Use Stock Images of Stars
If you don’t have any star photos of your own, you can always use stock images. Simply type “Stars” into your stock image website of choice and find your favorite photo.
To composite the star photo into your original image, use the move tool and click-and-drag the star image onto your original photo. Resize the stars if necessary, just be sure not to stretch them out.
If the star photo isn’t large enough to fill the entire sky, try duplicating it and moving the layer to fill in the gaps. As long as there aren’t any recognizable patterns in the stars, it shouldn’t be apparent that the photo has been duplicated.
How to Use Blending Modes and Clipping Masks
To make the stars blend into the night sky, change the blending mode of the star layer to “Screen.” Screen blend mode makes anything black invisible, leaving only the light visible, which is perfect for star photos.
If you find that the star image isn’t blending in well, try making the dark areas even darker – making them disappear.
Create a Levels Adjustment Layer and clip it to the star layer by right-clicking on the Adjustment Layer and selecting “Create Clipping Mask.” Now, move the mid-point slider in the Levels Adjustment layer until the stars blend perfectly into the original sky.
Be Sure to Mask the Foreground!
When adding stars to a sky, you want to be sure they don’t show up in front of trees, houses, and mountains.
Create a selection using the magic wand tool of the original sky. The important part of the selection is the area where the sky meets land, don’t worry about getting the rest of the selection perfect. Once the selection is active, click on the layer mask icon to add a layer mask to the star image or group.
Fill in the rest of the sky with on the layer mask and the stars will only be visible in the sky!