Stefan–Boltzmann constant

The Stefan–Boltzmann constant (also Stefan's constant), a physical constant denoted by the Greek letter σ, is the constant of proportionality in the Stefan–Boltzmann law: the total energy radiated per unit surface area of a black body in unit time is proportional to the fourth power of the thermodynamic temperature.

The value of σ is 5.670 400(40)×10⁻⁸ W·m^-2·K^-4. This value is derivable as well as experimentally determinable; see Stefan–Boltzmann law.

σ can be defined in terms of the Boltzmann constant $k B$ as:

$\sigma = \frac{2\pi^5k_{\mathrm B}^4}{15h^3c^2} = \frac{\pi^2k_{\mathrm B}^4}{60\hbar^3c^2},$