Hydrogen-burning

When the temperature of a contracting mass of hydrogen and helium atoms reaches about 10 K, a sequence of thermonuclear reactions is possible of which the most important are as shown in Table 1.2. 

The overall reaction thus converts 4 protons into 1 helium nucleus plus 2 positrons and 2 neutrinos  

Ahrens (ed.). Origin and Distribution of the Elements, Pergamon Press, Oxford, 1979, 920 pp. 

In fact, the sun is not a first-generation main-sequence star since spectroscopic evidence shows the presence of many heavier elements thought to be formed in other types of stars and subsequently distributed throughout the galaxy for eventual accretion into later generations of main-sequence stars. In the presence of heavier elements, particularly carbon and nitrogen, a catalytic sequence of nuclear reactions aids the fusion of protons to helium (H. A. Bethe 

The first major set of nuclear reactions in stellar evolution involves hydrogenburning through the pp chains and the CN cycle or CNO bi-cycle, which liberate 6.68 MeV of energy per proton minus neutrino losses (2 neutrinos are emitted for each 4He nucleus synthesized). The first two reactions of the pp chains are 

Reaction (5.66) involves the weak interaction through the /f+-decay and is accordingly exceedingly slow. The condition for an allowed decay is that the two 

The remaining reactions are much faster, e.g. the time for (5.67) is only 1.6 s, leading to a steady-state D/H ratio of the order of 10-17. 

After reaction (5.67), there are two alternatives 86 per cent of the time (in the case of the Sun), there is the final link of the pp — 1 chain 

Reaction Nature Expected flux3 Observed flux 

The first stage of stellar nucleosynthesis, which is still occurring in stars such as our sun, is hydrogen burning. In hydrogen burning, protons are converted to 4He nuclei. Since there are no free neutrons present, the reactions differ from those of Big Bang nucleosynthesis. The first reaction that occurs is 

There is an improbable (0.4%) variant of this reaction, called the pep reaction that also leads to deuteron production. It is 

This rare reaction is a source of energetic neutrinos from the sun. 

This reaction, when combined with the two previous reactions (p + p and d + p) corresponds to an overall reaction of 

In 14% of the cases, the 3He product undergoes the side reaction with an a particle  

The following types of nuclear reactions have been proposed to account for the various types of stars and the observed abundances of the elements  

Making allowance for the energy carried away by the 2 neutrinos (2 x 0.25 MeV) this leaves a total of 26.22 MeV for radiation, i.e. 4.20 pJ per atom of helium or 2.53 x 10 kJ mol . This vast release of energy arises mainly from the difference between the rest mass of the helium-4 nucleus and the 4 protons from which it was formed (0.028 atomic mass units). There are several other peripheral reactions between the protons, deuterons and He nuclei, but these need not detain us. It should be noted, however, that only 0.7% of the mass is lost during this transformation, so that the star remains approximately constant in mass. For example, in the sun during each second, some 600 x 10 tonnes (600 x 10 kg) of hydrogen are processed into 595.5 x 10 tonnes of helium, the remaining 

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The hydrogen contained in coke bums at a higher rate than carbon. Hydrogen-burning rates are four to five times greater than carbon-burning rates. 

Obviously the availability of a non-carbon fuel, usually hydrogen, would obviate the need for carbon dioxide extraction and disposal, and a plant with combustion of such a fuel becomes a simple solution (Cycle Cl, a hydrogen burning CBT plant, and Cycles C2 and C3, hydrogen burning CCGT plants). 

How much heat is released when two moles of hydrogen burn One-half mole ... 

The combustion of mixtures of hydrogen and air produces very few ions so that with only the carrier gas and hydrogen burning an essentially constant signal is obtained. When, however, carbon-containing compounds are present ionisation occurs and there is a large increase in the electrical conductivity of the flame. Because the sample is destroyed in the flame a stream-splitting device is employed when further examination of the eluate is necessary this device is inserted between the column and detector and allows the bulk of the sample to by-pass the detector. 

The color of a hydrogen-burning star depends on its mass the higher the mass, the higher the temperature of the star and the more blue it appears. 

A generality states that all compounds containing both carbon and hydrogen burn. Do octane and propane burn Each contains only carbon and hydrogen. 

Fig. 3. Abundances within the hydrogen-burning shell as function of relative mass 5m, defined as ranging from 0 to 1 between the bottom of the shell and the convective envelope. From observational evidence a penetration of extra-mixing down to 5m 0.15 has been inferred...

When solid body rotation is assumed in the CE (model B), the degree of differential rotation at its base is too low to trigger efficient shear-induced turbulence between the outer part of the hydrogen burning shell (HBS) and the CE (solid lines in Fig. la). On the contrary, in our model C the differential rotation... 

Big Bang nucleosynthesis (cosmic nucleosynthesis) Proton-proton cycle Triple He collisions Alpha capture CNO cycle Neutron capture High-energy photon collisions produce antimatter-matter pairs. This leads to H,D, He and some Li nuclei Hydrogen burning to produce He 12C production Addition of 4He to the nucleus Production of 13C, 13N, 14N and 150 Post-Fe nuclei... 

Fig. 5.7. Evolutionary tracks for Z = 0.02 (near solar metallicity) stars with different masses in the HR diagram. (Luminosities are in solar units.) Points labelled 1 define the ZAMS and points labelled 2 the terminal main sequence (TAMS), the point where central hydrogen is exhausted. The Schonberg-Chandrasekhar limit may be reached either before or after this (for M > 1.4 Af0). Points marked 3 show the onset of shell hydrogen-burning. Few stars are found in the Hertzsprung gap between point 4 and point 5 , where the surface convection zone has grown deep enough to bring nuclear processed material to the surface in the first dredge-up. Adapted from Iben (1967).

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