What event initiates the transition into the red giant phase for a Sun-like star?
Answer
Depletion of hydrogen fuel supply in the stellar center.
The life cycle shifts dramatically when the star exhausts its primary fuel source, hydrogen, deep within its core. This hydrogen depletion causes the fusion engine in that central region to stall. Without the outward thermal pressure generated by core hydrogen fusion to counteract the relentless inward force of gravity, the inert helium ash at the center begins a vigorous gravitational collapse. This contraction increases the core's temperature and pressure sufficiently to ignite hydrogen burning in a shell surrounding the contracting core, which is the defining event that pushes the outer layers outward, transforming the star into a red giant.
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