3D numerical simulations of an asymptotic giant branch (AGB) star
astro.uu.se
astro.uu.se
The website links to videos of simulations of an asymptotic giant branch (AGB) star of 1 solar mass with surface temperature of ~2700 K (therefore, roughly how our Sun would look like about 5–7 billion years from now). These were produced using CO5BOLD (COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions)[4], developed by a team in the University of Uppsala, led by Dr Bernd Freytag.
In such evolved stars, the outer layers are extremely turbulent and diffuse. The giant convection cells produce both large outflows of matter and inflows to the core with fresh hydrogen; fusion is mostly confined to a roughly Earth-sized region at the core (± one order of magnitude).
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One might have also seen the 'onion shell-burning model' for high-mass stars[5]; many times, such diagrams are also misleading, because they might lead one to think that the entire star is composed of these layers, when in fact, they represent only the (again, roughly Earth-sized) core and not the large, diffuse, and convective envelope of mostly hydrogen. This[6] is a more correct diagram, and even so, the central circle ought to be much smaller.
Very high-mass stars (worth noting that mass is not necessarily correlated with volume) frequently eject huge volumes of their hydrogen envelopes, as seen in the Homunculus nebula[7] surrounding the η Carinae system[8] (both stars are extremely massive: combined, they contain around a hundred solar masses).
In fact, the outer atmospheres of these stars are diffuse enough that they are significantly less dense than air at sea level
[1]: https://www.youtube.com/watch?v=M4M6wlBjU38
[2]: https://www.youtube.com/watch?v=i93Z7zljQ7I
[3]: https://www.youtube.com/watch?v=3mnSDifDSxQ
[4]: https://www.astro.uu.se/~bf/co5bold_main.html
[5]: https://commons.wikimedia.org/wiki/File:Evolved_star_fusion_...
[6]: https://supernova.eso.org/exhibition/images/0406_3_DUM/