Hydrostatic Equilibrium
During most of its life, a star maintains hydrostatic equilibrium: the outward pressure from nuclear fusion balances the inward pull of gravity. For massive stars (greater than 8 solar masses), this process continues through successive stages, fusing heavier elements up to iron.
The Iron Catastrophe
Iron fusion consumes energy rather than releasing it. Once the core is predominantly iron, fusion stops, pressure vanishes, and gravity causes the core to collapse catastrophically in milliseconds.