For top-left corner, repeat teh same steps: generate similar fractal with contextfree, remove white background, paste into wallpaper...

Christmas Night - making of - Step 11

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

Christmas Night - making of - Step 12

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%.

Christmas Night - making of - Step 13

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.

Christmas Night - making of - Step 14
 

Repeat several times to add stars here and there.

Christmas Night - making of - Step 15

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.

Christmas Night - making of - Step 16

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

Christmas Night - making of - Step 17

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.