Question

Why is the formation of iron a limiting factor? What happens when the core of a sun becomes iron?

Answer 1

As soon as the helium in the core runs out, the star collapses again, heats up, and starts fusing carbon and oxygen into larger atoms. If the star is massive enough, this keeps happening until it on is fused. At which point a hotter core still won't lead to fusion. The star collapses, becomes unstable, and POW.