We start with a blank, black canvas and create a circle selection with the Elliptical Marquee selection tool. Create a new layer, and then fill with black. Make sure your color palette is Black and White (press D to reset color palettes), and then goto Filter > Difference Clouds. We will want to repeat this process until we get something we like the look of for our fireball. You can press Ctrl+F to repeat the last used filter.
When you're happy with the difference clouds, press Ctrl+L or goto Image > Adjustments > Levels. We want to make the White areas whiter, so move the middle anchor to the left a little, and the same with the right anchor:
Next, goto Filter > Sharpen, Unsharp Mask. use Amount 500%, Radius 3.2px, and Threshold 10. We should be left with something like this:
Our next step is to make the fireball look more like a ball - it looks a little too flat right now. With the selection still made, goto Filter > Distort > Spherize, 100%:
Almost done now. Goto Image > Adjustments > Color Balance, and play around with the Color Levels of all three Tones (Shadows, Midtones, and Highlights). Aim for orange, yellow and red tones, like so:
You may notice that the edges of the fireball are a little rough, or pixelated. If so, Ctrl-click the layer to make a selection of it, then goto Select > Border, around 4 or 5px. Then goto Fitler > Blur > Gaussian Blur, and choose a small Radius (around 1-2px).
Finally, let's go ahead and add Layer Styles. We will want to add an Outer Glow with Color Orange, Spread 0 and Size around 25px. We also add an Inner Glow with Color Yellow, Choke 0%, Size 15px.
You should end up with something like this:
It is recommended that you experiment with the Layer Styles to achieve the result that you want. You could add an Inner Shadow to make it look more like a planet:
Or you can start all over again with a different Difference Cloud - you'll never come out with the exact same:
