The Tycho-2 catalogue of the 2.5 million brightest stars

The Tycho-2 Catalogue presented here is an astrometric reference catalogue containing positions and proper motions as well as two-colour photometric d ata for the 2.5 million brightest stars in the sky. The Tycho-2 positions a nd magnitudes are based on precisely the same observations as the original Tycho Catalogue (hereafter Tycho-l) collected by the star mapper of the ESA Hipparcos satellite, but Tycho-2 is much bigger and slightly more precise, owing to a more advanced reduction technique. Components of double stars w ith separations down to 0.8 arcsec are included. Proper motions precise to about 2.5 mas/yr are given as derived from a comparison with the Astrograph ic Catalogue and 143 other ground-based astrometric catalogues, all reduced to the Hipparcos celestial coordinate system. Tycho-2 supersedes in most a pplications Tycho-l, as well as the ACT and TRC catalogues based on Tycho-1 .