🚀Astrophysics II Unit 2 – Stellar Interiors and Nucleosynthesis

Key Concepts and Definitions

• Stellar interiors the inner regions of stars where energy is generated through nuclear fusion reactions
• Nucleosynthesis the process by which elements heavier than hydrogen and helium are created inside stars
• Hydrostatic equilibrium the balance between the inward force of gravity and the outward pressure gradient in a star
• Convection and radiation two primary modes of energy transport within stars
• Opacity a measure of how easily photons can travel through stellar material
• Equation of state describes the relationship between pressure, temperature, and density in stellar interiors
• Nuclear burning stages distinct phases of a star's life characterized by the dominant nuclear fusion reactions (hydrogen burning, helium burning)

Stellar Structure and Composition

• Stars are composed primarily of hydrogen and helium with trace amounts of heavier elements
• Stellar structure is divided into several layers: core, radiative zone, and convective zone
  • Core the central region where nuclear fusion reactions occur and energy is generated
  • Radiative zone a region where energy is transported outward by radiation
  • Convective zone a region where energy is transported by convective motions
• Chemical composition and physical properties vary with depth inside a star
• Stellar models use equations of stellar structure to describe the internal conditions and evolution of stars
• Mass is the primary factor determining a star's structure, composition, and evolutionary path
• Stellar atmospheres the outermost layers of stars that emit the light we observe

Energy Generation in Stars

• Nuclear fusion reactions in the core are the primary source of energy in stars
• Proton-proton chain dominates energy production in low-mass stars like the Sun
  • Involves the fusion of hydrogen nuclei into helium through a series of reactions
  • Highly temperature-sensitive with a strong dependence on the central temperature
• CNO cycle an alternative hydrogen-burning process that dominates in high-mass stars
  • Catalytic cycle involving carbon, nitrogen, and oxygen nuclei
  • More temperature-sensitive than the proton-proton chain
• Energy generation rate determines the luminosity and lifetime of a star
• Neutrinos are produced during nuclear reactions and can escape the star, carrying away energy
• Mass-luminosity relation a general trend where more massive stars have higher luminosities

Nuclear Reactions and Fusion Processes

• Nuclear binding energy the energy required to disassemble a nucleus into its constituent protons and neutrons
• Fusion of light elements releases energy because the products have higher binding energy per nucleon
• Coulomb barrier the electrostatic repulsion between positively charged nuclei that must be overcome for fusion to occur
• Quantum tunneling allows nuclei to fuse at lower temperatures than classically required
• Triple-alpha process the fusion of three helium-4 nuclei to form carbon-12, occurring in later stages of stellar evolution
• Helium flash a sudden onset of helium fusion in the core of low-mass stars
• Advanced nuclear burning stages (carbon, neon, oxygen, silicon) occur in high-mass stars near the end of their lives

Stellar Evolution Stages

• Main sequence the longest stage of a star's life when it fuses hydrogen in its core
  • Stars spend ~90% of their lives on the main sequence
  • Main sequence lifetime depends on a star's mass and luminosity
• Red giant phase a post-main sequence stage characterized by an expanded, cooler outer envelope and a contracting, hotter core
• Horizontal branch a stage in the evolution of low-mass stars where helium fusion occurs in the core
• Asymptotic giant branch (AGB) a late stage of stellar evolution with alternating hydrogen and helium shell burning
• Planetary nebula a glowing shell of gas ejected by low- to intermediate-mass stars in the AGB phase
• White dwarf the final evolutionary stage of low- to intermediate-mass stars, supported by electron degeneracy pressure
• Supernova a powerful explosion marking the end of a high-mass star's life, triggered by core collapse or thermonuclear runaway

Nucleosynthesis and Element Formation

• Big Bang nucleosynthesis produced hydrogen, helium, and trace amounts of lithium in the early universe
• Stellar nucleosynthesis is responsible for the creation of elements heavier than lithium
  • Main sequence stars primarily fuse hydrogen into helium
  • Red giant stars fuse helium into carbon and oxygen
  • Advanced nuclear burning stages in high-mass stars create elements up to iron
• S-process (slow neutron capture) occurs in AGB stars and produces elements heavier than iron
• R-process (rapid neutron capture) occurs during supernovae and neutron star mergers, producing the heaviest elements
• Supernova nucleosynthesis creates and disperses a wide range of elements into the interstellar medium
• Chemical evolution the gradual enrichment of the universe with heavier elements over cosmic time

Observational Evidence and Techniques

• Stellar spectra provide information about the chemical composition, temperature, and velocity of stars
• Spectral lines absorption or emission features in stellar spectra that correspond to specific elements and ionization states
• Asteroseismology the study of stellar oscillations to probe the internal structure and properties of stars
• Neutrino detection can provide direct evidence of nuclear reactions in the Sun and other stars
• Stellar color and brightness measurements help determine the temperature, luminosity, and evolutionary stage of stars
• Hertzsprung-Russell (HR) diagram a plot of stellar luminosity versus temperature that reveals distinct evolutionary stages
• Stellar population studies investigate the age, composition, and formation history of stars in galaxies

Applications and Current Research

• Solar neutrino problem the discrepancy between predicted and observed solar neutrino fluxes, resolved by neutrino oscillations
• Galactic chemical evolution models aim to understand the enrichment history of galaxies and the universe
• Stellar forensics using the chemical composition of stars to trace their formation and evolutionary history
• Exoplanet host star characterization studying the properties of stars hosting planetary systems
• Stellar activity and its impact on habitability investigating how stellar flares, winds, and radiation affect the potential for life on exoplanets
• Gravitational wave astronomy detecting mergers of compact objects (neutron stars, black holes) that may be sites of r-process nucleosynthesis
• Multimessenger astronomy combining observations of electromagnetic radiation, neutrinos, and gravitational waves to study stellar phenomena