Age and Morphology of the Main Sequence from Hipparcos Hr Diagram Data

With the help of new recent stellar models, we have studied the distribution in the HR diagram of 2170 stars from the Hipparcos database, subdivided into diierent groups according to their total velocities with respect to the local Keplerian reference frame and into diierent groups of metallicity. We discuss the morphology of the main sequence at the boundary between radiative and convective stellar envelopes (B ? V ' 0:3) and the age of the disk and halo stars.