Sounds like you have colour enabled for your surface layer or whatever you're doing. You want to use no colour (unchecked) and use the luminosity function. If you are using a default shader, make the colour absolute black (0) and then, again, use the luminosity function. I haven't experienced what you are talking about except with actual colour on the background sphere is above 0.
If all else fails you could use a default shader, and luminosity, in addition use a scalar to boolean (on same star mask for luminosity), and input into the opacity function. Then the background sphere won't be visible altogether except where stars are.