Data from the Global Oscillation Network Group (GONG) project and other helioseismic experiments provide a test for models of stellar interiors and for the thermodynamic and radiative properties, on which the models depend, of matter under the extreme conditions found in the sun. Current models are in agreement with the helioseismic inferences, which suggests, for example, that the disagreement between the predicted and observed fluxes of neutrinos from the sun is not caused by errors in the models. However, the GONG data reveal subtle errors in the models, such as an excess in sound speed just beneath the convection zone. These discrepancies indicate effects that have so far not been correctly accounted for; for example, it is plausible that the sound-speed differences reflect weak mixing in stellar interiors, of potential importance to the overall evolution of stars and ultimately to estimates of the age of the galaxy based on stellar evolution calculations.

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Abstract. We present stellar evolutionary models covering the mass range from 0.4 to 1 M calculated for metallicities Z = 0.020 and 0.001 with the MHD equation of state Mihalas et al. 1988;Däppen et al. 1988). A parallel calculation using the OPAL We use the radiative opacities by , completed with the atomic and molecular opacities by Alexander & Fergusson (1994). We follow the evolution from the Hayashi fully convective configuration up to the red giant tip for the most massive stars, and up to an age of 20 Gyr for the less massive ones. We compare our solarmetallicity models with recent models computed by other groups and with observations.The present stellar models complete the set of grids computed with the same up-to-date input physics by the Geneva group (Z = 0.020 and 0.001, Schaller et al. 1992;Bernasconi 1996, andCharbonnel et al. 1996; Z = 0.008, Schaerer et al. 1992; Z = 0.004, Charbonnel et al. 1993; Z = 0.040, Schaerer et al. 1993; Z = 0.10, Mowlavi et al. 1998; enhanced mass loss rate evolutionary tracks, Meynet et al. 1994).

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