The Asymptotic Giant Branch

The onset of asymptotic giant branch (AGB) evolution coincides with the end of the core helium source on the horizontal branch. AGB stars are powered by fusion in an inner helium and an outer hydrogen shell source. The AGB evolution is from point 14 onward in the following calculation for the evolution of a 5 solar mass star after the main sequence.

Helium Shell Flashes
The triple-alpha process is extremely sensitive to fluctuations in the temperature (much more so than even the CNO cycle). In the AGB phase, the helium burning shell becomes unstable to a series of explosive pulses called helium shell flashes because of small temperature fluctuations and the very high pressure. These pulses cause fluctuations in the intensity of emitted radiation and this disturbs the outer layers of the star (and may play a role in the large mass loss associated with the AGB phase).

Two Shell Sources

In some respects, the evolution is similar to that after the core hydrogen was depleted and hydrogen shell burning began, but now there are two shell sources and the corresponding increases in luminosity and radius take the star to higher luminosities than when it was on the earlier red giant branch.

The End of Core Burning

If the star is sufficiently massive, it may ignite more advanced core burning. But the next stage of burning would require a core temperature of about 600 million K. For the 5 solar mass example discussed here, attaining this temperature is unlikely and subsequent evolution will involve only shell burnings. At this stage, the core is sufficiently hot and the photon opacity sufficiently high that cooling by emission of neutrinos begins to be the main way to get energy out of the core. As we shall discuss further in the next two modules, the AGB phase typically corresponds to rapid mass loss from the envelope, leading to a planetary nebula from the ejected envelope and a white dwarf from the core left behind.