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The first step is to create a new image. I used 1024x768, but it doesn't really matter. Now fill it with black using the bucket tool.
Now we can add stars (optional). Go Filters>Noise>Scatter HSV. Use the 'Value' setting to make more stars.
You can make it a bit more realistic by going Filters>Light Effects>Sparkle and playing with the settings.
It should now look a bit like this:
Now create a new transparent layer through Layers>New Layer. Make it a good size. My image is 1024x768, so my layer is 800x800 or so. But make it square.
Now create the base of your galaxy by using Filters>Render>Nature>Flame. Make sure your gradient is the default Black-to-White. Set the color setting at the bottom to 'Custom Gradient'.
Now use the 'Edit' option to get a barred spiral. Use the 'Swirl' option.
Make it larger with the Camera options.
Now your screen should look like this:
Now go Filters>Distort>Whirl and Pinch. Whirl it so that it looks a bit like a barred spiral. Make sure it's spun the direction it is already spinning.
If you want, you can duplicate the layer and spin it the other way to fill it in.
Set the layer mode to 'Screen' or 'Overlay', whichever suits your tastes.
Merge the duplicate layer onto the original layer and whirl and pinch the layer again.
Repeat these steps as often as you like until you get something you like. I think I repeated three times for my image.
Now you should have something like this:
Now go to the GIMP main window and select the ‘shear’ tool.
Shear the layer to a side, whichever you like.
You should now have something like this:
Now for that ‘edge-on’ effect. Go to the GIMP main window and select the ‘perspective’ tool. Bring the bottom left corner nearer the middle until you have something you like.
Now—
Now you can go back to the background layer and do another Scatter HSV on the layer and sparkle that for a slightly more realistic effect.
Now, if you want, you can add a bulge in the center of the galaxy. Make a small new layer, for me, 100x100. Put a supernova in the center using Filters>Light Effects>Supernova. Make it white, 1 spoke, and about as big as the layer.
Shear the layer to the side so the supernova is no longer a circle.
Go Filters>Distort>IWarp.
Use the Grow tool to make the supernova bulge a little.
Position the bulge layer over the center of the galaxy and merge it down.
Now you can add color to your galaxy. Go Layers>Colors>Colorize. Select a hue you like.
Now select the galaxy layer and change the layer mode to ‘Screen’. A few stars should poke through the edges.
Now you can repeat all these steps to create smaller galaxies you can group around the center one.
You can also Gaussian Blur the galaxies to make them look a bit farther away or more realistic.
This was my final result, and yes, I did built this result as we went along.
Here’s my very first attempt at this:
And here are others:
