What event marks the main-sequence turnoff point on an H-R diagram for a cluster?

Answer

Stars beginning to evolve off the main sequence after exhausting core hydrogen

The main-sequence turnoff point identifies the most massive stars that have consumed all the hydrogen in their cores and have started evolving toward the red giant phase, signaling the cluster's age.

What event marks the main-sequence turnoff point on an H-R diagram for a cluster?

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