What event pushes a star's outer layers outward after core hydrogen is exhausted?

Answer

Intense hydrogen fusion in a shell surrounding the core.

The dramatic expansion characterizing the transition to a red giant or supergiant is a direct response to a catastrophic failure and subsequent reorganization of energy production within the star. After the stable hydrogen fuel in the star's center is used up, gravity forces the core to contract and heat up significantly. This intense heat ignites the hydrogen located just outside the inert core, initiating shell fusion. This new source of energy production is extremely vigorous and generates immense outward pressure. This outward push from the newly activated shell fusion overwhelms the star's remaining gravitational structure, forcing the outer layers to expand dramatically, which simultaneously causes the surface temperature to drop, yielding the characteristic reddish color.

What event pushes a star's outer layers outward after core hydrogen is exhausted?

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