A law relating the energy emitted by a black body (such as a star) to its temperature; also known as Stefan’s law. According to the law, the total energy radiated in watts per square metre is proportional to the fourth power of its thermodynamic temperature in kelvin; hence a doubling of temperature leads to a sixteen-fold increase in energy output. Expressed mathematically, E = σ‎T⁴, where σ‎ is the Stefan–Boltzmann constant. The total power per square metre can vary from 3 μ‎ W for the microwave background radiation, to 75 MW for the Sun, and thousands of gigawatts for hot stars such as white dwarfs. The law was discovered by Joseph Stefan (1835–93) and derived theoretically by Ludwig Edward Boltzmann (1844–1906).