# At what point in a star's life are they classified as AGB?

Jun 5, 2016

When the temperature reaches $3 \cdot {10}^{8} K$, helium fusion starts and the star becomes brighter. When the helium is depleted the expands again into a red giant. Fusion takes place in shells of hydrogen and helium around the core. The core being mostly oxygen and carbon which don't fuse because the temperature is too low. This the the beginning of then AGB stage.