What is "asymptotic giant branch"

n. (context astronomy English) The region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars.

Observationally, an asymptotic-giant-branch (AGB) star will appear as a bright red giant with a luminosity thousands of times greater than the Sun. Its interior structure is characterized by a central and inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning) and a very large envelope of material of composition similar to main-sequence stars.