Asymptotic giant branch

The asymptotic giant branch, or AGB, is a specific stage in stellar evolution. It occurs when stars with low to intermediate masses reach the end of their lives. These stars have masses between approximately 0.5 and 8 solar masses. During this phase, stars appear as extremely luminous red giants. They can shine thousands of times more brightly than our Sun. This stage is a vital part of how stars change and distribute material throughout the universe.

The internal structure of an AGB star is complex and layered. At the very center lies a largely inert core made of carbon and oxygen. Surrounding this core is a shell where helium undergoes fusion to form carbon, a process called helium burning. Outside that is another shell where hydrogen fuses into helium, known as hydrogen burning. The outermost part is a massive envelope of material. This envelope usually has a composition similar to main-sequence stars, though some become carbon stars.

To reach the AGB, a star must first exhaust its core hydrogen. This causes the core to contract and its temperature to rise. As a result, the outer layers expand and cool, turning the star into a red giant. Once the core reaches roughly 100 million Kelvin, helium burning begins. This causes the star to move down and leftward on the Hertzsprung–Russell diagram, entering the horizontal branch or a blue loop. After the core helium is exhausted, the star moves upward and to the right again. This path follows a track similar to its previous red-giant stage, which gives the AGB its name.

Astrophysicists divide the AGB phase into two distinct parts. The first is the early AGB, or E-AGB. During this stage, the primary energy source is helium fusion in a shell around the carbon-oxygen core. The star swells significantly, reaching radii as large as one astronomical unit. The second part is the thermally pulsing AGB, or TP-AGB. In this stage, the star derives energy from a thin hydrogen-burning shell. This thinness prevents the inner helium shell from fusing steadily, leading to instability.

Thermal pulses are a defining characteristic of the TP-AGB phase. Over periods of 10,000 to 100,000 years, helium builds up in a thin layer. Eventually, this helium ignites explosively in a helium shell flash. This flash can peak at thousands of times the star's observed luminosity. The flash causes the star to expand and cool, which temporarily shuts off hydrogen burning. As the helium burning nears the base of the hydrogen shell, the increased temperature reignites hydrogen fusion. This cycle repeats, creating a series of pulses.

These pulses trigger a process called dredge-up, which moves core material to the surface. This can change the star's surface chemistry, often creating S-type or carbon stars. There are different types of dredge-ups, including the first, second, and third dredge-ups. The third dredge-up follows thermal pulses and is often the deepest. During the AGB phase, stars also experience significant mass loss through stellar winds. A star may lose between 50% and 70% of its total mass. These winds move at velocities between 5 and 30 kilometers per second.

The lost material forms a circumstellar envelope, or CSE, around the star. These envelopes are major production sites for cosmic dust in the universe. The chemistry within the envelope changes as material moves away from the star. Near the star, the density is high, allowing for thermodynamic equilibrium. As the material expands and cools, the density drops, and kinetics become the dominant factor. In the outermost regions, ultraviolet radiation from interstellar space can ionize the gas. Eventually, the star loses its envelope entirely, leaving behind a core that becomes a planetary nebula.

Some stars exhibit even more unusual behavior during this time. Super-AGB stars are more massive, ranging from 8 to 9 solar masses or more. They may develop carbon-oxygen cores large enough to ignite carbon in a flash. Some of these stars might even explode as electron capture supernovae. Additionally, some post-AGB stars undergo a "born-again" episode. This happens if helium re-ignites, causing the star to return briefly to the AGB stage for about 200 years. These events, such as a late thermal pulse, show just how dynamic stellar death can be.