talogue comprises more than one million stars with 20–30 milliarc-sec astrometry and two-colour (B and V band) photometry.
A more extensive analysis of the star mapper (Tycho) data extracted additional faint stars from the data stream. Combined with old photographic plate observations made several decades earlier as part of the Astrographic Catalogue programme, the Tycho-2 Catalogue of more than 2.5 million stars (and fully superseding the original Tycho Catalogue) was published in 2000.
The Hipparcos and Tycho-1 Catalogues were used to create the Millennium Star Atlas: an all-sky atlas of one million stars to visual magnitude 11. Some 10,000 nonstellar objects are also included to complement the catalogue data.
Between 1997 and 2007, investigations into subtle effects in the satellite attitude and instrument calibration continued. A number of effects in the data that had not been fully accounted for were studied, such as scan-phase discontinuities and micrometeoroid-induced attitude jumps. A re-reduction of the associated steps of the analysis was eventually undertaken. This has led to improved astrometric accuracies for stars brighter than Hp=9.0 mag, reaching a factor of about three for the brightest stars (Hp<4.5 mag), while also underlining the conclusion that the Hipparcos Catalogue as originally published is generally reliable within the quoted accuracies.
All catalogue data are available online from the Centre de Données astronomiques de Strasbourg.
The Hipparcos results have affected a very broad range of astronomical research, which can be classified into three major themes:
(a) the provision of an accurate reference frame: this has allowed the consistent and rigorous re-reduction of historical astrometric measurements, including those from Schmidt plates, meridian circles, the 100-year old Astrographic Catalogue, and 150 years of Earth-orientation measurements. These, in turn, have yielded a dense reference framework with high-accuracy, long-term proper motions (the Tycho-2 Catalogue). Reduction of current state-of-the-art survey data has yielded the dense UCAC2 Catalogue of the US Naval Observatory on the same reference system, and improved astrometric data from recent surveys such as the Sloan Digital Sky Survey and 2MASS. Implicit in the high-accuracy reference frame is the measurement of General Relativistic light bending, and the detection and characterisation of double and multiple stars;
(b) constraints on stellar structure and stellar evolution: the accurate distances and luminosities of 100,000 stars has provided the most comprehensive and accurate data set of fundamental stellar parameters to date, placing constraints on internal rotation, element diffusion, convective motions, and asteroseismology. Combined with theoretical models and other data it yields evolutionary masses, radii, and ages for large numbers of stars covering a wide range of evolutionary states;
(c) Galactic kinematics and dynamics: the uniform and accurate distances and proper motions have provided a substantial advance in understanding of stellar kinematics and the dynamical structure of the solar neighbourhood, ranging from the presence and evolution of clusters, associations and moving groups, the presence of resonance motions due to the Galaxy's central bar and spiral arms, determination of the parameters describing Galactic rotation, discrimination of the disk and halo populations, evidence for halo accretion, and the measurement of space motions of runaway stars, globular clusters, and many other types of star.
Associated with these major themes, Hipparcos has provided results in topics as diverse as Solar System science, including mass determinations of asteroids, Earth's rotation and Chandler Wobble, the internal structure of white dwarfs, the masses of brown dwarfs, the characterisation of extra-solar planets and their host stars, the height of the Sun above the Galactic mid-plane, the age of the Universe, the stellar initial mass function and star formation rates, and strategies for the search for extraterrestrial intell