How does the turn-off point on an HR diagram correlate with the age of a star cluster?
Answer
A lower turn-off point indicates an older cluster
On an HR diagram, the turn-off point represents the moment stars evolve off the main sequence. In younger clusters, this point is higher and hotter because the massive stars are still on the main sequence. As a cluster ages, the most massive and hottest stars die off first, causing the turn-off point to shift down and to the right on the diagram, toward cooler and redder stars. Therefore, a lower turn-off point is a clear indicator that the cluster has aged significantly.
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