...and set its blend mode to "overlay" (and probably duplicate the layer).

For more fun and excitement, we'll need a bit of bigger stars. To create them, switch to Shape tool, choose Polygon, set number of sides to 4, and in Polygon options, check Star, and set indent to about 80%.

Then, when you draw a shape you will get a star you need (hold Shift while dragging mouse to align diagonally). You can also open Blending options (effects) and add a bit of Outer Glow.

Repeat several times to add stars here and there.

The next part of our artwork is..s constellations. Use google image search to get the idea of what constellations are there and their layout. Then, use Shape tool (first Line tool, then Ellipse tool) to create a constellation. For each shape in constellation, open blending options and add 2-px black stroke.

Here's how our constellation looks when pasted into wallpaper. Repeat these steps for each constellation you want to add.

Now the most exciting part - the earth. Create new layer (or better, create new layer set, and inside it, new layer; it will keep your file well organized),set color to blueish purple, use Shape tool (ellipse) to create VERY big circle (hold Shift to make sure you make circle, not ellipse). Then make sure only very top of circle is visible.