Start by creating a new document. I suggest you make it rather large (the example is 512px) since you don’t want the stars in the background to look repetitive. Then make a new layer and fill it with black.
Next we will create the stars. Go to Filter » Noise » Add Noise, and check the Gaussian and Monocromatic options. You don’t need too much noise—just enough to evenly cover the surface without overdoing it (for the example an amount of 12 was used).
Now, apply Filter » Blur » Gaussian Blur, to thicken up some of the soon-to-be stars (a setting of around 0.5 should work). Then, choose Image » Adjust » Threshhold, and drag the slider to the left to about 38 or so. Don’t worry! It won’t look terrible like this for long =)
Next we’ll soften it back up with another pass of Gaussian Blur. Depending on how you want your stars to come out, a setting of around 0.7 (but preferably not much over 1.0) should work well.
Now, we’ll add some detail… make a new layer, make white your foreground color, and choose the Airbrush tool. Drag the Pressure for the Airbrush down to a very low setting. Now pick a star and hold the Airbrush directly over it’s center and paint just a little bit of glow onto the star. Repeat this on a number of stars throughout your starscape, and be sure not to put the glowing stars too close to each other. If you finish and decide you’d like to reduce the amount of glow, you can lower the opacity for that layer a bit until you’re satisfied. When done, merge the two layers together.
The last step, if you plan to turn your starscape in to a seamless background, is to ensure that the four sides of your image will tile properly. To do this, go to Filter » Other » Offset, check the Wrap Around option, and input a horizontal and vertical value of about 1/2 the dimensions of your image. If you notice any places that don’t match up, correct those spots with your Airbrush (set to black). Once everything lines up, save your image and use it as a background for your web site, or whatever you like!