Why do stars become red giants?

Answer: Core fusion changes

Temperature increases with age — Wrong. Red giants are actually cooler (so red) despite expanding. Core heats up but outer layers cool as star expands enormously.

Gravity compresses them more — Wrong. Red giants expand, not compress. When core hydrogen exhausts, core contracts (heating up), but outer layers expand dramatically.

Core fusion changes ✓ — Correct! When stars exhaust core hydrogen fuel, fusion stops. Core contracts and heats up under gravity. This heats surrounding hydrogen shell, which begins fusing. Energy output increases, pushing outer layers outward—star expands 100-1000x! Surface cools (red color, ~3000-4000K). Our sun will become red giant in ~5 billion years, potentially engulfing inner planets!