Question

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

Answer 1

Stars are classified as AGB when they become red giants.

Explanation:

The Asymptotic Giant Branch (AGB) of the Herzsprung-Russell diagram is where all medium sized stars between 0.6 and 10 solar masses when they enter the red giant stage for the second time.

When a star uses up its supply of hydrogen in its core, the core collapses and its temperature increases. This causes the star to expand into a red giant. The temperature of the outer areas of the star cool which causes the red colour.

When the temperature reaches `3*10^8K`, 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.