How to make a nebula in few steps with Gimp
This is a very simple tutorial on creating a colored nebula using the Solid Noise filter of Gimp. The method is based on superimposing two textures generated with Perlin noise, using different frequencies/amplitudes.
The first image will contain a low-frequency Perlin noise: this will define the overall shape of the nebula; the second image will contain a higher frequency Perlin noise that will be used to give details to the first one.
First of all we have to create a new image of the size that we need, in this example I used a size of 480x320 pixels, which is the size of the iPhone screen. Now that we have our plain white picture, go to Filters and then select Render > Clouds > Solid Noise, a new window will appear with the filter options. The Random Seed contains the value used to initialize the random behavior of the noise texture, the Detail controls the amount of details of the filter and the X and Y size control the proportion and the size in both directions. Choose the values from below.
Open the layer dialog and add a new layer, name it "clouds2" and keep it active. Now select another time the Solid Noise filter and choose the values like the figure.
Change the layer mode in the Mode menu of the Layers dialog from Normal to Subtract, this will make your image darker because this method subtracts the pixel values of the upper layer from the pixel values of the lower layer. Now we have our black and white nebula, if you want to change its shape you can try with other Random Seed values of the Solid Noise filter.
Now it's time to color our nebula, so add a new layer and name it "color". Change the foreground color from black to the one that you like, I used a brilliant blue, and change the layer mode to Overlay. You will have something like the figure, mmm...perhaps you would like something brighter.
So add another layer, name it "gradient" and select the Blend Tool.
In the blend option change the default gradient clicking on the rectangle and select, or create, the one that best suits the color you choose in the "color" layer. For my brilliant blue I choose the Deep Sea gradient, in the picture you can see also that I've changed the shape from Linear to Radial and reverted the gradient (clicking on the double arrow near the rectangle).
Click in the middle of the nebula and drag the mouse to make your gradient, change the layer mode from Normal to Overlay. Now that you've done you probably have something like the figure, remember to save your work.
It would be nice to have also a star field! The easy way to create it is using Blender. Open Blender select the Shading button on the control panel below the 3D window, or press F5, and then select the World button.
In the Mist/Stars/Physics tab hit the Stars button and choose the settings for the stars size and distance checking the result on the preview image. Now select the Scene button on the panel, or press F10, and hit the Render button if it's not active, insert the size of the image of the nebula created before, select the file format PNG and hit RGBA to preserve the alpha channel.
Press F12 to render the star field and F3 to save the rendered image, call it "starfield.png".
Now return to Gimp. Add a new layer to the image of the nebula and name it "stars". Open the "starfield.png" image with Gimp copy it and past it in the "stars" layer. Change the layer mode from Normal to Value, this mode uses the value of the upper layer and the hue and saturation of the lower layer to form the resulting image, so the star field becomes colored as the nebula above it. That's it!