Hydrogen burning, nuclear

Chain reactions are a t3q)e of overall reactions, which require two or more steps to accomplish. They are also known as consecutive reactions or sequential reactions. Examples of chain reactions include nuclear hydrogen burning, nuclear decay chains, ozone production, and ozone decomposition. Some steps of a chain reaction may be rapid and some may be slow. The slowest step is the ratedetermining step. During a chain reaction, some intermediate and unstable species may be produced and consumed continuously. 

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).

Other nuclear reactions. The Sun is powered by nuclear hydrogen burning in the Sun s core ... 

If a reaction is not an elementary reaction, i.e., if the reaction does not occur at the molecular level as it is written, then it is called an overall reaction. An overall reaction may be accomplished by two or more steps or paths and/or with participation of intermediate species. For example, nuclear hydrogen burning Reaction 1-3, 4 H He, is an overall reaction, not an elementary reaction. There are several paths to accomplish the reaction, with every path still an overall reaction accomplished by three or more steps. One path is called aPP I chain and involves the following steps ... 

If a reaction can be accomplished by two or more paths, the paths are called parallel paths and the reaction is called a parallel reaction. The overall reaction rate is the sum of the rates of all the reaction paths. The fastest reaction path is the rate-determining path. For nuclear hydrogen burning, the PP I chain is one path, the PP II chain is another path, and the CNO cycle is yet another path. 

The nuclear reaction that finally stabilizes the structure of the protostar is the fusion of two protons to form a deuterium atom, a positron, and a neutrino (1 H(p,p+v)2D). This reaction becomes important at a temperature of a few million degrees. The newly produced deuterium then bums to 3He, which in turn bums to 4He in the proton-proton chain. The proton-proton chain is the main source of nuclear energy in the Sun. With the initiation of hydrogen burning... 

Champaign, A. E. and Wiescher, M. (1992) Explosive hydrogen burning. Annual Reviews of Nuclear and Particle Science, 42, 39-76. 

Figure 12.11 Three chains of nuclear reactions that constitute hydrogen burning and convert protons into 4He. The rate-limiting step in all reactions is the first reaction to create the deuterium.

We begin with a discussion of the poorly understood mechanisms for heavy-element nucleosynthesis and some of our efforts to understand these environments. Then we turn to a discussion of the exotic environments for hot hydrogen burning and some of our experimental and theoretical efforts to obtain the associated nuclear data. 

In the mix of interstellar atoms from which the solar system formed, 17O exists primarily owing to the hydrogen-burning process of stellar nucleosynthesis. 170 is made by converting a fraction of the 16 O to 170 by the nuclear reaction... 

Guided by early compilations of the cosmic abundances as reflected in solar system material (e.g., Suess and Urey, 1956), Burbidge cr a/. (1957) and Cameron (1957) identified the nuclear processes by which element formation occurs in stellar and supernova environments (i) hydrogen burning, which powers stars for —90% of their lifetimes (ii) helium burning, which is responsible for the production of and the two most abundant elements heavier than helium (iii) the a-process, which we now understand as a combination of... 

Figure 3 Evolution of the central temperature and density in stars of 15M and 25M from birth as hydrogen burning stars until iron core collapse (Table 1). In general, the trajectories follow a line of p oc but with some deviation downward (towards higher p at a given T) due to the decreasing entropy of the core. Nonmonotonic behavior is observed when nuclear fuels are ignited and this is exacerbated in the 5Mq model by partial degeneracy of the gas (source Woosley et al., 2002).

---