OK I did some testing last night again, here's what I found.
There are 2 "types" of nebula.
1. The usual type, described as "system wide storm", these are the ones you can destroy with a ND. there are 5 types(I think this was covered somewhere before). the red ones have a "maximum" obscuration of 5. I belive this means they can have anything up to that. The other four colors have 2 maxed at 3 and 2 maxed at 4. You cannot creat stars here untill you destroy it.
2. There are also some that are actually described as nebula, there are 4 types I believe. They are different from the five standard one everyone is used to. I think they are used only in the "ancient" quadrant type. You cannot detroy them but you can create stars in them. They have a max obscuration of 3 and cut shields by 5000(it says renders shield useless, but the systems text file says 5000). They are also much more colorful that the other kind.
I'll have to check it out to see if any of them hide stars or rings/spheres. I know they hide planets, don't hide warp points (you'd never find your way out, reminds me of a map I made once

Only bad part about it is the AI couldn't handle all reaserch being done by finding ruins). Anyway I'll check it out tonight unless someone else wants to confirm it before then.