system
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human
Why isn’t the Sun more uniform?

I’ve been seeing a lot of Sun photos lately- flares, spots, canyons, etc. After millions of years, why hasn’t gravity or diffusion or a similar process made the Sun homogeneous and uniform in appearance? Or is it, but it’s just at such a large scale that even small variations stand out?
gpt
Mostly because the fluid in the Sun is highly turbulent and this turbulence is constantly driven by the generation of heat through nuclear fusion. It is a very sensible question, given the fluid dynamical problem you are considering, to ask how strong the effects of diffusion are (since it is diffusion that acts to smooth out the small scales) and what timescales they operate on.

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Diffusion has not smoothed it simply because the timescales for diffusive processes in the Sun are extremely long (it is fairly straight forward to compute these timescales). The timescale for molecular viscosity is on the order of 10^6 billion years, the timescale for the diffusion of the magnetic field is 10^2 billion years. Both of these are longer than the current age of the universe let alone the age of the Sun. Thermal diffusion operates much faster but even then its timescale is of the order of 10 billion years which is the same timescale as the lifetime of the Sun. As such diffusive effects operate far too slowly to smooth things out which is why you see so much turbulence on the surface (which is driven by thermal convection) and various magnetic features (driven by the Suns internal dynamo which is driven by convection and differential rotation).

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The reason why gravity has not smoothed it out is a little more subtle. It is trying to and has done a very good job of it considering how spherical the Sun is. However, it has to compete against the buoyant rise of convective plumes due to the nuclear fusion occurring within the Suns deep interior as well as the buoyant rise of magnetic flux.