
Another way to do it is by just adding the stars:
If the image identifier of the image with the stars is "im_stars", and the one with the comet "im_comet", just put the following expression in PixelMath (and check the Rescale option if you want, to avoid clipping in the cores of the most brilliant stars):
im_comet + im_stars-Med(im_stars)
Replace the identifiers with the actual ones, or rename your images.