It's about 9 months ago that I last put this technique to use so it was a good idea to have a little refresher. This demo will show how to add an alpha channel onto an image in Photoshop to mask (remove) any unwanted area on a file texture.
This was the technique I used for the D&AD Black Hole Brief last year. I tried this but it didn't get the desired result, the image looked all scatty around the edges where the black fades into light.
I decided to make a transparency map by turning the image grey scale and mapping it to the transparency node in Maya. Apply the colour and transparency files to a new lambert the assigning the lambert materiel to a polyplane gave me a satisfiable result so I duplicated the polyplanes. Back in photoshop I varied the colour by adjusting the hue and saturation, saving several colour variants.
Assigning different coloured galaxies to polyplanes and randomizing the size and position made my galaxies look as though they are within a Universe. The bottom image shows the transparency against a white background.