you'll have to do something like this: create the starless image, then load both the starless image and the original image, and subtract the starless image from the original using pixelmath. if you are using mono images, now you have the best starmask that could ever be created. if using RGB images, you'll probably want to extract the L* from the difference image and use that as a star mask. maybe for good measure you might blur the star mask just a little bit with Atrous.
then you can do any number of things including the typical morphological transformation to dim down stars, or perhaps do a different stretch of the original image where the stars are not pushed as much, and then use pixelmath to replace the stars in the original image, thru the mask, with the dimmer stars.
rob
edit: also i forgot that of course you could blend the starless and original images together with pixelmath. although there are probably some screen-type formulae that might work best, doing a weighted average of the two images would probably be a reasonable start.
