Overview
This lecture explains how elements heavier than beryllium are formed in stars through stellar nucleosynthesis, including the fusion processes in different star life stages.
Formation and Evolution of Stars
- After the Big Bang, a nebula (cloud of gas and dust) collapses under gravity to form a protostar.
- The protostar's increasing pressure and temperature lead to nuclear fusion, marking the star's birth.
- Stellar nucleosynthesis is the process by which elements are formed inside stars via nuclear fusion.
Nuclear Fusion in Stars
- Nuclear fusion combines lighter nuclei to form heavier nuclei, releasing energy.
- Stars are mostly hydrogen, so fusion begins with hydrogen nuclei in the core.
- The primary hydrogen fusion process is the proton-proton chain, which creates deuterium, helium-3, and then helium-4.
- Fusion reactions release energy as heat, light, and gamma rays.
Formation of Heavier Elements
- As hydrogen is depleted, the core accumulates helium and heats up further, turning the star into a red giant.
- The triple-alpha process fuses three helium-4 nuclei to form carbon-12.
- The CNO cycle (carbon-nitrogen-oxygen cycle) uses carbon as a catalyst to help produce helium.
- Alpha ladder: Helium nuclei fuse with heavier elements, forming oxygen, neon, magnesium, and up to iron.
- The atomic number determines the element formed by fusion (e.g., carbon-12 + helium-4 = oxygen).
Formation of Elements Heavier Than Iron
- Fusion in stars only creates elements up to iron due to energy constraints.
- Elements heavier than iron (like copper, gold, and silver) form during supernova explosions.
- Supernova nucleosynthesis involves rapid neutron capture (r-process) and, in red giants, slow neutron capture (s-process).
Fate of Stars
- Medium-sized stars (like the Sun) become planetary nebulae, then white dwarfs, and finally black dwarfs.
- Massive stars may become neutron stars or black holes after a supernova.
Key Terms & Definitions
- Big Bang Nucleosynthesis — formation of light elements (hydrogen, helium, lithium, beryllium) in the early universe.
- Stellar Nucleosynthesis — creation of heavier elements inside stars via nuclear fusion.
- Nuclear Fusion — joining of lighter nuclei to form a heavier nucleus, releasing energy.
- Proton-Proton Chain — series of reactions fusing hydrogen into helium in stars.
- Triple Alpha Process — fusion of three helium-4 nuclei to form carbon-12.
- CNO Cycle — process where carbon acts as a catalyst to fuse hydrogen into helium.
- Alpha Ladder — sequential fusion of helium nuclei with heavier elements.
- Supernova — massive star explosion, creating elements heavier than iron via r-process.
- r-process — rapid neutron capture during supernova to form heavy elements.
- s-process — slow neutron capture in red giants to form heavy elements.
Action Items / Next Steps
- Review the fusion processes (proton-proton chain, triple alpha, CNO cycle, alpha ladder).
- Understand the difference between elements formed in stars vs. supernovae.
- Prepare for questions on how atomic and mass numbers determine element identity.