The Origin of The Heavier Elements

The Origin of The Heavier Elements

• Hydrogen atoms first produced in the big bang would need additional protons and neutrons to become helium and lithium.

• In a nucleus, the strong force holds protons and neutrons together, which overcomes and opposes the repulsion between protons.

• Protons can also be combined with neutrons to form heavier nuclei in a process called nuclear fusion.

Nuclear fission

• refers to the splitting of a heavy nucleus into lighter nuclei This reaction sustains nuclear power plants as well as some nuclear weapons.

• this reaction sustains nuclear power plants as well as some nuclear weapons

Nuclear fusion

• a reaction through which light nuclei combine to form a heavier nucleus

  
  

• this reaction produces less radioactive products and is thus safer than nuclear fission and can generate fewer amounts of nuclear waste that must be disposed of properly

Stellar Nucleosynthesis

• Nucleosynthesis in stars.

• Responsible for the formation of heavy elements.

• The two key processes of stellar nucleosynthesis are nuclear fusion and neutron capture. 

• Nuclei of different elements from boron to iron formed from nuclear fusion.

• Nuclei heavier than iron emerged from neutron capture in conjunction with beta decay.

• Nuclear fusion and neutron capture release and requires energy, respectively. 

Classification of Nuclear Fusion Reaction

• Exothermic reactions that form helium

  
  

• Burning phases form heavier nuclei.

EXOTHERMIC REACTIONS: HYDROGEN TO HELIUM

• These reactions release energy in the form of heat or light.

  
  

  
  

• These reactions include the proton-proton chain reaction (p-p chain), hydrogen burning, and carbon-nitrogen-oxygen cycle (CNO Cycle).

BURNING PHASES INVOLVING HEAVIER NUCLEI

• Helium Burning

• Carbon and Oxygen Burning

• Silicon Burning

Hydrogen Burning

• Once the star has used up the hydrogen in its core, the p-p chain and the CNO cycle stop, and the helium previously produced starts to be expended and burned.

• Helium burning occurs in the core of stars once the temperature reaches about 108 Kelvin.

• The main nuclear reactions in this process produce carbon-12 via a triple alpha process.

Hydrogen Burning

• This covers an overall (net) reaction of

  3 42 He ------>126 C + energy

  
  

Triple Alpha

• derived from essentially three alpha particles (42 He) that interact since beryllium-8 (84 Be ), which formed from the first reaction, is unstable.

  
  

• Beryllium-8 merely acts as an intermediate that usually decays back into two helium-4 atoms.

Triple Alpha

• derived from essentially three alpha particles (42 He) that interact since beryllium-8 (84 Be ), which formed from the first reaction, is unstable.