This question was triggered by a discussion regarding the computer game Elite: Dangerous, where spaceships routinely operate in close proximity to stars (two or three light seconds away), at which point heat becomes an issue. Someone claimed that you would hardly feel any heating effect since there is so little matter around, indeed that you couldn't really tell if you are inside a star or not. I found that very dubious.
So let us assume we have spaceships and could fly around the solar system at will. Our ships have a certain degree of ruggedness to them, so people might get adventurous. What would it actually be like to approach, or even enter, a star?
We know that the corona is hot, hotter even than the surface. But it is also much less dense.
Q1: Would there actually be enough matter in the corona to make our ship heat up, or would we be able to ignore the very hot but few particles around us? What of the photosphere, or the upper convection zone — those are still much less dense than earth's atmosphere at sea level. How much would the heating affect us?
Assuming we could pass through the corona, we would reach the photosphere, the point at which the (mostly hydrogen) body of the sun becomes opaque.
Q2: Does this opaqueness only apply to a certain layer (i.e. would we be able to "see" again once we go below the photosphere), or are all lower layers of the sun opaque? How opaque is that — would we be able to see a couple of meters, some kilometers, or nothing at all?
Q2b: Would the matter directly in front of our cockpit window be bright, or dark?
Best Answer
The photosphere of our sun is somewhere on the order of 500 km thick. For a quick ballpark, you can imagine an exponential decrease in the transmission of light which about this characteristic thickness. It might be a little less, but it's still order-of-magnitude accurate. That means that you would (theoretically) be able to see local objects just fine within the photosphere.
It's difficult to compare this to Earth's atmosphere, because scattering of light is both Raleigh and aerosol, and the concentration of aerosols differs at different times. However, even Raleigh scattering attenuates more than half of the light within 100 km. That means that you can actually see better in the sun's photosphere than you can in Earth's atmosphere.
Now for the qualifier, being inside the photosphere would render human eyes useless instantly, because it would be like painting the sun's brightness over your entire field of vision. A spacecraft 100 km away could be identified with a telescope (if this telescope is not actively melting), but it would not be any brighter than the background of photosphere gas. Nonetheless, it would still be identifiable by color, contrast, and so on. In fact, if the spaceship was mostly absorbing of radiation, it would be a distinct black spot among a bright background.
I could get more specific to speculation about spacecraft. They will need to have stored thermal mass in order to absorb more heat than they emit. Reflection is a good option, but it's not perfect. Active cooling of the spacecraft's surface is likely in order to avoid melting. Thus, spacecraft would be slightly "camouflaged" by thermal necessity, but not perfectly.
Your vision will certainly decrease as you reach lower layers - since they have higher density and give off more radiation. The photosphere is only unique in that the radiation emitted commonly makes it out into space where it can continue indefinitely. For the lower convection layer zones, radiation is given off at a much higher rate, but it is almost entirely reabsorbed.
Stars above a certain mass threshold actually transmit their heat through the interior zones through a radiation gradient. So I hope this helps to illustrate that the inner zones are not "less bright". They would be blindingly bright (well the photosphere is already blindingly bright, but these would be worse), but possibly at higher temperatures, which go beyond the visible range.