Audio Transcript Excerpt:
PROFESSOR: 'In stellar astrophysics, the balance of a main sequence star is governed by hydrostatic equilibrium—the outward

Question

Audio Transcript Excerpt:
PROFESSOR: 'In stellar astrophysics, the balance of a main sequence star is governed by hydrostatic equilibrium—the outward thermal pressure generated by core nuclear fusion precisely offsets the inward compressive pull of gravity. But consider what happens when a medium-mass star exhausts the hydrogen fuel in its core. With fusion halting in the core, outward radiation pressure drops, causing the helium core to contract under gravity. As the core contracts, gravitational potential energy is converted into heat, intensely heating an overlying shell of unburned hydrogen. This hydrogen shell ignites fusion at a furious rate, generating far more radiation pressure than the original core. This immense outward pressure forces the star's outer gaseous envelope to expand by orders of magnitude, transforming the star into a red giant.'

What causal chain causes a star to expand into a red giant after core hydrogen is depleted?

Accepted Answer

Core contraction generates heat that triggers explosive fusion in an overlying hydrogen shell, producing radiation pressure that inflates the outer envelope.

Answer

The star absorbs massive amounts of interstellar gas from adjacent nebula clouds.

Answer

Nuclear fusion ceases entirely, allowing cold outer space to expand the star's surface.

Answer

The core explodes in a supernova, blowing the outer layers away into a planetary nebula.

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