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The Origin of Element Abundances in the Universe and Nuclear Reactions in Stars

Explore the primordial composition and current composition of elements in the universe, and discover how nuclear reactions in stars are responsible for the element abundance and energy generation. Understand the four forces of nature and the interactions involving nuclei, including fusion and fission. Learn about the conditions needed for nuclear fusion and how it powers stars. Discover the potential of controlled fusion as a long-term energy solution on Earth.

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The Origin of Element Abundances in the Universe and Nuclear Reactions in Stars

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  1. Ch 9: Origin of Element Abundances in the Universe: Primordial Composition: 77% Hydrogen, 23% Helium, 3 parts per billion Lithium Current Composition: 74% Hydrogen, 24% Helium, 2% elements number 3-92 (everything else) Where did the extra 1% Helium and 2% everything else come from?

  2. Nuclear reactions in stars are responsible for the current element abundance of our universe. • They are also responsible for the generation of energy by stars (sunlight). • You need to understand something about nuclear reactions to understand the presence of chemical elements and energy in the universe!

  3. Four Forces of Nature: Force Acts On Range Gravity all mass long Electromagnetic p, e long Strong Nuclear p, n short Weak Nuclear p, n short p = proton, n = neutron, e = electron long range = infinite short range = 10-15 m (size of a nucleus)

  4. Four Forces of Nature (Cont): Force Effect Gravity always attractive Electromagnetic opposite charges attract/like charges repel Strong Nuclear strongly attractive over short range Weak Nuclear changes protons to neutrons & vice versa

  5. Two Kinds of interactions involving Nuclei: Fusion: Two small nuclei are drawn together by the strong force and release energy Fusion will release energy for nuclei lighter than Iron (element 26). Fission: A large nucleus falls apart due to electric repulsion and releases energy Fission will release energy for nuclei heavier than Iron (example: uranium).

  6. Nuclear fusion of Hydrogen into Helium is what powers 90% of all stars (including the Sun). Conditions Needed: Need plentiful hydrogen Need high temperatures (>10 million Kelvin) These conditions occur naturally in the cores of stars. High temperature fusion = Thermonuclear fusion Controlled fusion is probably the only viable long-term solution to energy problems on Earth!

  7. Where does the energy in fusion come from? • It comes from the conversion of a small amount of mass directly into energy. • A consequence of Theory of Special Relativity: • Energy can be converted directly into mass and mass can be converted directly into energy. • Formula for the amount of energy produced: • E = mc2 (equation 9.7) c = speed of light (big number) c2 = a really big number (lots and lots of energy)

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