 The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius , and is often estimated from the star 's color index . The temperature is normally given in terms of an effective temperature , which is the temperature of an <unk> black body that radiates its energy at the same luminosity per surface area as the star . Note that the effective temperature is only a representative of the surface , as the temperature increases toward the core . The temperature in the core region of a star is several million <unk> . 
