My usual contribution to simialr posts:
Atmospheric effects are chaotic. If your shot is long enough, any repetitive movement in your mask will be discernible. That may not be a problem. If you watch any animation that has large crowd scenes in it you will notice cycling.
What specifically are you looking for? More like the fake camera glints, glow, atmospheric turbulence or just some generic brightness variation? The last could be done using a second instance of Fractal Noise using Multiply blendiung mode, turbulences would probably use Turbulent Displace with tiny settings. glints would require specific effects or multiple layers with glows and directional blur... Also keep in mind that most of that is actually camera artifacts or atmospheric disturbances and depending on your actual shot none of them may apply...
the stars are basically tiny white dots against black. I don't need anything super fancy, i would just like them to twinkle a little bit to give it more of a variety.
TimeRemapper's post above is a good suggestion.
To break it down:
• Create a new comp, same size as your star comp.
• Add a solid.
• Apply Fractal Noise.
• Keyframe the evolution and/or transform properties so you have a fairly rapidly changing noise cluster.
• Add the Noise comp as a layer to the Star comp, immediately above the star layer.
• Set the star layer's Track Matte switch to Luma Matte.
• Refine the speed and scale of the fractal noise until you get the look you want.