Asymptotic Giant Branch

After the horizontal-branch phase, low-mass stars enter the Asymptotic Giant Branch (AGB). The star now has layered shells — a carbon-oxygen core, a helium-burning shell, and a hydrogen-burning shell. These shells ignite in alternating bursts called thermal pulses, making the star's brightness swing wildly.

Each pulse dredges up elements from deep inside and carries them to the surface, enriching the star with carbon, nitrogen, and heavy elements like barium. This material is later cast into space, seeding the gas between stars with the building blocks for new stars and planets.