The formation of iron in a massive star's core acts as a sudden, lethal trigger that causes the star to collapse.

Massive stars fuse lighter elements into heavier ones like carbon and silicon. But iron is different. Fusing iron does not produce energy; it absorbs it. When the core turns to iron, the star's nuclear engine shuts down instantly. Without outward pressure, gravity wins in a fraction of a second. The entire star collapses in freefall, bouncing off the dense core and exploding.