And the nebula is pretty cool but it could use some more defined shapes, check out this dudes gallery for some cool images
This is the bane of my existence. I can't seem to get sharp, defines shapes and lines without it looking absolutely retarded. Any tips?
Try to keep from using sharp, defined shapes and lines when you want to draw nebulae? Sorry, but I think what Jopax means is that you need to use more detail, or more accurate detail, rather than trying to create the oxymoron "a sharply defined nebula". Take a few moments to focus individually on small (200x100), random sections of your drawing (using window resizing if necessary), then compare each ~wisp to various shapes of vapor/cloud/smoke/fog in photos (Hubble or otherwise); do those pieces really look like gas?* Because to me, they all tend to look a lot more like opaci-brush squiggles surrounded by blur; they lack the suggestion of 3d shape that comes with a 3d object. Might want to spend a while looking at photos you've dug up and imagining what shapes produced them.
Now, in the nebula photos you might have noticed that the stars tend to act as both foreground and background; where sometimes the nebulae occlude them, contain them, or get lit up by them. It helps especially here to remember that those gases do not actually glow visibly; all that colored light comes from stars in or around the nebulae.
(This is all completely ignoring that without the preface the drawing looks like it was supposed to be a comet in the middle of breaking to pieces.)
Also, take a look at the times that guy said he took for his nebulae; 4-7 hours each work— or in other words, he's probably spent about 120 hours on just that first deviantart page of paintings. Practice power.
*Or, in a different spot, do those •'s and randomly oriented +'s look like stars? Remember that the + is a result of the camera taking the image or artificial lens effects, so them being different orientations implies a bunch of different images stitched together. Also, said stars look like they would through an atmosphere over a city, instead of from a space telescope.