Here are all the polishes I used! From left to right: NYC "Grand Central Station" (top coat), Sally Hansen "Complete Care" (base coat), Sally Hansen "Midnight In NY" (base color), Sinful Colors "Let Me Go", Orly "Chantilly Peach", Sally Hansen "Marine Scene", Milani "Green Glow", Sinful Colors "Tokyo Pearl", Orly "Love Each Other", and last but DEFINITELY not least, Wet N' Wild "Kaleidoscope". You can really use whatever colors you want for the "nebula" and/or base but the trick is that the base must be very dark (hence why black works well) but most importantly, all the "nebula" colors MUST be sheer/pale shimmers. Opaque or cream colors would not layer well and would break up the blending very quickly. The only other thing I used besides these polishes is a makeup sponge that I cut into smaller pieces to get more use out of.
The layer that really gives the nails the galactic feel, however, is the glitter topcoat. Although I'm normally really finicky about polishes and tend to steer clear of Wet N' Wild and the other $1 brands, I've found no glitter polish can really compete with "Kaleidoscope" for creating well spaced and appropriately sized/glittering stars. A thin coat is all you need, these nails are thick enough as is. After the glitter dries for just a teeny bit, just slap on a top coat and you're done!
See, wasn't that easy? Enjoy, and good luck!
PS: Please forgive me for the horrible picture quality today, for some reason the lighting is really odd in my living room and so I was in a rush to get these pictures done while natural light still remained. Hopefully you still get the gist of everything!