Question

Explain how and why the turnoff point on the H-R diagram of a star cluster is related to the cluster's age.

Answer 1

"Why" first. Almost all stars in a cluster shaped about a similar time. High mass stars, in the upper piece of the Main Sequence, have shorter lifetimes. Lifetime on the Main Sequence increments easily as mass reductions. So: the principal stars to transform into Red Giants (and pass quickly through different stages) are the high-mass ones. So "How:" the side road point is resolved (L and T). These qualities are corresponded with a lifetime, and that is the age of the cluster.