This is possibly one of many ways to create basic star skies:
Create a new layer
Fill the layer with a black.
Use the noise filter to create monochromatic, gausian noise (10-20%).
Open the IMAGE/ADJUST/THRESHOLD palette and use the slider to fine tune the "star" density.
Use the magic wand and select the black area
SELECT/INVERSE
EDIT/FILL/WHITE (your stars)
COPY
PASTE to new layer (on this layer you can choose individual stars using SHIFT/LASSO and trasform them to make them bigger or to create star formations)
EDIT/FILL the first layer with the desired colour or delete it you have a background you want to use.
Repeat the process on new layers with varying star densities. Delete unwanted areas of these new layers with a mask that has feather on it to create varied star groupings
Here is a simple example:
Ron Winslow
A good plan today is better than a perfect plan tomorrow.