Formation of the light elements

Formation of the light elements - Formation of the light...

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Formation of the light elements Primordial nucleosynthesis led to to chemical properties of the first generation of stars, perhaps also affecting the cooling rate at early times. Production of heavy elements (mostly He) resulted from competition between fusion, the expansion, and neutron decay. At very high temperatures, thermodynamic equilibrium and detailed balance give equal numbers of protons and neutrons. Calculations of his kind date to Gamow (1948 Phys. Rev. 74, 505), Alpher and Herman (1949 Phys. Rev. 75, 1089), and Hayashi (1950 Prog. Theor. Phys. 5, 224). A recent critical comparison of reaction rates and results is given by Walker et al 1991 (ApJ 376, 51); see also the compact treatment in section 7.5 of Zel'dovich and Novikov. At high temperatures, such that the rates for n<-->p reactions are faster than the expansion timescale t = 1/(2H), the neutron/proton ratio follows its equilibrium value At temperatures below about 1 MeV, the ratio freezes as the expansion decreases the density of available particles for two-body processes. To calculate expected abundances, one uses the expansion history for a particular set of cosmological parameters and follows the possible reactions for mass number 7 and less, such as As in stellar cores, the mass gap at mass number 8 is bridged in negligibly small numbers. The
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This note was uploaded on 11/10/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.

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Formation of the light elements - Formation of the light...

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