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effective temperatureSymbol T eff, T e. The surface temperature of a star, expressed as the temperature of a black body (i.e. a perfect radiator) having the same radius as the star and radiating the same total amount of energy (E) per unit area per second. A star is thus being considered a black body and its effective temperature is thus given by Stefan's law:
σ is Stefan's constant and L and R are the star's luminosity and radius. Although accurate determinations of effective temperature are difficult, each spectral type has a characteristic range of T eff; the total range is from about 2500 to over 40 000 kelvin. The Sun's effective temperature is 5770 K. The effective temperature is the best measure of the actual temperature of the gas in a star's outer layers.
Radiation temperature, T r, is equivalent to the effective temperature measured not over the whole spectral range but over a narrow portion of the spectrum, for example at visible wavelengths, which gives the optical temperature.