When a star reaches the Zero-Age Main Sequence (ZAMS), what mechanism ultimately stops the rapid gravitational contraction?
Answer
Outward radiation pressure from fusion photons balancing gravity
Once sustained hydrogen fusion ignites, the massive energy output creates strong outward radiation pressure. This pressure perfectly counters the inward crush of gravity, halting the rapid contraction and allowing the star to settle into the stable hydrostatic equilibrium of the Main Sequence.
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