Spectral radiation density of
(effective temperature around 5780 K
) compared to that of a black body of the same size (labeling in English)
The effective temperature of a star is the temperature of its surface that a black body would have to have in order to shine with the same brightness per area . The effective temperature of an object deviates from the kinetically defined temperature, the less the spectrum of the object corresponds to that of a black body.
According to the Stefan-Boltzmann law, the following applies
with the Stefan-Boltzmann constant
This gives the bolometric brightness to
- the star's surface , where is the radius of the star.
Since the stellar radius cannot be clearly defined, the optical density is used to calculate the effective temperature
The effective temperature and the bolometric brightness are the two physical parameters with which a star can be classified in the Hertzsprung-Russell diagram .