Long exposures of the night sky is easy using filters and the Levels adjustment
On a blank canvas, fill the entire space with black. This is our night sky backdrop, which can be transported on to a photo later on.
How to add stars? Well, there are a few ways but we’ve reached for the Noise filter! Go to Filter>Noise>Add Noise. Set Amount to around 150%, Distribution to Uniform and tick the Monochromatic option. Hit OK to see a mess of white noise.
Go to Filter>Blur>Radial Blur and set Method to Spin. Change Amount to 30 and hit OK. This should create a circular movement of white lines, but they’ll be too faint for star trails at the moment.
To make these white lines look more like stars, go to Layer>New Adjustment Layer>Levels. Squash together the three sliders under the Levels graph until the black shows through and there are more distinct lines of white. Instead of Flattening the whole image, press Cmd/Ctrl+E to merge the adjustment layer into the Background.