The sun is a gravitationally-bound ball of gas (or plasma) so it’s density should decrease continuously with distance from the center. But visually it looks like there is a boundary - and astronomers refer to the “surface” of the sun without hemming or hawing. What is that surface and why does it look so distinct?
The “surface” of the sun is just the radius at which the gas becomes opaque. Or conversely, the radius where outwardly-emitted photons are unlikely to be reabsorbed by the gas before making it a significant distance.
There is something of a fuzzy boundary, but the “fuzzy region” is so small compared to the radius of the sun that you can think of it as a firm radius.
There’s a point where the density drops to a point where the mean free path of a photon exceeds the distance between atoms and we can “see” it.
Analogously, the microwave background is the surface of the visible universe when it transitioned from opaque to transparent.
But there’s still a fairly sharp density gradient at right around the same level as the photosphere, which is still worth asking about.
IIRC - not an astronomer - the “smooth gradient” doesn’t apply. At a certain point, the gas reaches a density equivalent to liquid, where it can really compress no further; it is only kept fluid by heat. The pressure and heat is so great as to cause fusion, replenishing for escaped heat. The transition from high density to relative transparency happens quickly. After all, the transition from pretty dense to almost nothing is only about 50 miles here on earth with 1 gravity. Similarly, the transition on Jupiter with about 2.7 gravities, is pretty sharp too. The only difference is that the sun’s surface glows, so the transition is also marked by a distinct drop-off in light emission.
An aside, but I was utterly shocked by the comment in the remake of Cosmos that photons generated in the heart of the sun can take tens of thousands of years to escape, and then mere minutes later help glare out your iPhone screen. I mean, I’m not ignorant of general astrophysics, but it never occurred to me that the forces within the sun are so titanic they can trap light for eons.
My understanding is that it is the density that is so titanic … there’s not enough room for the photons to travel very far before being reabsorbed in the inner parts of the sun. There’s a horizon someplace in there when the energy begins it’s convective travel which is what we see on the surface as granulation. Chronos mentioned the photosphere and I believe this is the level where density is low enough for photons to escape finally such that this is what appears to be the surface.
I agree, pretty amazing stuff to be sure …
So you’re saying the sun eats its brightest young?
Not really. The bright ones just rattle around in there for a very long time before they happen to rattle the right way up near the surface and escape.
Sort of like a lot of kids with vastly overprotective smothering parents.
It’s not really meaningful to speak of “a photon” taking ten thousand years to make it out. There are many photons, which are absorbed and emitted many times, and there’s no basis for saying that the photon that’s emitted is the “same photon” as the one that was recently absorbed.
The question is, as always (I think) with this question: there is nothing to walk on, except hydrogen, and later, iron…?
Well, nothing’s in it’s solid state, nor it’s liquid state. I’m not sure if it could even be called the gaseous state. More of the plasma state, where the electrons are stripped off the atomic nuclei, kinda of a free flowing mix of random protons and electrons with a smidgen of helium nuclei and a passing metal here and there.
If this plasma is too dense, then any emitted photon will collide with another particle of this plasma soup and be reabsorbed. This particle emits a photon and that photon is reabsorbed, etc etc etc. At some magical level away from the center, I don’t know the scientificy name, the density is low enough the photon will have “free path” to our eyeballs and eight minutes later the photon hits our retinas. It is where this photon finally escapes the sun that we perceive the surface to be. It’s where the photon originated from.
The photosphere …
So you didn’t know the scientific name for that layer, even though it was mentioned previously in the thread, and you then say it yourself at the end of the post?
And for what it’s worth, plasmas are a subset of gases, and the Sun, except possibly at its very deepest layers, is well-described as a gas. Though this is not true of all stellar objects: White dwarfs and neutron stars are both composed mostly of fluids which are neither liquid nor gas.
Oh … it’s nothing like the Ellsworth Limit of Retardation or anything like that? Maybe we can sell ad space and call it the Photosphere brought to you by Burger King™ … jazz it up a little bit …
Certainly the density of the photosphere decreased smoothly with distance. However, it decreases exponentially (as does the Earth’s atmosphere, and indeed most ideal or quasi-ideal systems in a constant force gradient), and a characteristic feature of exponential curves is that they look very much like nothing is happening when you zoom in but like hard vertical lines when you zoom out. Plot a few exponentials and see for yourself.
The same thing is true about atoms, incidentally. The density of electrons falls of exponentially, which is why from a distance atoms look like hard little spheres, and can be well approximated as such.
The sun will never produce iron. A star has to be much larger than the sun to reach the point where it produces iron. Our sun is fusing hydrogen in to helium, which accumulates in the core. At some point after it becomes a red giant, the helium will probably ignite in a helium flash and be converted into carbon. But that’s it; the sun will never produce any elements except helium and carbon.
Larger stars will eventually, when they get hot enough, start fusing the helium core into heavier elements, like neon, oxygen, silicon and iron. Fusing iron can’t release energy, so that’s the end of the line, unless the star explodes in a supernova, during which event heavier elements can be produced by the enormous energy available during the explosion.
BTW, don’t get the idea that because hydrogen and helium are gaseous in terrestrial conditions, that you’d be able to float around throughout the interior of the sun (assuming you had some magical protection from being vaporized by the heat). The density of the sun at its core is 150 times the density of water on earth. There’s a nice graph of the sun’s density profile here. According to that, an object the density of the human body (about 1 kg/liter) would sink to about half the sun’s radius before reaching a point where the density is equal to that of water. Below that, it gets denser and at about 1/3 the radius reaches a density that we would call “solid” if it were on earth. Of course, there’s no point where there’s a sharp discontinuity in density like that between ground and air on earth, so movement would be more like swimming or burrowing rather than walking.
–Mark
We don’t call things “solid” based on their density, but on their rigidity, and the Sun is nowhere rigid.
I believe Dr. Manhattan can walk on the surface of the Sun. 
Even during the daytime?