akin to the solar system but on a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the [[nebula]]e visible in the night sky might be separate galaxies.<ref name="our_galaxy">{{cite web
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| last = Evans | first = J. C.
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|date= [[November 24]] [[1998]]
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| url = http://physics.gmu.edu/~jevans/astr103/CourseNotes/ECText/ch20_txt.htm
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| title = Our Galaxy | publisher = George Mason University
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| accessdate = 2007-01-04 }}</ref>
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[[Image:Herschel-Galaxy.png|thumb|250px|left|The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the solar system was assumed to be near the center.]]
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The first attempt to describe the shape of the Milky Way and the position of the [[Sun]] in it was carried out by [[William Herschel]] in 1785 by carefully counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the solar system close to the center.<ref>{{cite web
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| last=Marschall | first=Laurence A.
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| date=October 21, 1999
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| url=http://www.sciam.com/space/article/id/how-did-scientists-determ/topicID/2/catID/3
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| title=How did scientists determine our location within the Milky Way galaxy--in other words, how do we know that our solar system is in the arm of a spiral galaxy, far from the galaxy's center?
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| publisher=Scientific American
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| accessdate=2007-12-13 }}
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</ref><ref>{{cite book
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| first=Karl F. | last=Kuhn | coauthors=Koupelis, Theo
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| year=2004 | title=In Quest of the Universe
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| publisher=Jones and Bartlett Publishers
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| id=ISBN 0763708100 }}</ref> 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 coordinate system|galactic plane]], but after [[Robert Julius Trumpler]] quantified this effect in 1930 by studying [[open cluster]]s, the present picture of our galaxy, the Milky Way, emerged.<ref>{{cite journal | last = Trimble | first = V. | title=Robert Trumpler and the (Non)transparency of Space | journal=Bulletin of the American Astronomical Society | year=1999 | issue=31 | pages=1479 | url=http://adsabs.harvard.edu/abs/1999AAS...195.7409T | accessdate = 2007-01-08 }}</ref>
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===Other nebulae===
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[[Image:M51Sketch.jpg|thumb|right|250px|Sketch of the [[Whirlpool Galaxy]] by [[William Parsons, 3rd Earl of Rosse|Lord Rosse]] in 1845]]
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Toward the end of the 18th century, [[Charles Messier]] compiled a [[Messier object|catalog]] containing the 109 brightest nebulae (celestial objects with a nebulous appearance), later followed by a larger catalog of 5,000 nebulae assembled by William Herschel.<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>{{cite web | last = Abbey | first = Lenny | url = http://labbey.com/Telescopes/Parsontown.html | title = The Earl of Rosse and the Leviathan of Parsontown | publisher = The Compleat Amateur Astronomer | accessdate = 2007-01-04 }}</ref>
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In 1917, [[Heber Curtis]] had observed the nova [[S Andromedae]] within the "Great [[Andromeda (constellation)|Andromeda]] Nebula" ([[Messier object]] [[Andromeda Galaxy|M31]]). 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|parsecs]]. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.<ref>{{cite journal | author=Heber D. Curtis | authorlink=Heber Doust Curtis | title=Novae in Spiral Nebulae and the Island Universe Theory | journal=Publications of the Astronomical Society of the Pacific | year=1988 | volume=100 | pages=6 | url=http://adsabs.harvard.edu/abs/1988PASP..100....6C }}</ref>
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[[Image:Pic iroberts1.jpg|thumb|250px|left|Photograph of the "Great Andromeda Nebula" from 1899, later identified as the [[Andromeda Galaxy]]]]
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In 1920 the so-called [[The Great Debate|Great Debate]] took place between [[Harlow Shapley]] and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant [[Doppler effect|Doppler shift]].<ref>{{cite web | first=Harold F. | last=Weaver | url = http://www.nap.edu/readingroom/books/biomems/rtrumpler.html | title = Robert Julius Trumpler | publisher = National Academy of Sciences | accessdate = 2007-01-05 }}</ref>
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The matter was conclusively settled by [[Edwin Hubble]] in the early 1920s using a new telescope. He 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 | author=E. P. Hubble | authorlink=Edwin Hubble | title=A spiral nebula as a stellar system, Messier 31 | journal=Astrophysical JournalEngl | year=1929 | volume=69 | pages=103–158 | url=http://adsabs.harvard.edu/cgi-bin/bib_query?1929ApJ....69..103H}}</ref> In 1936 Hubble produced a classification system for galaxies that is used to this day, the [[Galaxy morphological classification|Hubble sequence]].<ref>{{cite journal | last = Sandage | first = Allan | title=Edwin Hubble, 1889–1953 | journal=The Journal of the Royal Astronomical Society of Canada | year=1989 | volume=83 | issue=6 | url=http://antwrp.gsfc.nasa.gov/diamond_jubilee/1996/sandage_hubble.html | accessdate = 2007-01-08 }}</ref>
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===Modern research===
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In 1944 [[Hendrik C. van de Hulst|Hendrik van de Hulst]] predicted [[microwave]] radiation at a [[hydrogen line|wavelength of 21 cm]] resulting from interstellar atomic [[hydrogen]] gas;<ref>{{cite web | first=Joe | last=Tenn | url = http://www.phys-astro.sonoma.edu/BruceMedalists/vandeHulst/ | title = Hendrik Christoffel van de Hulst | publisher = Sonoma State University | accessdate = 2007-01-05 }}</ref> this radiation was observed in 1951. The radiation allowed for much improved study of the Milky Way Galaxy, since it is not affected by dust absorption and its Doppler shift can be used to map the motion of the gas in the Galaxy. These observations led to the postulation of a rotating [[barred spiral galaxy|bar structure]] in the center of the Galaxy.<ref>{{cite journal | author=M. López-Corredoira, P. L. Hammersley, F. Garzón, A. Cabrera-Lavers, N. Castro-Rodríguez, M. Schultheis, T. J. Mahoney | title=Searching for the in-plane Galactic bar and ring in DENIS | journal=Astronomy and Astrophysics | year=2001 | volume=373 | pages=139–152 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001A%26A...373..139L | accessdate = 2007-01-08 }}</ref> With improved [[radio telescope]]s, hydrogen gas could also be traced in other galaxies.
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[[Image:GalacticRotation2.svg|right|thumb|280px|[[Galaxy rotation curve|Rotation curve]] of a typical spiral galaxy: predicted (A) and observed (B). The distance is from the galactic core.]]
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In the 1970s it was discovered in [[Vera Rubin]]'s study of the [[galaxy rotation curve|rotation speed]] of gas in galaxies that the total visible mass (from the stars and gas) does not properly account for the speed of the rotating gas. This galaxy rotation problem is thought to be explained by the presence of large quantities of unseen [[dark matter]].<ref>{{cite web | url = http://www.petergruberfoundation.org/Rubin.htm | title = 2002 Cosmology Prize Recipient&mdash;Vera Rubin, Ph.D. | publisher = Peter Gruber Foundation | accessdate = 2007-01-05 }}</ref>
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Beginning in the 1990s, the [[Hubble Space Telescope]] yielded improved observations. Among other things, it established that the missing dark matter in our galaxy cannot solely consist 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/ | accessdate = 2007-01-08 }}</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 galaxies in the universe.<ref>{{cite web |date= [[November 27]] [[2002]] | url = http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/021127a.html | title = How many galaxies are there? | publisher = NASA | accessdate = 2007-01-08 }}</ref> Improved technology in detecting the [[electromagnetic spectrum|spectra]] invisible to humans (radio telescopes, infrared cameras, and [[x-ray astronomy|x-ray telescopes]]) allow detection of other galaxies that are not detected by Hubble. Particularly, galaxy surveys in the [[Zone of Avoidance|zone of avoidance]] (the region of the sky blocked by the Milky Way) have revealed a number of new galaxies.<ref>{{cite journal | author=R. C. Kraan-Korteweg, S. Juraszek | title=Mapping the hidden universe: The galaxy distribution in the Zone of Avoidance | journal=Publications of The Astronomical Society of Australia | year=2000 | volume=17 | issue=1 | pages=6–12 | url=http://adsabs.harvard.edu/abs/1999astro.ph.10572K }}</ref>
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==Types and morphology==
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{{Main|Galaxy morphological classification}}
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[[Image:Hubble sequence photo.png|thumb|360px|Types of galaxies according to the Hubble classification scheme. An ''E'' indicates a type of elliptical galaxy; an ''S'' is a spiral; and ''SB'' is a barred-spiral galaxy.{{Ref_label|A|a|none}}]]
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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 [[Galaxy morphological classification|Hubble sequence]]. Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as [[star formation]] rate (in starburst galaxies) and activity in the core (in [[active galaxy|active galaxies]]).<ref name="IRatlas" />
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===Ellipticals===
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{{main|Elliptical galaxy}}
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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 the galaxy is dominated by generally older, more [[stellar evolution|evolved stars]] that are orbiting the common center of gravity in random directions. In this sense they have some similarity to the much smaller [[globular cluster]]s.<ref name="elliptical">{{cite web | name = M.A. Barstow | year = 2005 | url = http://www.star.le.ac.uk/edu/Elliptical.shtml | title = Elliptical Galaxies | publisher = Leicester University Physics Department | accessdate = 2006-06-08 }}</ref>
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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]] | url = http://curious.astro.cornell.edu/galaxies.php | title = Galaxies | publisher = Cornell University | accessdate = 2006-08-10 }}</ref> [[Starburst galaxies]] are the result of such a galactic collision that can result in the formation of an elliptical galaxy.<ref name="elliptical" />
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===Spirals===
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{{main|Spiral galaxy|Barred spiral galaxy}}
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[[Image:M104 ngc4594 sombrero galaxy hi-res.jpg|thumb|250px|The [[Sombrero Galaxy]], an example of an unbarred spiral galaxy.  Credit:[[Hubble Space Telescope]]/[[NASA]]/[[ESA]].]]
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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'') that 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 | last = Smith | first = Gene |date= [[March 6]] [[2000]] | url = http://casswww.ucsd.edu/public/tutorial/Galaxies.html | title = Galaxies — The Spiral Nebulae
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| publisher = University of California, San Diego
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Center for Astrophysics & Space Sciences
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| accessdate = 2006-11-30
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}}</ref>
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In spiral galaxies, the spiral arms 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 also rotate around the center, but they do so with constant [[angular velocity]]. That means that stars pass in and out of spiral arms, with stars near the galactic core orbiting faster than the arms are moving while stars near the outer parts of the galaxy typically orbit more slowly than the arms. The spiral arms are thought to be areas of high density matter, or "density waves". 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.
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[[Image:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg|left|thumb|300px|[[NGC 1300]], an example of a barred spiral galaxy.  Credit:[[Hubble Space Telescope]]/[[NASA]]/[[ESA]].]]
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A majority of spiral galaxies 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 | author=P. B. Eskridge, J. A. Frogel | title=What is the True Fraction of Barred Spiral Galaxies? | journal=Astrophysics and Space Science | year=1999 | volume=269/270 | pages=427–430 | url=http://adsabs.harvard.edu/abs/1999Ap&SS.269..427E }}</ref> In the Hubble classification scheme, these are designated by an ''SB'', followed by a lower-case letter (''a'', ''b'' or ''c'') that 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 | author=F. Bournaud, F. Combes | title=Gas accretion on spiral galaxies: Bar formation and renewal | journal=Astronomy and Astrophysics | year=2002 | volume=392 | pages=83–102 | url=http://adsabs.harvard.edu/abs/2002A&A...392...83B }}</ref> Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.<ref>{{cite journal | author=J. H. Knapen, D. Pérez-Ramírez, S. Laine | title=Circumnuclear regions in barred spiral galaxies — II. Relations to host galaxies | journal=Monthly Notice of the Royal Astronomical Society | year=2002 | volume=337 | issue=3 | pages=808–828 | url=http://adsabs.harvard.edu/abs/2002MNRAS.337..808K }}</ref>
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Our own galaxy is a large disk-shaped barred-spiral galaxy<ref>{{cite journal | author= C. Alard | title=Another bar in the Bulge | journal=Astronomy and Astrophysics | year=2001 | volume=379 | issue=2 | pages=L44-L47 | url=http://adsabs.harvard.edu/abs/2001A&A...379L..44A }}</ref> about 30&nbsp;kiloparsecs in diameter and a kiloparsec in thickness. It contains about two hundred billion (2×10<sup>11</sup>)<ref>{{cite news | first = Robert | last = Sanders | title=Milky Way galaxy is warped and vibrating like a drum  | publisher=UCBerkeley News | date=[[January 9]] [[2006]] | url=http://www.berkeley.edu/news/media/releases/2006/01/09_warp.shtml | accessdate=2006-05-24 }}</ref> stars and has a total mass of about six hundred billion (6×10<sup>11</sup>) times the mass of the Sun.<ref>{{cite journal | author=G. R. Bell, S. E. Levine | title=Mass of the Milky Way and Dwarf Spheroidal Stream Membership | journal=Bulletin of the American Astronomical Society | year=1997 | volume=29 | issue=2 | pages=1384 | url=http://adsabs.harvard.edu/abs/1997AAS...19110806B }}</ref>
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===Other morphologies===
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[[Image:Hoag's object.jpg|thumb|[[Hoag's Object]], an example of a [[ring galaxy]]. Credit:[[Hubble Space Telescope]]/[[NASA]]/[[ESA]].]]
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Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies. An example of this is the [[ring galaxy]], which possesses 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 | author=R. A. Gerber, S. A. Lamb, D. S. Balsara | title=Ring Galaxy Evolution as a Function of "Intruder" Mass | journal=Bulletin of the American Astronomical Society | year=1994 | volume=26 | pages=911 | url=http://adsabs.harvard.edu/abs/1994AAS...184.3204G }}</ref> Such an event may have affected the [[Andromeda Galaxy]], as it displays a multi-ring-like structure when viewed in [[infrared]] radiation.<ref>{{cite press release | publisher=Esa Science News |date=[[October 14]] [[1998]] | title=ISO unveils the hidden rings of Andromeda | url=http://www.iso.vilspa.esa.es/outreach/esa_pr/andromed.htm | accessdate=2006-05-24 }}</ref>
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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]] | url = http://www.cfa.harvard.edu/press/pr0419.html | title = Spitzer Reveals What Edwin Hubble Missed | publisher = Harvard-Smithsonian Center for Astrophysics | accessdate = 2006-12-06 }}</ref> ([[Barred lenticular galaxy|Barred lenticular galaxies]] receive Hubble classification SB0.)
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[[Image:Ngc5866 hst big.jpg|thumb|left|200px|[[NGC 5866]], an example of a [[lenticular galaxy]]. Credit:[[Hubble Space Telescope]]/[[NASA]]/[[ESA]]]]
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In addition to the classifications mentioned above, there are a number of galaxies that can not be readily classified into an elliptical or spiral morphology. These are categorized as irregular galaxies. 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 | last = Barstow | first = M.A. | year = 2005 | url = http://www.star.le.ac.uk/edu/Irregular.shtml | title = Irregular Galaxies | publisher = University of Leicester | accessdate = 2006-12-05 }}</ref> Nearby examples of (dwarf) irregular galaxies include the [[Magellanic Clouds]].
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===Dwarfs===
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{{main|Dwarf galaxy}}
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Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. These tiny galaxies are about one hundredth the size of the Milky Way, containing only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100&nbsp;parsecs across.<ref>{{cite journal | author=S. Phillipps, M. J. Drinkwater, M. D. Gregg, J. B. Jones | title=Ultracompact Dwarf Galaxies in the Fornax Cluster | journal=The Astrophysical Journal | year=2001 | volume=560 | issue=1 | pages=201–206 | url=http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/322517 }}</ref>
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Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300&ndash;500 yet to be discovered.<ref>{{cite news | first=Kimm | last=Groshong | title=Strange satellite galaxies revealed around Milky Way | publisher=NewScientist | date=[[April 24]] [[2006]] | url=http://space.newscientist.com/article/dn9043 | accessdate=2007-01-10 }}</ref> Dwarf galaxies may also be classified as [[dwarf elliptical galaxy|elliptical]], [[dwarf spiral galaxy|spiral]], or [[dwarf irregular galaxy|irregular]]. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called [[dwarf spheroidal galaxy|dwarf spheroidal galaxies]] instead.
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==Unusual dynamics and activities==
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===Interacting===
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{{main|Interacting galaxy}}
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The average separation between galaxies within a cluster is a little over an [[order of magnitude]] larger than their diameter. Hence interactions between these galaxies are relatively frequent, and play an important role in their [[galaxy formation and evolution|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 | accessdate = 2006-12-19 }}</ref><ref name="suia">{{cite web | url = http://cosmos.swin.edu.au/entries/interactinggalaxies/interactinggalaxies.html?e=1 | title = Interacting Galaxies | publisher = Swinburne University | accessdate = 2006-12-19 }}</ref>
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[[Image:Antennae galaxies xl.jpg|left|280px|thumb|The [[Antennae Galaxies]] are undergoing a collision that will result in their eventual merger.  Credit:[[Hubble Space Telescope]][[NASA]]/[[ESA]].]]
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Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars within these interacting galaxies will typically pass straight through without colliding. However, the gas and dust within the two forms will interact. This can trigger bursts of star formation as the interstellar medium becomes disrupted and compressed. A collision can severely distort the shape of one or both galaxies, forming bars, rings or tail-like structures.<ref name="umda" /><ref name="suia" />
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At the extreme of interactions are galactic mergers. In this case the relative momentum of the two galaxies is insufficient to allow the galaxies to pass through each other. Instead, they gradually merge together to form a single, larger galaxy. Mergers can result in significant changes to morphology, as compared to the original galaxies. In the case where one of the galaxies is much more massive, however, the result is known as [[Interacting galaxy#Galactic cannibalism|cannibalism]]. In this case the larger galaxy will remain relatively undisturbed by the merger, while the smaller galaxy 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" />
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===Starburst===
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{{main|Starburst galaxy}}
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[[Image:M82_HST_ACS_2006-14-a-large_web.jpg|right|thumb|280px|[[Messier 82|M82]], the archetype starburst galaxy, has experienced a 10-fold increase<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 | accessdate = 2006-08-10 }}</ref> in star formation rate as compared to a "normal" galaxy.  Credit:[[Hubble Space Telescope]][[NASA]]/[[ESA]]//[[STScI]].]]
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Stars are created within galaxies from a reserve of cold gas that forms into giant [[molecular cloud]]s. Some galaxies have been observed to form stars at an exceptional rate, known as a starburst. Should they continue to do so, however, they would consume their reserve of gas in a time frame lower than the lifespan of the galaxy. Hence starburst activity usually lasts for only about ten million years, a relatively brief period in the history of a galaxy. Starburst galaxies were more common during the early history of the universe,<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 | accessdate = 2006-08-10 }}</ref> and, at present, still contribute an estimated 15% to the total star production rate.<ref>{{cite conference | author=R. C. Kennicutt Jr., J.C. Lee, J.G. Funes, S. Shoko, S. Akiyama | title = Demographics and Host Galaxies of Starbursts | booktitle = Starbursts: From 30 Doradus to Lyman Break Galaxies | pages = 187- | publisher = Dordrecht: Springer |date= 6–10 September 2004 | location = Cambridge, UK | url = http://adsabs.harvard.edu/abs/2005sdlb.proc..187K | accessdate = 2006-12-11 }}</ref>
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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 | last = Smith | first = Gene |date= 2006-07-13 | url = http://casswww.ucsd.edu/public/tutorial/Starbursts.html | title = Starbursts & Colliding Galaxies | publisher = University of California, San Diego Center for Astrophysics & Space Sciences | accessdate = 2006-08-10
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}}</ref> These massive stars also produce [[supernova]] explosions, resulting in 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 starburst activity come to an end.<ref name="chandra" />
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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 | last = Keel | first = Bill |date= September 2006 | url = http://www.astr.ua.edu/keel/galaxies/starburst.html | title = Starburst Galaxies | publisher = University of Alabama | accessdate = 2006-12-11 }}</ref>
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===Active nucleus===
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{{main|Active galactic nucleus}}
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A portion of the galaxies we can observe are classified as active. That is, a significant portion of the total energy output from the galaxy is emitted by a source other than the stars, dust and [[interstellar medium]].
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The standard model for an [[active galactic nucleus]] is based upon an [[accretion disc]] that forms around a [[supermassive black hole]] (SMBH) at the 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 | last = Keel | first = William C. | year = 2000 | url = http://www.astr.ua.edu/keel/galaxies/agnintro.html | title = Introducing Active Galactic Nuclei | publisher = The University of Alabama | accessdate = 2006-12-06 }}</ref> In about 10% of these objects, a diametrically opposed pair of energetic jets ejects particles from the core at velocities close to the [[speed of light]]. The mechanism for producing these jets is still not well-understood.<ref name="monster">{{cite web | author = J. Lochner, M. Gibb | url = http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html | title = A Monster in the Middle | publisher = NASA | accessdate = 2006-12-20 }}</ref>
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[[Image:M87 jet.jpg|left|thumb|280px|A jet of particles is being emitted from the core of the elliptical radio galaxy [[Messier 87|M87]]. Credit:[[Hubble Space Telescope]][[NASA]]/[[ESA]].]]
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Active galaxies that emit high-energy radiation in the form of [[x-ray]]s are classified as [[Seyfert galaxy|Seyfert galaxies]] or [[quasar]]s, depending on the luminosity. [[Blazar]]s are believed to be an active galaxy with a [[relativistic jet]] that is pointed in the direction of the 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 viewing angle of the observer.<ref name="monster" />
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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.<ref name="heckman1980">{{cite journal | author= T. M. Heckman | title=An optical and radio survey of the nuclei of bright galaxies — Activity in normal galactic nuclei | journal=Astronomy and Astrophysics | year=1980 | volume=87 | pages=152–164 | url=http://adsabs.harvard.edu/abs/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 | author= L. C. Ho, A. V. Filippenko, W. L. W. Sargent | title=A Search for "Dwarf" Seyfert Nuclei. V. Demographics of Nuclear Activity in Nearby Galaxies | journal=Astrophysical Journal | year=1997 | volume=487 | pages=568–578 | url=http://adsabs.harvard.edu/abs/1997ApJ...487..568H}}</ref>
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==Formation and evolution==
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{{Main|Galaxy formation and evolution}}
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The study of galactic formation and evolution attempts to answer questions regarding how galaxies formed and their evolutionary path over the history of the universe. Some theories in this field have now become widely accepted, but it is still an active area in [[astrophysics]].
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===Formation===
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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]]. 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>{{cite web | date = November 18, 1999 | url = http://cfa-www.harvard.edu/~aas/tenmeter/proto.htm | title = Search for Submillimeter Protogalaxies | publisher = Harvard-Smithsonian Center for Astrophysics | accessdate = 2007-01-10 }}</ref> These primordial structures would eventually become the galaxies we see today.
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Evidence for the early appearance of galaxies 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 primordial galaxy yet seen.<ref>{{cite journal | last = McMahon | first = R. | title=Journey to the birth of the Universe | journal=Nature | year=2006 | volume=443 }}</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. The existence of such early [[protogalaxy|protogalaxies]] suggests that they must have grown in the so-called "Dark Ages".<ref>{{cite web | date = November 18, 1999 | url = http://cfa-www.harvard.edu/~aas/tenmeter/proto.htm | title = Search for Submillimeter Protogalaxies | publisher = Harvard-Smithsonian Center for Astrophysics | accessdate = 2007-01-10 }}</ref>
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The detailed process by which such early galaxy formation occurred is a major open question in astronomy. Theories could be divided into two categories: top-down and bottom-up. In top-down theories (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 | author=O. J. Eggen, D. Lynden-Bell, A. R. Sandage | title=Evidence from the motions of old stars that the Galaxy collapsed | journal=Reports on Progress in Physics | year=1962 | volume=136 | pages=748 | url=http://adsabs.harvard.edu/abs/1962ApJ...136..748E }}</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 | author=L. Searle, R. Zinn | title=Compositions of halo clusters and the formation of the galactic halo | journal=Astrophysical Journal | year=1978 | volume=225 | issue=1 | pages=357–379 | url=http://adsabs.harvard.edu/abs/1978ApJ...225..357S }}</ref> Modern theories must be modified to account for the probable presence of large dark matter halos.
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Once protogalaxies began to form and contract, the first [[halo star]]s (called [[metallicity|Population III stars]]) appeared within them. These were composed almost entirely of hydrogen and helium, and may have been massive. 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 | author=A. Heger, S. E. Woosley | title=The Nucleosynthetic Signature of Population III | journal=Astrophysical Journal | year=2002 | volume=567 | issue=1 | pages=532–543 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002ApJ...567..532H }}</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 | author=R. Barkana, A. Loeb | title=In the beginning: the first sources of light and the reionization of the universe | journal=Physics Reports | year=1999 | volume=349 | issue=2 | pages=125–238 | url=http://adsabs.harvard.edu/abs/2000astro.ph.10468B }}</ref>
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===Evolution===
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[[Image:Hubble - infant galaxy.jpg|right|thumb|280px|[[I Zwicky 18]] (lower left) resembles a newly-formed galaxy.<ref>{{cite news
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| author=R. Villard, F. Samarrai, T. Thuan, G. Ostlin
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| title=Hubble Uncovers a Baby Galaxy in a Grown-Up Universe
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| publisher=HubbleSite News Center
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| date=[[December 1]] [[2004]]
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| url=http://hubblesite.org/newscenter/archive/releases/2004/35/text/
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| accessdate=2007-01-11 }}</ref>.<ref>{{cite news
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| author=Weaver, D.; Villard, R.
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| title=Hubble Finds 'Dorian Gray' Galaxy
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| publisher=HubbleSite News Center
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| date=[[October 16]] [[2007]]
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| url=http://hubblesite.org/newscenter/archive/releases/2007/35/full/
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| accessdate=2007-10-16 }}</ref> Credit:[[Hubble Space Telescope]]/[[NASA]]/[[ESA]].]]
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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 | title=Simulations Show How Growing Black Holes Regulate Galaxy Formation | publisher=Carnegie Mellon University |date=[[February 9]] [[2005]] | url=http://www.cmu.edu/PR/releases05/050209_blackhole.html | accessdate=2007-01-07 }}</ref> During this early epoch, galaxies undergo a major burst of star formation.<ref>{{cite news
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  | first=Robert
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  | last=Massey
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  | title=Caught in the act; forming galaxies captured in the young universe
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  | publisher=Royal Astronomical Society
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  | date=April 17, 2007
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  | url=http://www.ras.org.uk/index.php?option=com_content&task=view&id=1190&Itemid=2
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  | accessdate=2007-04-20 }}</ref>
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During the following two billion years, the accumulated matter settles into a [[disc (galaxy)|galactic disc]].<ref>{{cite journal | last = Noguchi | first = Masafumi | title=Early Evolution of Disk Galaxies: Formation of Bulges in Clumpy Young Galactic Disks | journal=Astrophysical Journal | year=1999 | volume=514 | issue=1 | pages=77–95 | url=http://adsabs.harvard.edu/abs/1999ApJ...514...77N | accessdate = 2007-01-16 }}</ref> A galaxy will continue to absorb infalling material from [[interstellar cloud|high velocity clouds]] and [[dwarf galaxy|dwarf galaxies]] throughout its life.<ref>{{cite web | author=C. Baugh, C. Frenk | date = May 1999 | url = http://physicsweb.org/articles/world/12/5/9 | title = How are galaxies made? | publisher = Physics Web | accessdate = 2007-01-16 }}</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 | first = G. | last = Gonzalez | title = The Stellar Metallicity — Planet Connection | booktitle = Proceedings of a workshop on brown dwarfs and extrasolar planets | pages = 431 | year = 1998 | location = Puerto de la Cruz, Tenerife, Spain | url = http://adsabs.harvard.edu/abs/1998bdep.conf..431G | accessdate = 2007-01-16 }}</ref>
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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 journal | first=Christopher J. | last=Conselice | title=The Universe's Invisible Hand | journal=Scientific American | pages=35–41 | date=February 2007 | volume=296 | issue=2 }}</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 | author=H. Ford ''et al'' | title=Hubble's New Camera Delivers Breathtaking Views of the Universe | publisher=Hubble News Desk | date=April 30, 2002 | url=http://hubblesite.org/newscenter/archive/releases/2002/11/image/d | accessdate=2007-05-08 }}</ref> or the [[Antennae Galaxies]].<ref>{{cite journal | last = Struck | first = Curtis | title=Galaxy Collisions | journal=Galaxy Collisions | year=1999 | volume=321 | url=http://xxx.lanl.gov/html/astro-ph/9908269/homepage.html }}</ref>
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As an example of such an interaction, the Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130&nbsp;[[metre per second|km/s]], and&mdash;depending upon the lateral movements&mdash;the two may 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 | first=Janet | last=Wong | title=Astrophysicist maps out our own galaxy's end | publisher=University of Toronto | date=[[April 14]] [[2000]] | url=http://www.news.utoronto.ca/bin/000414b.asp | accessdate=2007-01-11 }}</ref>
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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 also peaked approximately five billion years ago.<ref>{{cite journal | author=Heavens, Panter, Jimenez and Dunlop|title=The star-formation history of the Universe from the stellar populations of nearby galaxies|journal=Nature|year=2004|volume=428|Issue=6983|pages=625–627|url=http://arxiv.org/abs/astro-ph/0403293}}</ref>
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====Future trends====
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At present, most star formation occurs in smaller galaxies where cool gas is not so depleted.<ref name="sa296" /> Spiral galaxies, like the Milky Way, only produce new generations of stars as long as they have dense [[molecular cloud]]s of interstellar hydrogen in their spiral arms.<ref>{{cite journal | author=R. C. Kennicutt Jr., P. Tamblyn, C. E. Congdon | title=Past and future star formation in disk galaxies | journal=Astrophysical Journal | year=1994 | volume=435 | issue=1 | pages=22–36 | url=http://adsabs.harvard.edu/abs/1994ApJ...435...22K }}</ref> Elliptical galaxies are already largely devoid of this gas, and so form no new stars.<ref>{{cite book | author=G. R. Knapp | year=1999 | title=Star Formation in Early Type Galaxies | url=http://adsabs.harvard.edu/abs/1998astro.ph..8266K | id=ISBN 1-886733-84-8 }}</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 | author=Fred Adams, Greg Laughlin | date = 2006-07-13 | url = http://www.astrosociety.org/pubs/mercury/0001/cosmic.html | title = The Great Cosmic Battle | publisher = Astronomical Society of the Pacific | accessdate = 2007-01-16 }}</ref>
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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 (10<sup>13</sup>–10<sup>14</sup>&nbsp;years), as the smallest, longest-lived stars in our astrosphere, 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 time|gravitational relaxation]], all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.<ref>{{cite web | last = Pobojewski | first = Sally | date = January 21, 1997 | url = http://www.umich.edu/~urecord/9697/Jan21_97/artcl17.htm | title = Physics offers glimpse into the dark side of the universe | publisher = University of Michigan | accessdate = 2007-01-13 }}</ref><ref name="cosmic_battle" />
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==Larger scale structures==
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{{main|Large-scale structure of the cosmos|Groups and clusters of galaxies}}
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Deep sky surveys show that galaxies are often found in relatively close association with other galaxies. Solitary galaxies that have not significantly interacted with another galaxy of comparable mass during the past billion years are relatively scarce. Only about 5% of the galaxies surveyed have been found to be truly isolated; however, these isolated formations may have interacted and even merged with other galaxies in the past, and may still be orbited by smaller, satellite galaxies. Isolated galaxies{{Ref_label|B|b|none}} can produce stars at a higher rate than normal, as their gas is not being stripped by other, nearby galaxies.<ref>{{cite web | last = McKee | first = Maggie | date = June 7, 2005 | url = http://www.newscientist.com/article.ns?id=dn7478 | title = Galactic loners produce more stars | publisher = New Scientist | accessdate = 2007-01-15 }}</ref>
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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 in the universe, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This on-going merger process (as well as an influx of infalling gas) heats the inter-galactic gas within a cluster to very high temperatures, reaching 30&ndash;100 million [[Kelvin|K]].<ref>{{cite web | url = http://chandra.harvard.edu/xray_sources/galaxy_clusters.html | title = Groups & Clusters of Galaxies | publisher = NASA Chandra | accessdate = 2007-01-15 }}</ref> About 70&ndash;80% of the mass in a cluster is in the form of dark matter, with 10&ndash;30% consisting of this heated gas and the remaining few percent of the matter in the form of galaxies.<ref>{{cite web | last = Ricker | first = Paul | url = http://www.npaci.edu/enVision/v15.2/ricker.html | title = When Galaxy Clusters Collide | publisher = National Partnership for Advanced Computational Infrastructure | accessdate = 2007-01-15 }}</ref>
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[[Image:Seyfert Sextet full.jpg|left|thumb|280px|[[Seyfert's Sextet]] is an example of a compact galaxy group.  Credit:[[Hubble Space Telescope]]/[[NASA]]/[[ESA]].]]
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Most galaxies in the universe are gravitationally bound to a number of other galaxies. These form a [[fractal]]-like hierarchy of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster, and these formations contain a majority of the galaxies (as well as most of the [[baryon]]ic mass) in the universe.<ref>{{cite web | first=Michael | last=Dahlem | date=[[November 24]] [[2006]] | url = http://www.atnf.csiro.au/people/mdahlem/sci/SCGs.html | title = Optical and radio survey of Southern Compact Groups of galaxies | publisher = University of Birmingham Astrophysics and Space Research Group | accessdate = 2007-01-15 }}</ref><ref>{{cite web | first=Trevor | last=Ponman | date=[[February 25]] [[2005]] | url = http://www.sr.bham.ac.uk/research/groups.html | title = Galaxy Systems: Groups | publisher = University of Birmingham Astrophysics and Space Research Group | accessdate = 2007-01-15 }}</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 | author=M. Girardi, G. Giuricin | title=The Observational Mass Function of Loose Galaxy Groups | journal=The Astrophysical Journal | year=2000 | volume=540 | issue=1 | pages=45–56 | url=http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/309314 }}</ref>
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Larger structures containing many thousands of galaxies packed into an area a few megaparsecs across are called clusters. 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 = John | title=The Origin of the Brightest Cluster Galaxies | journal=Astrophysical Journal | year=1998 | volume=502 | issue=2 | pages=141–149 | url=http://www.cita.utoronto.ca/~dubinski/bcg/ }}</ref>
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[[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 | last = Bahcall | first = Neta A. | title=Large-scale structure in the universe indicated by galaxy clusters | journal=Annual review of astronomy and astrophysics | year=1988 | volume=26 | pages=631–686 | url=http://adsabs.harvard.edu/abs/1988ARA&A..26..631B }}</ref> Above this scale, the universe appears to be [[isotropy|isotropic]] and [[wiktionary:Homogeneity|homogeneous]].<ref>{{cite journal | author=N. Mandolesi, P. Calzolari, S. Cortiglioni, F. Delpino, G. Sironi | title=Large-scale homogeneity of the Universe measured by the microwave background | journal=Letters to Nature | year=1986 | volume=319 | pages=751–753 | url=http://www.nature.com/nature/journal/v319/n6056/abs/319751a0.html }}</ref>
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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 galaxies.<ref>{{cite journal | last=van den Bergh | first=Sidney | title=Updated Information on the Local Group | journal=The Publications of the Astronomical Society of the Pacific | year=2000 | volume=112 | issue=770 | pages=529–536 | url=http://adsabs.harvard.edu/abs/2000astro.ph..1040V }}</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 around the [[Virgo Cluster]].<ref name="tully1982">{{cite journal | author= R. B. Tully | title=The Local Supercluster | journal=Astrophysical Journal | year=1982 | volume=257 | pages=389–422 | url=http://adsabs.harvard.edu/abs/1982ApJ...257..389T}}</ref>
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==Multi-wavelength observation==
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[[Image:Centaurus A Galaxy.VLA and Optical.jpg|thumb|250px|left|A radio map of the galaxy [[Centaurus A]] (upper left and lower right) is overlaid across the optical image (center), showing two lobes from the jets being generated by an active nucleus.  Credit:[[NASA]].]]
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After galaxies external to the Milky Way were found to exist, initial observations were made mostly using [[visible spectrum|visible light]]. The peak radiation of most stars lies here, 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.
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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 galactic cores in great detail.<ref>{{cite web | url = http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html | title = Near, Mid & Far Infrared | publisher = IPAC/NASA | accessdate = 2007-01-02 }}</ref> Infrared is also used to observe distant, [[redshift|red-shifted]] galaxies that were formed much earlier in the history of the universe. 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]].
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The first non-visual study of galaxies, particularly active galaxies, was made using [[radio astronomy|radio frequencies]]. The 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 | url = http://radiojove.gsfc.nasa.gov/education/educ/radio/tran-rec/exerc/iono.htm | title = The Effects of Earth's Upper Atmosphere on Radio Signals | publisher = NASA | accessdate = 2006-08-10 }}</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;centimetre radiation]]), including, potentially, the non-ionized matter in the early universe that later collapsed to form galaxies.<ref>{{cite news | title=Giant Radio Telescope Imaging Could Make Dark Matter Visible | publisher=ScienceDaily |date=[[December 14]] [[2006]] | url=http://www.sciencedaily.com/releases/2006/12/061214135537.htm | accessdate=2007-01-02 }}</ref>
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[[UV astronomy|Ultraviolet]] and [[X-ray astronomy|X-ray telescopes]] can observe highly energetic galactic phenomena. An ultraviolet flare was observed when a star in a distant galaxy was torn apart from the tidal forces of a black hole.<ref>{{cite news | title= NASA Telescope Sees Black Hole Munch on a Star | publisher=NASA |date=[[December 5]] [[2006]] | url=http://www.nasa.gov/mission_pages/galex/galex-20061205.html | accessdate=2007-01-02 }}</ref> The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of super-massive black holes at the cores of galaxies was confirmed through X-ray astronomy.<ref>{{cite web | first=Robert | last=Dunn | url=http://www-xray.ast.cam.ac.uk/xray_introduction/ | title=An Introduction to X-ray Astronomy | publisher=Institute of Astronomy X-Ray Group | accessdate=2007-01-02 }}</ref>
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{{see also|Observational astronomy}}
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==See also==
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*[[List of galaxies]]
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*[[List of nearest galaxies]]
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*[[Timeline of galaxies, clusters of galaxies, and large scale structure]]
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{{clear}}
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==Notes==
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<div class="references-small">
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<ol type="a">
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<li>{{Note_label|A|a|none}}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".</li>
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<li>{{Note_label|B|b|none}}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.</li>
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</ol>
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</div>
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==References==
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{{reflist|2}}
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'''General references:'''
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* {{cite book
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| first=Terence
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| last=Dickinson
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| year=2004
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| title=The Universe and Beyond
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| edition=4th
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| publisher=Firefly Books Ltd.
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| id=ISBN 1552979016 }}
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* {{cite book
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| author=James Binney, Michael Merrifield
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| year=1998
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| title=Galactic Astronomy
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| publisher=Princeton University Press
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| id=ISBN 0691004021 }}
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{{commonscat|Galaxies}}
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==External links==
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*[http://www.seds.org/messier/galaxy.html Galaxies, SEDS Messier pages]
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*[http://www.atlasoftheuniverse.com/ An Atlas of The Universe]
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*[http://www.nightskyinfo.com/galaxies Galaxies — Information and amateur observations]
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*[http://science.nasa.gov/headlines/y2002/08feb_gravlens.htm The Oldest Galaxy Yet Found]
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*[http://www.bbc.co.uk/radio4/history/inourtime/inourtime_20060629.shtml Galaxies — discussed on BBC Radio 4's "In Our Time" programme]
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*[http://www.galaxyzoo.org Galaxy classification project, harnessing the power of the internet and the human brain]
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{{featured article}}
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[[Category:Galaxies| ]]
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Latest revision as of 01:57, 5 July 2008

A galaxy is a very large group of stars.

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