In nuclear physics, the astrophysical S-factor "S"("E") is a rescaling of a nuclear reaction's total cross section "σ"("E") to account for the Coulomb repulsion between the charged reactants. It determines the rates of nuclear fusion reactions that occur in the cores of stars.
Definition.
The quantity is defined as
 formula_1,
where E is the energy, σ is the cross section, η is the dimensionless Sommerfeld parameter:
 formula_2,
"Z"1 "Z"2 "e"2 is the product of the charges of the reactants, formula_3 is the permittivity of free space, formula_4 is the reduced Planck constant and v is the relative incident velocity.
Motivation.
The Coulomb barrier causes the cross section to have a strong exponential dependence on the center-of-mass energy "E". The S-factor remedies this by factoring out the Coulomb component of the cross section.
References.
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