Description:
<jats:p>The Hipparcos data are providing a dramatic increase, qualitatively and quantitatively, of the basic available distance information. For example, the numbers of stars for which a relative accuracy better than 10 % is available from Hipparcos data and from ground-based data are respectively 22 396and about 1000. Moreover, the range of spectral types and luminosity classes for which precise parallaxes are available is considerably enlarged, including many stars in open clusters and a small number of Cepheids and RR Lyrae. The bottom of the main sequence is populated down to absolute <jats:italic>Hp</jats:italic>magnitude 14, including a number of subdwarf stars essential to derive globular clusters distances and ages (Pont <jats:italic>et al</jats:italic>, 1997a).</jats:p><jats:p>Finally, the Hipparcos data show how difficult are the calibration of photometric distances and the transformation of relative trigonometric parallaxes to absolute parallaxes. This is illustrated by the comparison of distances given in the last edition of the Catalogue of Nearby Stars (CNS3, Gliese & Jahreiß 1991), which is the best available compilation of stars said to be closer than 25 pc from ground-based data. About a third of them are found by Hipparcos to be (much) further than this limit (Perryman <jats:italic>et ai</jats:italic>, 1995). A second example is given by the study of (Binney <jats:italic>et al</jats:italic>, 1997).</jats:p>