I paint a lot of galaxies, and this is one of the paintings i originally made for the class. However, the filming didn't go exactly as planned, so I didn't use it for the video.
First I used masking fluid for the stars.
After the masking fluid had dried, I added some light pink in the middle of the painting. For this piece, I also used some black ink along the sides of the painting, to add more dimension. Mixing watercolors and ink create a cool effect.
When the first layer was dry, I added a bit more ink to make the contrasts between the light and the dark bigger. I also added a bit more shadow to the planets after it was dry.