Adding texture and glow to the moon would help it, as for the stars, you don't need a star shape. Just use the airbrush on different opacity and different sizes with white instead of yellow.
You should also add different hues of blue in the sky, ranging from the colour you have now to almost black.
Yes, now try adding more, smaller, brighter dots as a stars, and for the moon glow, I mean, make the airbrush bigger, on a soft brush, and use a pale white around the moon (faintly, and getting fainter as you move away from the moon itself.)