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Unformatted text preview: Key Concepts: Lecture 22: The Sun as a Star Nuclear Fusion: the source of the Sun’s energy Structure of the Sun and other stars Where does the Sun get its Energy? • Lifetime for the Sun to shine is related to source of energy: lifetime = (energy available) / (luminosity) • Gravitational Contraction? – Contraction due to gravity releases energy • Total energy due to contraction 10 42 Joules (from Newton’s Laws) – lifetime = (10 42 Joules)/(4x10 26 Joules/sec) = (1/4)x10 16 sec ~ 10 8 years (short compared to age of solar system) – So gravitational contraction cannot be supplying Sun’s energy. • Radioactive decay, similar to heating of Earth? – Sun is mostly H and He, which do not decay. Need heavy nuclei – Even if solid uranium it would only produce 1/2 of energy seen. So not enough radioactivity to supply Sun’s power. Nuclear Fusion • Several light nuclei (H) collide and combine to form a heavier nucleus (He) • Energy is released • Need high temperatures and densities – Must overcome electrostatic repulsion of positively charged atomic nuclei – Core of Sun is 15,000,000 K – Density is 150,000 kg/m 3 1st step towards He Nuclear Fusion: The Proton-Proton Chain Note this process also produces neutrinos. Where Does the Energy Come From?...
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This note was uploaded on 09/07/2011 for the course AST 1002 taught by Professor Dr.haywoodsmith,jr during the Spring '07 term at University of Florida.
- Spring '07