A Comparison Between Homogeneous Stellar Models and the Observations.

To achieve consistency between models and the observational data for stars in the Hyades and Pleiades clusters, it is necessary to abandon the assumption that Hyades and Pleiades stars have the same helium content. It is therefore incorrect to fix the exact position of Pleiades stars in the color-magnitude diagram by superimposing the unevolved portion of the Pleiades cluster on the unevolved portion of the Hyades cluster. If the parameter (l/H) = (mixing length/scale height) in the standard treatment of convective flow is not a function of composition, then the Pleiades must be farther away (higher in the color-magnitude plane) than has been assumed in the past If the Pleiades distance is less than, or equal to, that which has been formerly assumed, then one must conclude that (t/H) is a function of composition increasing with hydrogen content. The possibility that (t/H) varies with composition suggests that one should examine further assumptions of the mixing-length theory. It is found that a basic requirement for the validity of the standard recipe-that quantities pertinent to convection vary slowly over the distance of a mixing length-is in practice violated over that portion of the convective envelope where the choice of assumed mixing length influences model surface temperature most sensitively.