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Portrait of the Austrian physicist Josef Stefan, 1835-1893. Stefan gained a Ph.D at Vienna University, and joined its faculty in 1863. He was interested in the rate of cooling of hot bodies. After a wide range of observations he stated in 1879 that the total radiation of a hot body was proportional to the fourth power of its absolute temperature. This law has proved of great importance in the study of stellar evolution. In 1884 his ex-student Boltzmann showed that this law could be derived from thermodynamic principles, and that it held only for bodies radiating perfectly at all wavelengths, so-called black bodies.
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