Is a thermal emitter the same thing as a blackbody? It seems that both have the property to absorb all light and re-emit them in the form of thermal radiation. Is a thermal emitter the same thing as a blackbody?
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No, they are not the same thing. A thermal emitter has a source function that is a function of a well-defined temperature. That is, the object can be considered in local thermodynamic equilibrium where the speeds and distribution of energies and energy levels of particles/atoms/molecules are determined by the temperature. Such objects do not emit blackbody radiation unless they are also in equilibtrium with the radiation field.
A classic example of a thermal emitter that does not emit blackbody radiation would be the Sun's corona. It emits thermal bremsstrahlung, but since it is "thin" (almost transparent) to that radiation, the radiation is not in equilibrium with the matter and the spectrum is most certainly not a blackbody.
I guess others might define a "thermal emitter" in a different way, but certainly in astrophysics, there is a big distinction between thermal emission and blackbody emission.
ProfRob
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Could you clarify what you mean by "thin"? As in a small absorption cross-section or otherwise transparent to the radiation? Or that the amount emitted is so small as to be thermodynamically negligible? – Buck Thorn Nov 20 '19 at 13:07
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1@BuckThorn I mean almost transparent. – ProfRob Nov 20 '19 at 20:50