The CNO Cycle

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. It is called the CNO cycle, which stands for the carbon-nitrogen-oxygen 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. 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 adjacent right. In this diagram the symbol β⁺ 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."

Steps of the Cycle

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 ten 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 carbon-12.

Notice that a total of four protons (hydrogen nuclei) are captured and one alpha particle (helium nucleus) is produced by the cycle, so it converts hydrogen to helium. 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. Although we have illustrated the above cycle starting with carbon, the cycle is a closed loop so one can think of it starting with any of the C, N, or O isotopes in the above figure. Here is an animation illustrating the CNO cycle.

Calculation of element production in the CNO cycle.