Trumpler classification

Trumpler classification is a classification scheme for open clusters devised by the American astronomer Robert Trumpler. It uses three criteria: degree of central concentration (from I, most concentrated, to IV, least); range of brightness of member stars (from 1, narrow range, to 3, wide range); and total number of stars in the cluster (p for poor, less than 50; m for moderately rich, 50 to 100; r for rich, more than 100). The addition of "n" indicates that the cluster has some nebulosity; for example, the Trumpler classification of the Pleiades is I3rn.