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.