Formation of Elements in Big Bang and Stars

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 and subatomic particles formed, it entered the phase of Big Bang nucleosynthesis.
• This process strongly favors very light isotopes of hydrogen and helium, due to their lower atomic mass compared to other elements.

Nucleosynthesis

• Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons, primarily protons and neutrons.

Big Bang Nucleosynthesis Details

• Proton + neutron = deuterium
• Deuterium + neutron = tritium
• Big bang nucleosynthesis strongly favors light isotopes like hydrogen (1.008) and helium (4.003).

Subatomic Particles

• Atom: The smallest unit of matter.
• Proton: Positively charged subatomic particle.
• Neutron: Subatomic particle with no charge.
• Electron: Negatively charged subatomic particle.

Nuclear Fusion

• Nuclear fusion is the process where light nuclei fuse together to form a heavier nucleus.
• Isotopes are forms of an element with the same atomic number but have different atomic masses or mass numbers.
• Fusion means combination or to fuse together
• Fission means separation or splitting of particles

Origin of Heavier Elements

• Elements heavier than iron form only billions of years after stars are created because the density within stars needs to be high enough to sustain fusion for a long enough time
• Stellar nucleosynthesis is the formation of heavy elements by fusion of lighter nuclei in the interior of stars.

Star Fusion Processes

• Stars are hot enough to burn hydrogen (1H) to helium (4He).
• There are multiple nuclear pathways to produce heavier elements including:
• Carbon-Nitrogen-oxygen cycle
• Proton-proton fusion
• Triple alpha process
• Layers near the star's core have tremendously high temperatures to nucleosynthesize heavy elements like silicon and iron.

Elements Heavier than Iron

• Elements heavier than iron are created in a supernova, a massive star explosion.
• Fusion processes for elements heavier than iron require tremendous amounts of energy.
• Neutron capture reaction in supernovae creates heavier elements by adding neutrons to existing nuclei.

Neutron Capture Reactions

• Adding neutrons to a nucleus doesn't change the element, but rather creates a more massive isotope.
• Elements higher than iron require tremendous energy to form.

Summary

• Three processes lead to the formation of elements: nucleosynthesis, fusion, and neutron capture.
• Energy from the continuously expanding universe forms these elements, which make up matter today.
• Nucleosynthesis formed light elements; fusion in stars formed elements with atomic masses in a certain range; neutron-capture reactions in supernovae created elements heavier than iron.