If I understand what you mean, I think you must use the spherical gradient texture at the top as the ‘factor’ in the mix, that would indicate the voronoi textures where to put the stars and where to ignore the texture.
What you have now is saying something like this:
“Ok my two voronoi textures, I want #1 to be 100% and I want #2 to be added only at 25%, then this new result, I want you to combine with that white sphere, but only at 23%”
