I got about half way through the first step in writing up the directions before realizing I could just do a proper tutorial and link to that. Ha

This tutorial will help you make a stylized star system map like the ones I have posted here. As I have not prepared anything ahead of time (and am writing this more or less on the spur of the moment) I can't guarantee it won't be a little messy, but I'll do my best. This tutorial is for the GIMP, and I have no experience with other graphics programs, so I don't know how well it can be applied to others.

The first step is to make your sun. I used another tutorial for this, which can be found on Youtube, How to make a sun with gimp. The tutorial is in, I think Italian, but you can see what's going on in the GIMP, and every tool is more or less in the same place. The gist of it is that you need to make a 500x500 transparent image, give it three layers, make a coloured circle on each one (red, orange, and yellow in the tutorial), give the top two layers a mask, render plasma noise on the masks, flatten the image, map it to a sphere (remember to give it a transparent background and no light source), put a layer behind it, select the transparency on the top layer, invert the selection, use a radial gradient (the German Flag gradient is the best) inside the circle, Gaussian blur it by about 90 in both directions, recolour the blur, flatten, colourize the whole thing, and save as either a .png or a .xcf.
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The star I'm using for this map, Modra.

That's rather quick and dirty, and I may have left out steps, but I did every star I needed almost a year ago, so I don't really remember it all. If you watch the tutorial, however, it should be easy enough to see what's being done, and follow it from there.

Anyway, step two is making the map file itself. Be prepared to make a large .xcf file with a lot of layers, as I find that gives you the best control over things. As I'm making my maps to completely fill my laptop screen, the size I use is 1366x768. This is pretty useful if you have a television for a monitor, because it appears modern TVs all use roughly that resolution as well. Make the background of your image pure black.

The background layer will be a starfield. There's a simple trick to making a good one easily. First apply HSV Noise [Filters->Noise->HSVNoise] with these values:
Holdness: 7
Hue: 4
Saturation: 16
Value 190
This will add tiny specks to your plain background.
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It looks okay, but it can be a lot better. Add Sparkle [Filters->Light and Shadow-> Sparkle] with these values:
Luminosity Threshold: 0.014
Flare Intensity: 0.33
Spike Length: 11
Spike Points: 3
Spike Angle: 51
Spike Density: 1.00
Transparency: 0.00
Random Hue: 0.13
Random Saturation: 0.22
Leave the boxes unchecked, and choose "natural color" for the radial button. This gives you a much nicer background overall, in my opinion.
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Next, create a new layer, call it "Star" or whatever, and copy your star(s) onto the map. Best to stick it in a corner or centered against one of the long sides or something. I don't think one of these maps can be made well with a full-size star in the center, but it's up to you. You'll also want to take this opportunity to add another layer, "markers," on top of the star, placing a little Visible mark exactly in the center of the star. I add the star to the image by dragging it straight from the folder, so it first shows up as a separate, 500x500 layer. With this I can easily find the exact center by selecting the top left corner with a 250x250 box. Then I place a little red dot there on the "markers" layer. From there you can safely merge your star picture onto the "star" layer.
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I use the most visible colour possible.

With the star in place, it's time for the orbits. Make a new layer "orbits" above the star layer. Make sure your marker is visible in the center of the star. Select the "Ellipse Select Tool" and make sure Antialiasing is checked. Also check "Expand from center" and "Fixed." Make sure Aspect ratio is selected in the drop-down box, and place a ratio that you would like in the box. Sirius, I believe, was done with 1.33:1, and anything between 1.5:1 and 1:1 looks pretty good.

Now, select a large ellipse, starting from the marker and expanding outward, until you have an orbital path you like. Make sure the Paths dialog is opened in your Layers box, and click the little button towards the bottom called "Selection to Path." This will give you a path which you can stroke using the adjacent button. Repeat the process for each planet in orbit. When you're done, make sure the "orbits" layer is active, make sure your colour is white (or whatever colour you want the orbit lines to be) and go down the list of paths, Stroking each one in turn. I generally use a line width of 1 pixel, and the Short Dashes preset with the rounded cap style, but most settings can be made to look good.
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Modra has five planets, one of which is artificial and follows a different (1:1 instead of 1.45:1) orbit.

Continued in my next post.