Your starfield depends on the size of a pixel. To see the problem try rendering
the image at different resolution and you'll see that suddenly more stars appear
and they're in different places.

The problem is as follows: At present certain pixels turn white because they
happen to sample your star pattern on a white point. But when you anti-alias it
the other samples for that pixel will probably sample black, so you'll end up
with all your stars vanishing.

There's 2 possible ways to solve this: Either use bigger stars (which is what I
tend to do), or alternatively use *brighter* stars. So the other anti-aliasing
samples don't reduce the colour so much. If you use stars with a colour like rgb
20 you should still be able to see some after anti-aliasing. But they will look
terrible if you try to animate them.

I use this pattern for larger stars:
sky_sphere {
 pigment {
  //create impression of multiple stars of different sizes.
  pigment_pattern {
   crackle form <1,0,0>
   pigment_map {
    [0 crackle solid pigment_map { [0 rgb 1][1 rgb 0] } ]
    [.4 rgb 0]
   }
  }

  colour_map {
   [.4 rgb 0]
   [.6 rgb 1]
  }
  scale 0.02
 }
}

Hope that helps,
-- 
Tek
www.evilsuperbrain.com

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