Proton-Proton Fusion

This is the nuclear fusion process which fuels the Sun and other stars which have core temperatures less than 15 million Kelvin. A reaction cycle yields about 25 MeV of energy.

Some details of the nuclear reactions involved
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Proton-Proton Cycle

The fusion of hydrogen in lower temperature stars like our Sun involve the following reactions yielding positrons, neutrinos, and gamma rays.

The solar neutrino problem

which can be followed by either

The latter of these reactions is part of what is usually called the proton-proton cycle, which yields about 25 MeV and can be combined to the form

Illustration of proton-proton cycle
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Step 1. Proton Fusion

The fusing of two protons which is the first step of the proton-proton cycle created great problems for early theorists because they recognized that the interior temperature of the sun (some 14 million Kelvins) would not provide nearly enough energy to overcome the coulomb barrier of electric repulsion between two protons.

With the development of quantum mechanics, it was realized that on this scale the protons must be considered to have wave properties and that there was the possibility of tunneling through the coulomb barrier.

Proton-proton cycle
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Eddington and his fusion critics

Arthur Eddington thought that nuclear processes must be involved to account for the radiant energy of the sun, but was criticized because the temperature was seen to be not hot enough when considered by classical physics alone. His tongue-in-cheek reply to his critics: "I am aware that many critics consider the stars are not hot enough. The critics lay themselves open to an obvious retort; we tell them to go and find a hotter place."

Proton-proton cycle
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Step 2. Deuterium formation

The second step of the proton-proton cycle . This step involves the weak interaction because it involves the transmutation of one of the protons to a neutron in order to form deuterium. This process requires energy and produces a positron and an electron neutrino.

In the proton-proton fusion process, deuterium is produced by the weak interaction in a quark transformation which converts one of the protons to a neutron. The neutrinos quickly escape the sun, requiring only about 2 seconds to exit the sun compared to perhaps a million years for a photon to traverse from the center to the surface of the sun. The neutrino flux can be calculated, but earlier measurements of the neutrino flux measured only about a third of the expected number. This is called the solar neutrino problem. It is now presumed to be solved with the evidence for neutrino oscillation at the Sudbury Neutrino Observatory and at the Super Kamiokande neutrino detector.

Proton-proton cycle
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Step 3. Deuterium-proton fusion

The third step of the proton-proton cycle .

Proton-proton cycle
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Step 4. Helium-3 fusion

The fourth step of the proton-proton cycle .

Proton-proton cycle
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Step 5. Alpha particle formation

The fifth step of the proton-proton cycle . Alpha particles are the end product of the proton-proton fusion cycle. They are formed by the fusion of two helium-3 nuclei.

Proton-proton cycle
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