Processes underlying the stellar evolution are characterized by a wide variety of time scales, such as hydrodynamical (т h ), thermal (т th ) and nuclear (т n ) times and a time characterizing the weak interaction rate (т β ). Throughout most of the evolution, from the phase of the young contracting star to the late evolutionary phases, T h remains the smallest of all the time scales. In massive presupernova models, under conditions of nuclear equilibrium, the time scale T n is the smallest one (see Sect. 10.1). In an ordinary star there is a rough equilibrium with respect to rapid processes (static equilibrium, for example), while the evolutionary lifetime is determined by one of the slower processes, as follows
KeywordsVariational Principle Dynamic Stability White Dwarf Trial Function Thermal Time
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