The Rayleigh-Taylor instability forms massive complexes. When 1021 atoms cm−2 are gathered, X-rays which heat the gas and UV-rays which ionize carbon are absorbed. A layer should appear with temperatures as low as 6 K and density to 4×103 cm−3. Finally the layer is fragmented into stars whose masses may even be less than one solar mass. The temperature of the layer should increase with time because part of free carbon is gradually absorbed by dust. Therefore more massive stars should appear after less massive stars. The stars which are formed kept near the layer by its gravitation. When their total mass becomes comparable with the mass of the layer, they should fall to the galactic plane in agreement with observed proper motions of several studied stellar systems.