I've had pretty good success in using the 2D backgrounds for space animations. I'll often make several layers(each on its own 2D pic, each with its own alpha map. I'll make the stars on the "foreground"  layer bigger, and that layer moves faster, and each successive map has smaller and smaller stars which move a bit slower the farther away they are from the "camera". I'll put the nebulas and galaxys on the back layer because they are the farthest away from the camera. This can give a very satisfying traveling "star matte" behind spaceships.