The Hipparcos and Tycho catalogs are the primary products of the European Space Agency's astrometric mission, Hipparcos. The satellite, which operated for four years, returned high quality scientific data from November 1989 to March 1993. The Tycho-2 catalog is an astrometric reference catalog containing positions and proper motions as well as two-color photometric data for the 2.5 million brightest stars in the sky. The Tycho-2 positions and magnitudes are based on precisely the same observations as the original Tycho catalog (hereafter Tycho-1; see <a href="https://cdsarc.cds.unistra.fr/ftp/cats/I/239">CDS Cat. I/239</a>) 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 with 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 Astrographic Catalogue and 143 other ground-based astrometric catalogs, all reduced to the Hipparcos celestial coordinate system. Tycho-2 supersedes in most applications Tycho-1, as well as the ACT (<a href="https://cdsarc.cds.unistra.fr/ftp/cats/I/246">CDS Cat. I/246</a>) and the TRC (CDS Cat. I/250) catalogs based on Tycho-1. Supplement-1 (not part of this HEASARC database but available at <a href="https://cdsarc.cds.unistra.fr/ftp/cats/I/259/suppl_1.dat.gz">https://cdsarc.cds.unistra.fr/ftp/cats/I/259/suppl_1.dat.gz</a>) lists stars from the Hipparcos and Tycho-1 catalogs which are not in Tycho-2. Supplement-2 (not part of this HEASARC database but available at <a href="https://cdsarc.cds.unistra.fr/ftp/cats/I/259/suppl_2.dat.gz">https://cdsarc.cds.unistra.fr/ftp/cats/I/259/suppl_2.dat.gz</a>) lists 1146 Tycho-1 stars which are probably either false or heavily disturbed. The principal characteristics of the Tycho-2 catalog are summarized below. By means of proper motions the positions are transferred to the year 2000.0, the epoch of the catalog. The median values of internal standard errors are given: <pre> Mean satellite observation epoch ~J1991.5 Epoch of the Tycho-2 catalog J2000.0 Reference system ICRS coincidence with ICRS (1) +/-0.6 mas deviation from inertial (1) +/-0.25 mas/yr Number of entries 2,539,913 Astrometric standard errors (2) V_T < 9 mag 7 mas all stars, positions 60 mas all stars, proper motions 2.5 mas/yr Photometric std. errors (3) on V_T V_T < 9 mag 0.013 mag all stars 0.10 mag Star density b= 0 deg 150 stars/sq.deg. b= +/-30 deg 50 stars/sq.deg. b= +/-90 deg 25 stars/sq.deg. Completeness to 90 per cent V ~ 11.5 mag Completeness to 99 per cent V ~ 11.0 mag Number of Tycho observations ~300 10<sup>6</sup> Note (1): about all 3 axes Note (2): ratio of external to internal standard errors is ~1.0 for positions and for proper motions. Systematic errors are less than 1 mas and 0.5 mas/yr Note (3): ratio of photometric external to internal standard errors at V_T > 9 mag is below 1.5 </pre> For more information on the original catalog, please consult the Tycho-2 home page at <a href="http://www.astro.ku.dk/~erik/Tycho-2">http://www.astro.ku.dk/~erik/Tycho-2</a> For more information on the HEASARC implementation of the Tycho-2 catalog, please consult <a href="#heasarc_implementation">the "HEASARC Implementation" section</a> of this help. This database table was created at the HEASARC in June 2000 based on the ADC/<a href="https://cdsarc.cds.unistra.fr/ftp/cats/I/259">CDS Catalog I/259</a>, using the file tyc2.dat. Galactic coordinates (calculated by converting the observed ICRS Equatorial positions) were added to this HEASARC database table in August 2005. This is a service provided by NASA HEASARC .