Question

b. Why do stars evolve?
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Answer 1

Explanation:

Nuclear fusion powers a star for most of its existence. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. ... This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase.,

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Answer 2

Answer:

Because when you dump energy into a normal gas (in this case the star's envelope), the pressure that gas exerts increases. ... And so generally speaking, stars evolve from the main sequence over toward the upper right quadrant of the H-R diagram, eventually becoming giant or supergiant stars

Explanation:

Nuclear fusion powers a star for most of its existence. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. ... This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase.