SAT OwlAstronomers can estimate the age of a star cluster by examining which of its stars have begun to swell into red giants. Massive stars burn through their fuel quickly and become red giants sooner than less massive stars do. In a cluster whose stars all formed at roughly the same time, then, the most massive stars that are still burning normally reveal the cluster's age: the less massive those remaining stars are, ______
Which choice most logically completes the text?
the younger the cluster must be.
the more red giants the cluster will eventually lose.
the older the cluster must be.
the closer the cluster must be to Earth.
Show answer and explanation
Answer: C
Massive stars become red giants first, and progressively less massive stars follow as time passes. If even fairly low-mass stars have already become red giants, so that only lower-mass stars are still burning normally, a lot of time must have passed. So the cluster is older, choice C.
Choice A reverses the reasoning: lower-mass survivors mean more of the heavier stars have already changed, which takes more time.
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