Solar cosmic abundances of the elements h and he are

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Unformatted text preview: ns in the solar photosphere) • Not really “cosmic”, because precise concentrations vary among stars • H and He are most abundant elements (primordial H, some He) Cosmic Abundances of the Elements U B Nucleosynthesis ! Solar (cosmic) abundances of the elements • • H and He are most abundant elements (primordial H, some He) First 50 elements show exponentially decreasing abundances • • After first 50 elements, abundances are very low Even-number nuclides are more abundant than odd-numbered nuclides • Oddo-Harkins rule, a result of nuclear “magic numbers” • Li, Be and B are very low in abundance • nuclei are destroyed in stars or are weakly stable • Fe is of anomalously high abundance • 56Fe is the most stable of all possible nuclear configurations • • Tc and Pm do not occur naturally in our solar system Z > 83 (Bi) have no stable isotopes • Once thought Bi209 was heaviest stable, but in 2002 was shown to have a half-life of 1.9 x 1019 years… Nucleosynthesis in the Sun & smaller stars ! Proton-Proton chain reaction • Proton-Proton chain requires high Tº, low interaction probability, regarded as principal reaction in stars of Solar mass or smaller Step 1: 1H Step 2: e- + #+ $ = gamma ray Step 3: 2H Step 4: 3He Overall: 2H + #+ + " + 1H #+ = positron, " = neutrino + 1H + 3He (releases 0.422 MeV) (releases 1.02 MeV) $ 3He +$ (releases 5.49 MeV) 4He + 1H + 1H (releases 12.86 MeV) 4H form one He + neutrino + gamma ray + 19.794 MeV Nucleosynthesis in massive stars ! The CNO cycle is the dominant mechanism of nucleosynthesis in The stars heavier than the Sun (1.5 solar mass and up) Step 1: Step 2: Step 3: Step 4: Step 5: Step 6: 12C + 1H Overall: 4 protons form one He, C acts as a catalyst… 13N 13C + 1H 14N + 1H 15O 15N + 1H 13N +$ 13C + #+ + " 14N + $ 15O + $ #+ + " + 15N 12C + 4He He Fusion in Red Giants ! Triple alpha process • A two-step reaction, both steps must occur in rapid succession, because the 8Be nucl...
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This document was uploaded on 04/05/2014.

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