Overveiw of the proton-proton chain.

In the first step in the proton-proton chain, two protons fuse to form a deuterium nucleus. The reaction results in the emission of a positron and a neutrino.

In the second step of the chain, a single proton fuses with a deuterium nucleus, resulting in ³He and a gamma ray

In the third step of the chain, two ³He nucli fuse, forming ⁴He and two protons

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 dominates in stars the size of the Sun or less. Image File history File links Download high resolution version (600x680, 23 KB) Please see the file description page for further information. ... Image File history File links Download high resolution version (600x680, 23 KB) Please see the file description page for further information. ... Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Wpdms_physics_proton_proton_chain_2. ... Image File history File links Wpdms_physics_proton_proton_chain_2. ... Image File history File links Wpdms_physics_proton_proton_chain_3. ... Image File history File links Wpdms_physics_proton_proton_chain_3. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... The Pleiades star cluster A star is a massive, compact body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... This article does not cite its references or sources. ... The Sun is the star at the center of our solar system. ...

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 at the temperature of the Sun's core. 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. Temperature is also the name of a song by Sean Paul. ... Kinetic jkljfkdffmdklcjenergy (SI unit: the [[klof its motion. ... In physics, Coulombs law is an inverse-square law indicating the magnitude and direction of electrical force that one stationary, electrically charged substance of small volume (ideally, a point source) exerts on another. ... One of Sir Arthur Stanley Eddingtons papers announced Einsteins theory of general relativity to the English-speaking world. ... It has been suggested that this article or section be merged with Social issues of the 1920s. ... For a non-technical introduction to the topic, please see Introduction to Quantum mechanics. ... In the most restricted usage in quantum mechanics, the wavefunction associated with a particle such as an electron, is a complex-valued square integrable function ψ defined over a portion of space normalized in such a way that In Max Borns probabilistic interpretation of the wavefunction, the amplitude squared... Classical physics is physics based on principles developed before the rise of quantum theory, usually including the special theory of relativity and general theory of relativity. ...

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. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Properties In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of one atom in 6400 of hydrogen (see VSMOW; the abundance changes slightly from one kind of natural water to another). ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ... Properties In physics, the neutron is a subatomic particle with no net electric charge and a mass of 939. ... The neutrino is an elementary particle. ...

¹H + ¹H → ²H + e⁺ + ν_e

with the neutrinos released in this step carrying energies up to 0.42 MeV. Properties The electron is a lightweight fundamental subatomic particle that carries a negative electric charge. ... The neutrino is an elementary particle. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ...

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 weak nuclear force or weak interaction is one of the four fundamental forces of nature. ...

The positron immediately annihilates with one of the hydrogen's electrons, and their mass energy is carried off by two gamma ray photons. Properties The electron is a lightweight fundamental subatomic particle that carries a negative electric charge. ... This article is about electromagnetic radiation. ... In quantum physics, the photon (from Greek φως, phōs, meaning light) is the quantum of the electromagnetic field (light). ...

e⁺ + e⁻ → 2γ + 1.02 MeV

After this the deuterium produced in the first stage can fuse with another hydrogen to produce a light isotope of helium, ³He: This article is about electromagnetic radiation. ... Isotopes are forms of an element, therefore their nuclei have the same atomic number — the number of protons in the nucleus — but different mass numbers because they contain different numbers of neutrons. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... Helium-3 is a non-radioactive and light isotope of helium. ...

²H + ¹H → ³He + γ + 5.49 MeV

From here there are three posible paths to generate helium isotope ⁴He. In pp1 helium-4 comes from fusing two of the helium-3 nuclei produced; the pp2 and pp3 braches fuse ³He with a pre-existing ⁴He to make Beryllium-7. In the Sun, branch pp1 takes place with a frequency of 86%, pp2 with 14% and pp3 with 0.11%. There is also an extremely rare pp4 branch. This article is about electromagnetic radiation. ... Helium-4 is a non-radioactive and light isotope of helium. ...

The pp I branch

³He +³He → ⁴He + ¹H + ¹H + 12.86 MeV

The complete pp I chain reaction releases a net energy of 26.7 MeV. The pp I branch is dominant at temperatures of 10 to 14 megakelvins (MK). Below 10 MK, the PP chain does not produce much ⁴He. The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ...

The pp II branch

The pp II branch is dominant at temperatures of 14 to 23 MK. General Name, Symbol, Number beryllium, Be, 4 Chemical series alkaline earth metals Group, Period, Block 2, 2, s Appearance white-gray metallic Atomic mass 9. ... This article is about electromagnetic radiation. ... General Name, Symbol, Number lithium, Li, 3 Chemical series alkali metals Group, Period, Block 1, 2, s Appearance silvery white/gray Atomic mass 6. ... The neutrino is an elementary particle. ...

90% of the neutrinos produced in the reaction ⁷Be(e⁻,ν_e)⁷Li* carry an energy of 0.861 MeV, while the remaining 10% carry 0.383 MeV (depending on whether lithium-7 is excited or in the ground state).

The pp III branch

The pp III chain is dominant if the temperatures exceeds 23 MK. This article is about electromagnetic radiation. ... General Name, Symbol, Number boron, B, 5 Chemical series metalloids Group, Period, Block 13, 2, p Appearance black/brown Atomic mass 10. ... This article is about electromagnetic radiation. ... The neutrino is an elementary particle. ...

The pp III chain is not a major source of energy in the Sun (only 0.11%), but was very important in the solar neutrino problem because it generates very high energy neutrinos (up to 14.06 MeV). The solar neutrino problem was a major discrepancy between measurements of the neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002. ...

The pp IV or Hep

This reaction is predicted but has never been observed due to its great rarity (about 0.3 parts per million in the Sun). In this reaction, Helium-3 reacts directly with a proton to give helium-4, with an even higher possible neutrino energy (up to 18.8 MeV). Parts per million (ppm) is a measure of concentration that is used where low levels of concentration are significant. ...

³He + ¹H → ⁴He + ν_e + e⁺

Energy release

Comparing the mass of the final helium-4 atom with the masses of the four protons reveals that 0.007 or 0.7% of the mass of the original protons has been lost. This mass has been converted into energy, in the form of gamma rays and neutrinos released during each of the individual reactions. The total energy we get in one whole chain is 26.73 MeV.

Only energy released as gamma rays will interact with electrons and protons and heat the interior of the Sun. This heating supports the Sun and prevents it from collapsing under its own weight.

Neutrinos do not interact significantly with matter and do not help support the Sun against gravitational collapse. The neutrinos in the ppI, ppII and ppIII chains carry away the 2.0%, 4.0% and 28.3% of the energy respectively.^[1]

The pep reaction

Deuterium can also be produced by the rare pep (proton-electron-proton) reaction (electron capture): Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of one atom in 6400 of hydrogen (see VSMOW; the abundance changes slightly from one kind of natural water to another). ... Electron capture is a decay mode for isotopes that will occur when there are too many protons in the nucleus of an atom, and there isnt enough energy to emit a positron; however, it continues to be a viable decay mode for radioactive isotopes that can decay by positron...

¹H + e⁻ + ¹H → ²H + ν_e

In the Sun, the frequency of pep reaction versus pp reaction is 1:400. However the neutrinos released are far more energetic: while neutrinos produced in the first step of the pp reaction range in energy up to 0.42 MeV, the neutrinos from the pep reaction produce sharp-energy-line neutrinos of 1.44 MeV.

References

1. ^ Claus E. Rolfs, William S. Rodney: Cauldrons in the Cosmos, The University of Chicago Press, 1988, 354. pp

See also

• Triple-alpha process
• CNO cycle