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| [[Image:Abell S740, cropped to ESO 325-G004.jpg|thumb|The giant elliptical galaxy ESO 325-G004]]
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| An '''elliptical galaxy''' is a [[galaxy]] having an approximately [[ellipsoid]]al shape and a smooth, nearly featureless brightness profile. They are one of the three main [[galaxy morphological classification|classes of galaxy]] originally described by [[Edwin Hubble]] in his 1936 work ''The Realm of the Nebulae'',<ref>{{cite book |last=Hubble |first=E. P. |authorlink=Edwin Hubble |title=The Realm of the Nebulae |year=1936 |publisher=Yale University Press |location=New Haven |isbn=978-0300025002}}</ref> along with [[spiral galaxy|spiral]] and [[lenticular galaxy|lenticular]] galaxies. They range in shape from nearly spherical to highly flat and in size from tens of millions to over one trillion [[star]]s. Originally, Edwin Hubble thought that elliptical galaxies may evolve into [[spiral galaxy|spiral galaxies]], which later turned out to be false.<ref name="author">John, D, (2006), ''Astronomy'', ISBN 1-4054-6314-7, p. 224-225</ref> Stars found inside of elliptical galaxies are very much older than stars found in spiral galaxies.<ref name="author" />
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| Most elliptical galaxies are composed of older, [[stellar evolution#Low-mass stars|low-mass stars]], with a sparse [[interstellar medium]] and minimal [[star formation]] activity, and they tend to be surrounded by large numbers of [[globular cluster]]s. Elliptical galaxies are believed to make up approximately 10–15% of galaxies in the [[Virgo Supercluster]], and they are not the dominant type of galaxy in the universe overall.<ref>{{cite journal |author=Loveday, J. |date=February 1996 |title=The APM Bright Galaxy Catalogue. |journal=Monthly Notices of the Royal Astronomical Society |volume=278 |issue=4 |pages=1025–1048 |bibcode=1996MNRAS.278.1025L|arxiv = astro-ph/9603040 |doi=10.1093/mnras/278.4.1025}}</ref> They are preferentially found close to the centers of [[galaxy cluster]]s.<ref>{{cite journal |author=Dressler, A. |date=March 1980 |title=Galaxy morphology in rich clusters - Implications for the formation and evolution of galaxies. |journal=The Astrophysical Journal |volume=236 |pages=351–365 |bibcode=1980ApJ...236..351D |doi=10.1086/157753}}</ref> Elliptical galaxies are (together with [[Lenticular galaxy|lenticular galaxies]]) also called "early-type" galaxies (ETG), due to their location in the [[Hubble sequence#Physical significance|Hubble sequence]], and are found to be less common in the early [[Universe]].
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| ==General characteristics==
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| [[File:Development of massive elliptical galaxies.jpg|thumb|Development of massive elliptical galaxies]]
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| Elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. They are spherical or ovoid masses of stars, starved of star-making gases. The smallest known elliptical galaxy is about one-tenth the size of the [[Milky Way]]. The motion of stars in elliptical galaxies is predominantly radial, unlike the disks of [[Spiral galaxy|spiral galaxies]], which are dominated by [[rotation]]. Furthermore, there is very little [[interstellar matter]] (neither gas nor dust), which results in low rates of [[star formation]], few [[open cluster|open star clusters]], and few young stars; rather elliptical galaxies are dominated by [[Population II|old stellar populations]], giving them red colours. Large elliptical galaxies typically have an extensive system of [[globular cluster]]s.<ref>{{cite book |last=Binney |first=J. |coauthors=Merrifield, M. |title=Galactic Astronomy |year=1998 |publisher=Princeton University Press |location=Princeton |isbn=978-0-691-02565-0 |oclc=39108765}}</ref>
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| The dynamical properties of elliptical galaxies and the [[Bulge (astronomy)|bulges]] of [[Disc galaxy|disk galaxies]] are similar,
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| <ref name="DM1999"> | |
| {{Cite journal
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| | last = Merritt | first = D.
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| | authorlink = David Merritt
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| | title = Elliptical galaxy dynamics
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| | journal = The Astronomical Journal
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| | volume = 756
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| | issue = 756 | pages = 129–168
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| | date = February 1999
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| | bibcode = 1999PASP..111..129M
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| | doi = 10.1086/316307 |arxiv = astro-ph/9810371
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| }}</ref> suggesting that they are formed by the same physical processes, although this remains controversial. The [[Luminosity#In_astronomy|luminosity]] profiles of both elliptical galaxies and bulges are well fit by [[Sersic profile|Sersic's law]].
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| Elliptical galaxies are preferentially found in [[galaxy clusters]] and in compact groups of galaxies.
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| ==Star formation==
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| The traditional portrait of elliptical galaxies paints them as galaxies where [[star formation]] finished after an initial burst at high-redshift, leaving them to shine with only their aging stars. Elliptical galaxies typically appear yellow-red, which is in contrast to the distinct blue tinge of most [[Spiral galaxy|spiral galaxies]]. In spirals, this blue color emanates largely from the young, hot stars in their spiral arms. Very little [[star formation]] is thought to occur in elliptical galaxies, because of their lack of gas compared to spiral or irregular galaxies. However, in recent years, evidence has shown that a reasonable proportion (~25%) of these galaxies have residual gas reservoirs<ref name=Young2011>{{cite journal|authors=Young, L. M. et al.|title=The Atlas3D project -- IV: the molecular gas content of early-type galaxies|journal=Monthly Notices of the Royal Astronomical Society|date=June 2011|volume=414|issue=2|pages=940–967|arxiv= 1102.4633|doi=10.1111/j.1365-2966.2011.18561.x|bibcode=2011MNRAS.414..940Y}}</ref>
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| and low level star-formation.<ref name=Crocker2011>{{cite journal|last=Crocker|first=A. F. et al.|title=Molecular gas and star formation in early-type galaxies|journal=Monthly Notices of the Royal Astronomical Society|date=January 2011|volume=410|issue=2|pages=1197–1222|arxiv=1007.4147|doi=10.1111/j.1365-2966.2010.17537.x|bibcode=2011MNRAS.410.1197C}}</ref>
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| ==Sizes and shapes==
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| [[File:Galactic fireflies.jpg|thumb|The central galaxy in this image is a gigantic elliptical galaxy designated 4C 73.08.<ref>{{cite news|title=Galactic fireflies|url=http://www.spacetelescope.org/images/potw1246a/|accessdate=13 February 2013|newspaper=ESA/Hubble Picture of the Week}}</ref> ]]
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| [[File:MACSJ1423.8+2404.jpg|thumb|The brilliant central object is a supergiant elliptical galaxy, the dominant member of a [[galaxy cluster]] with the name MACSJ1423.8+2404.]]
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| Elliptical galaxies vary greatly in both size and mass, from as little as a tenth of a [[kiloparsec]] to over 100 kiloparsecs, and from 10<sup>7</sup> to nearly 10<sup>13</sup> solar masses.{{Citation needed|date=July 2011}} This range is much broader for this galaxy type than for any other. The smallest, the [[Dwarf elliptical galaxy|Dwarf elliptical galaxies]], may be no larger than a typical [[globular cluster]], but contain a considerable amount of [[dark matter]] not present in clusters. Most of these small galaxies may not be related to other ellipticals.
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| The [[Galaxy morphological classification|Hubble classification]] of elliptical galaxies contains an integer that describes how elongated the galaxy image is.
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| The classification is determined by the ratio of the major (''a'') to the minor (''b'') axes of the galaxy's [[Contour_line#Other_phenomena|isophotes]]:
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| :<math>10 \times \left(1 - \frac{b}{a}\right)</math>
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| Thus for a spherical galaxy with ''a'' equal to ''b'', the number
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| is 0, and the Hubble type is E0. The limit is about E7, which is believed to be due to a [[firehose instability|bending instability]] that causes flatter galaxies to puff up. The most common shape is close to E3. Hubble recognized that his shape classification depends both on the intrinsic shape of the galaxy, as well as the angle with which the galaxy is observed. Hence, some galaxies with Hubble type E0 are actually elongated.
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| There are two physical types of ellipticals; the "boxy" giant ellipticals, whose shapes result from random motion which is greater in some directions than in others (anisotropic random motion), and the "disky" normal and low luminosity ellipticals, which have nearly isotropic random velocities but are flattened due to rotation.
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| [[Dwarf elliptical galaxy|Dwarf elliptical galaxies]] have properties that are intermediate between those of regular elliptical galaxies and [[globular clusters]]. [[Dwarf spheroidal galaxy|Dwarf spheroidal galaxies]] appear to be a distinct class: their properties are more similar to those of irregulars and late spiral-type galaxies.
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| At the large end of the elliptical spectrum, there is further division, beyond Hubble classification. Beyond ''gE'' giant ellipticals, lies [[Type-D galaxy|D-galaxies]] and [[Type-cD galaxy|cD-galaxies]]. These are similar to their smaller brethren, but more diffuse, with larger haloes. Some even appear more akin to [[Lenticular galaxy|lenticular galaxies]].
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| ==Evolution==
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| Current thinking is that an elliptical galaxy may be the result of a long process where two or more galaxies of comparable mass, of any type, collide and [[Galaxy merger|merge]].{{Citation needed|date=November 2009}}
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| Such major galactic mergers are thought to have been common at early times, but may carry on more infrequently today. Minor galactic mergers involve two galaxies of very different masses, and are not limited to giant ellipticals. For example, our own [[Milky Way]] galaxy is known to be consuming a couple of small galaxies right now.{{Citation needed|date=January 2010}} The [[Milky Way]] galaxy is also, depending upon an unknown tangential component, on a collision course in 4–5 billion years with the [[Andromeda Galaxy]]. It has been theorized that an elliptical galaxy will result from a merger of the two spirals.<ref>{{cite web|title=Milky Way Galaxy Doomed: Collision with Andromeda Pending|url=http://apod.nasa.gov/apod/ap120604.html}}</ref>
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| Every bright elliptical galaxy is believed to contain a [[supermassive black hole]] at its center. The mass of the black hole is tightly correlated with the mass of the galaxy, via the [[M–sigma relation]]. It is believed that black holes may play an important role in limiting the growth of elliptical galaxies in the early universe by inhibiting [[star formation]].{{Citation needed|date=January 2010}}
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| == Examples ==
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| [[File:SDSS J162702.56+432833.9.jpg|thumb|SDSS J162702.56+432833.9 is an elliptical galaxy.<ref>{{cite news|title=The Calm after the Galactic Storm|url=http://spacetelescope.org/images/potw1148a/|accessdate=1 December 2011|newspaper=ESA/Hubble Picture of the Week}}</ref>]]
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| * [[Messier 32|M32]]
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| * [[Messier 49|M49]]
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| * [[Messier 59|M59]]
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| * [[Messier 60|M60 (NGC 4649)]]
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| * [[Messier 87|M87 (NGC 4486)]]
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| * [[Messier 89|M89]]
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| * [[Messier 105|M105 (NGC 3379)]]
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| * [[IC 1101]], one of the largest galaxies in the [[observable universe]], with more than 100 trillion stars.
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| * [[Maffei 1]], the closest giant elliptical galaxy.
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| * [[Centaurus A|Centaurus A (NGC 5128)]], a [[radio galaxy]], elliptical/lenticular disputed.
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| == See also ==
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| * [[Firehose instability]]
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| * [[Galaxy color–magnitude diagram]]
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| * [[Galaxy morphological classification]]
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| * [[Hubble sequence]]
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| * [[Lenticular galaxy]]
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| * [[M–sigma relation]]
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| * [[Osipkov–Merritt model]]
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| * [[Sersic profile]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| {{Commons category|Elliptical galaxies}}
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| *[http://www.seds.org/messier/elli.html Elliptical Galaxies], SEDS Messier pages
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| *[http://csep10.phys.utk.edu/astr162/lect/galaxies/elliptical.html Elliptical Galaxies]
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| {{Galaxy}}
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| [[Category:Elliptical galaxies| ]]
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| [[Category:Galaxy morphological types]]
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Not much to tell about myself really.
Feels good to be a part of wmflabs.org.
I just hope Im useful in one way .
Feel free to visit my page hemorrhoid surgery