First Results from HIPPARCOS

After recalling the principle of the Hipparcos mission and of the data reduction, the main statistical features of the final Hipparcos and Tycho catalogues are presented. The results, characterized by accuracies in annual proper motions and parallaxes of the order of one milliarcsecond have been already widely used for astrophysical investigations described in this review. In the field of galactic kinematics, new results were obtained in the analysis of the motions of nearby stars, the evolution of OB associations and the modeling of the Gould Belt. An important result is that, on average, stars are farther apart than previously thought. Among the consequences is a reconciliation between the ages of the Universe and globular clusters. In particular, the evolutionary sequences of metal-poor stars need to be reassessed. The HR diagram is now extended to the giant horizontal branch and A stars in the main sequence. The Hyades are seen for the first time in three dimensions, allowing a detailed description of their dynamics. Some unexplained inconsistencies between the HR diagrams of open clusters were found. Other results in galactic dynamics, open clusters, variable stars, unseen companions of stars, and the post-Newtonian parameters g are also presented.