Module 1: Big Bang and Stellar Evolution

Module 1: Formation of Elements in the Big Bang and Stellar Evolution

• Objectives:
  • Explain and provide evidence for the formation of light elements in the Big Bang theory.
  • Describe and provide evidence for the formation of heavier elements during star formation and evolution.
  • Write the nuclear fusion reactions in stars that lead to the formation of new elements.

Big Bang Nucleosynthesis

• As the universe cooled, subatomic particles formed, initiating the Big Bang nucleosynthesis phase.

Nucleosynthesis

• Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons (primarily protons and neutrons) .

Big Bang Nucleosynthesis: Processes

• Big Bang nucleosynthesis strongly favors light isotopes like hydrogen (1.008) and helium (4.003), as they are denser compared to elements with larger atomic mass.
• Processes involve:
  • Proton + neutron = deuterium.
  • Deuterium + neutron = tritium.

Atoms: Subatomic Particles

• Atom: The smallest unit of matter.
• Proton: Positively charged subatomic particle.
• Neutron: Uncharged subatomic particle.
• Electron: Negatively charged subatomic particle.

Nuclear Fusion

• The process where light nuclei combine to form heavier nuclei.
• Isotopes: Forms of an element with the same atomic number but different atomic masses (or mass numbers). Examples of isotopes produced during Big Bang nucleosynthesis are H-1, H-2, He-3, He-4, and Li-7.

Origin of Heavier Elements

• Heavier elements formed billions of years after stars were formed.
• The density inside a star is sufficient to maintain fusion for extended periods, necessary to synthesize heavier elements.

Stellar Nucleosynthesis

• The formation of heavy elements by the fusion of lighter nuclei within stars.

Stellar Fusion Processes

• Stars burn hydrogen (1H) to helium (4He) due to high temperatures and density.
• Stars employ various nuclear fusion pathways to create heavier elements like:
  • Carbon-Nitrogen-Oxygen cycle
  • Proton-proton fusion
  • Triple alpha process

Elements Heavier than Iron

• Elements heavier than iron cannot be formed through fusion as it requires substantial amounts of energy.
• Heavy elements are formed in supernova explosions.

Supernova

• A supernova is an explosive stellar death.
• Heavy elements are created during a neutron capture reaction within the supernova, by adding neutrons to existing nuclei rather than merging light nuclei.

Neutron Capture

• Adding neutrons to a nucleus does not alter the element but produces a more massive isotope of the element.
• Elements heavier than iron are formed due to neutron capture during supernovae.

Summary of Element Formation

• Element formation involves three processes: nucleosynthesis, fusion, and neutron capture.
• Reactions are powered by energy from the expanding universe.
• The atomic mass of the elements dictates the required energy for formation of elements, where fusion in stars creates elements with mass ranging from beryllium to iron.
• Elements with atomic masses greater than iron require the tremendous energy released in supernova events.