Doubling the Hipparcos star count: the Tycho-2 Catalogue
11 Feb 2000The success of ESA's Hipparcos satellite in mapping many stars with amazing accuracy takes another stride this week. The Tycho-2 Catalogue, giving positions, motions, brightness and colours of 2 539 913 stars, more than doubles the number of stars in the original Tycho Catalogue. Included are 99 per cent of all stars down to magnitude 11, which means almost 100 000 times fainter than the brightest star, Sirius.
During its operational life in space, 1989-93, Hipparcos surveyed the whole sky many times. Computations from observations by the main instrument generated the Hipparcos Catalogue of 118 218 stars charted with the highest precision. An auxiliary star mapper pinpointed many more stars with lesser but still unprecedented accuracy, in the Tycho project led by Erik Høg of Copenhagen, Denmark. The target was 400 000 objects, but the Tycho Catalogue published in 1997 already contains 1 058 332 stars.
To generate Tycho-2, a multinational team in the Copenhagen University Observatory, US Naval Observatory (Washington DC), Astronomisches Rechen-Institut (Heidelberg, Germany) and European Southern Observatory (Garching, Germany) has worked through the data again. The astronomers re-computed 300 million Tycho observations from Hipparcos and combined them with 7.7 million ground-based observations taken over the past 100 years. The Tycho-2 Catalogue is being made available to the world's astronomers on a CD-ROM, and via the Internet from the Centre de Donnies astronomiques de Strasbourg.
"Even this is not the end of the story," Erik Høg comments. "We're now examining our 2.5 million stars to see how they can enrich our knowledge of double stars and variable stars. We call this new phase of our work Tycho-3."
A technical letter on "The Tycho-2 Catalogue of the 2.5 Million Brightest Stars" by E. Høg and others will appear in the next issue of the journal Astronomy & Astrophysics.