How is the age of an open cluster reliably estimated using the H-R diagram?

Answer

By observing the position where stars begin to peel away from the main sequence.

The age estimation for open clusters is intrinsically linked to the concept of stellar evolution on the Hertzsprung-Russell (H-R) diagram. Since all members of the cluster formed simultaneously, they share an identical age and chemical composition. Massive stars burn their fuel quickly and evolve off the main sequence sooner, becoming red giants. The specific point on the H-R diagram where the cluster's population begins to deviate or 'peel away' from the main sequence line directly corresponds to the lifespan of stars at that critical mass point. By knowing the theoretical lifespan associated with that cutoff mass, astronomers can accurately determine the cluster's age, making this feature a reliable chronological marker.

How is the age of an open cluster reliably estimated using the H-R diagram?

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