Hydrogen Shell Burning

The period of time after the main sequence is marked by episodes of shell burning in a star, where nuclear fuel is being burned in concentric shells around the core. Such shell burnings are initiated when contraction of the core generates increased temperature in the center of the star so that regions further out than before become hot enough to initiate fusion reactions.

The adjacent figure illustrates the conditions necessary for hydrogen shell burning. The hydrogen is depleted in the center, so only at a large enough radius is much hydrogen available. The temperature decreases as we move away from the center, so only at a small enough radius will the temperature be high enough to fuse hydrogen (the critical temperature marked on the diagram). Only in the narrow band marked in orange are the two conditions for hydrogen fusion (presence of hydrogen and high enough temperature) fulfilled. So hydrogen fusion is restricted to this narrow region, which corresponds to a shell surrounding the core. This shell is termed a hydrogen shell source.

Animation: Shell Burning