What ongoing process causes a stable main sequence star like the Sun to slowly evolve into a red giant later in its life?
Answer
Core hydrogen depletion leading to core contraction and outer layer expansion
The Hydrostatic and Thermal Equilibrium achieved at ZAMS represents a dynamic balance, not a static endpoint. This balance is maintained as long as hydrogen fuel is available for core fusion. Over billions of years, the star consumes its core hydrogen, converting it into helium ash. As the helium accumulates in the center, the core contracts slightly, which increases the central temperature further. This slight increase in core energy generation forces the star's outer layers to expand significantly, causing the star to swell into a red giant, demonstrating that stability is constantly maintained against the inevitable change in core composition.
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