Nuclear reactions in stars

No Be is synthesized by nuclear reactions in stars. Thus the Be in the cosmic rays has been produced by the cosmic rays themselves during their collisions with interstellar atoms. The fragments then move along with the cosmic rays as new nuclei of somewhat smaller energies. 

Other Elements Form by Nuclear Reactions in Stars... 

CAMERON, A.G.W. 1957 Nuclear reactions in stars and nucleogenesis. PASP 69, 201. 

Salpeter, E. (1952). Nuclear reactions in stars without hydrogen. Astrophysical Journal, 115, 326-8. Repr. (1979) in A Source Book in Astronomy and Astrophysics, 1900-1975, ed. K. R. Lang and O. Gingerich. Cambridge, MA Harvard University Press. 

Trends in the stability of nuclei are important not only in determining the number of elements and their isotopes (see below) but also in controlling the proportions in which they are made by nuclear reactions in stars. These determine the abundance of elements in the Universe as a whole (see Topic... 

Abundant elements on Earth are therefore ones which were both made efficiently in nuclear reactions in stars, and also formed involatile metals or compounds when the Solar System was formed. Subsequent heating by radioactive decay allowed the denser metals (Fe, Co and Ni combined with some S) to melt and sink towards the center, forming the core. Silicates and other complex oxides remained as the dominant constituents of the outer layers. 

Helium was still being formed after the first few moments of the universe s evolution. For example, it is produced as a by-product of nuclear reactions in stars. The amount produced by such reactions is thought to be very small compared with the amount formed as a result of the big bang. Thus, if observers were to examine the abundance of helium in various parts of the universe today, they would expect to find quantities very close to 25 percent. 

G. R. and E. M. Burbidge, W. A. Fowler and F. Hoyle (B2FH) publish influential article describing nuclear processes which generate essentially all nuclear species by reactions in stars or interstellar medium. Similar ideas put forward by A. G. W. Cameron in a Chalk River internal report. 

Physics is also concerned with the very large think about cosmology and astrophysics. Issues include the beginning of the universe, known as the Big Bang, which occurred some 13.7 billion years ago, the expansion of the universe, formation and evolution of stars and galaxies, and properties of black holes. Here too there are connections between physics and chemistry the origin of the atoms in nuclear reactions within stars and the nature of molecules found in interstellar space, for example. 

Fig. 5.4. Schematic evolution of the internal structure of a star with 25 times the mass of the Sun. The figure shows the various combustion phases (shaded) and their main products. Between two combustion phases, the stellar core contracts and the central temperature rises. Combustion phases grow ever shorter. Before the explosion, the star has assumed a shell-like structure. The centre is occupied by iron and the outer layer by hydrogen, whilst intermediate elements are located between them. CoUapse followed by rebound from the core generates a shock wave that reignites nuclear reactions in the depths and propels the layers it traverses out into space. The collapsed core cools by neutrino emission to become a neutron star or even a black hole. Most of the gravitational energy liberated by implosion of the core (some 10 erg) is released in about 10 seconds in the form of neutrinos. (Courtesy of Marcel Amould, Universite Libre, Brussels.)...

Released in phenomenal quantities, only 1 % of their energy need be communicated to the envelope in order to shatter it to smithereens. The revived shock wave reignites nuclear reactions in its wake and modifies the deep isotopic composition of the star (Fig. 7.5). 

The rise in temperature that follows in the wake of the shock wave triggers a series of nuclear reactions in the central regions of the star. It resuscitates... 

The nuclear reactions in which tighter nuclei fuse together to form a heavier nuclear are called nuclear fusion reactions. Such reactions, occur at very high temperature (of the order of > 10 K) which exist only in the sun or interior of stars therefore, such reactions are also called thermonuclear reactions. 

Smaller elements found in nature, such as hydrogen and helium, formed shortly after the birth of the universe, some 14 billion years ago. Heavier elements such as oxygen, iron, and gold formed in the nuclear reactions of stars such as the Sim during their lifetimes or, in the case of the heaviest elements, in nuclear reactions that occur at the end of a large star s lifetime, when it explodes and becomes what astronomers call a supernova. 

This chapter concerns the fields that use inorganic mass spectrometry to investigate the composition and evolution of matter in the universe and in the solar system. Cosmochemistry is related to nuclear astrophysics, because almost all the chemical elements were synthesized by nuclear reactions in the interior of stars.1 Mass spectrometric analyses of elemental composition, the distribution and variation of isotope abundances are very helpful, especially for cosmochronological studies, in order to explain the formation, history and evolution of stars in our universe and to understand the chemical and nuclear processes. 

Stellar nucleosynthesis No production of 6Li seems possible in stars, other than a very small surface abundance that can be established by nuclear reactions in solar flares. Even with that small production, stars are net destroyers of 6Li, so when their ejecta return to the interstellar material it is 6Li-poor. So stars are not its source. 

Boron is one of the three light elements (Li, Be, B) that are not effectively synthesized by nuclear reactions in stable stars. Its origin in nature must be sought in other astrophysical processes. These involve cosmic-ray collisions with interstellar atoms and neutrino-burst nucleosynthesis in supernova matter. Transient production at the solar center of one of its radioactive isotopes, however, produces energetic neutrinos that have been the easiest of the solar neutrinos to detect. 

The lion s share of fluorine is produced by the intense burst of neutrinos that occurs when the Type II supernova core collapses. Although neutrinos interact only infrequently with matter, a tiny fraction of their intense flux during a 10-second burst drives a proton or neutron from the 20Ne nucleus, in either case resulting in 19F. This occurs where both 20Ne and the neutrino flux are most abundant, near the core of the exploding massive star. Much of this 19F is subsequently destroyed by nuclear reactions in the heated gas when the shock wave passes, but enough survives to account for the 19F/2°Ne abundance ratio in the Sun. 

Neon is the fifth most abundant gas in the Earth s atmosphere, after N2, 02, Ar, and C02 and just before He. But it is only 18 parts per million in air. And because there is precious little Ne within the Earth, it is even more rare on Earth in comparison with rock-forming elements. The interstellar gas tells a quite different story, however. Coincidentally, Ne is also the fifth most abundant element in the universe, following H, He, O, and C, and ranking just before N. It is a very abundant element, much more so than terrestrial scientists suspected. This is because the most abundant of its three isotopes is created directly by one of the most common power-generating nuclear reactions within stars. 

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