The Asymptotic Giant Branch is the name given to a region of the Hertzsprung-Russell Diagram populated by evolving low to medium-mass stars. When a star exhausts the supply of hydrogen in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star's luminosity increases greatly, and it becomes a red giant, following a track leading into the upper-right hand corner of the HR diagram.

Eventually, once the temperature in the core has reached approximately 3x10⁸K, helium burning begins. The onset of helium burning in the core halts the star's cooling and increase in luminosity, and the star instead moves back towards the left hand side of the HR diagram. Its path is almost aligned with its previous red giant track, hence the name of Asymptotic Giant Branch. Stars at this stage of stellar evolution are known as AGB stars, and are important astrophysically as they create a lot of dust, and are also the precursors to planetary nebulae.