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In a plasma, or wherever, do the completely ionized nuclei commonly absorb much EM radiation? Or any free neutrons or protons? Can astronomers detect this? Enough so that astronomers take it into account in their observations?

I've read elsewhere that protons, neutrons and nuclei, even when free of any electrons, usually (or only, depending on who you read) aborb and/or emit only hard X-rays and gamma rays...

Is this true?

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    $\begingroup$ Here's an answer on Physics.SE (also from Rob Jeffries) that explains why a free electron cannot absorb a photon: physics.stackexchange.com/a/225538/123208 $\endgroup$
    – PM 2Ring
    Jul 24 '20 at 21:41
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    $\begingroup$ Free electrons DO absorb and emit photons! It is called Thomson and/or Compton scattering.... $\endgroup$
    – Kurt Hikes
    Jul 24 '20 at 22:10
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    $\begingroup$ Also, if free electrons didn't emit light, why does the sun shine? It is made of mostly free protons and electrons... $\endgroup$
    – Kurt Hikes
    Jul 24 '20 at 22:15
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    $\begingroup$ Scattering is not absorption. "Free" electrons that are accelerating in the electric field of a positive nucleus can result in the emission or absorption of light. It's called bremsstrahlung. $\endgroup$
    – ProfRob
    Jul 24 '20 at 23:06
  • $\begingroup$ Do you feel the current answer is sufficient? I've been going through my old questions and looking for answers I'd forgotten to accept and I ran across this post by accident. $\endgroup$
    – uhoh
    Aug 6 at 0:04
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You cannot have free protons without electrons. Plasmas, in general, are electrically neutral.

It is usually electrons that dominate the scattering (note that a point-like charge cannot absorb a photon and conserve energy and momentum) in a plasma at low photon energies. That is basically due to their much lower masses (classically you can think of the electric field of the incoming light accelerating the particles). NB. Electrons in the electric field of ions can emit or absorb photons in a process called (inverse) bremsstrahlung.

High energy (>MeV) photons are capable of causing transitions in the energy levels of nuclei, so can be absorbed. At even higher energies the photons can interact with a nucleus and create particle/anti-particle pairs - pair production.

Very high energy (>100 MeV) photons can scatter from protons and neutrons because these particles have internal quark structure. It is even possible for photons to be absorbed, creating short-lived 3 quark resonances.

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  • $\begingroup$ If protons and nuclei do not generally absorb and/or emit most photons, how do we know which elements are in which stars? The emissions from all the atoms in a star, from hydrogen and helium to the 'metals', are from completely naked, fully ionized nuclei, correct? $\endgroup$
    – Kurt Hikes
    Jul 24 '20 at 22:14
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    $\begingroup$ @kurthikes Spectral absorption and emission lines are due to the transitions of bound electrons in neutral or partially ionised atoms. $\endgroup$
    – ProfRob
    Jul 24 '20 at 23:01
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    $\begingroup$ @kurthikes The photosphere of the Sun is not completely ionised. $\endgroup$
    – ProfRob
    Jul 24 '20 at 23:10
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    $\begingroup$ Free neutrons (also mentioned in the question) aren't very abundant because they are unstable - they decay into other particles with a half-life of about 10 minutes. $\endgroup$ Jul 25 '20 at 0:28

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