Perez function with alpha ?

I don't know the perez function, but if you're using shaders, apply your alpha depending on the color/brightness of a pixel or vertex. Remember that color is a 4 component (rgba) vector (float4 in Cg). The 4th component (a) is your alpha component.

just a quick thought but do you even need the alpha component to accomplish this? If your starmap is essentially black, you could use additive blending based on source and destination color to add the sky color on top of the star map. As the sky color becomes darker by time of day or altitude, more of the stars will show through.