The study of equilibrium and stability of different gravitating systems must evidently play an important role in the construction of the picture of their evolution. The instants when the system loses its stability are defined by the critical points of the line of evolutionary development, when the smooth evolution of the system should be replaced by a rapid reconstruction. Some authors  indeed observe some traces of jumplike transformations which have occurred in our Galaxy. On the other hand, the smooth evolution must evidently follow the sequence of quasistationary states. On the line of evolutionary development, the quasistationary states present such points, to which all the stellar systems observed in sufficient abundance must correspond. This is similar to the situation in the world of stars , where the spectrum-luminosity diagram shows the presence, in appreciable quantities, of only stars in the regions of the diagram familiar in its stationarity (above all, on the Main Sequence): the regions corresponding to strongly nonstationary states are very rapidly “rushed” by the evolving system.
KeywordsVelocity Dispersion Globular Cluster Stellar Evolution Elliptical Galaxy Stellar System
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