The fringing on the star is the problem. From everything I've done, this should work correctly. What finally made me realize that this must be a bug is that the star looks perfectly fine on the new map screen, and the image shown on the mini-map has the same glitch as the main screen!
Additionally (and not pictured due to it not really being noticeable from a screenshot) is that the star's corona, the transparent part, seems to shimmer and change. Little speckles appear in some of the areas when moving. It may have something to do with the glow from the star changing the background BEFORE the star is drawn.
Am I doing something wrong, I wonder? Or is this really a bug with the way the game handles transparency? All masks are being done as 8 bit Greyscale BMP images, and the textures are JPG. All the settings seem to be identical to those from the vanilla game, which also appear to have this bug (but I couldn't snap a screenshot).
I think something is terribly wrong with the way the game handles transparency, and it's old, legacy code from a decade ago. This would all be so much easier if the game just supported transparent PNG images in a sane way.
As an experiment, I removed spaceColor and maxLightRadius from the star's station type, and this is what I got:
So that's confirmed. The spaceColor attribute absolutely destroys transparency. It's a game bug, after all.