Stefan, Josef (yōˈzĕf shtĕfˈän) [key], 1835–93, Austrian physicist. At the Univ. of Vienna he became a professor of physics and later director of the Physical Institute. From his observations on the relationship between radiant heat emitted by a body and its temperature, Stefan concluded that the total radiation of a body was proportional to the fourth power of its absolute temperature. One of his students, Austrian physicist Ludwig Boltzmann, later derived the same relationship from a thermodynamic viewpoint. This principle, called the Stefan-Boltzmann law, played an important part in considerations leading to Max Planck's quantum theory.