Results of an n-body calculation, containing about 120000 particles, were shown as a motion picture. Some of the particles are treated as ‘gas’, obeying a special dissipative dynamics, the rest as ‘stars’. The system was started as pure ‘gas’, and ‘stars’ were made out of the ‘gas’ in a manner closely mimicking real galaxies. Spiral density wave patterns appear in the ‘gas’, and last for about 3 ‘galactic rotations’ without substantial change of form. Various experiments are described that have been undertaken in an attempt to learn the roles of various parts of the system in the maintenance of spiral patterns.