catalyst for this cycle. (A catalyst helps something happen, but is not itself changed in the process.) There are more complicated versions of this cycle involving even heavier elements, but the basic ideas are the same.

As we try to build up heavier and heavier elements through fusion, the electrical repulsion becomes stronger. This becomes an effective barrier to the formation of heavier elements. However, no matter how high the Z a nucleus has, we can always get a neutron near it with no electrical repulsion. If the neutron is moving slowly, it can be captured by the nucleus. This can be important in stars, because some reactions provide free neutrons, so there are generally some free neutrons available. We can schematically represent what happens when the nucleus (Z, A) captures a neutron:

particle before breaking up. If this did not happen, the buildup of heavier elements would be blocked. The combination of the 4He and 8Be gives

The triple-alpha process is also important as solar mass stars age and leave the main sequence (discussed in Chapter 10).

In massive stars, there is another scheme that is important in converting four protons into one 4He nucleus. It is called the CNO cycle. The cycle is indicated graphically in Fig. 9.6, and the steps are:

What happens next depends on whether the rate of neutron capture is slow or rapid, compared to the rate of beta decay. If neutron capture is slow, we call the sequence of reactions an s-process (Fig. 9.7). In this situation the new nucleus

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