I want to be there

Cool!!
Looks like the real thing!!
GOOD JOB!!

xD You have to have that "DUDE! LOOKIT THAT'S SO KEWL!" kind of aspect to do nebulas. That's how I go about it all... I let the "OMG COOL!" affect that slowly stirs up as you do it start to take control. If you need more inspiration on this front, then I can give you quite a few links. Also, please don't feel like there's a cut and dry way to make nebulas. For all intensive purposes your method is just as valid as any other and I doubt many people go about it the same unless they are really... well... need I say more?

Have fun... mix it up... go bonkers... and try not to kill too many nuns in the process (they don't appreciate it... I asked).

Just a couple opinionated comments:
You can let those rays go out further, or you can more uniformly fade them so they purposefully end... otherwise, I'm sorta sitting here yelling "Be FREE my masked nebula rays! Be freeeee!" (I'm only talking about the bright one in the middle left, btw).

The stuff in the mid right is FLIPPIN SCHWEEET and saucy. I want to dip my tongue on the screen and show off the colors to people walking past my room.

Yeah... that was nearly the 'line that ends the critique'.

I like also what you did in the upper right, and looking at the lower right corner, I wonder if you wanted to do the same there. I mean, the smidge of stars that's there right now sorta unhinges some of the design appeal. But not a big deal. I say just maybe make it go all black if you like. There's nothing wrong with empty space... and many times people don't use empty space enough.

Now there's only one serious note, and I choose to end on it because I want it to be the one thing you for sure leave with. I really really wanna stress that it'd be nice if you clean up the jpeg compression stuff like what's at the upper left and sometimes smitten randomly in other spots. I'm not sure of how many ways you can go about doing that, but it sure is noticeable to me. This is still an opinion, but I consider it the only thing out of this entire critique that you should consider carefully. Source images are hard to come by, and perhaps that's all you had to work with, but then you can always try seeing if you can use the source as inspiration and try to hand paint some stuff along. There are tons of tutorials on it, and like you said, it mainly takes patience, but to be honest, it can't take longer than 15 minutes to do a corner like the one in the upper left.

Man, I still feel that's a little harsh of me. Please, understand this thing is a good piece of work and you put this together in a unique and attractive manner... I just assume that, getting over that, you'll definitely need pointers on where to go from here.

Sorry there aren't more people around to critique this for you. Man. If only, eh?

I'll try to do my best to be unbiased, but that's unrealistic. Use your best judgement, and please adopt your own style over mine. I'm happy to disagree, but I don't want you putting blinders to details.

dudy... has left the text box *zooms out, holding onto his pants for dear life...* .... *wait, he's actually wearing shorts...*