Turnoff Mass

When stars age, they run out of hydrogen in their cores. When that happens they begin to change and they move off the main sequence toward the red giant branch. When a cluster of stars forms, all the stars will be on the main sequence. The more massive stars evolve faster and leave the main sequence. The point on the main sequence where stars have just left (defining the end of the main sequence for that cluster) is the main sequence turnoff point. The mass of these stars is the turnoff mass for the cluster.

Old Cluster

Figure: A HR-Diagram of an old globular cluster showing the location of the main sequence turnoff. Only the dim and red (low mass) portion of the main sequence remains. More massive stars have evolved into red giants and stellar remnants.