Why does saturation of a massive star's core with iron guarantee gravitational collapse?
Iron fusion consumes energy rather than releasing it, eliminating outward pressure.
The core-collapse pathway for massive stars hinges critically on the formation of an iron core. Unlike lighter elements which release tremendous amounts of energy when fused (providing the necessary outward pressure to counteract gravity), the fusion process involving iron demands an energy input; it consumes energy rather than releasing it. Once the core is saturated with iron, the star loses its internal energy source that sustains its structure against the overwhelming force of gravity. This immediate cessation of outward thermal pressure guarantees a rapid, catastrophic gravitational collapse of the core, leading directly to the rebound explosion known as a supernova.
