The Sun generates heat via fusion. The heat from this reaction gets distributed around the solar system and beyond. This process of spewing heat and radiation all over the place doesn't immediately seem to be reversible. "Everyday" processes that generate and spew heat and radiation (such as burning a log in a fireplace) generally increase entropy, but these processes are molecular in nature and not nuclear, so it is not obvious to me that the analogy is apt. Does the Sun increase entropy in its neighborhood in the way that a burning log increases entropy in its neighborhood?
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2$\begingroup$ The "spewing heat all over the place" part certainly increases entropy, regardless of the source of the heat, since it moves heat from a hot location to a cold location. $\endgroup$– The PhotonCommented Feb 2, 2023 at 1:24
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2$\begingroup$ Every real process generates entropy. The entropy production rate scales with the temperature difference squared, so yes, it’s quite large for sunshine. $\endgroup$– ChemomechanicsCommented Feb 2, 2023 at 1:32
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$\begingroup$ "Does the Sun increase entropy?" Of what? Of itself? Of some isolated system of which it is a part? Or what? $\endgroup$– hftCommented Feb 2, 2023 at 3:54
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$\begingroup$ @hft is the Sun part of an isolated system? $\endgroup$– HimCommented Feb 2, 2023 at 4:12
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1$\begingroup$ I am not necessarily suggesting that. I'm not going to answer this question. However, I will say that the thermodynamics of systems that are dominated by gravity (e.g., a solar system) have some counter-intuitive properties as compared to "normal" "everyday" systems (e.g., a slab of meat on a grill). So, be careful. $\endgroup$– hftCommented Feb 2, 2023 at 17:53
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Yes, the processes in the Sun result in increasing overall entropy of the universe. Entropy in neighborhood is a poorly defined quantity - the closest to this that I could think of is the local entropy - that is entropy average over a small volume, such as appears when we reduce Boltzmann equation to hydrodynamic approximation. However in case of the Sun this would probably mean averaging over a volume much larger than the Solar system.
In general, entropy production necessarily accompanies all the physical processes (happening on macroscopic scale). See, e.g.,
Entropy production in non-equilibrium systems: physical interpretation?
Maximum Principle vs. Minimum Principle in Non-equilibrium Thermodynamics
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$\begingroup$ Are "total entropy of the universe" and "increase in total entropy of the universe" sensible things to talk about? I'm happy to edit the question to use this language if this is what makes the most sense. $\endgroup$– HimCommented Feb 2, 2023 at 9:55
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$\begingroup$ "However in case of the Sun this would probably mean averaging over a volume much larger than the Solar system." I wonder that, even taking the whole Milky Way into account, probably a very large proportion of the Sun's radiation doesn't get absorbed by anything, and just goes out into the void. In that sense, would totaling over any volume and just ignoring the energy lost to the void (nearly all of it) still show an increase in entropy? $\endgroup$– HimCommented Feb 2, 2023 at 10:02