3.2 Red Giants and Asymptotic Giant Branch Stars
4 min read•august 9, 2024
Red giants and AGB stars mark crucial late stages in stellar evolution. These phases see stars dramatically expand, with cores contracting and outer layers swelling. Fusion moves to shells around the core, causing wild changes in size, brightness, and composition.
During these stages, stars experience thermal pulses and dredge-up events, mixing newly-made elements to their surfaces. This alters their chemistry, creating stars and other exotic types. Massive stellar winds shed outer layers, shaping the star's final fate.
Red Giant and AGB Evolution
Red Giant Branch and AGB Stages
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- branch marks the beginning of a star's late evolutionary stages
- Stars expand dramatically, increasing in size by up to 100 times their original radius
- Core hydrogen exhaustion triggers helium and shell hydrogen burning
- Asymptotic giant branch (AGB) phase follows the horizontal branch stage
- AGB stars have inert carbon- cores surrounded by helium and hydrogen burning shells
- Luminosity increases significantly during AGB phase, reaching up to 10,000 times that of the Sun
Thermal Pulses and Dredge-up Events
- Thermal pulses occur in AGB stars due to unstable helium
- Pulses cause periodic expansion and contraction of the star's outer layers
- Helium shell ignites explosively, leading to rapid energy release and convection
- Dredge-up events bring newly synthesized elements from the core to the surface
- First dredge-up occurs during red giant phase, mixing CNO-processed material
- Second and third dredge-ups happen during AGB phase, enriching the surface with carbon and
- Dredge-ups significantly alter the star's chemical composition and spectral characteristics
Evolution Timescales and Stellar Lifetimes
- Red giant phase lasts approximately 1 billion years for a solar-mass star
- AGB phase is relatively short, typically lasting a few million years
- Thermal pulse cycles occur every 10,000 to 100,000 years during AGB phase
- Total stellar lifetime depends on initial mass (larger stars evolve faster)
- Sun-like stars spend about 10% of their lives as red giants and AGBs
- Massive stars () may skip the AGB phase entirely, proceeding directly to supernova
AGB Star Composition
Carbon Stars and Their Characteristics
- Carbon stars form when carbon abundance exceeds oxygen at the stellar surface
- C/O ratio due to third dredge-up events bringing carbon-rich material to surface
- Exhibit strong molecular absorption bands of carbon compounds (CN, C2, CH)
- Appear distinctly red due to carbon dust in their atmospheres
- Examples include R Coronae Borealis variables and carbon Miras (TX Piscium)
- Carbon stars are important sources of carbon dust in the interstellar medium
S-type Stars and Transitional Objects
- S-type stars represent an intermediate stage between oxygen-rich and carbon-rich AGB stars
- C/O ratio close to unity, typically between 0.95 and 1.0
- Characterized by strong zirconium oxide (ZrO) bands in their spectra
- Enhanced s-process element abundances (strontium, yttrium, zirconium)
- MS and SC stars serve as transitional objects between M-type and carbon stars
- Notable examples include χ Cygni and R Andromedae
Chemical Evolution and Nucleosynthesis in AGB Stars
- AGB stars are primary sites for s-process nucleosynthesis
- Produce approximately half of all elements heavier than iron in the universe
- Neutron capture reactions occur during thermal pulses, creating heavy elements
- Lithium production through hot bottom burning in more massive AGB stars
- Isotopic ratios (12C/13C, 14N/15N) provide insights into AGB nucleosynthesis
- AGB stars contribute significantly to galactic chemical evolution
Mass Loss Mechanisms
Stellar Winds and Their Driving Forces
- Mass loss rates in AGB stars can reach up to per year
- Radiation pressure on dust grains drives strong stellar winds
- Pulsations and convection contribute to mass loss by lifting material into the outer atmosphere
- Cool, extended atmospheres of AGB stars facilitate dust formation and wind acceleration
- Mass loss increases dramatically during final AGB stages, leading to formation
- Stellar wind velocities typically range from 5 to 30 km/s in AGB stars
Mira Variables and Pulsation-driven Mass Loss
- Mira variables are long-period pulsating AGB stars
- Pulsation periods range from 100 to 1000 days
- Large-amplitude variations in visual magnitude (up to 8 magnitudes)
- Pulsations enhance mass loss by creating shock waves and extending the atmosphere
- Examples include Mira (ο Ceti) and R Leonis
- Mira variables often exhibit OH maser emission, indicating strong mass loss
Consequences of Mass Loss on Stellar Evolution
- Mass loss determines the final fate of AGB stars (white dwarf vs. supernova)
- Influences the initial-final mass relation for white dwarfs
- Affects the chemical composition of the interstellar medium
- Contributes to the formation of circumstellar envelopes and planetary nebulae
- Modifies stellar rotation rates and magnetic fields
- Mass loss history can be traced through observations of circumstellar shells and detached envelopes (AFGL 3068)
Key Terms to Review (19)
Asymptotic Giant Branch Star: An asymptotic giant branch star is a late stage in the evolution of a star that has exhausted the hydrogen and helium fuel in its core and is now undergoing helium fusion in a shell surrounding the core. During this phase, these stars expand significantly and exhibit high luminosity, becoming some of the brightest stars in the universe. They represent a critical transition in stellar evolution, leading to eventual shedding of outer layers and the formation of planetary nebulae.
C. I. Lewis: C. I. Lewis was an influential American philosopher known for his work in pragmatism and logic, particularly during the 20th century. He emphasized the importance of the interplay between logic and empirical science, which has implications in various fields, including astrophysics as it relates to the understanding of stellar phenomena such as red giants and asymptotic giant branch stars.
Carbon: Carbon is a chemical element with the symbol C and atomic number 6, serving as a fundamental building block of life and the universe. It plays a crucial role in stellar nucleosynthesis, where it is produced in the cores of stars during fusion processes. As stars evolve into red giants and undergo significant changes, carbon becomes essential in the formation of heavier elements and is integral to the life cycles of stars.
Core contraction: Core contraction refers to the process where the central region of a star, particularly during its later evolutionary stages, decreases in size and increases in temperature due to gravitational collapse. This phenomenon occurs in red giants and asymptotic giant branch stars as they exhaust their nuclear fuel, leading to changes in energy generation and the subsequent development of a new burning shell around the core.
Expanded outer envelope: The expanded outer envelope refers to the outer layers of a star that have significantly increased in size and volume as a result of nuclear fusion processes and changes in the star's life cycle. This phenomenon is particularly relevant for red giants and asymptotic giant branch stars, as they exhibit dramatic expansions due to helium burning and the exhaustion of hydrogen in their cores.
Helium burning: Helium burning refers to the process in which helium nuclei, or alpha particles, fuse to form heavier elements, primarily carbon and oxygen, through nuclear fusion. This phase occurs in the later stages of stellar evolution, particularly in red giants and asymptotic giant branch stars, and is a crucial step in stellar nucleosynthesis as stars evolve beyond hydrogen burning.
Henrietta Leavitt: Henrietta Leavitt was an American astronomer whose work in the early 20th century focused on variable stars, particularly Cepheid variables. Her groundbreaking discoveries laid the foundation for understanding the structure of the Milky Way and the measurement of astronomical distances, connecting her work to various stellar phenomena and cosmic scale measurements.
Hertzsprung-Russell Diagram: The Hertzsprung-Russell Diagram is a scatter plot that shows the relationship between the absolute magnitude (or luminosity) of stars versus their stellar classifications (or temperatures). This diagram is crucial for understanding stellar evolution, illustrating how different types of stars, such as red giants and main-sequence stars, occupy specific regions based on their properties, making it an essential tool in the study of cosmic distances and standard candles.
Increased Luminosity: Increased luminosity refers to the greater brightness or energy output of a star compared to its previous state, often observed during specific stages of stellar evolution. This phenomenon is especially significant in red giants and asymptotic giant branch stars, where changes in nuclear fusion processes lead to a dramatic rise in a star's overall brightness, making them some of the most luminous objects in the universe.
Nuclear fusion: Nuclear fusion is the process by which two light atomic nuclei combine to form a heavier nucleus, releasing a tremendous amount of energy in the process. This reaction is the primary source of energy for stars, where hydrogen nuclei fuse to create helium, and it plays a vital role in stellar evolution, particularly during specific stages such as the red giant phase and in various nucleosynthesis processes.
Oxygen: Oxygen is a chemical element with the symbol O and atomic number 8, essential for life as it plays a crucial role in processes such as respiration and combustion. In stellar contexts, oxygen is produced during the burning phases of stars and is a significant element in the nucleosynthesis processes that occur in their cores. As stars evolve, they generate heavier elements through fusion, and oxygen becomes a key byproduct that influences later stages of stellar evolution.
Planetary nebula: A planetary nebula is a luminous shell of ionized gas ejected from red giant stars at the end of their evolution, marking a transitional phase between the dying star and the formation of a white dwarf. As these stars exhaust their nuclear fuel, they expand and shed their outer layers, which then illuminate through ultraviolet radiation from the remaining hot core. This process is crucial in recycling materials back into the interstellar medium, contributing to the formation of new stars and planets.
Red Giant: A red giant is a late-stage star that has expanded and cooled after exhausting the hydrogen fuel in its core, resulting in a characteristic reddish appearance. These stars are significant in the life cycle of stars as they mark the transition from the main sequence phase to the more advanced stages of stellar evolution, leading to phenomena such as planetary nebulae or supernovae, depending on their initial mass.
S-process elements: s-process elements are a group of chemical elements that are produced through slow neutron capture during the nucleosynthesis processes in stars, particularly in red giants and asymptotic giant branch stars. These elements include many of the heavy elements found on the periodic table, such as silver, gold, and lead. The s-process occurs in environments where neutrons are available in moderate quantities, allowing for a gradual accumulation of neutrons by atomic nuclei.
Shell burning: Shell burning refers to the process of nuclear fusion occurring in a shell surrounding the core of a star, particularly during its late evolutionary stages. This process occurs when the core has exhausted its nuclear fuel and the surrounding layers begin fusing lighter elements into heavier ones. Shell burning plays a crucial role in the evolution of stars, especially as they transition from the main sequence to red giants and asymptotic giant branch stars.
Stellar evolution tracks: Stellar evolution tracks are graphical representations that illustrate the life cycle of a star on the Hertzsprung-Russell diagram as it evolves through different stages of its existence. These tracks show how a star's temperature and luminosity change over time, providing insight into the star's physical properties during various evolutionary phases, including the critical transitions into red giant and asymptotic giant branch stages.
Stellar nucleosynthesis: Stellar nucleosynthesis is the process by which elements are created within stars through nuclear fusion reactions. This process not only produces new elements but also influences the composition of stars and the interstellar medium, playing a key role in the evolution of galaxies and the universe as a whole.
Supernova progenitor: A supernova progenitor is a massive star that is in the final stages of its life cycle before exploding as a supernova. These stars undergo significant changes as they exhaust their nuclear fuel, leading to their eventual collapse under gravity and subsequent explosion. Understanding supernova progenitors is crucial for exploring the different types of supernovae and their role in cosmic evolution.
Thermal pulsing: Thermal pulsing refers to a process occurring in asymptotic giant branch (AGB) stars, characterized by periodic increases in temperature and luminosity due to helium shell flashes. These events happen when the star experiences unstable nuclear fusion in its outer layers, causing significant changes in its structure and energy output. This pulsation is essential for understanding stellar evolution, especially in the later stages of a star's life cycle, as it contributes to mass loss and the chemical enrichment of the interstellar medium.