Horizontal Branch Stars and Helium Fusion

There is a problem with He He Be 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 seconds. So, 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.

• Horizontal Branch Stars have a structure that is analogous to main-sequence stars.

  

• The fusion rates are running fast and the structure of the HB stars changes relatively quickly. The carbon core starts to shrink and release energy, the He fusion moves into a shell (now we have a ``double shell" source) and this starts to look like the situation when a main-sequence star became a Red Giant.

• This time, the star ascends the Asymptotic Giant Branch.

• For a 1 Star, it never gets hot enough to fuse carbon together. Now the star is running out of fuel for good

• The triple-alpha reaction is extremely sensitive to temperature: Triple-Alpha rate which leads to instabilities called Helium shell flashes.