A
galaxy cluster or
cluster of galaxies is a structure that consists of anywhere from hundreds to thousands of
galaxies bound together by
gravity with typical masses ranging from 10
14–10
15 solar masses. They are the largest known
gravitationally bound structures in the universe and were believed to be the
largest known structures in the universe until the 1980s when
superclusters were discovered. One of the key features of clusters is the
intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with
star clusters such as
open clusters, which are structures of stars
within galaxies, as well as
globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as
groups of galaxies rather than clusters of galaxies. The
groups and clusters can themselves cluster together to form
superclusters.