I agree with treeform. Create a cube map by rendering the sky objects at the beginning of the level. Use a simple LOD system where most objects in the distance (most stars and far off planets) can simply be rendered as a screen sprite with modulated brightness, color, size, and alpha value.
If some objects need to move in real time, I would isolate these and render the other objects into the cube map, then render the moving objects in real time.