How do astronomers use the H-R diagram to accurately age an open cluster of stars?

Answer

By observing the "turnoff point" where massive stars peel off the Main Sequence

The H-R diagram serves as an excellent chronometer for stellar populations that formed simultaneously, such as those found in open clusters. When a cluster is plotted, all stars begin on the Main Sequence. Since stellar evolution proceeds based on mass—the most massive stars exhaust their fuel and evolve away first—astronomers look for the point where the stars are beginning to leave the stable sequence, moving up and right towards the giant branch. This specific point on the Main Sequence, called the 'turnoff point,' corresponds precisely to the mass of the stars that have just reached the end of their hydrogen-burning lives. By correlating this observed turnoff mass with established theoretical stellar evolution models, astronomers can accurately determine the age of the entire cluster.

How do astronomers use the H-R diagram to accurately age an open cluster of stars?

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HR Diagram Explained - Star Color, Temperature and Luminosity

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