Asymptotic Giant Branch Star
A dying low-to-intermediate mass star in its final expansion phase, fusing elements in shells and producing heavy cosmic building blocks before shedding its layers to become a white dwarf.
An asymptotic giant branch star is a late-evolution phase of low-to-intermediate mass stars (0.8–8 solar masses) occurring after the helium-burning phase, where the star expands significantly, fuses hydrogen and helium in shells, and produces heavy elements before shedding its outer layers. This stage bridges the main sequence and white dwarf formation, playing a critical role in cosmic nucleosynthesis and supernova precursor activity.
What this means in real life
Betelgeuse, the red supergiant visible in Orion's shoulder, is nearing or in this phase—it has swollen so large that if placed at our Sun's position, its surface would extend past Mars's orbit.
What it isn’t
This is not a red giant; while both are bloated and cool, AGB stars are a distinct later phase with active shell burning and higher luminosity, occurring after the red giant phase ends.
Commonly misused online
People often use 'AGB star' interchangeably with 'red giant,' but AGB stars are specifically the final evolutionary stage before planetary nebula ejection, not simply any large, cool star.