What happens during the fusion of iron in the core of a massive star?

It consumes energy rather than releasing it

The sequence of nuclear fusion in massive stars ends catastrophically when the core begins producing iron. All elements up to iron in the periodic table release energy when fused, which provides the thermal pressure needed to counteract gravity and maintain hydrostatic equilibrium. However, iron represents the peak of binding energy per nucleon. Consequently, when the stellar core attempts to fuse iron, the reaction requires an energy input rather than producing an energy output. This sudden switch from generating outward pressure to consuming internal energy causes the thermal support to fail instantaneously, leading directly to the core's gravitational collapse and subsequent supernova event.