Why will fusion wait until hydrogen is gone before starting to fuse heavier atoms?
Why will endothermic fusions wait until iron is everywhere? For the fusion of any element you have to have sufficiently high temperature. This high temperature comes along with sufficiently high density and pressure. Inside a star, only the core (innermost 10% or so of the star) has sufficient density and pressure to start fusing Hydrogen. When stars switch from fusing one element to another they have a sort of a hiccup. I’ll explain. As a star burns the hydrogen in its core, the Helium produced in the reaction sinks to the center because it is heavier. Over time you have a successfully larger Helium core with a hydrogen shell. Hydrogen burning is not energetic enough to start off the Helium burning on its own. Instead what happens is that once all the burnable Hydrogen (only the hydrogen within the innermost 10% or so of the star) is used up fusion temporarily ceases, the core cools and contracts (the contraction is primarily due to the fact that when you convert H to He you have a fe
