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
- 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
- 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.
Tue Feb 17 13:32:05 PST 1998