I took a series of photos tonight for a star trail shot over downtown Seattle but since the city lights are so bright the stars are somewhat dark. Once I separate the sky from the city, what is the best way to brighten the stars? If I just use levels it brightens the sky too which I don't want. Boosting the contrast brightens some stars and makes others disappear. Basically what I want to do is only brighten parts over a certain level (the stars) and leave the almost black sky completely alone. Any easy way?