Through nuclear fusion, the Sun can (or at the very least, someday will) produce atoms of all elements up to and including oxygen. And in terrestrial chemistry at least, when you combine oxygen, hydrogen, and a small amount of heat you get water. Or lithium and oxygen and heat will produce lithium oxide. Those individual components are all readily available in a star like the Sun.

So my questions is, does the oxygen produced by the Sun (or any other star, now, in the future, or in the past) react chemically (for instance, through oxidation or combustion processes) with the other elements present in the star's atmosphere?

Or I suppose more generally, do the elements formed through nuclear fusion in a star chemically interact to produce more complex molecules (and if not, then why not)?

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    $\begingroup$ Please note that elements heavier than oxygen will be produced in the Sun (in the AGB phase) via the s-process (neutron capture). $\endgroup$
    – ProfRob
    Nov 4 '15 at 10:34

The Sun is a small main sequence star. It does not produce oxygen via fusion. It can't. The temperature and pressure in the Sun's core are too low. Fusion in the Sun is currently limited to production of helium. This will remain the case for several billion years.

That said, there is oxygen in the Sun, about 1% by mass. This oxygen was produced long ago by other stars at the end stages of their lives. Our Sun is a third generation (or more) star. Most of the Sun is far too hot for those oxygen atoms to combine chemically. One exception is sunspots, relatively cool areas on the Sun's photosphere. (Relatively cool means less than 4500 kelvins, so still quite hot.) Molecules can form at these lowish temperatures, and scientists do see signatures of many different molecules in the light coming from the Sun.

**Update, in response to edits to the question**

Molecules cannot form inside of a star. The temperatures are just too high. Molecules decompose (split apart) into their constituent parts at high temperatures. The Sun's photosphere is about 5800 kelvins, which is already too hot to sustain very many molecules. Temperature rises rapidly with increasing depth below the photosphere. The Sun's core temperature is about 15 million kelvins (27 million Fahrenheit), and the Sun is a small star. Larger stars have even higher core temperatures. At 15 million kelvins, there aren't even atoms, let alone molecules. There are instead atomic nuclei and electrons. Atoms are stripped of their electrons at those extreme temperatures.

In five to seven billion years, our Sun will have fused all of the hydrogen in the core into helium. That's when our Sun will become a red giant. Even then, it still will not produce oxygen. The first stage a one solar mass star experiences after leaving the main sequence is the red giant phase, where the core is an inert mass of helium surrounded by a shell of fusing hydrogen.

Eventually (after another billion years or so), the temperature of that helium core will rise to the point where the helium starts fusing into carbon, plus a little bit of oxygen via the first step on the alpha ladder. At this point, the Sun will leave the red giant phase and join the horizontal branch of the Hertzsprung–Russell diagram. This is a rather short-lived phase of a star's life. The carbon and oxygen produced by helium fusion quickly (in stellar timeframes) form an inert core. At that point, our sun will become an asymptotic red giant.

The red giant and asymptotic red giant phases are rather messy affairs, wracked by convulsions where the star expels lots of gas. Our Sun will lose about half its mass to such convulsions. Molecules do form when this expelled gas cools. This results in some of the prettiest pictures in astronomy, shown below.

(source: inspirehep.net)

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    $\begingroup$ Not sure if the first point is relevant, or even accurate. According to the information here, here, and here the Sun is massive enough to produce oxygen. If not today then at some point in its lifecycle. $\endgroup$
    – aroth
    Nov 4 '15 at 4:50
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    $\begingroup$ @aroth: The first point is entirely relevant and is complete accurate. Read your second and third references carefully. I'm not a fan of wikipedia, but it's certainly better than yahoo answers. The Sun will eventually produce oxygen, but first it has to deplete all of the hydrogen in the core. Then it will become a red giant (and it still won't be producing oxygen). The Sun will only produce oxygen when it leaves the red giant stage to join the horizontal branch. That won't happen for several billion years. $\endgroup$ Nov 4 '15 at 5:08
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    $\begingroup$ Yes, which is why the "It can't" isn't accurate. It can and will, just not for awhile yet. A long, long while, true. But the Sun several billion years from now is still "the Sun", just as it was still "the Sun" some ~4 billion years ago when it started its existence. As for relevance...the substance of the question was about what happens to chemically reactive elements in a star and why (which the second half of your answer addresses), not about whether or not the Sun is producing oxygen right now. $\endgroup$
    – aroth
    Nov 4 '15 at 5:15
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    $\begingroup$ @aroth: Your question was phrased in the present tense at the time I wrote this answer. (The second paragraph in your question is still phrased in the present tense.) I answered the question you originally wrote, not the question as currently edited. $\endgroup$ Nov 4 '15 at 5:35
  • $\begingroup$ In the present tense, but also with a general summary that was meant to make it clear that the subject was "what happens to reactive elements when they exist". Anyways, I've edited the question to hopefully make it more clear what was actually being asked. $\endgroup$
    – aroth
    Nov 4 '15 at 5:54

The answer is yes. Molecule formation is common in the outer photospheres of cool stars, and those molecules frequently contain oxygen. Obvious and common examples are TiO, VO.

This chemistry almost entirely happens when temperatures fall below 5000K, because otherwise the molecules are dissociated. Therefore it never occurs in stellar interiors.

The Sun will form molecules (involving O) in the later stages of its life, first in its atmosphere when it becomes a red giant (in small quantities). The oxygen in these molecules was not produced in the Sun, it was already there. More extensive molecule formation happens when it ascends the Asymptotic Giant Branch, prior to losing about half its envelope to thermal pulsations and a strong stellar wind. Here, some of the oxygen was made in a helium burning shell inside the Sun and transported to the surface by convective mixing.


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