Why does nuclear fusion stop abruptly when a massive star's core produces iron?
Answer
Fusing iron consumes energy rather than releasing it.
The process of nuclear fusion continues sequentially, building up heavier elements until the core is composed entirely of iron. Iron represents the terminal element in this chain because it possesses the most stable nucleus among all elements. Unlike the fusion of lighter elements (like hydrogen into helium or helium into carbon), fusing iron nuclei does not release the necessary thermal energy required to counteract the immense, relentless inward crush of gravity. Instead, attempting to fuse iron requires an input of energy, meaning the core loses its primary source of outward pressure support, leading inevitably to catastrophic collapse.
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