Why does nuclear fusion cease in the Iron Core of a massive star?

Fusing iron consumes energy instead of releasing it.

Nuclear fusion operates by combining lighter atomic nuclei into heavier ones, releasing vast amounts of energy in the process, which provides the outward pressure necessary to counteract gravity. However, iron represents a fundamental endpoint in stellar nucleosynthesis because fusing iron atoms does not generate energy; rather, it requires an input of energy to force the fusion reaction. Once the central core is composed predominantly of iron and accumulates past the Chandrasekhar limit, this energy-consuming reaction means the core instantaneously loses its primary source of outward pressure support. Consequently, fusion effectively ceases as a viable energy generation mechanism, paving the way for catastrophic gravitational collapse.