Final answer:
Star clusters are important for understanding stellar evolution because they provide the opportunity to observe multiple stages of stellar evolution within a single cluster. By comparing observations with models and analyzing H-R diagrams, astronomers can gain insights into the different ages and evolutionary effects of stars within the cluster.
Step-by-step explanation:
Star clusters are important for understanding stellar evolution because they allow astronomers to observe multiple stages of stellar evolution within a single cluster. Since stars with higher masses evolve more quickly, clusters can contain stars that have completed their main-sequence phase and become red giants, while other stars in the same cluster are still on the main sequence or undergoing pre-main-sequence gravitational contraction. By studying star clusters, astronomers can compare the observations with their models and analyze the differences in the Hertzsprung-Russell (H-R) diagrams of clusters of different ages.