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Galaxy clusters, grouping of galaxies. Galaxies, like stars, tend to unite in various size systems: multiple systems; groups; clusters; superclusters. The multiple systems can be constituted by two or more galaxies, linked by an intense gravitational interaction. An example of multiple systems is the Milky Way, with its two satellite galaxies, the Large and Small Magellanic Cloud.
The groups are combinations of a few tens of galaxies: the Milky Way, for example, is part of the Local Group, it consists of thirty members and with a diameter of about 1.5 Mpc. The two largest in the Local Group galaxies Andromeda and the Milky Way; among the minor components there is another spiral, some irregular galaxies, and many dwarf elliptical galaxies.
A group of at least 50 very luminous galaxies is called heap: the typical radius of a cluster is 2 to 5 Mpc. They distinguish at least two types of clusters: the regular clusters, characterized by a strong central concentration of galaxies and a structure rather regular and irregular clusters, without an appreciable central concentration of galaxies and irregularly shaped. Regular clusters always contain thousands of members, almost all elliptical and lenticular galaxies; in this type of clusters, we have been recently discovered ray emissions X. The irregular clusters may, however, contain a very variable number of galaxies of all types; often, the brightest galaxies in these clusters are spirals. The cluster is the nearest Virgo Cluster, irregular type, which has at least 2,500 members. Galaxy clusters can form even larger clusters, which are called superclusters.
The Local Group, for example, is part of the Local Supercluster, whose center is formed by the Virgo cluster. The diameters of the superclusters are on the order of 10 to 20 Mpc. The first cluster of galaxies was discovered, by chance, by the famous comet finder Charles Messier, back on April 15, 1779, when he noticed three huge gauzy patches, which although resembling comets, showed a diverging characteristic of the latter, such as lack of mobility.
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