Helium asymptotic giant branch

Nichols R. H., Jr., Kehm K., Hohenberg C. M., Amari S., and Lewis R. S. (2003) Neon and helium in single interstellar Sic and graphite grains asymptotic giant branch, Woh-Rayet, supernova and nova sources. Geochim. Cosmochim. Acta (submitted). 

Fig. 9. A simplified view of the internal chemical structure of a star during the major phases of stellar evolution. The panels represent the main-sequence, through the giant-branch, helium-burning, asymptotic-giant branch and white dwarf phases. Filled circles and thick lines represent nuclear-burning regions. Not to scale...

Intermediate-mass stars with initial masses roughly between 3 and 8M ignite helium in a non-degenerate core, thus avoiding both the helium core flash and the core-collapse supernova fate. These stars will all evolve to the asymptotic-giant branch phase and subsequently to become CO white dwarfs. 

In low-mass stars with initial masses less than about 3 M , helium-ignition occurs in a degenerate core - usually when the helium-core mass has reached about 0.48 M . The star subsequently evolves as a horizontal-branch star (Sect. 14). With a sufficiently massive hydrogen envelope (the exact value depends strongly on metallicity), it will evolve up the asymptotic giant branch after core helium exhaustion. Otherwise it will evolve directly to the white-dwarf cooling track. 

Stars less massive than about 8 M0 will avoid the supernova fate. In the 3-8 Mq range, helium ignition occurs in a non-degenerate core, so is not explosive. Core helium burning is associated with a blueward loop through the Cepheid instability strip, after which the star develops a double shell structure and becomes an asymptotic giant branch star. 

Following core-helium exhaustion, there being no H-burning shell, the mass of the helium-rich intershell remains fixed. A few mild helium-shell ignitions may occur11, but the star is ultimately doomed not to reach the asymptotic giant branch. Instead it will contract directly to become a hybrid He/CO white dwarf. 

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