= Star cluster
{wiki}
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>.<Gaia Data Release 2: Observational Hertzsprung-Russell diagrams by Gaia Collaboration>[Abstract and cluster analysis: astrometric membership, open and globular cluster sequences, and comparisons with evolutionary tracks]{ref}
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