Asymptotic giant branch

Some stars grow very big.

They turn bright red. These stars are very large. They can be thousands of times brighter than our Sun.

Inside the star, different layers make energy. One layer burns helium. Another layer burns hydrogen. This helps the star stay bright.

Sometimes the star has a big flash. This makes the star grow even more. The star can also lose much of its weight. It blows out gas and dust.

This dust stays around the star. It makes a big cloud.

This cloud can eventually become a planetary nebula.

These stars are a special part of a star's life. They are very interesting to see.

Some stars go through a special stage late in life. This is called the asymptotic giant branch, or AGB.

Stars in this stage are very big. They look like bright red giants. They can be thousands of times brighter than our Sun.

Inside these stars, many things happen at once. The center has a core of carbon and oxygen. Around this core, two shells make power. One shell burns helium to make carbon. Another shell burns hydrogen to make helium.

This stage has two parts. In the first part, the star swells up. It can grow as large as one astronomical unit.

In the second part, the star has pulses. Helium builds up in a thin layer. Then, it ignites in a helium shell flash. This is a sudden, bright burst of energy. These pulses can mix core material to the surface. This is called a dredge-up.

These stars also lose much of their mass. They blow out gas and dust in a stellar wind. This dust can form a big cloud. This cloud may become a planetary nebula.

Some stars go through a very bright and busy stage late in their lives. This stage is called the asymptotic giant branch, or AGB.

It happens to stars with a mass between 0.5 and 8 times that of our Sun. These stars look like huge, glowing red giants in the sky. They are incredibly bright and can shine thousands of times more than the Sun. This period is a key part of how stars change over time. It marks a major transition before they reach their final stages.

Inside an AGB star, energy is made in a very specific way. The center has a core of carbon and oxygen that does not burn. Around this core, two different shells are working at once. One shell uses helium to make carbon, which is called helium burning. The other shell uses hydrogen to make helium, which is called hydrogen burning.

This structure allows the star to stay bright even as it changes. The star's outer layers can grow to be as large as one astronomical unit.

The AGB stage is divided into two main parts. First, the early AGB phase happens as the star swells up. Next comes the thermally pulsing AGB, or TP-AGB, phase. During this second part, helium builds up in a thin layer. Eventually, this helium ignites in a sudden burst called a helium shell flash.

This flash can be thousands of times brighter than the star's normal light. These pulses happen every 10,000 to 100,000 years.

These pulses do more than just change the brightness. They cause a process called a dredge-up. This happens when material from the deep core moves up to the surface.

This can change what the star is made of, sometimes creating carbon stars. The star also loses a lot of its weight through a stellar wind. This wind can blow away 50 to 70 percent of the star's mass. The wind moves at speeds between 5 and 30 kilometers per second.

You can see the results of this stage in the dust they create. The stellar winds are the main places where cosmic dust is made in the universe. This dust forms a large cloud around the star called a circumstellar envelope.

Eventually, the star loses its outer layers and leaves behind a core. This core can become a planetary nebula. These beautiful clouds show us the history of the star's life. They are a way for stars to share their material with space.

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.