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The Vogt-Russell theorem is the proposition that a star's structure and evolution (its structure over time) depend only on its mass and chemical makeup. Despite the term "theorem", it is not "proved" as would be a mathematical theorem. However, it is accepted as typically approximately true for main sequence stars in hydrostatic equilibrium, based on observation and current understanding of stellar structure. The "theorem" was devised when the mechanics of binary stars revealed stellar masses, which could be compared with spectrography and determinations of their luminosity. Among the confounding factors are the effects of interactions with companion stars, such as mass transfer (e.g., Algol).