As a subgiant star becomes a red giant, its luminosity increases while its temperature remains approximately constant.What does this mean?

Name: As a subgiant star becomes a red giant, its luminosity increases while its temperature remains approximately constant.What does this mean?
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Question

Quizplus · Accepted Answer

As a star's luminosity increases while its temperature remains constant, the increase in luminosity must be due to an increase in the star's radius, according to the Stefan-Boltzmann law.

As a Subgiant Star Becomes a Red Giant, Its Luminosity