some tips.
your starfield has good depth, but i think what i would try is with a soft round brush (whatever size needed + whatever opacity) lightly fade out the stars behind and around the stars and planets. enough so you can see them, but it focuses more and its more realistic.
your nebular has great colours just it looks like only clouds. if you keep it cloduy then try and add some dark clouds in the foreground so you can see the dark dust to add depth.
there may be a few too many stars. just my opinion though, great start for sure.