proton-proton chain reaction

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Proton-proton chain reaction

proton-proton chain reaction

(p-p chain) A series of nuclear fusion reactions by which energy can be generated in the dense cores of stars. The overall effect of the chain reaction is the conversion of hydrogen nuclei to helium nuclei with the release of an immense amount of energy. The energy maintains the core temperature, some of it flowing to the stellar surface. The p-p chain is an efficient energy source even at core temperatures of only 15 million K; it is therefore believed to be the major source of energy in the Sun and in all main-sequence stars that are cooler (less massive) than the Sun. The carbon cycle predominates in the hotter, somewhat more massive stars.

The sequence of steps of the p-p chain is shown in the illustration, with the principal sequence outlined heavily in black. The possible chains occur simultaneously but in what is thought to be 99.75% of the reactions, the sequence begins with two hydrogen nuclei (i.e. protons) combining to emit a positron and a neutrino and forming a nucleus of deuterium. The deuterium then combines with a proton to yield a nucleus of helium–3 and a photon. About 95% of the helium–3 nuclei will combine together to give helium–4, liberating two protons. Overall, each cycle thus converts four protons into one helium–4 nucleus with the release of energy. High collision speeds (plus a quantum-mechanical process known as tunneling) are required for the protons to fuse, since they have to overcome the mutual repulsion arising from their positive charge.

References in periodicals archive ?
1a, the creep rate of PP1 increased rapidly with increasing temperature.
Similar to the PP1 and PP2 composites, for all filler particles, the flexural strength and modulus of the PP3 composites also increased.
PP1, which does not show any strain hardening, deforms very inhomogeneously, PP2 with pronounced strain hardening gives very homogeneous samples over a wide range of elongational rates (cf.
PP1 (which initially does not contain any polar additives) was melt mixed with OMMT and AA in the absence of compatibilizer to investigate the intercalation behavior of this additive into clay galleries.
Group 2(a) was observed mostly in foaming of homo-polypropylenes, PP1, and PP2 while Group 2(b) was observed in foaming of cross-linked polypropylene PP3.
it also has a significantly lower cell concentration than either the linear PP1 or branched PP2.
The content of the [gamma] crystals of samples of PP1 and PP3 melt at 170[degrees]C prepared by method 2 was calculated with the method of Turner-Jones (29) and listed in Table 1.
Typical Resin Properties ASTM Method PP1 PP2 PP3 PP4 Melt flow rate, 230[degrees]C/2.
The length of the capillary had no evident effect on the onset of gross volume distortions for PP1 and LDPE1, whereas the onset for PE1 and PE2 shifts to lower flow rates with longer capillaries.
Norway, whereas the bimodal PE1 and PP1 materials were provided by Borealis Polymers Oy, Finland.
HDPE1 exhibits a higher viscosity than three grades of polypropylene over the whole shear rate range and PP1 has higher viscosity than PP2 and PP3.