Nuclear Reactions in Stars

The energy of the stars comes from nuclear fusion processes. For stars like the sun which have internal temperatures less than fifteen million Kelvin, the dominant fusion process is proton-proton fusion. For more massive stars which can achieve higher temperatures, the carbon cycle fusion becomes the dominant mechanism. For older stars which are collapsing at the center, the temperature can exceed one hundred million Kelvin and initiate the helium fusion process called the triple-alpha process.

Another class of nuclear reactions is responsible for the nuclear synthesis of elements heavier than iron.

Nuclear Fusion in Stars

The enormous luminous energy of the stars comes from nuclear fusion processes in their centers. Depending upon the age and mass of a star, the energy may come from proton-proton fusion, helium fusion, or the carbon cycle. For brief periods near the end of the luminous lifetime of stars, heavier elements up to iron may fuse, but since the iron group is at the peak of the binding energy curve, the fusion of elements more massive than iron would soak up energy rather than deliver it. While the iron group is the upper limit in terms of energy yield by fusion, heavier elements are created in the stars by another class of nuclear reactions. This nuclear synthesis of elements heavier than iron uses energy rather than supplies it.