Stefan-Boltzmann lawStefan-Boltzmann law (also Stefan's law) states that the total energy radiated per unit surface area of a blackbody in unit time (blackbody irradiance), (or the energy flux density (radiant flux) or the emissive power), j* is directly proportional to the fourth power of its thermodynamic temperature T:
Temperature of the Sun
With his law Stefan also determined the temperature of the Sun's surface. He learnt from the data of Charles Soret (1854–1904) that the energy flux density from the Sun is 29 times greater than the energy flux density of a warmed metal lamella. A round lamella was placed at such a distance from the measuring device that it would be seen at the same angle as the Sun. Soret estimated the temperature of the lamella to be circa 1900 °C to 2000 °C. Stefan surmised that 1/3 of the energy flux from the Sun is absorbed by the Earth's atmosphere, so he took for the correct Sun's energy flux a value 3/2 times greater, namely 29 × 3/2 = 43.5. Precise measurements of atmospheric absorption were not made until 1888 and 1904. The temperature Stefan obtained was a median value of previous ones, 1950 °C and the absolute thermodynamic one 2200 K. As 2.574 = 43.5, it follows from the law that the temperature of the Sun is 2.57 times greater than the temperature of a lamella, so Stefan got a value of 5430 °C or 5700 K (modern value is 5780 K). This was the first sensible value for the temperature of the Sun. Before this, values from circa 1800 °C to 13,000,000 °C were claimed. The first value of 1800 °C was determined by Claude Servais Mathias Pouillet (1790-1868) in 1838 using the Dulong-Petit law. Pouilett also took just half the value of the Sun's correct energy flux. Perhaps this result reminded Stefan that the Dulong-Petit law could break down at large temperatures. If we collect the Sun's light with a lens, we can warm a solid to much higher temperature than 1800 °C.
The Stefan-Boltzmann law is an example of a power law.
With the Stefan-Boltzmann law, astronomers can easily infer the radii of stars. The law is also met in the thermodynamics of black holes. Similarly we can calculate the temperature of the Earth TE:
Some physicists have criticized Stefan for using a theoretically unsound method to determine the law. It is true that he was helped by some fortunate coincidences, but this does not mean that he found the law blindly.