Stefan's Law and Wien's Law

A perfect black body is a theoretical object that absorbs all the EM radiation that falls on it. Perfect absorbers are also perfect emitters.

Stars are modelled as Black bodies, and it is assumed that no light is absorbed or scattered by material between the star and the observer.

Hotter stars produce blackbody curves that peak at shorter wavelengths according to Wien’s displacement law:

Formula:

= peak wavelength

= temperature

Hotter stars will therefore emit most of their light in the blue region of the spectrum, whilst cooler stars appear red as they emit most light at longer wavelengths. This is how stars were mainly classified, see the table below:

Stefan’s Law relates the total power output of a star to its black body temperature, , and surface area, .

Formula:

= power

= the Stefan constant

= area

= temperature

We can use these two laws to identify whether a star is more luminous due to its temperature or its size.