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Stellar nucleosynthesis is the process by which the natural abundances of the chemical elements within stars change due to nuclear fusion reactions in the cores and overlying mantles of stars. Stars are said to evolve (age) with changes in the abundances of the elements within. Core fusion increases the atomic weight of its gaseous elements, causing pressure loss and contraction accompanied by increase of temperature.〔Donald D. Clayton, Principles of Stellar Evolution and Nucleosynthesis, Mc-Graw Hill, New York (1968) Chapter 6〕 Stars lose most of their mass when it is ejected late in their stellar lifetimes, thereby increasing the abundance of elements heavier than helium in the interstellar medium. The term supernova nucleosynthesis is used to describe the creation of elements during the evolution and explosion of a presupernova star, as Fred Hoyle advocated presciently in 1954.〔F. Hoyle, Synthesis of the elements between carbon and nickel, Astrophys. J. Suppl., 1, 121 (1954)〕 One stimulus to the development of the theory of nucleosynthesis was the variations in the abundances of elements found in the universe. Those abundances, when plotted on a graph as a function of atomic number of the element, have a jagged sawtooth shape that varies by factors of tens of millions. This suggested a natural process other than a random distribution. Such a graph of the abundances can be seen at History of nucleosynthesis theory. Stellar nucleosynthesis is the dominating contributor to several processes that also occur under the collective term nucleosynthesis. A second stimulus to understanding the processes of stellar nucleosynthesis occurred during the 20th century, when it was realized that the energy released from nuclear fusion reactions accounted for the longevity of the Sun as a source〔Donald D. Clayton, ''Principles of stellar Evolution and Nucleosynthesis''. McGraw-Hill, New York (1968); reissued by University of Chicago Press (1983)〕 of heat and light. The fusion of nuclei in a star, starting from its initial hydrogen and helium abundance, provides that energy and synthesizes new nuclei as a byproduct of that fusion process. This became clear during the decade prior to World War II. The fusion product nuclei are restricted to those only slightly heavier than the fusing nuclei; thus they do not contribute heavily to the natural abundances of the elements. Nonetheless, this insight raised the plausibility of explaining all of the natural abundances of elements in this way. The prime energy producer in the sun is the fusion of hydrogen to form helium, which occurs at a solar-core temperature of 14 million kelvin. ==History== In 1920, Arthur Eddington, on the basis of the precise measurements of atomic masses by F.W. Aston and a preliminary suggestion by Jean Perrin, proposed that stars obtained their energy from nuclear fusion of hydrogen to form helium and raised the possibility that the heavier elements are produced in stars.〔A.S. Eddington, The Internal Constitution of the Stars, ''The Observatory'', 43, 341 (1920) http://adsabs.harvard.edu/abs/1920Obs....43..341E〕〔A.S. Eddington, The Internal Constitution of the Stars, ''Nature'', 106, 106 (1920) http://adsabs.harvard.edu/abs/1920Natur.106...14E〕〔(Why the Stars Shine ) D.Selle, Guidestar (Houston Astronomical Society), October 2012, p.6-8〕 This was a preliminary step toward the idea of nucleosynthesis. In 1928, George Gamow derived what is now called the Gamow factor, a quantum-mechanical formula that gave the probability of bringing two nuclei sufficiently close for the strong nuclear force to overcome the Coulomb barrier. The Gamow factor was used in the decade that followed by Atkinson and Houtermans and later by Gamow himself and Edward Teller to derive the rate at which nuclear reactions would proceed at the high temperatures believed to exist in stellar interiors. In 1939, in a paper entitled "Energy Production in Stars", Hans Bethe analyzed the different possibilities for reactions by which hydrogen is fused into helium.〔(Energy Production in Stars ) by Hans Bethe〕 He defined two processes that he believed to be the sources of energy in stars. The first one, the proton–proton chain reaction, is the dominant energy source in stars with masses up to about the mass of the Sun. The second process, the carbon-nitrogen-oxygen cycle, which was also considered by Carl Friedrich von Weizsäcker in 1938, is most important in more massive stars. These works concerned the energy generation capable of keeping stars hot. A clear physical description of the p-p chain and of the CNO cycle appears in a 1968 textbook.〔Donald D. Clayton, Principles of Stellar Evolution and Nucleosynthesis, McGraw-Hill, New York (1968)〕 Bethe's two papers did not address the creation of heavier nuclei, however. That theory was begun by Fred Hoyle in 1946 with his argument that a collection of very hot nuclei would assemble into iron. Hoyle followed that in 1954 with a large paper describing how advanced fusion stages within stars would synthesize elements between carbon and iron in mass.〔F. Hoyle, Synthesis of the elements between carbon and nickel, ''Astrophys. J. Suppl.'', 1, 121 (1954)〕 This is the dominant work in stellar nucleosynthesis.〔D. D. Clayton, Hoyle's equation, ''Science'', 318, 1876–77 (2007)〕 It provided the roadmap to how the most abundant elements on earth had been synthesized from initial hydrogen and helium, making clear how those abundant elements increased their galactic abundances as the galaxy aged. Quickly, Hoyle's theory was expanded to other processes, beginning with the publication of a celebrated review paper in 1957 by Burbidge, Burbidge, Fowler and Hoyle (commonly referred to as the B2FH paper). This review paper collected and refined earlier research into a heavily cited picture that gave promise of accounting for the observed relative abundances of the elements; but it did not itself enlarge Hoyle's 1954 picture for the origin of primary nuclei as much as many assumed, except in the understanding of nucleosynthesis of those elements heavier than iron. Significant improvements were made by Alastair GW Cameron and by Donald D. Clayton. Cameron presented his own independent approach〔A. G. W. Cameron, Stellar Evolution, Nuclear astrophysics and nucleogenesis, Chalk River (Canada) Report CRL-41 (1957)〕 (following Hoyle's approach for the most part) of nucleosynthesis. He introduced computers into time-dependent calculations of evolution of nuclear systems. Clayton calculated the first time-dependent models of the S-process〔Donald D. Clayton, W. A. Fowler, T. E. Hull, and B. A. Zimmerman, "Neutron capture chains in heavy element synthesis", ''Annals of Physics'', 12, 331–408, (1961)〕 and of the R-process,〔Seeger, P. A., W. A. Fowler, and Donald D. Clayton, "Nucleosynthesis of heavy elements by neutron capture", ''Astrophys. J. Suppl'', XI, 121–66, (1965)〕 as well as of the burning of silicon into the abundant alpha-particle nuclei and iron-group elements,〔Bodansky, D., Donald D. Clayton, and W. A. Fowler, "Nucleosynthesis during silicon burning", ''Phys. Rev. Letters'', 20, 161–64, (1968); Bodansky, D., Donald D. Clayton, and W. A. Fowler, Nuclear quasi-equilibrium during silicon burning, ''Astrophys. J. Suppl.'' No. 148, 16, 299–371, (1968)〕 and discovered radiogenic chronologies〔Donald D. Clayton, "Cosmoradiogenic chronologies of nucleosynthesis", ''Astrophys. J.'', 139, 637–63, (1964)〕 for determining the age of the elements. The entire research field expanded rapidly in the 1970s. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「stellar nucleosynthesis」の詳細全文を読む スポンサード リンク
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