[[Image:NGC 4650A I HST2002.jpg|right|thumb|140px|NGC 4650A, an example of a polar-ring galaxy.]]
A '''polar-ring galaxy''' is a relatively rare type of galaxy, with only around 100 examples known. The best known polar-ring galaxy is [[NGC 4650A]]. It is theorized that the structure of these galaxies result from the collision of two galaxies. The collision results in a structure composed of two rings of material rotating at an approximate right angle to each other.
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. These polar rings are thought to form when two galaxies 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.
==Formation of Polar-ring galaxies==
The first four galaxies that were identified as polar-ring galaxies were NGC 2685, While these galaxies have been extensively studied, many other polar-ring galaxies have since been identified.
{{cleanup-image}}[[Image:Form-PRG.jpg|thumb|right|250px]]It is theorized that the structure of these galaxies result from the collision of two galaxies. The collision results in a structure composed of two rings of material rotating at an approximate right angle to each other. It is important to understand that the collision occurs so slowly that even over a lifetime, the movement is almost imperceptible. Also important is that once what is left of the interacting galaxy has gone on after the collision, the remaining static galaxy does not simply stop moving. The stars in the galaxy will continue on their new paths until they are dispersed and it is no longer a galaxy. What one sees when they look at a polar-ring galaxy is a freeze-frame of a former galaxy spilling out around itself, its stars destined to disperse over many light-years.
==List of Polar-ring galaxies==
==External links==
{| class="wikitable"
* [http://antwrp.gsfc.nasa.gov/apod/ap990510.html APOD: May 10, 1999 - Polar Ring Galaxy NGC 4650A]
<tr><th> Category</th><th>Names of objects</th></tr>
* [http://hubblesite.org/newscenter/archive/releases/1999/16/ Internet Voters Get Two Galaxies in One from Hubble]
* [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
*[http://antwrp.gsfc.nasa.gov/apod/ap990510.html An image of NGC 4650A, a Polar Ring Galaxy]
[[Category:Galaxy morphological types]]
[[Category:Galaxy morphological types]]
[[Category:Polar-ring galaxies| ]]
{{ntnes}}
[[de:Polarring-Galaxie]]
Latest revision as of 00:31, 11 April 2009
NGC 4650A, an example of a polar-ring galaxy.
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. These polar rings are thought to form when two galaxies 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.
The first four galaxies that were identified as polar-ring galaxies were NGC 2685, While these galaxies have been extensively studied, many other polar-ring galaxies have since been identified.
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
