Stars falling up

I am trying to figure out how to make a simple animation of stars rising (similar to bubbles in other tutorials or snow, but it needs to rise rather than fall). Every time I try to modify a different tutorial, I run into the same problems. What I need is for the stars to raise in the background but I find that in most tutorials I can’t get the animation to stay in the background layer. The stars keep coming in front of the subject even though my movie clip is several layers underneath.

The other big problem is that I am working with a 946 px wide movie and I only want the stars to rise in the right third. I can’t seem to contain the width and placement of the animation. I’ve tried masking, but again, the animated stars continue to raise, it seems, on on every layer. Any ideas or tutorials?