nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies th ...

, the astrophysical ''S''-factor is a rescaling of a

nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two atomic nucleus, nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a t ...

's total cross section to account for the Coulomb repulsion between the charged reactants. It determines the rates of

nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...

reactions that occur in the cores of

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

Definition

The quantity is defined as :

S(E) \equiv \frac \sigma(E)

, where is the energy, is the cross section, is the dimensionless Sommerfeld parameter: :

\eta \equiv \frac

, is the product of the charges of the reactants,

\epsilon_0

is the

permittivity of free space Vacuum permittivity, commonly denoted (pronounced "epsilon nought" or "epsilon zero"), is the value of the absolute dielectric permittivity of classical vacuum. It may also be referred to as the permittivity of free space, the electric const ...

\hbar

is the

reduced Planck constant The Planck constant, or Planck's constant, denoted by h, is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and the wavelength of a ...

and is the relative incident

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

Motivation

The Coulomb barrier causes the cross section to have a strong exponential dependence on the center-of-mass energy . The ''S''-factor remedies this by factoring out the Coulomb component of the cross section.

References

{{reflist Concepts in astrophysics Equations of astronomy Nuclear physics