On an H-R diagram of a cluster of stars, which characteristic of the diagram do astronomers use as a good indicator of the cluster's age?

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Quizplus · Accepted Answer

The point on the main sequence where stars begin to "turn off" -- to move toward the red giant region is known as the main sequence turnoff point. This point is a good indicator of the cluster's age, as stars of different masses evolve at different rates and stars near the turnoff point have just exhausted their core hydrogen and are beginning to evolve off the main sequence. The exact location of the turnoff point depends on the age of the cluster, with older clusters having turnoff points at lower temperatures and lower luminosities compared to younger clusters.

On an H-R Diagram of a Cluster of Stars, Which