Figure 19-13: Age of a Globular Cluster This color-magnitude diagram is equivalent to an H-R diagram and reveals the age of the cluster. Below the turnoff point, stars are still on the main sequence. Above the turnoff point, higher mass stars have already moved off the main sequence. The turnoff point occurs for stars around 0.8 M⊙; since stars of this mass are only on the main sequence for about 12 billion years, this tells us the age of the cluster. Each dot in this diagram represents the apparent visual magnitude (a measure of the brightness as seen through a V filter) and surface temperature (as measured by the color ratio bV/bB) of a star in the globular cluster M55 in Sagittarius.
(Adapted from D. Schade, D. VandenBerg, and F. Hartwick)