At what point in a star's life are they classified as AGB?
Stars are classified as AGB when they become red giants.
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