Final answer:
The HR diagram can provide an estimate of the age of star clusters by studying the position of the main-sequence turnoff. By comparing the luminosities of the brightest stars still on the main sequence in observed HR diagrams to calculated HR diagrams of different ages, astronomers can determine the age of a star cluster.
Step-by-step explanation:
The HR diagram can provide an estimate of the age of star clusters. The diagram shows the locations of stars in different stages of their evolution, such as the main sequence, red giants, and white dwarfs. By studying the position of the main-sequence turnoff in the HR diagram, astronomers can determine the age of a star cluster. This is done by comparing the luminosities of the brightest stars still on the main sequence in observed HR diagrams to the calculated HR diagrams of different ages.