They use CMPs, at which you can even look with HardCMP. Thgey are different, some are just a bunch of small stars scattered aroubd an imaginary sphere, some are real spheres with nebula texture etc., a lot of stuff is not just on a sphere but for example a wide band with the barrier texture going past the sphere, which is in the end projected onto the screen etc. Just install HardCMP, then you can open the starsphere CMPs with doubleclick and look at them (zoom out!).