asymptotic giant branch (AGB)

A region in the Hertzsprung-Russell diagram delineated by points representing stars that initially had masses no greater than ten times the Sun's, and are currently in a late phase of their evolution. For a second time since they ceased to be main-sequence stars, vast expansion has resulted in a combination of high luminosity and low temperature that places them among the coolest of the red giant stars. All stars of under ten solar masses pass through this phase of stellar evolution. AGB stars have exhausted both hydrogen and helium as nuclear fuels in their cores. The core has become a very hot white dwarf composed of carbon and oxygen. A thin shell of helium and carbon overlying the core undergoes recurrent helium "flash" burning. Between flash episodes, hydrogen burning continues in a shell layer further out. The whole is surrounded by a greatly extended envelope formed from the star's outer layers, with a radius of between a hundred and a thousand times that of the. The internal structure is unstable and these stars form the long-period variables. Ultimately, the envelope separates from the core and is ejected to form a planetary nebula. A star initially of ten solar masses ejects all but 1.4 solar masses in this way.