Star formation
In the figure you can see a simulation of a simple model for SSPSF on a circular grid. It is generated by randomly starting star formation in certain boxes of the grid, which propagates to nearby boxes in the grid while time progresses. Star formation dies out with time and a box has a certain regeneration time which prevents it from starting new star formation just after it was active. Adding (differential) rotation to the disk during propagation creates spiral patterns that are of the same nature of those in actual spiral galaxies. Dark spots are areas of active star formation, lighter spots are areas of recent star formation/areas in regeneration.