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galaxy groups and clusters
Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation.〔Voit, G.M.; "Tracing cosmic evolution with clusters of galaxies"; Reviews of Modern Physics, vol. 77, Issue 1, pp. 207-258〕 They form the densest part of the large-scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain ten to thousands of individual galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called superclusters.
== Groups of galaxies ==
(詳細はgalaxies are the smallest aggregates of galaxies. They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs (Mpc)(see 1022 m for distance comparisons). Their mass is approximately 1013 solar masses. The spread of velocities for the individual galaxies is about 150 km/s. However, this definition should be used as a guide only, as larger and more massive galaxy systems are sometimes classified as galaxy groups. Groups are the most common structures of galaxies in the universe, comprising at least 50% of the galaxies in the local universe. Groups have a mass range between those of the very large elliptical galaxies and clusters of galaxies.
Our own Galaxy, the Milky Way, is contained in the Local Group of more than 40 galaxies.〔(【引用サイトリンク】url= http://www.ast.cam.ac.uk/~mike/local_more.html )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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