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The definition of thermal radiation given by Zemansky ("Heat and thermodynamics", pag 95) is the following:

"The radiation emitted by a solid or a liquid by virtue of its own teperature."

What is the meaning of the expression "by virtue of its own temperature", exactly? What is exactly the menaning of the expression "non-thermal radiation"?

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Thermal radiation is emitted from bodies/gases/objects where the distribution of energy states and particle speeds has reached an equilibrium and these distributions are determined solely by the temperature of the body/gas/object.

In non-thermal radiation, the emission may come from charged particles whose speeds are not determined by temperature (e.g. particles accelerated by magnetic fields - synchrotron radiation, or by electric fields- non-thermal bremsstralung from an X-ray target medium) or where the populations of energy states are not determined by temperature (e.g. a population inversion in a laser).

EDIT: There is some confusion about the first paragraph. What depends solely on the temperature is the distribution of energy states and particle speeds, not the radiation spectrum. The radiation spectrum can also depend on the density, composition and geometry of the thermal emitter. This is largely how we are able to find out information about the universe from radiation - for example the composition of stars, which are to an excellent approximation sources of thermal radiation. Note also that thermal radiation isn't a synonym for black body radiation. For the latter, no information can be obtained from its featureless spectrum other than the source temperature and an emitting area.

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  • $\begingroup$ +1 and for further reference: physics.stackexchange.com/q/536924/226902 (since it depends only on $T$, it contains no information on the system like composition or history). $\endgroup$
    – Quillo
    Commented May 21, 2023 at 17:49
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    $\begingroup$ @Quillo thermal radiation can of course also depend on density and composition. E.g. The spectrum of a thermal coronal plasma tells you what it is made of and how dense it is. $\endgroup$
    – ProfRob
    Commented May 21, 2023 at 18:16
  • $\begingroup$ So it's strange to say that it "depends only on temperature". Maybe it' s a semantic problem: for sure pure black body spectrum tells you nothing about composition and black body is often used to model the purely thermal component. About this point, the discussion here is interesting: Why is the black body radiation independent of composition and incident radiation? $\endgroup$
    – Quillo
    Commented May 21, 2023 at 21:33
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    $\begingroup$ @Quillo what do you mean by "it"? Thermal radiation $\neq$ blackbody radiation. Stars emit thermal radiation. We know what they are made of. A gas of pure hydrogen has a rather different spectrum to say Argon. Both can have their energy levels populated according to their temperature. $\endgroup$
    – ProfRob
    Commented May 21, 2023 at 21:44
  • $\begingroup$ I agree with you, it' s just a semantic misunderstanding (thank you for the clarification). Your first sentence starts with "thermal radiation" and continues by saying that temperature is the only relevant parameter. This is why I incorrectly assumed that you're using "thermal radiation" and "blackbody" as synonyms. $\endgroup$
    – Quillo
    Commented May 21, 2023 at 21:52

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