Why does the formation of an iron core halt the star's ability to generate outward pressure through standard fusion?

Answer

Fusing iron consumes energy rather than releasing it

Iron is uniquely stable in this context because fusing iron atoms together requires an input of energy (is endothermic) rather than releasing energy, causing the star to lose the internal pressure supporting its mass.

Why does the formation of an iron core halt the star's ability to generate outward pressure through standard fusion?
Elementstarsynthesissuper giant