A blackbody is defined as an object that absorbs all light incident upon it when it is cold. The radiation emitted by such object depends only on the temperature, and not on any other property. The spectrum is a continuum with the peak moving toward longer wavelength as the temperature decreases. Most solid objects are good approximations to blackbodies. Stars (which are essentially dense balls of hot gas) also behave like blackbodies. The figure shows four graphs of blackbody radiation at different temperature corresponding to different kind of object.

The temperature is expressed in degrees Kelvin ^oK. It is related to the Celsius scale by the formula:

T (in ^oC) = T (in ^oK) - 273.15

Thus T = 273.15 ^oK corresponds to the freezing point temperature of T = 0 ^oC for water.