The amount of energy being generated each second at any point in the interior of the star is determined by how much hydrogen is being converted into helium each second per unit mass. This process is called the nuclear reaction rate. The reaction rate depends on the temperature, density, and chemical composition. How fast helium is produced by the carbon‐nitrogen‐oxygen cycle is very different from the reaction rate of the proton‐proton cycle. As a result, the CNO cycle dominates the total energy production in stars more massive than twice the solar mass. For lower‐mass stars, the proton‐proton cycle dominates the generation of energy.
The difference in temperature dependency of these two forms of energy generation not only affects which cycle dominates the total energy generation but also has an immediate effect on the internal structure of main sequence stars. Because of its extreme dependence on temperature, the CNO cycle dumps most of the energy generated in a high‐mass star into a very tiny region about the star's center. Radiation cannot move this energy away fast enough, but convection can. In the outer part of the star where the temperature gradient is more gentle, radiation is adequate to move the energy farther out to the star's visible surface layer. On the other hand, the proton‐proton cycle has a reaction rate that varies relatively gently with temperatures. As a direct consequence, the energy produced in a low‐mass star occurs over a large part of the interior of the star. The temperature gradient is low, and radiation is able to carry away the energy. In the outer, cooler parts of the star, however, photons are absorbed:
Only part of the energy of an absorbed photon goes into breaking the bond between the atomic nucleus and the electron; the rest becomes energy of motion. Faster moving atoms mean a higher temperature; the matter thus expands producing the condition for convection. In the outer layers of a low‐mass star, the dominant mode of energy transport becomes convective motion. The internal structures of high‐mass and low‐mass stars are thus essentially reversed from each other (see Figure 1).
- High‐mass versus low‐mass main sequence structure.