Figure 19-14: An H-R Diagram for Open Star Clusters The black bands indicate where stars from various open clusters fall on the H-R diagram. The age of a cluster can be estimated from the location of the cluster’s turnoff point, where the cluster’s most massive stars are just now leaving the main sequence. The times for these turnoff points are listed alongside the main sequence. For example, the Pleiades cluster turnoff point is near the 108-year point, so this cluster is about 108 years old.
(Adapted from A. Sandage)