Why Does Iron Mark the Limit for Energy-Producing Fusion in Stars?

[ay2k]

The buildup of heavier elements in the nuclear fusion processes in stars is limited to elements below iron, since the fusion of iron would subtract energy rather than provide it



The fusion of two nuclei lighter than iron generally releases energy while the fusion of nuclei heavier than iron absorbs energy; vice-versa for the reverse process, nuclear fission.

These two statements semm contradictory. Which one is actually correct? If both are actually correct, can someone please explain that what is the point that I am missing?

 

[mathman]

They are not contradictory. The basic idea is that ratio of atomic mass to number of nucleons is at the minimum for iron.

The point is that fusion of elements lighter than iron releases energy, while fission of elements heavier than iron releases energy. For iron both processes require energy input.

 

[sir-pinski]

Both statements are actually correct and they are not contradictory. This is because the energy released or absorbed during fusion or fission reactions is determined by the concept of binding energy.

Binding energy is the amount of energy required to hold the nucleus of an atom together. It is the result of the strong nuclear force, which is the strongest force in nature and holds protons and neutrons together in the nucleus. The stronger the binding energy, the more stable the nucleus is.

In nuclear fusion, lighter elements combine to form heavier elements, releasing energy in the process. This is because the resulting nucleus has a higher binding energy than the individual nuclei that fused together. This is why fusion reactions up to iron release energy.

However, when heavier elements are fused, the resulting nucleus has a lower binding energy than the individual nuclei that fused together. This means that energy is actually required to hold the nucleus together, resulting in a net energy loss. This is why fusion reactions beyond iron absorb energy.

On the other hand, in nuclear fission, a heavy nucleus splits into smaller nuclei, releasing energy. This is because the resulting nuclei have a higher binding energy than the original nucleus. However, when lighter elements undergo fission, the resulting nuclei have a lower binding energy and energy is required to hold them together, resulting in a net energy loss.

Therefore, it is not a matter of one statement being correct and the other being incorrect, but rather understanding that the concept of binding energy plays a crucial role in determining the energy released or absorbed during nuclear fusion and fission reactions.