Why does fusion halt when the core becomes dominated by Iron ($ ext{Fe}$)?

Fusing iron consumes energy instead of releasing it

Iron ($ ext{Fe}$, atomic number 26) represents the thermodynamic endpoint for energy generation via nuclear fusion in stellar cores. Iron possesses the most tightly bound nucleus among all elements, meaning it sits at the peak of the nuclear binding energy curve. Fusion reactions involving elements lighter than iron are exothermic, releasing the energy necessary to support the star against gravitational collapse. However, attempting to fuse iron nuclei together requires an input of energy rather than releasing it; this endothermic reaction means the core loses its primary source of outward pressure, leading to immediate, catastrophic collapse because there is no remaining thermonuclear mechanism to counteract the overwhelming gravitational force.