What causes the gravitational collapse trigger in massive stars after fuel exhaustion?

Iron fusion consumes energy rather than releasing it, removing outward pressure.

The catastrophic gravitational collapse in a massive star is initiated when its core progresses through successive burning stages and eventually begins fusing elements up to iron. Iron fusion is fundamentally different from the preceding stages because it consumes energy instead of releasing it. This process eliminates the necessary outward thermal pressure that had been counteracting the immense inward crushing force of the star's own gravity. Without this thermal support, gravity instantly dominates, leading to the core's rapid inward collapse, which is the trigger point for the star's death sequence.