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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?

Qmechanic
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Him
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    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. – The Photon Feb 02 '23 at 01:24
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    Every real process generates entropy. The entropy production rate scales with the temperature difference squared, so yes, it’s quite large for sunshine. – Chemomechanics Feb 02 '23 at 01:32
  • "Does the Sun increase entropy?" Of what? Of itself? Of some isolated system of which it is a part? Or what? – hft Feb 02 '23 at 03:54
  • @hft is the Sun part of an isolated system? – Him Feb 02 '23 at 04:12
  • I am asking you to clarify your own question, not ask additional questions. – hft Feb 02 '23 at 16:15
  • @hft Sure. It seems that you maybe have some possibilities in mind? I would specify some isolated system of which the sun is a part if such a system existed. It's not clear to me that such a thing is possible. If you have suggestions for options that would make the question more clear, I am open to them. AFAIK, basically 100% of the radiation from the sun doesn't "end up somewhere", it just races out to infinity at the speed of light. Possibly "a spherical bubble the size of $c*\text{Age}_{Sun}$"? This is a bit weird, though, because the size of the system increases with time. – Him Feb 02 '23 at 17:36
  • @hft if you are suggesting that this question cannot be made to refer to a well-defined quantity because of our inability to define an "isolated system" in this context, I would upvote an answer that went into details about that. – Him Feb 02 '23 at 17:38
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    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. – hft Feb 02 '23 at 17:53
  • @hft "have some counter-intuitive properties as compared to 'normal everyday' systems". Hence my question. My question is in some sense whether the intuition that a burning log increases the entropy of its environment applies to the Sun. If it does not, because of reasons, then this is a valid answer, and those reasons are probably super interesting. Possibly someone else will find your comments informative and provide a "no" answer in this vein. – Him Feb 02 '23 at 18:05

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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

Roger V.
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  • 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. – Him Feb 02 '23 at 09:55
  • "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? – Him Feb 02 '23 at 10:02