# Under what conditions can a body can be approximated as a black body?

I have read this post and from it was my understanding that the definition of a blackbody is:

Black body means a body which ABSORBS all wavelengths completely.

I gained the understanding that a $$\color{green}{\text{black body doesn't have to be in thermal equilibrium.}}$$

Wikipedia agrees with both the statements above.

So all is well, but then I read this (from the Imperial College London, Department of Physics):

Stars have high densities -> frequent collisions that can lead to thermodynamic equilibrium where all particles (electrons, ions, photons) have a single temperature

Can often approximate stars as black bodies!

This has got to be a joke right? So does it seem that any object in thermal equilibrium with its surroundings will emit as a black body? I don't see how thermal equilibrium implies a black body (even if it is an approximation).

This also contradicts the line in green above.

• A black body can both absorb and emit radiation.
– MaxW
Jan 25, 2020 at 21:31
• This isn't an implication, it's just two unrelated things. Stars can be approximated as being at a single temperature, and they can be approximated as blackbodies. Jan 25, 2020 at 22:10
• Individual particles do not have a temperature. The formulation of the anonymous slide is a bit awkward. Jan 25, 2020 at 22:37
• @BLAZE I edited the text into the question -- keeping it as an image isn't a good idea for accessibility reasons. Screen-readers won't be able to translate the image into something audible. In the future, please use native Markdown for text and equations, and only use images for things that cannot be expressed that way. Also, it would be good if you can update with a link/title/something for where you read this at the Imperial College. Jan 27, 2020 at 6:09
• Where emissivity approaches 1. Jan 27, 2020 at 7:51