Black Body Radiation, and Stellar Spectra
      "Black body" radiation is the light (including invisible forms of electromagnetic radiation) given off by a "perfect" radiator. In general, the better things are at absorbing light, the better they are at emitting it, so a perfect absorber would be the most efficient radiator possible; but if something is a perfect absorber, it will not reflect any radiation, and will look black. Hence the odd name of the radiation given off by a perfect radiator.
      Since stars have no surface, in the ordinary sense of the term, any radiation which enters a star is very likely to be scattered and absorbed until it is completely lost; making stars very close to perfectly black bodies, as far as absorption is concerned; and as a result, it would be reasonable to suppose that they would be perfect, or black body radiators, as well.
      At the surface, stars are not actually perfect radiators, because the absorption of light at individual wavelengths by various atoms and ions blocks part of the outward flow of radiation, and this has to be made up for by emitting more light at other wavelengths (the gas heats up, emitting enough extra light at unabsorbed wavelengths to make up for the line absorptions). As an example, the spectrum of the Sun is shown in red, below, in comparison with the spectrum of a black-body of 5780 Kelvins, shown in orange. By and large, the spectra are the same, but the absorption of light at various wavelengths (where the red spectrum of the Sun dips below the orange spectrum of a black body with the same temperature as the "surface" of the Sun) is balanced by an excess of light at other wavelengths (where the red spectrum of the Sun lies above the orange spectrum of the black body).

Black Body Radiation Laws
      The diagrams and equations below summarize the appearance of black body spectra of various temperatures, and the relationship between the temperature of a black body radiator, and the color and intensity of its radiation.