R contains all the nuclear physics of the specific reaction and takes very different values depending on the nature of the interaction. However, for most reactions, the variation of R ( ϵ ϵ ) is small compared to the variation from the Gamow factor and so is approximated by a function called the astrophysical S-factor, S ( ϵ ϵ ) , which is weakly varying in energy. Putting these dependencies together, one approximation for the fusion cross section as a function of energy takes the form: