Galaxy cluster

Composite image of five galaxies clustered together just 600 million years after the Universe's birth^[1]

A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity,^[1] with typical masses ranging from 10¹⁴ to 10¹⁵ solar masses. Clusters consist of galaxies, heated gas, and dark matter.^[2] They are the second-largest known gravitationally bound structures in the universe after superclusters. They were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered.^[3] Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. Together, galaxy groups and clusters form superclusters.

^ ^a ^b "Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen". ESA/Hubble Press Release. Retrieved 13 January 2015.
^ Cite error: The named reference :0 was invoked but never defined (see the help page).
^ Kravtsov, A. V.; Borgani, S. (2012). "Formation of Galaxy Clusters". Annual Review of Astronomy and Astrophysics. 50: 353–409. arXiv:1205.5556. Bibcode:2012ARA&A..50..353K. doi:10.1146/annurev-astro-081811-125502. S2CID 119115331.

[Hubble_protocluster-1] "Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen". ESA/Hubble Press Release. Retrieved 13 January 2015.

[:0-2] Cite error: The named reference :0 was invoked but never defined (see the help page).

[Kravtsov2012-3] Kravtsov, A. V.; Borgani, S. (2012). "Formation of Galaxy Clusters". Annual Review of Astronomy and Astrophysics. 50: 353–409. arXiv:1205.5556. Bibcode:2012ARA&A..50..353K. doi:10.1146/annurev-astro-081811-125502. S2CID 119115331.

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