TITLE: Why we apply Planck's formula to the Sun?
QUESTION [4 upvotes]: It isn't clear for me. Planck's formula is applicable only to equilibrium radiation. But radiation of the Sun isn't in thermodynamic equilibrium with itself because Sun radiates energy into space. So why we use Plancks's formula?

REPLY [7 votes]: Planck's formula is for black body radiation.
Although in reality the sun's radiation isn't exactly a black body spectrum it is close.
The term black body was originally used to describe radiation from an object that doesn't reflect any radiation - i.e. the radiation that comes from it is due to its own temperature.
Since the sun isn't reflecting any radiation and all of the radiation emitted from it is due to its temperature, Planck's law describes the frequency distribution of the radiation.
It's true that the sun is radiating energy into space, but since energy is constantly being produced, each part of the sun is in equilibrium in the sense that it remains at constant temperature, so the approximation works reasonably well.
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