Post Main-Sequence Evolution: The Red Giant Branch

Horizontal Branch Stars and Helium Fusion

There is a problem with 4He2 + 4He2 implies 8Be4 because this reaction is endothermic. It does not release energy, but rather requires energy for it to happen.

So, He and He atoms DO fuse to Be sometimes, but the Be decays in ~10-12 seconds. So, you need to have the temperature and density be high enough so that before this decay happens another He collides with the Be to produce Carbon. This is called the Triple Alpha reaction and it is the energy source for horizontal branch stars.