Lowering the opacity won't give you smooth edges.
Create a black/dark blue filled layer. Add yellow/white/blue dots on it, then smoothen the result, then add dots in the center of the stars for those that you want to clarify.
Also, don't overdo the stars. A sky usually only have a few stars. You need to look somewhere else to find more stars. Its not that if you look into a direction you see hundreds. They are there, but due to the distance, you can't see them.