Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. 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.
Read more about Galaxy Groups And Clusters: Groups of Galaxies, Clusters of Galaxies, Observational Methods, Temperature and Density, List of Groups and Clusters
Famous quotes containing the words galaxy, groups and/or clusters:
“for it is not so much to know the self
as to know it as it is known
by galaxy and cedar cone,
as if birth had never found it
and death could never end it:”
—Archie Randolph Ammons (b. 1926)
“Only the groups which exclude us have magic.”
—Mason Cooley (b. 1927)
“Where we such clusters had,
As made us nobly wild, not mad;
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Outdid the meat, outdid the frolic wine.”
—Robert Herrick (1591–1674)