What characteristic of a star cluster is used to determine its age?

Question

Quizplus · Accepted Answer

The main sequence turnoff is the point on the Hertzsprung-Russell diagram where the bulk of a star cluster's stars are no longer burning hydrogen in their cores and are instead evolving off the main sequence. The position of the turnoff on the diagram is strongly correlated with the age of the cluster, so by measuring the turnoff point, astronomers can estimate the cluster's age. The main sequence turnoff is the point on the Hertzsprung-Russell diagram where stars, having exhausted all of their core hydrogen fuel, begin to evolve off the main sequence and into the giant phase. The position of the turnoff on the diagram is related to the age of the cluster. Younger clusters will have a main sequence turnoff at hotter, bluer temperatures, while older clusters will have a turnoff at cooler, redder temperatures. Therefore, determining the main sequence turnoff of a star cluster is a key way to estimate its age.

What Characteristic of a Star Cluster Is Used to Determine