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 10¹⁴–10¹⁵ 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.