Once you have generated your favorite fractal image, you can convert it into a fractal starfield with the 'a' transformation (for 'astronomy'? - once again, all of the good letters were gone already). Stars are generated on a pixel-by-pixel basis - the odds that a particular pixel will coalesce into a star are based (partially) on the color index of that pixel.
(The following was supplied by Mark Peterson, the starfield author).
If the screen were entirely black and the 'Star Density per Pixel' were set to 30 then a starfield transformation would create an evenly distributed starfield with an average of one star for every 30 pixels.
If you're on a 320x200 screen then you have 64000 pixels and would end up with about 2100 stars. By introducing the variable of 'Clumpiness' we can create more stars in areas that have higher color values. At 100% Clumpiness a color value of 255 will change the average of finding a star at that location to 50:50. A lower clumpiness values will lower the amount of probability weighting. To create a spiral galaxy draw your favorite spiral fractal (IFS, Julia, or Mandelbrot) and perform a starfield transformation. For general starfields I'd recommend transforming a plasma fractal.
Real starfields have many more dim stars than bright ones because very few stars are close enough to appear bright. To achieve this effect the program will create a bell curve based on the value of ratio of Dim stars to bright stars. After calculating the bell curve the curve is folded in half and the peak used to represent the number of dim stars.
Starfields can only be shown in 256 colors. Fractint will automatically try to load ALTERN.MAP and abort if the map file cannot be found.