# 2.8: Stefan's Law (The Stefan-Boltzmann Law)

The total exitance integrated over all wavelengths or frequencies can be found by integrating Equations 2.6.1 - 2.6.4. Integration of 2.6.1 over wavelengths or of 2.6.3 over frequencies each, of course, gives the same result:

\[M= \sigma T^4 \label{Stefan}\]

where

\[\sigma = \dfrac{2\pi^5 k^4}{15h^3 c^2} = 5.6705 \times 10^{-8} \ \text{W m}^{-2} \text{K}^4 \]

Equation \ref{Stefan} is Stefan's Law, or the Stefan-Boltzmann law, and \(\sigma\) is *Stefan's constant.*

Integration of Equation 2.6.2 over wavelengths or of 2.6.4 over frequencies each, of course, gives the same result:

\[N = \rho T^3\]

where

\[\rho = \dfrac{4\pi \zeta (3) k^3}{h^3 c^2} = 1.5205 \times 10^{-8} \ \text{ph s}^{-1} \text{m}^{-2} \text{K}^{-3}\]

Here \(\zeta(3)\) is the Riemann zeta-function:

\[\zeta(3) = 1 + \left(\dfrac{1}{2}\right)^3+ \left(\dfrac{1}{3} \right)^3 + \left(\dfrac{1}{4} \right)^3 + ... = 1.202057\]