On an H-R Diagram of a Cluster of Stars, Which

Multiple Choice

On an H-R diagram of a cluster of stars, which characteristic of the diagram do astronomers use as a good indicator of the cluster's age?

A) the number of M stars on the main sequence
B) the lowest luminosity star that is visible in the cluster
C) the point on the main sequence where stars begin to "turn off" -- to move toward the red giant region
D) how high up on the main sequence M type stars are found
E) the coolest surface temperature for a star that they can measure