This is a technique attributed to Russ Croman, that I found documented on the Star Shadows Remote Observatory.  At the end of the procedure is a link to download a PhotoShop Action. The file contains two actions... one for an RGB image and the other for gray scale. I couldn't get one action to work for both images. I guess I need to work on my PhotoShop development skills. <g>

  1. Star Selection (from Russ Croman):
  2. Make a grayscale copy of the image. I'll call this image #2.
  3. High-pass filter image #2 with a radius of one pixel.
  4. Apply a Gaussian blur to image #2 with a radius of one pixel.
  5. Invoke Image->Adjust->Threshold.
  6. Adjust the Threshold Level one click at a time until just the
      stars are white and everything else is black.
  7. In the original image, in the Channels Palette, create a new
     channel. Name it "Stars." Choose "color indicates masked areas."
  8. Paste image #2 into this channel.
  9. Make just the RGB channels visible (i.e. make the Stars
     channel invisible).
10. Discard image #2.
11. In the original image, invoke Select->Load Selection. Choose
     the Stars channel you just created.
12. Invoke Select->Expand and expand the selection by a few
     pixels (e.g., three).
13. Voila!