The PP chain is the most important energy production mechanism for normal stars having masses comparable to the Sun or less. For more massive stars the PP chain can still occur, but there is another sequence of reactions that becomes more favorable for converting hydrogen to helium that is called the CNO cycle.

The Reactions of the CNO Cycle

In stars the primary constituents are hydrogen and helium, but there are usually (much) smaller amounts of heavier elements present. In particular there can be Carbon (C), Nitrogen (N), and Oxygen (O) ions. If these are present, they can participate in the sequence of reactions illustrated in the figure below.

The CNO cycle.

In this diagram beta+ indicates a beta decay and the notation (a,b) means that the nucleus captures the particle labeled by "a" and emits the particle labeled by "b".

This Carbon-Nitrogen-Oxygen or CNO cycle converts hydrogen to helium according to the following sequence of reactions:

  1. The mass-12 isotope of Carbon captures a proton and emits a gamma-ray, producing the mass-13 isotope of Nitrogen.
  2. Nitrogen-13 is unstable and beta decays to the mass-13 isotope of Carbon with a half-life of approximately 10 minutes.
  3. The mass-13 isotope of Carbon captures a proton and emits a gamma-ray to become the mass-14 isotope of Nitrogen.
  4. The mass-14 isotope of Nitrogen captures another proton and emits a gamma-ray to become the mass-15 isotope of Oxygen.
  5. The mass-15 isotope of Oxygen undergoes a beta decay to become the mass-15 isotope of Nitrogen.
  6. The mass-15 isotope of Nitrogen captures a proton and emits an alpha-particle (that is, a nucleus of helium) to close the cycle and return to C-12.
Here is an illustration of this sequence. Although we have illustrated the above cycle starting with Carbon, similar sequences are possible starting with isotopes of Nitrogen or Oxygen.

Carbon-Nitrogen-Oxygen as Catalysts

The net effect of the CNO cycle is to convert hydrogen to helium (the alpha particle emitted in the last step). It has the peculiarity that the Carbon that initiates the sequence in the above example is necessary to the reactions, but is not consumed in them since the last step returns a Carbon-12 nucleus. Borrowing from terminology originating in chemistry, an ingredient such as this which is necessary for the reaction to proceed but is not itself consumed in the reaction is called a catalyst.

As for the PP chain, the energy released in the CNO cycle is contained in the energy of the particles and gamma-rays produced in the steps of the cycle.

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