I'm seeing stars!

I’m working on a space scene, using 2 render layers. Layer 1 has a sun and in the world settings, I have stars checked. Layer 2 has one spaceship chasing another. When I render the final image, the background stars show up in front of the ships. I’ve tried the following:

  1. Unchecking sky for Layer 1 (and when that didn’t work, I did the same for Layer 2)
  2. Increasing the stars’ distance from the camera.
  3. Swapping the two layers.

No matter what I try, my final render has background stars appearing in front of my ships. I’m at a loss. Does anyone have a solution for me? I’m using 2.57.

I fooled around in the compositer, and upon adding an Alpha Overlay node to the spaceship layer, the problem was solved. I don’t understand why or how, but it worked.