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Answers to Hydrogen burning vs Hydrogen fusing explain that in astrophysics "burning" generally refers to nuclear fusion or at least nuclear reactions1, and information at

explains that those are astronomical nicknames for very small bright bits of plasma recently discovered on the Sun, not actual fires.

But in the Sun's atmosphere there may (or may not) be opportunities for chemical reactions. Certainly in some cool stars the atmosphere even makes dust.

Question: The sun "burns" hydrogen and even has "campfires" on it, but has anyone calculated a rate of some actual chemical burning on the Sun? Any oxidation reactions at all? If not our Sun, is oxidation simulated for some other stellar atmospheres?

1it's hard for me to think of two protons "fusing into a diproton", later beta-decaying for example; feels more like a nuclear reaction, but maybe that's just me.

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    $\begingroup$ 1. Although atoms are separated & fully ionised deep inside the Sun, there are some molecules (eg carbon monoxide) in the upper layers. See aanda.org/articles/aa/abs/2007/06/aa6173-06/aa6173-06.html 2. Stellar dust is mostly emitted by red giants. It's dredged up at various stages, by convection. The metals in the dust can be primordial, but in larger stars they can be freshly produced. $\endgroup$
    – PM 2Ring
    Commented May 22, 2021 at 4:14
  • $\begingroup$ @PM2Ring "Arrrrr... there be chemistry in them thar starrrrrrs!" (spoken in 'pirate dialect') $\endgroup$
    – uhoh
    Commented May 22, 2021 at 7:51
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    $\begingroup$ Note that I was very careful in my "hydrogen burning" answer to avoid saying that chemical reactions don't happen in stars. ;) $\endgroup$
    – PM 2Ring
    Commented May 22, 2021 at 7:59
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    $\begingroup$ @PM2Ring Yes, but the point is that it's the elemental constituents (e.g., C, O, Si, Fe) that eventually make up grains which are "dredged-up", not the actual dust grains or even their simple molecular precursors. Those are formed in the very outer atmosphere or in outflows and winds. (For one thing, the temperatures at the base of the convection zone are millions of K, while the evaporation temperature for dust grains is of order 1000 or 2000 K.) $\endgroup$
    – Peter Erwin
    Commented May 22, 2021 at 10:02
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    $\begingroup$ @PeterErwin Ah, right. That's what I meant to imply in part 2 of my 1st comment. (Oops). As I said in part 1, it's certainly far too hot near the core for atoms to be combined into molecules, what to speak of dust grains. $\endgroup$
    – PM 2Ring
    Commented May 22, 2021 at 10:11

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I just became curious enough to search a few keyphrases.

This article talks about hydrogen molecules acting as energy "sink" in the sunspots - much like water phase changes on Earth create climate features. The heat capacity, mass density, adiabatic properties of the molecular hydrogen are pretty much different from the atomic one and these are important for the convection.

Hydrogen changing between atomic and molecular form is clearly a chemical process.

The article talks even about water molecules there. Given the high temperature and low pressure, they are expected to be pretty much short-lived and oxygen constantly moving between water, hydroxyl ion or radical and oxygen atom or ion.

Related matter here

The formation of H2 can significantly alter the thermodynamic properties of the sunspot atmosphere and may play a significant role in sunspot evolution. In addition to the survey observations, we have performed detailed chemical equilibrium calculations with full consideration of radiative transfer effects to establish OH as a proxy for H2, and demonstrate that a significant population of H2 exists in the coolest regions of large sunspots.

Looks like chemical processes not only do happen on the Sun, but are of great importance for the photospheric features we see.

Do these qualify as "burning" is a matter of oppinion.

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    $\begingroup$ If there's something in the "related matter" that addresses the question please mention the title, and a quick summary or include a block quote. Every reader shouldn't have to go off-site to find out what's there. Thanks! $\endgroup$
    – uhoh
    Commented May 23, 2021 at 13:12

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