Why can space telescopes see through a planetary nebula? I recently read the book &quot;An Introduction to Planetary Nebulae&quot; by Jason J. Nishiyama. Although I'm not an astronomy student, I could at least understand the written texts and less the physical formulas. So, I got a nice basis knowledge now -- or that's what I like to think.
Now, I browsed through some pictures of planetary nebulae. Mostly from Hubble and such. I know that they synthesize their images by combining infrared + visible light + uv-light, but I still can't comprehend why these telescopes are able to see INSIDE a nebulae when the shape of mentioned nebulae are spherical? For example, the planetary nebula: NGC 3242 (Ghost of Jupiter); NGC 7293 (Helix Nebula); NGC 6543 (Cat's Eye).
  
You can see the exposed white dwarf. And it looks like a thin shell envelopes that star, while the space between the star and the shell is scarce, like see-through. But wouldn't that image imply that the planetary nebula is disk-shaped? If the nebulae is spherical, then shouldn't there be &quot;more nebulae&quot; between the star and the shell? Or shouldn't the inner space be tinted in the color of the nebulae, because we look at a white dwarf that's behind a shell? I've drawn my problem with Paint too, sry for my artistic skills: Pattern A is what I thought it should be; Pattern B is what I think it looks like.
Let's say we have a spherical shell of some material emitting light, much bigger than the star it surrounds. If we look right down the center of the shell, our line of sight takes us through only a small amount of gas - not enough for there to be significant emission. On the other hand, if we look at the nebula near the outer edge, our line of sight takes us through substantially more material:


Inspired in part by an image in this piece by Phil Plait.
If you look through more light-emitting material, you'll see stronger emission than if you look through less material, so in this toy model, we would perceive the edges to have strong emission. So a spherical nebula wouldn't lead to uniform emission throughout our cross-sectional view of it.
This simple spherical model holds for some cases, but certainly not all, particularly because many nebulae aren't spherical (Wikipedia makes the claim that only ~20% are truly spherical, which I haven't been able to verify.) For example, we know that the Ring Nebula isn't actually a sphere because the central surface brightness is much lower than the projection effect I described above would predict. Something else clearly must be going on. In this case, the discrepancy is solved by the fact that the Ring Nebula is a bipolar nebula, with its axis along our line of sight, with a dense ring of gas around its equator. Therefore, there's more material near its edges than its center from our perspective, and so we see more emission.