Competition between the P-P
Chain and the CNO Cycle

Since both PP chains and CNO cycles can produce fusion energy by converting hydrogen to helium, they compete with each other. What controls which dominates in a given situation?

Factors Controlling the Fusion Rates

There are two points. First, there must be C, N, or O present for the CNO cycle to occur, so it can only happen for stars where this is true. However, only a very small amount is required, so this condition is often fulfilled. Second, the reactions have very different temperature dependences, as illustrated in the following figure.

Energy production rates for the PP chain and CNO cycle as a function of temperature (units of 106 K).

In this figure the horizontal axis is the temperature in units of millions of K (thus 10 means 10,000,000 K), and the vertical axis is a quantity that measures the rate of energy production plotted on a logarithmic scale.

Thus, at lower temperatures the PP chain dominates, but with rising temperatures there is a sudden transition to dominance by the CNO cycle, which has an energy production rate that varies strongly with temperature. This is why the CNO cycle is more important for heavier stars: their interior temperatures are higher, thus favoring the CNO cycle.

Energy Production in the Sun

In the case of the Sun, detailed considerations suggest that it is producing about 98-99% of its energy from the PP chain and only about 1% from the CNO cycle. However, if the Sun were but 10-20% more massive, its energy production would be dominated by the CNO cycle.

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