Why does the fusion process halt when the core becomes predominantly iron ($^{56} ext{Fe}$) or nickel?

Answer

Fusing iron requires a net input of energy rather than releasing it

The process of stellar nucleosynthesis, which generates the energy that supports the star against gravitational collapse, relies on exothermic reactions—reactions that release energy. Iron ($^{56} ext{Fe}$) and nickel occupy a unique position in nuclear physics because they possess the highest binding energy per nucleon among all elements. This means that attempting to fuse iron into even heavier elements consumes energy (it is an endothermic reaction) rather than releasing the thermal pressure needed to counteract the immense gravity. Once the core is saturated with iron, the star abruptly loses its primary energy source, leading to an inevitable, catastrophic gravitational collapse.

Why does the fusion process halt when the core becomes predominantly iron ($^{56} ext{Fe}$) or nickel?

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