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 fewer number of atoms left over thus decreasing the pressure), and the core begins to collapse in on itself. The collapse quickly increases the temperature, pressure, and density in the core. IF the star is massive enough to produce sufficient pressure, Helium will start burning. The heat released from the reaction re-expands the core and is sufficient to increase the temperature in the core to the point where helium burning can be sustained. However, only a certain portion of the Helium will be burnable (an even smaller region than that for the burnable hydrogen), and once that it burnt up, if the star is massive enough, the process will repeat itself with successively higher elements.
Due to the violence of each hiccup the star will lose some of its outer hydrogen envelope. We've actually seen stars surrounded by several expanding shells of gas which correspond to each hiccup.
As for iron, the fusion burning of lighter elements does not produce not nearly enough energy to start fusing iron in any appreciable quantities. For that you need a much more energetic event such as a supernova explosion. The difference between the energies involved in fusion and a supernova is several orders of magnitude.
This page was last updated June 27, 2015.