I created the worlds using Mathmap Sphere filter and my ruby texture (both diamondworld and ruby moon). I used GIMP's Supernova filter for the Planet's sun (added lens flare effect for the sun too) as well as some of the lighting effects on the planet and moon. I also added sparkles as needed. Borrowed this image for the backdrop. Created the galaxy using a raindrop brush and GIMP's Whirl and Pinch filter. I then duplicated the layer (raindrops have transparencies) as needed and made one of the layer properties set to dissolve for the star effect. Added Supernova again for the Galaxy's Quasar. Look forward to seeing some of your results.
Below is my result that I call Diamond World just outside of the Crystalline Galaxy

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