# blackbody

Practical approximation to a blackbody. Image credit: NASA.

Variation in blackbody curves with temperature. Image credit: ESA.

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* = 4*σR*^{2}*T*_{e}^{4}

where *L* is the luminosity of the
body, *R* is its radius, and σ (= 5.67 × 10^{-8} W/m^{2}/K^{4}) 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.