Why is iron special in stellar evolution?

Answer 1

Energy is liberated through fusion reactions, producing heavier and heavier elements. There are two transient elements heavier than iron which are produced by standard stellar nucleosynthesis, but these are short lived and decay into lighter elements. Iron is the heaviest element forged in the heart of a star via standard stellar evolution.

All elements heavier than iron are the byproduct of a supernova, wherein atomic nuclei are smashed together with such force energy is consumed in the nuclear reaction. This is why there tends to be an abundance of stable isotopes as light as iron, but elements heavier than iron are much more rare. Lead is an exception to this general rule as it is the end product of a long radioisotope decay sequence.