The Proton-Proton Chain

There are two sequences of reactions that can convert hydrogen to helium and thereby release energy in stars.

  • The Proton-Proton or PP chain, which is important in stars the mass of the Sun or less.
  • The CNO cycle, which is important in more massive stars.

    • We discuss the PP chain in this section and the CNO cycle in the next.

    Reactions of the PP Chain
    The primary reactions in the main branch of the PP chain are illustrated in the following figure. (There are other less important branches of this chain that we shall ignore in our discussion.)

    The main branch of the PP chain consists of the following reactions:

    1. Two mass-1 isotopes of hydrogen undergo a simultaneous fusion and beta decay to produce a positron, a neutrino, and a mass-2 isotope of hydrogen (deuterium). A positron is the antiparticle of the electron (antielectron). A neutrino is a nearly massless fundamental particle. They are denoted by the symbols:

    positron = antielectron = β+       neutrino = ν

    See the further discussion of weak interations and beta decay in the box below.

    2. The deuterium reacts with another mass-1 isotope of hydrogen to produce helium-3 and a gamma ray.
    3. Two helium-3 isotopes produced in separate implementations of the first two steps fuse to form a helium-4 nucleus plus two protons.

    The net effect is to convert hydrogen to helium, with the energy released going into the particles and gamma rays produced at each step of the sequence. This animation illustrates the steps of the proton-proton chain.

    Beta Decay

    In the proton-proton chain, and in various other processes of importance in astrophysics, we encounter the form of radioactive decay called beta decay. This is a particular example of a class of reactions called weak interactions that involve the elusive fundamental particles that we call neutrinos. Beta decay either converts a neutron to a proton, or a proton to a neutron. There are three kinds of beta decay, as illustrated in the adjacent figure (in this diagram, a bar over the neutrino symbol indicates an antineutrino).

    1. In β+ decay a proton is converted to a neutron with the emission of an antielectron (positively charged antiparticle of the electron, also called a positron) and a neutrino (an electron neutrino, to be precise, since we will see later that there are different types of neutrinos). The emitted positron in this case is called a β+ particle.

    2. In β- decay a neutron is converted to a proton with the emission of an electron and antineutrino (the antiparticle of the electron neutrino). The emitted electron is indistinguishable from a normal electron, but when it is produced in a beta decay the electron is called a β- particle.

    3. There is a third class of beta decay called electron capture. In electron capture an electron from the atomic cloud surrounding the nucleus is captured by a proton in the nucleus, converting the proton to a neutron and a neutrino is emitted. Because the electron is captured from the normal cloud of atomic electrons, the symbol e- is commonly used for the captured electron rather than β-.