Nuclear Fusion Reactions in Stars

Nuclear Fusion Reactions in Stars

• Stellar nucleosynthesis is the process where elements are formed in the core and surrounding layers of stars through nuclear fusion reactions.
• Hydrogen burning is a set of stellar reactions that produce energy in stars.
• Helium burning is a set of stellar nuclear reactions using helium to produce heavier elements (e.g., beryllium, oxygen, neon, and iron).

Stellar Nucleosynthesis

• Refers to the fusion reactions in a star's core and the layers above.
• Responsible for creating elements heavier than those formed during the Big Bang, such as hydrogen (H), helium (He), lithium (Li), and beryllium (Be).

Stellar Nucleosynthesis Theory - Arthur Eddington

• Used atomic mass measurements from F.W. Aston.
• Stars derive energy from the nuclear fusion of hydrogen nuclei.
• Heavier elements form within stars.

Stellar Nucleosynthesis Theory - George Gamow

• Formulated the formula for mutual electrostatic repulsion.
• Examines the probability of getting two nuclei close enough to overcome repulsion forces.
• Determined the rate of high-temperature reactions.

Stellar Nucleosynthesis Theory - Hans Bethe

• Described how stars produce energy through hydrogen burning.

Hydrogen Burning

• Refers to a set of reactions that produce helium (He-4) from hydrogen (H).
• Responsible for star energy generation.
• Two main processes:
  • Proton-proton chain reaction: Transforms hydrogen into helium; happens when there's mutual electrostatic repulsion.
  • Carbon-nitrogen-oxygen (CNO) cycle.

Proton-Proton Chain Reaction

• A chain reaction in stars where hydrogen is converted to helium.
• Occurs only when there's electrostatic repulsion between nuclei.

Proton-Proton Chain Reaction Steps

• Beta-plus decay: Two protons fuse to form a deuteron, a positron, and a neutrino.
• Deuterium burning: Deuteron fuses with a proton to create helium-3 and a gamma ray.
• Fusion of two helium-3: Two helium-3 nuclei fuse to form helium-4 and two protons.

Carbon-Nitrogen-Oxygen (CNO) Cycle

• Proton capture: Carbon-12 fuses with a proton to create nitrogen-13 and a gamma ray.
• Beta-plus decay: Nitrogen-13 decays into carbon-13, a positron, and a neutrino.
• Fusion of carbon-13: Carbon-13 fuses with a proton to create nitrogen-14 and a gamma ray.
• Proton capture: Nitrogen-14 fuses with a proton to form oxygen-15 and a gamma ray.
• Beta-plus decay: Oxygen-15 decays into nitrogen-15, a positron, and a neutrino.
• Fusion of nitrogen-15: Nitrogen-15 fuses with a proton to produce carbon-12 and helium-4, plus a gamma ray.

Helium Burning

• The use of helium to produce energy and heavier elements in stars.
• Two dominant processes:
  • Triple-alpha process
  • Alpha process

Triple-Alpha Process

• A two-stage nuclear fusion reaction that changes three helium-4 nuclei to carbon-12.
• Forms an inert carbon core in white dwarfs and larger stars.

Triple-Alpha Process Steps

• Two helium nuclei fuse to create beryllium-8 and a gamma ray.
• Beryllium-8 fuses with a third helium nucleus to produce carbon-12 and a gamma ray.

Alpha Processes

• A set of reactions that converts helium into heavier elements.
• The reactions use helium and ultimately end with iron (Fe).
• Iron-56 is the most stable element with the lowest mass-to-nucleon ratio.

Alpha Processes – Additional Details

• Increases the core size and density by forming heavier elements, vital in transforming main sequence stars to supergiants.
• Reactions capture an alpha particle and release a gamma ray (e.g., carbon-12 captures a helium-4 nucleus to form oxygen-16).
• The process continues capturing alpha particles until it produces the final element.
• All the produced atoms are from even-numbered elements.

Key Points

• Stellar nucleosynthesis: Process where stars form elements through nuclear fusion (in cores and surrounding layers).
• Hydrogen burning: Set of processes in stars that creates energy using hydrogen.
• Helium burning: Series of reactions in stars that produce heavier elements using helium.

Check Your Understanding Questions

• List the products of the following reactions:
  • 36Si + 4He →
  • 15N + 1H →
  • 36Ar + 4He →

Challenge Yourself Questions

• Explain the formation of carbon-12 (12C) through the triple-alpha process.

Explore the fascinating world of stellar nucleosynthesis, where elements are formed through nuclear fusion reactions in stars. Learn about processes like hydrogen and helium burning, and the contributions of theorists like Arthur Eddington and George Gamow. Test your knowledge on how stars create heavier elements and the fundamental theories behind these cosmic processes.