What structural configuration defines the Asymptotic Giant Branch (AGB) phase?
Answer
A helium-fusing shell over a carbon-oxygen core, beneath a hydrogen-fusing shell
After helium is exhausted, the core becomes inert carbon and oxygen. This triggers a second shell burning phase: a helium-fusing shell resides over this core, and beneath that lies the still-burning hydrogen shell, placing the star on the AGB.
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