Why does the fusion process abruptly stop sustaining the star when the core material transforms into iron (Fe)?

Answer

Fusing iron consumes energy instead of releasing it.

Iron (Fe) occupies a unique and terminal position within the sequence of stellar nucleosynthesis because it represents the most tightly bound nucleus known in the universe. Consequently, any attempt to force iron nuclei to fuse together into even heavier elements requires a net input of external energy to drive the reaction forward; energy is absorbed rather than liberated. Since the star relies on the release of energy from fusion reactions to generate the outward thermal pressure needed to counteract gravity, reaching an iron core means the star has lost its central thermal energy generation mechanism, creating an immediate crisis.

Why does the fusion process abruptly stop sustaining the star when the core material transforms into iron (Fe)?

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