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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) 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 as: where h is Planck's constant, is Dirac's form of Planck's constant, and c is the speed of light in a vacuum. A related constant is the radiation constant (*) which is equal to .
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