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An open cluster is a relatively low-mass group of stars formed together, typically in a galactic disk. Two-body relaxation, stellar evaporation, and external tidal disturbances disperse many open clusters on much shorter times than the lifetimes of their low-mass member stars.

Many open clusters are weakly bound and are dispersed by two-body relaxation, stellar evaporation and tidal heating, including encounters with molecular clouds in the galactic disk, so their present age histogram preferentially retains young survivors. More massive compact globular clusters can survive much longer, while the released low-mass stars remain as old field stellar populations: disruption removes clusters from the catalogue without removing their stars from the disc.