Evolution of stars decreasing in mass

Abstract
By a ‘steady’ evolution of a star is understood such a change of its main parameters with time which occurs in the range of an assumed equilibrium model and is caused by regular interior processes. So, for instance, the gradual conversion of hydrogen into helium in the convective core of a star, whose substance is not mixed fully, leads to a continuous change of its luminosity, radius T c , ρ c and convective core dimensions. However, all these changes occur all the time in the limits of the model’s state of equilibrium, and the structure of the star (radiative envelope and convective core) remains unchanged until a certain limit is reached in the ratio μ (core)/ μ (envelope) = y crit . After the value y crit , corresponding to the almost full exhaustion of hydrogen X in the convective core, is reached, further ‘steady’ evolution becomes impossible, since for the further decrease of X in the range of the assumed model there are no equilibrium configurations. The subsequent development is already connected with the reconstruction of the star—with an unstable condition.

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