1. Open your image in Photoshop
and convert to 8 bit mode (necessary for using the "distort" function).
Orient your image vertically, so that it is taller than it is
Duplicate your image into another layer.
Increase your canvas size to accomodate the two images, and
place them side by side.
You will now only work on the right hand image, leaving the
left hand (original) image untouched.
To bring a star into the foreground, it must be shifted to the
right. To place it into the background, it must be shifted to the
left. The use of the "distort" function makes this easy to
accomplish (see next step).
Identify a star that you wish to shift. Encircle it
using the lasso tool with a feather of 5 pixels (experiment with
this). Go to "Filter, Distort, Shear". Adjust the vertical
shear line as desired (i.e., move to right or left). The more you
move the line, the more shift you will create. Experiment with
this to achieve the desired effect for a given star or nebula field.
You may wish to encircle the perimeter of a shifted nebula
region and apply a less aggressive shift to this area, in order to
achieve a pleasing transition between shifted and non-shifted regions.
technique takes practice but is not difficult to master. Have
fun with it!