What fundamental characteristic defines the Iron (Fe) core boundary regarding energy production in stellar nucleosynthesis?
Answer
Fusing iron requires an input of energy (endothermic)
The formation of an iron or nickel core marks the absolute end of core-driven stellar energy generation via fusion. Atomic nuclei up to iron are arranged such that fusing them together releases energy, an exothermic process that generates the outward thermal pressure necessary to counteract the crushing force of gravity, thus stabilizing the star. Iron-56 is the most stable atomic nucleus known. Therefore, attempting to fuse iron or any element heavier than iron requires an external input of energy (endothermic reaction) rather than releasing it, causing the star to instantly lose its internal energy source and leading to gravitational collapse.
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