Photoshop does have a layer Blending Mode that is equivalent to "Add," however this will brighten everything. In CS4, Adobe introduced a feature called "Stacks." This lets you create a "stack" out of any number of layers, and then it will FINALLY do image arithmetic on the stack. Once you have your layers in a stack, you can set the stack mode to "Max," which will produce a final product where it will look through every pixel position across all the images and only pull out the maximum pixel value. So where there's a star, it will have a star as opposed to sky because the star is brighter than the sky.
I realize I haven't explained this very well, but that's because there are so many different guides online on how to use Stacks in CS4 ... my purpose was really to give you the terminology so that you can search for it.