Звёзды асимптотической ветви гигантов: структура и эволюция.

Stars powered by fusion of hydrogen and helium in shell with an inactive core of carbon and oxygen

The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars (about 0.5 to 8 solar masses) late in their lives. Observationally, an asymptotic giant branch star will appear as a bright red giant with a luminosity ranging up to thousands of times greater than the Sun. Its interior structure is characterized by a central and largely 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 (except in the case of carbon stars).

When a star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star becomes a red giant, following a track towards the upper right hand corner of the HR diagram. Eventually, once the temperature in the core has reached approximately 3, helium burning (fusion of helium nuclei) begins. The onset of helium burning in the core halts the star's cooling and increase in luminosity, and the star instead moves down and leftwards in the HR diagram. This is the horizontal branch (for population II stars) or a blue loop for stars more massive than about .

The AGB phase is divided into two parts, the early AGB (E AGB) and the thermally pulsing AGB (TP AGB). During the E AGB phase, the main source of energy is helium fusion in a shell around a core consisting mostly of carbon and oxygen. During this phase, the star swells up to giant proportions to become a red giant again. The star's radius may become as large as one astronomical unit
During the thermal pulses, which last only a few hundred years, material from the core region may be mixed into the outer layers, changing the surface composition, in a process referred to as dredge up. Because of this dredge up, AGB stars may show S process elements in their spectra and strong dredge ups can lead to the formation of carbon stars. All dredge ups following thermal pulses are referred to as third dredge ups, after the first dredge up, which occurs on the red giant branch, and the second dredge up, which occurs during the E AGB. In some cases there may not be a second dredge up but dredge ups following thermal pulses will still be called a third dredge up. Thermal pulses increase rapidly in strength after the first few, so third dredge ups are generally the deepest and most likely to circulate core material to the surface. AGB stars are typically long period variables, and suffer mass loss in the form of a stellar wind. For M type AGB stars, the stellar winds are most efficiently driven by micron sized grains. Thermal pulses produce periods of even higher mass loss and may result in detached shells of circumstellar material. A star may lose 50 to 70% of its mass during the AGB phase. The mass loss rates typically range between 10−8 to 10−5 M⊙ year−1, and can even reach as high as 10−4 M⊙ year−1.

As many as a quarter of all post AGB stars undergo what is dubbed a "born again" episode. The carbon–oxygen core is now surrounded by helium with an outer shell of hydrogen. If the helium is re ignited a thermal pulse occurs and the star quickly returns to the AGB, becoming a helium burning, hydrogen deficient stellar object. The outer atmosphere of the born again star develops a stellar wind and the star once more follows an evolutionary track across the Hertzsprung–Russell diagram. However, this phase is very brief, lasting only about 200 years before the star again heads toward the white dwarf stage. Observationally, this late thermal pulse phase appears almost identical to a Wolf–Rayet star in the midst of its own planetary nebula. Stars such as Sakurai's Object and FG Sagittae are being observed as they rapidly evolve through this phase. Mapping the circumstellar magnetic fields of thermal pulsating (TP ) AGB stars has recently been reported using the so called Goldreich Kylafis effect.

Stars close to the upper mass limit to still qualify as AGB stars show some peculiar properties and have been dubbed super AGB stars. They have masses above and up to 9 or (or more). They represent a transition to the more massive supergiant stars that undergo full fusion of elements heavier than helium. During the triple alpha process, some elements heavier than carbon are also produced: mostly oxygen, but also some magnesium, neon, and even heavier elements. Super AGB stars develop partially degenerate carbon–oxygen cores that are large enough to ignite carbon in a flash analogous to the earlier helium flash. The second dredge up is very strong in this mass range and that keeps the core size below the level required for burning of neon as occurs in higher mass supergiants. The size of the thermal pulses and third dredge ups are reduced compared to lower mass stars, while the frequency of the thermal pulses increases dramatically. Some super AGB stars may explode as an electron capture supernova, but most will end as oxygen–neon white dwarfs. Since these stars are much more common than higher mass supergiants, they could form a high proportion of observed supernovae. Detecting examples of these supernovae would provide valuable confirmation of models that are highly dependent on assumptions.