Stellar Evolution
Asymptotic Giant Branch
Late stellar phase — helium and hydrogen shell burning, intense mass loss, dredge-ups
The asymptotic giant branch (AGB) is the late evolutionary phase of low-to-intermediate mass stars (1-8 M_sun). After helium core burning ends, star has both helium and hydrogen shells burning. Star contracts and expands repeatedly — thermal pulses. Intense mass loss via stellar wind ("superwind"). Convective dredge-ups bring fusion products to surface. Ends with planetary nebula formation. Sun's future for ~few hundred Myr after red giant branch. AGB stars contribute ~30% of galactic CNO elements.
- Mass range1-8 M_sun progenitor
- Lifetime~10⁵-10⁶ years (relatively brief)
- BurningHydrogen + helium shells (alternating)
- Mass loss~10⁻⁵ M_sun/yr (very high)
- Surface T~3000-3500 K (cool)
- Dredge-upsBring C, N, O to surface
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Why AGB matters
- Late stellar evolution. Critical phase.
- Element distribution. Major source of CNO.
- Mass loss. Drives ISM enrichment.
- S-process. Heavy element production.
- Sun's future. Sun's fate in ~5 Gyr.
- Variable stars. Mira-type pulsations.
- Galactic chemistry. Significant nucleosynthesis.
Common misconceptions
- AGB stable. Highly variable; thermal pulses.
- AGB long-lived. ~10⁶ years (relatively brief).
- AGB stars are massive. Up to ~8 M_sun progenitor.
- AGB doesn't contribute elements. ~30% of CNO production.
- AGB just one phase. Multiple sub-phases.
- AGB doesn't pulsate. Mira-type pulsations standard.
Frequently asked questions
What's the asymptotic giant branch?
Late phase for low/intermediate mass stars. After helium core depletion, star burns hydrogen and helium in shells. Two shell-burning regions: H-shell, He-shell. Star expands again — re-becomes a red giant. Path on HR diagram approaches "asymptotically" the original red giant branch.
What's a thermal pulse?
He-shell ignites suddenly; releases energy fast. Star expands and cools. H-shell extinguishes. After He-shell exhausts, H-shell reignites. Cycle: typical period 10⁴-10⁵ years. ~10-100 thermal pulses during AGB lifetime. Each pulse is a "thermal pulse" (TP).
What's "dredge-up"?
During or after thermal pulse, convective zone in star reaches deep into burning regions. Brings products of fusion to surface. First dredge-up: brings He and N to surface during H-shell burning (early). Second: post-He core. Third: during AGB thermal pulses, brings C from triple-alpha and s-process elements.
What about mass loss?
AGB has extreme mass loss. Late AGB: 10⁻⁵ to 10⁻⁴ M_sun/yr. "Superwind" — dust-driven wind. Loses outer envelope. Some stars lose >50% of mass in this phase. Sets stage for planetary nebula formation. Major contributor to galactic gas and dust.
What's a Mira variable?
Long-period pulsating variable star. Most are AGB stars. Period: 80-1000 days. Brightness varies by 1-10 magnitudes. Cool, expanded outer atmosphere. Pulsations driven by hydrogen ionization. Mira itself: prototype, period 332 days, brightness varies by ~7 magnitudes.
What about s-process?
Slow neutron capture nucleosynthesis. Occurs in AGB stars during thermal pulses. Neutrons captured by Fe nuclei one at a time, with time to decay between captures. Produces stable heavy isotopes. Examples: Ba, Pb, several others. Different from r-process (rapid). Both contribute to heavy elements.
How does AGB end?
Mass loss removes outer envelope. Hot core (white dwarf) exposed. Surrounding ejected material becomes planetary nebula. Brief planetary nebula phase (~10⁵ yr). Then white dwarf cools forever. Sun's eventual fate.