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Let's say we are studying the integrated near-infrared (NIR) light of a distant spiral galaxy. We would expect most of this light to be dominated by red giants stars and dwarfs. I assumed these stars would radiate thermally but a source online states only the mid (MIR) and far (FIR) infrared radiate thermally. Can anyone clarify how objects in the NIR radiate? I would think it would be blackbody radiation but perhaps I am wrong.

Source

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    $\begingroup$ Someone can correct me if I'm wrong, but the way that paragraph is written, I think they're only referring to the region between 0.7 and 1.1 microns, which is only a small part of the near-infrared region. It's also worth noting that you still get thermal emission in this region; it's not like a blackbody spectrum has a gap there. My guess is they mean that the emission in the 0.7-1.1 micron range is dominated by non-thermal sources, but I'm not certain. $\endgroup$
    – HDE 226868
    Jun 18 at 23:49
  • $\begingroup$ Right, it certainly wouldn't make sense to have a gap in the blackbody spectrum. I'd still like to know how red giants and dwarfs stars radiate in the NIR. Is it primarily thermal emission or spectral line emission or both? $\endgroup$
    – Astroturf
    Jun 19 at 1:52
  • $\begingroup$ @HDE226868 “Near-IR” in astronomical contexts usually means 1-2.5 microns, or sometimes 1-5 microns. $\endgroup$ Jun 19 at 9:12
  • $\begingroup$ The "0.7 to 1.1 micron" remarks are about a subset of the near-IR. They then move on to talk about longer wavelengths, while still in the "Near Infrared" section. (The idea that stellar radiation in this region is "non-thermal" is still wrong, of course.) $\endgroup$ Jun 19 at 12:23
  • $\begingroup$ @PeterErwin I know; I was talking about the subset of the near-IR which the page claims emission is dominated by non-thermal sources. $\endgroup$
    – HDE 226868
    Jun 19 at 14:56
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Your source is mistaken. Almost all the radiation we see from stars is "thermal". That is, the radiation arises from material where the occupation of energy states in atoms, molecules and particle speeds is characterised by a temperature. This includes emission from the photosphere, chromospheres and coronae of stars.

In the case of the 1-5 micron emission referred to as "near infrared", that emission is predominantly from the photospheres of stars (both hot stars and cool stars) and it can also arise in warm dust surrounding a star (e.g. a thick, protoplanetary disc).

The only "non-thermal" radiation you get from stars can come at radio and hard X-ray wavelengths. This is associated with the acceleration of charged particles by magnetic fields in the corona.

I also noted another strange statement to the effect that stars fade in the mid and far infrared. The basic physics of blackbody radiation (stars approximate to blackbodies, especially in the infrared) is that they emit at all wavelengths. Whilst the peak of a hotter blackbody would be at shorter wavelengths than a cooler object, the hotter body will be brighter (per unit area) than a cooler body at all wavelengths. i.e., A star is brighter than a planet at all wavelengths, although the contrast will be lower in the mid and far infrared.

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    $\begingroup$ The only thing I can think of is that the source is for some reason referring to thermal radiation from dust, which would indeed dominate (spiral galaxy) light in the mid- and far-IR. $\endgroup$ Jun 19 at 9:16
  • $\begingroup$ @PeterErwin certainly that is (or could be, depending on the galaxy) true, but all this radiation is thermal. Maybe if one was talking about observing the cores of some active galaxies (BL Lac objects) there could be instances where the NIR emission was non-thermal, but the post is clearly talking about radiation from star in the NIR. $\endgroup$
    – ProfRob
    Jun 19 at 10:10
  • $\begingroup$ But then the source also says that mid-IR can only be done from space, which isn't true either. The whole thing is littered with errors (or at least gross simplifications). $\endgroup$
    – ProfRob
    Jun 19 at 10:11
  • $\begingroup$ I see that I initially missed the "Source" link. Yes, the bit implying that stellar radiation is "non-thermal" is certainly wrong. $\endgroup$ Jun 19 at 12:20
  • $\begingroup$ While it's certainly true that there are a few mid-IR windows in the atmosphere (I've worked with such data!), most of the mid-IR can only be seen from space, which is why satellites (IRAS, Spitzer, Herschel) are so important. $\endgroup$ Jun 19 at 12:20

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