== "Black body radius" == 

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 The black body radius is defined by Stefan–Boltzmann law. It is the radius of an object given its temperature and its total absolute power of Energy radiated: 

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 # What is ε here? Emissivity? Then why is it called black body radius, not grey body radius? 
 # Is this the radius of a black body disk radiating the same amount of power with the same temperature as the object? If that is the case, then it should be  since every disk has two sides that both emit radiation. 
 # It is claimed that "the black body radius is a concept that is excessively encountered in astrophysical science". Give a reference.