Radiation Laws
Objects in the universe give off electromagnetic radiation according to well understood radiation laws. If an object is giving off electromagnetic radiation because of its temperature, then the color depends on the temperature. The amount of energy being released also depends on the temperature.
These laws certainly apply to stars, which is why we study them here.
Wien's law states that the peak wavelength given off by a black body radiator depends on the temperature of that object. We can fit the data from a star to a black body curve, giving us the temperature of that star. Try it! Stars are different colors for this reason. Blue stars are hotter than red stars. Here is a nice demonstration of Wien's Law.
The Stefan-Boltzman law relates the amount of radiation given off by an object to that object's temperature
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Updated February 14, 2011