What is the energy consequence when a massive star's core fuses iron (Fe)?
It consumes energy.
The fusion chain reaction that powers a star releases energy by fusing lighter elements into heavier ones, providing the outward thermal pressure necessary to counteract gravity. However, this energy generation mechanism fails completely when the fusion process reaches iron ($ ext{Fe}$) in the core of a very massive star. Iron is the heaviest element that can be formed through fusion processes that release energy; attempting to fuse iron requires an input of energy rather than producing it. This fundamental thermodynamic barrier means that once the core is predominantly iron, the star instantaneously loses its central energy source, leading to immediate gravitational collapse because the thermal pressure supporting the star's massive outer layers vanishes.
