Does this really look like the galaxy or the skies to you?
Anyways, I've used a fan brush to come up with the "ombre" effect. I applied the black first using the usual nail polish brush. And then using a fan brush, I applied the light blue first, waited for it to dry and then using the fan brush again, I applied the purple. And then topped everything with glitter polish. And of course, a top coat.
I hope you like it!