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Power output of a star Stars emit massive amounts of energy per second and so the power of a star is enormous. We assume that a star behaves as a perfectly ‘black body’ in other words it is a perfect radiator of radiation at its surface temperature. The Stefan-Boltzmann law states that the power emitted by a black body of surface area A and with a surface temperature T (K) is given by the equation: Power = AT4 where is a constant (5.7x10-8 Wm-2K-4). (Note: we are assuming here that the temperature of the surroundings (deep space) has a temperature of 0 K) If we assume that a star is roughly spherical then A = 4r2 for a star of radius r. The power of a star is therefore 4r2T4 = 7.16x10-7r2T4. Consider our Sun. It is a star of surface temperature 6000 K, and a radius 6.96x108 m. Using the preceding equation we can calculate its power output: Power output of the Sun = 7.16x10-7r2T4 = 7.16x10-7x[6.96x108]2x[60004] = 7.16x10-7x 4.84x1017x1.296x1015 = 4.5x1026 W An alternative way of finding out the power output of the Sun is to use the solar constant. (See: 16-19/Thermal physics/Transfer of heat/Text/Solar constant) It is interesting to compare this power output with that of Canopus ( Carinae). Canopus has a surface temperature of 7500 K and a radius of 2x1011 m. Using these figures it is possible to calculate its power output as being about 9x1031 W, about 200 000 times greater than that of the Sun!