Low-Mass stars take longer to form. High mass run out of hydrogen in their cores first, evolving off into supergiants. As successively lower mass stars run out of hydrogen in their cores, they too evolve off. Peel off the Main-Sequence from the top. As cluster ages, the stars at the turn-off are lower mass Low mass stars have redder colors.
Older Clusters have redder turn-off points.
A prominent Horizontal Branch Slightly bluer and fainter Main Sequence due to having less metals than nearby stars Conclusions Cluster H-R Diagrams give us a snapshot of stellar evolution. Observations of clusters with ages from a few Million to 15 Billion years confirms much of our picture of stellar evolution. Remaining challenges are in small details, but the big picture seems secure. 1998 January 28 Copyright 1998 Richard W. Pogge, All Rights Reserved.