Non-stable motions and instability in stellar associations

The results are discussed of investigations carried out since the discovery of stellar associations. It is shown that both proper motions, provided they have been accurately determined, and radial velocities of stars in the associations indicate the expansion and subsequent disintegration of stellar associations. Results concerning multiple systems of the Trapezium type, which occur almost exclusively in associations as their cores, are discussed separately. Statistical investigations and investigations of their dynamic evolution by means of a computer, have brought evidence of the dynamic instability of these systems. An analysis of the proper motions of cores of Herbig-Haro objects entering associations, also indicates non-stationary (disintegrating) motions associated with evolution. The expansion and disintegration of stellar associations and Trapezium-type systems, the receding of Herbig-Haro objects from parental T-Tauri type stars characterize the continuous diminishing of mass density, i.e., the disintegration and dispersion of matter. This is also brought out by observations of flow or emission processes of matter from many young stars (of the R Cygni, Wolf-Rayet, Be, T Tauri, etc. types). Recently, evidence was obtained of considerable mass loss of OB stars. Finally, the results of radio-astronomical observations of diffuse matter and cosmic masers have also indicated expansive motions. Results of various observations of existing stellar associations made it possible to draw the conclusion that the trend of evolutionary processes in the present Galaxy corresponds to the disintegration and dispersion of matter, at least in the early stages of evolution