A blackbody is a theoretical object that is both a perfect absorber and a perfect radiator of electromagnetic radiation.

 

A blackbody absorbs all the radiation that falls on it, converts it into internal energy (heat), and then re-radiates this energy into the surroundings. The re-radiated thermal energy, known as blackbody radiation, has a continuous spectrum governed solely by the body's temperature.

 

For any given temperature, there is a specific wavelength at which radiation emission is greatest (see diagram labeled "Variation in blackbody curves with temperature").

 

The effective temperature (T_e), or blackbody temperature, is the surface temperature that an object, such as a star, would have if it were a blackbody that radiated the same amount of energy per unit area. This is a useful and widely employed measure of stellar surface temperature. T_e can be calculated from the Stefan-Boltzmann law, which states that the total energy radiated by a blackbody varies as the fourth power of its absolute temperature. This law leads to the formula:

 

    L = 4σR²T_e⁴

 

where L is the luminosity of the body, R is its radius, and σ (= 5.67 × 10^-8 W/m²/K⁴) is the Stefan-Boltzmann constant. (A simplified form appears in the box on blackbody radiation laws in the bottom illustration.)

 

Substituting solar values for L and R gives a value for the effective temperature of the Sun of about 5,780 K.