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
indicates a beta decay
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:
Here is an
of this sequence. Although we have illustrated the above cycle starting with
Carbon, similar sequences are possible starting with isotopes of Nitrogen or
The mass-12 isotope of Carbon captures a proton and emits a gamma-ray, producing
the mass-13 isotope of Nitrogen.
Nitrogen-13 is unstable and beta decays to the mass-13 isotope
of Carbon with a half-life of approximately 10 minutes.
The mass-13 isotope of Carbon captures a proton and emits a gamma-ray to become the
mass-14 isotope of Nitrogen.
The mass-14 isotope of Nitrogen captures another proton and emits a gamma-ray to
become the mass-15 isotope of Oxygen.
The mass-15 isotope of Oxygen undergoes a beta decay to become the mass-15 isotope
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.
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.