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A star cluster is a gravitationally associated group of stars formed in a relatively compact region. Open clusters and globular clusters provide populations whose members have broadly related formation histories, making them useful laboratories for stellar evolution. Membership is tested using parallax, proper motion and, where available, radial velocity, rather than apparent proximity on the sky alone. A cluster's Hertzsprung-Russell diagram then compares stars of different masses at a shared distance; its main-sequence turnoff helps estimate stellar age. Space-based astrometry sharpens both membership selection and these evolutionary sequences, although some clusters contain multiple stellar populations.^[ref]