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| [[Image:NGC 4650A I HST2002.jpg|right|thumb|140px|[[NGC 4650A]], an example of a polar-ring galaxy. Credit:[[Hubble Space Telescope]]/[[NASA]]/[[ESA]].]]
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| A '''polar-ring galaxy''' is a type of [[galaxy]] in which an outer ring of gas and stars rotates over the poles of the galaxy.<ref name="binneymerrifield1998">{{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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| | location=Princeton, New Jersey
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| | id=ISBN 0-691-00402-1}}</ref> These polar rings are thought to form when two galaxies [[interacting galaxy|gravitationally interact]] with each other. One possibility is that a material is tidally stripped from a passing galaxy to produce the polar ring seen in the polar-ring galaxy. The other possibility is that a smaller galaxy collides orthogonally with the plane of rotation of the larger galaxy, with the smaller galaxy effectively forming the polar ring structure.<ref name="schweizeretal1983">{{cite journal
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| | author=F. Schweizer, B. C. Whitmore, V. C. Rubin
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| | title=Colliding and merging galaxies. II - S0 galaxies with polar rings
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| | journal=Astronomical Journal
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| | year=1983
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| | volume=88
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| | pages=909-925
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| | url=http://adsabs.harvard.edu/abs/1983AJ.....88..909S}}</ref>
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| The first four galaxies that were identified as polar-ring galaxies were [[NGC 2685]],<ref name="schectergunn1978">{{cite journal
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| | author=P. L. Schecter, J. E. Gunn
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| | title=NGC 2685 - Spindle or pancake
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| | journal=Astronomical Journal
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| | year=1978
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| | volume=83
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| | pages=1360-1362
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| | url=http://adsabs.harvard.edu/abs/1978AJ.....83.1360S}}</ref> [[NGC 4650A]],<ref name="sersic1967">{{cite journal
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| | author=J. L. Sérsic
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| | title=Southern Peculiar Galaxies III
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| | journal=Zeitschrift für Astrophysik
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| | year=1967
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| | volume=67
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| | pages=306-311
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| | url=http://adsabs.harvard.edu/abs/1967ZA.....67..306S}}</ref><ref name="whitmoreetal1987">{{cite journal
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| | author=B. C. Whitmore, D. B. McElroy, F. Schweizer
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| | title=The shape of the dark halo in polar-ring galaxies
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| | journal=Astrophysical Journal
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| | year=1987
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| | volume=314
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| | pages=439-456
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| | url=http://adsabs.harvard.edu/abs/1987ApJ...314..439W}}</ref> [[A 0136 -0801]],<ref name="schweizeretal1983">{{cite journal
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| | author=F. Schweizer, B. C. Whitmore, V. C. Rubin
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| | title=Colliding and merging galaxies. II - S0 galaxies with polar rings
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| | journal=Astronomical Journal
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| | year=1983
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| | volume=88
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| | pages=909-925
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| | url=http://adsabs.harvard.edu/abs/1983AJ.....88..909S}}</ref> and [[ESO 415 -G26]].<ref name="whitmoreetal1987" /> While these galaxies have been extensively studied, many other polar-ring galaxies have since been identified.<ref name="whitmoreetal1990">{{cite journal
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| | author= B. C. Whitmore, R. A. Lucas, D. B. McElroy, T. Y. Steiman-Cameron, P. D. Sackett, R. P. Olling
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| | title=New observations and a photographic atlas of polar-ring galaxies
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| | journal=Astronomical Journal
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| | year=1990
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| | volume=100
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| | pages=1489-1522, 1721-1755
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| | url=http://adsabs.harvard.edu/abs/1990AJ....100.1489W}}</ref> Polar rings may be found around 0.5% of all nearby [[lenticular galaxies]], and it is possible that 5% of lenticular galaxies may have had polar rings at some point during their lifetimes.<ref name="whitmoreetal1990"/>
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| ==References==
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| <div class="references-small"><references/></div>
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| ==External links==
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| * [http://antwrp.gsfc.nasa.gov/apod/ap990510.html APOD: May 10, 1999 - Polar Ring Galaxy NGC 4650A]
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| * [http://hubblesite.org/newscenter/archive/releases/1999/16/ Internet Voters Get Two Galaxies in One from Hubble]
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| * [http://space.newscientist.com/article/mg19726455.500-x-marks-the-spot-in-dark-matter-web.html?feedId=online-news_rss20 X marks the spot in dark matter web] - Polar ring galaxies offer first-hand evidence of the existence of the cosmic web, ''New Scientist'', 29 February 2008
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| [[Category:Galaxy morphological types]]
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| [[de:Polarring-Galaxie]]
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