Creating a starfield is easy!

Press the D key and fill the layer with black (Option-Delete [PC: Alt-Backspace]) and then apply the Add Noise Filter (Filter> Noise>Add Noise). You'll need to enter a large Amount (we used 230%) in the dialog to produce a lot of noise. To introduce some gray values to the noise, apply the Blur More filter (Filter>Blur>Blur More).

Now comes the trick to make the noise behave so you have a star field: Choose Image>Adjustments>Levels. In the Levels dialog, drag the dark value (black) slider almost entirely to the right and move the highlight slider (white) slightly to the left, with the mid-tone slider gray) between the two. Click OK and your star field appears.

from Bert Monroy