Diameters of Open Star Clusters

Abstract

The present paper presents a tabulation of data on all 600 Galactic open clusters for which it is currently possible to calculate linear diameters. As expected, the youngest ``clusters,'' with ages <15 Myr, contain a significant (>=20%) admixture of associations. Among intermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, the median cluster diameter is found to increase with age. Small, compact clusters are rare among objects with ages >1.5 Gyr. Open clusters with ages >1 Gyr appear to form what might be termed a ``cluster thick disk,'' part of which consists of objects that were probably captured gravitationally by the main body of the Galaxy.