Stx Proton
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Wpdms Physics Proton Proton Chain 2.Png
In the second step of the Proton-proton chain, a proton undergoes fusion with a deuterium nucleus, resulting in He3 (an isotope of Helium) as well as a gamma ray.
© 2004 Matthew Trump.
Proton-Proton Chain Reaction
The proton-proton chain reaction is one of two fusion reactions by which stars convert hydrogen to helium, the other being the CNO cycle. The proton-proton chain is more important in stars the size of the Sun or less.
To overcome the electromagnetic repulsion between two hydrogen nuclei requires a large amount of energy, and this reaction takes an average of 109 years to complete. Because of the slowness of this reaction the Sun is still shining; if it were faster, the Sun would have exhausted its hydrogen long ago.
In general, proton-proton fusion can occur only if the temperature (i.e. kinetic energy) of the protons is high enough that they can overcome the mutual Coulomb force repulsion. The theory that proton-proton reactions were the basic principle by which the Sun and other stars burn was advocated by Arthur Eddington in the 1920s. At the time, the temperature of the Sun was considered too low to overcome the Coulomb-force barrier. After the development of quantum mechanics, it was discovered that the tunneling of the wave functions of the protons through the repulsive barrier allowed for fusion at a lower temperature than the classical prediction.
The pp chain reaction
The first step involves the fusion of two hydrogen nuclei 1H (protons) into deuterium 2H, releasing a positron as one proton changes into a neutron, and a neutrino.
:1H + 1H → 2H + e+ + νe
with the neutrinos released in this step carrying energies up to 0.42 MeV.
This first step is extremely slow, because it depends on the weak interaction to convert one proton into a neutron. In fact this is the limiting step, with a proton waiting an average of 109 years before fusing into deuterium.
The positron immediately annihilates with one of the hydrogen's electrons, and their mass energy
Proton
:''For alternative meanings see proton (disambiguation).''
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In physics, the proton (Greek ''proton'' = first) is a subatomic particle with an electric charge of one positive fundamental unit (1.602 × 10−19 coulomb) and a mass of 938.3 MeV/''c''2 (1.6726 × 10−27 kg, or about 1836 times the mass of an electron). The proton is observed to be stable, with a lower limit on its half-life of about 1035 years, although some theories predict that the proton may decay.
The nucleus of the most common isotope of the hydrogen atom is a single proton. The nuclei of other atoms are composed of protons and neutrons held together by the strong nuclear force. The number of protons in the nucleus determines the chemical properties of the atom and which chemical element it is.
Protons are classified as baryons and are composed of two up quarks and one down quark, which are also held together by the strong nuclear force, mediated by gluons. The proton's antimatter equivalent is the antiproton, which has the same magnitude charge as the proton but the opposite sign.
Because the electromagnetic force is many orders of magnitude stronger than the gravitational force, the charge on the proton must be equal and opposite to the charge on the electron, otherwise the net repulsion of having an excess of positive or negative charge would cause a noticeable expansion effect on the universe, and indeed any gravitationally aggregated matter (planets, stars, etc.).
In chemistry and biochemistry, the term proton may refer to the hydrogen ion, H+. In this context, a proton donor is an acid and a proton acceptor a base (see acid-base reaction theories).
History
The proton was discovered in 1918 by Ernest Rutherford. He noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors
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