Ooh, nice! I'm going to start learning OpenGL and in turn shaders some time, so it'll be useful. Thanks!
Side note: The shader seems to be 80% noise functions. The main function is actually quite digestible. Nice.
Yup, the noise functions are the biggest chunk of it. Basically all you're doing for the stars is intelligently shaping the noise. A big reason the noise functions are so large is that they are quite optimized to run on the GPU by their authors. The planets are far
more involved, due to their more complex topology and variation, and because they rotate, but I'm going to hold on to that code for the moment.