Exam-style question
Try this first
On the HR diagram, the "turn‑off point" of a star cluster indicates what about the cluster’s stars?.
- A.The age at which the cluster formed.
- B.The mass of the most massive star still on the main sequence.
- C.The temperature of the coolest star in the cluster.
- D.The luminosity of the brightest red giant in the cluster.
Model answer
What a good answer should say
- The mass of the most massive star still on the main sequence.
Explanation
Why this works
The turn‑off point marks where stars of a given mass have exhausted core hydrogen and begun to leave the main sequence. Its position therefore reflects the mass (and hence age) of the most massive stars still on the main sequence, allowing cluster age estimation.
Common mistake
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