2.4 Stellar Nucleosynthesis Processes

Stellar nucleosynthesis is the cosmic kitchen where elements are cooked up inside stars. From hydrogen fusion to the creation of heavy elements, these processes shape the chemical makeup of the universe. Understanding them is key to grasping how stars evolve and produce the building blocks of life.

This topic dives into the various ways stars forge elements. We'll explore hydrogen burning in low-mass stars, advanced burning stages in massive stars, and neutron capture processes that create heavy elements. These mechanisms explain the abundance of elements we observe in the universe today.

Hydrogen Burning Processes

Proton-Proton Chain Reaction

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• Initiates stellar nucleosynthesis in low-mass stars (0.08 to 1.5 solar masses)
• Consists of three main branches PP-I, PP-II, and PP-III
• PP-I branch dominates in stars like the Sun and cooler stars
  • Begins with two protons fusing to form deuterium
  • Releases a positron and an electron neutrino
  • Deuterium then captures another proton to form helium-3
  • Two helium-3 nuclei combine to produce helium-4 and release two protons
• PP-II and PP-III branches become more significant in hotter, more massive stars
  • Involve the production of beryllium-7 and lithium-7 as intermediate steps
• Energy release primarily in the form of gamma rays and kinetic energy of particles

CNO Cycle

• Dominates hydrogen burning in stars more massive than 1.3 solar masses
• Uses carbon, nitrogen, and oxygen as catalysts to convert hydrogen into helium
• Consists of two main cycles CNO-I and CNO-II
• CNO-I cycle steps
  • Carbon-12 captures a proton to form nitrogen-13
  • Nitrogen-13 beta decays to carbon-13
  • Carbon-13 captures another proton to form nitrogen-14
  • Nitrogen-14 captures a proton to form oxygen-15
  • Oxygen-15 beta decays to nitrogen-15
  • Nitrogen-15 captures a proton to form carbon-12 and helium-4
• CNO-II cycle becomes important at higher temperatures
  • Involves the production of oxygen-17 and fluorine-17 as intermediate steps
• More temperature-sensitive than the proton-proton chain
• Produces a different neutrino spectrum compared to the proton-proton chain

Advanced Burning Stages

Helium Burning

• Occurs when hydrogen fuel depletes in the core (core temperature ~100 million K)
• Triple-alpha process forms carbon-12 from three helium-4 nuclei
  • Two helium-4 nuclei fuse to form unstable beryllium-8
  • Beryllium-8 quickly captures another helium-4 to form carbon-12
• Alpha process continues beyond carbon-12
  • Carbon-12 captures helium-4 to form oxygen-16
  • Oxygen-16 can capture helium-4 to form neon-20
• Produces energy and synthesizes elements up to oxygen and neon
• Occurs in red giant stars and horizontal branch stars

Carbon and Oxygen Burning

• Carbon burning initiates at core temperatures ~600-900 million K
  • Two carbon-12 nuclei fuse to form various products (magnesium-24, sodium-23, neon-20)
  • Produces energy and synthesizes elements up to silicon and sulfur
• Oxygen burning begins at core temperatures ~1.5-2.6 billion K
  • Two oxygen-16 nuclei fuse to form various products (sulfur-32, phosphorus-31, silicon-28)
  • Synthesizes elements up to calcium and argon
• Both processes occur in massive stars (>8 solar masses) during late evolutionary stages
• Duration of these burning stages decreases as star mass increases

Silicon Burning

• Final stage of nuclear burning in massive stars (core temperature ~2.7-3.5 billion K)
• Silicon-28 undergoes photodisintegration, breaking into helium nuclei
• Helium nuclei recombine to form heavier elements through a complex network of reactions
• Produces iron-peak elements (chromium, manganese, iron, cobalt, nickel)
• Marks the end of exothermic nuclear fusion in stellar cores
• Leads to the formation of an iron core, triggering core-collapse supernova in massive stars

Neutron Capture Processes

Slow Neutron Capture (s-process)

• Occurs in low to intermediate-mass stars during asymptotic giant branch phase
• Neutron flux lower than r-process, allowing beta decay between neutron captures
• Produces about half of the elements heavier than iron
• Key steps in s-process
  • Seed nuclei (usually iron) capture neutrons slowly
  • Unstable isotopes have time to beta decay before capturing another neutron
  • Process continues along the valley of stability in the chart of nuclides
• Main neutron sources include C13(α,n)O16 and Ne22(α,n)Mg25 reactions
• Synthesizes elements like strontium, barium, and lead

Rapid Neutron Capture (r-process)

• Occurs in extreme environments with high neutron flux (supernovae, neutron star mergers)
• Neutron captures happen faster than beta decay
• Produces about half of the elements heavier than iron, including many rare earth elements
• Key steps in r-process
  • Seed nuclei rapidly capture many neutrons, forming very neutron-rich unstable isotopes
  • Neutron capture stops at the neutron drip line
  • Resulting nuclei undergo a series of beta decays to form stable isotopes
• Responsible for the production of uranium and thorium
• Site of r-process nucleosynthesis still debated (core-collapse supernovae, neutron star mergers)

Neutron Capture Dynamics

• Neutron capture cross-section varies with neutron energy and target nucleus
• s-process path determined by competition between neutron capture and beta decay rates
• r-process path influenced by nuclear shell structure and neutron separation energies
• Branching points occur where neutron capture and beta decay rates are comparable
• Neutron capture processes explain the observed abundance peaks in the solar system
• s-process peaks occur at neutron magic numbers (N = 50, 82, 126)
• r-process peaks shifted to lower atomic numbers due to the waiting point approximation

Additional Nucleosynthesis Processes

Proton Capture Process (p-process)

• Occurs in explosive stellar environments (supernovae, novae)
• Produces proton-rich isotopes that cannot be formed by neutron capture processes
• Key steps in p-process
  • Seed nuclei (usually s-process or r-process products) undergo photodisintegration
  • Resulting nuclei capture protons or undergo beta decay
  • Forms proton-rich isotopes on the proton-rich side of the valley of stability
• Responsible for the production of rare, proton-rich isotopes (p-nuclei)
• Examples of p-nuclei include Molybdenum-92, Ruthenium-96, and Samarium-144
• Accounts for less than 1% of heavy element abundances in the solar system

Alpha Process and Photodisintegration

• Alpha process involves the capture of helium nuclei (alpha particles) by heavy elements
• Occurs in high-temperature environments (T > 1 billion K)
• Produces elements with even atomic numbers up to calcium
• Photodisintegration becomes important at very high temperatures
  • Energetic photons break apart heavy nuclei into lighter elements
  • Competes with alpha capture reactions
  • Plays a crucial role in silicon burning and the formation of iron-peak elements
• Balance between alpha capture and photodisintegration determines final elemental abundances
• Important in understanding the nucleosynthesis of elements in massive stars and supernovae