Astronomers only live for a few 10s of years. Star Clusters Groups of 100's to 1000's of stars. All stars in a cluster are at the same distance easy to measure relative Luminosity don't need distances to individual stars have the same age. Low-mass stars are cooler and fainter . 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.