Asymptotic Giant Branch (AGB)

Definition

The Asymptotic Giant Branch (AGB) is a late stage in the evolution of stars that are typically between 0.6 to 10 solar masses, characterized by a significant increase in size and luminosity. During this phase, stars undergo thermal pulses and heavy mass loss, leading to the formation of planetary nebulae as they transition to the end of their life cycles. The AGB phase is crucial as it contributes to the chemical enrichment of the interstellar medium through the ejection of enriched material.

5 Must Know Facts For Your Next Test

1. The AGB phase lasts for a relatively short time in cosmic terms, typically around a few hundred thousand years.
2. Stars on the AGB lose a significant amount of their mass, sometimes up to 50% or more, mainly through stellar winds.
3. The outer layers of an AGB star expand dramatically, causing it to become one of the largest types of stars in terms of volume.
4. Asymptotic Giant Branch stars are often surrounded by a circumstellar envelope composed of dust and gas created from their intense mass loss.
5. The chemical processes during AGB can create heavier elements, such as carbon and nitrogen, which are later released into space when the star sheds its outer layers.

Review Questions

• What are thermal pulses and how do they affect the evolution of a star during the AGB phase?
  • Thermal pulses are explosive events that occur in AGB stars when helium fusion resumes in shells surrounding the core. These pulses lead to dramatic changes in luminosity and can cause the star to expand and contract periodically. This process not only affects the physical structure of the star but also triggers significant mass loss as outer layers are expelled into space, ultimately contributing to planetary nebula formation.
• Discuss how the mass loss in AGB stars contributes to the formation of planetary nebulae and affects interstellar chemistry.
  • Mass loss in AGB stars occurs through strong stellar winds that expel the outer layers of the star. This process is essential for forming planetary nebulae, which emerge when these outer layers are ejected at the end of a star's life. The ejected material is enriched with elements created during stellar nucleosynthesis and contributes to interstellar chemistry by recycling these elements back into the galaxy, aiding in the formation of new stars and planets.
• Evaluate the significance of AGB stars in cosmic evolution and how their lifecycle influences future stellar generations.
  • AGB stars play a pivotal role in cosmic evolution as they not only enrich the interstellar medium with heavy elements but also influence star formation processes. Their lifecycle serves as a bridge between stellar evolution stages, providing necessary materials for future stellar generations. By returning processed materials back into space, AGB stars help create an enriched environment for subsequent stars, affecting their chemical compositions and potentially altering planetary system development across galaxies.

Related terms

Thermal Pulses:

Short bursts of energy during the AGB phase that occur due to helium shell burning, resulting in variations in luminosity and expansion of the star.

Planetary Nebula: A luminous shell of gas ejected from a red giant star late in its life cycle, often resulting from the AGB phase, where the outer layers are expelled into space.

Stellar Nucleosynthesis: The process by which elements are formed within stars through nuclear reactions, significantly contributing to the chemical composition of the universe.