Different Blend modes can help you to blend different objects together for compositing images. Recently, I was playing around with a few space images and came across a comet that I wanted to blend into a space background. This was quick and easy:
Since the background is dark and the comet is white, I set the blend mode of the comet layer to screen. This makes anything that is black disappear, leave white and different shades and tones.
Then, easily, with a mask I used black to conceal, and painted around the edges to blend the comet a little more. This was fairly convincing, and without screen, would have been difficult to convincingly mask or extract without harsh edges. The best part is that I used all non-destructive techniques in order to blend the two images together. This technique makes it much easier to blend and mask white or bright objects and combine them with other objects for a composite.
You don’t need to be exact, just as long as you take care of the biggest part. Screen mode itself takes care of the rest. Here is the end result: