The Triple Alpha Process is a Fusion reaction in stars turning Helium into Carbon. This happens in older stars with abundant helium when the core collapses to produce sufficient heat. Helium fuses into Beryllium-8 which then fuses with another helium atom to produce carbon.
4 4 8 He + He -> Be (-93.7keV) 2 2 4 8 4 12 Be + He -> C (+7.367 MeV) 4 2 6
At about 100,000,000K, helium fuses faster than beryllium-8 decays and this process progresses. After this, some Carbon and Helium can fuse into Oxygen.
Horizontal Branch (HB)