Which element represents the end point for energy-releasing fusion in a massive stellar core?
Answer
Iron
The entire process of stellar nucleosynthesis, where lighter elements are built up to sustain the star against gravity, reaches an abrupt dead end when iron is formed from the fusion of silicon in the core. Iron occupies a unique position as the most stable atomic nucleus. Fusing elements lighter than iron releases net energy, which powers the outward thermal pressure supporting the star. However, fusing iron requires an input of energy rather than releasing it; it is an endothermic reaction. Once the core is saturated with iron, the central energy generation ceases instantaneously, removing the crucial outward pressure that gravity relies upon overcoming.
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