Paris, 19 February 1998
The brightest members of Hyades are visible to the naked eye, in the constellation Taurus. As the nearest moderately rich star cluster, the Hyades have loomed large in astrophysics for more than a century. Contradictory results for the distance of the star cluster left big question marks for the theorists, and even recent observations with the Hubble Space Telescope seemed only to deepen the mystery.
Astronomers from ESA, Leiden Observatory, Observatoire de Paris-Meudon, University of Lausanne and Observatoire de la Cote d'Azur joined forces to analyse the data on the Hyades cluster contained in the Hipparcos Catalogue published last year. Their results will appear in the March issue of the European journal Astronomy & Astrophysics.
The distance to the centre of the Hyades cluster is 151 light-years (46.34 parsecs) with an uncertainty of less than one light-year (0.27 parsec). From astrophysical theory the astronomers can date the birth of the Hyades at 625 million years ago, when only the most primitive animals lived on the Earth. The cluster has done well to survive so long.The individual stars of the Hyades are bound together by the gravity of the cluster as a whole, and their collective and individual motions are also plotted by Hipparcos. The result is a crisp 3-D motion picture of the cluster. An animation is available on the Internet (see note below).
Outlying members sharing the same general motion can now be added to the Hyades tribe, while other candidate members are rejected on grounds of distance or track. Almost imperceptible internal motions are revealed by Hipparcos. Relatively massive stars have sunk towards the cluster's centre of gravity, but some other stars are quitting the Hyades. They slowly "evaporate" from the cluster's gravitational field as a result of near-collisions with other stars in the cluster, or because the Hyades have been stressed by gravitational encounters with other massive objects in the Galaxy.
The multinational research team has found out why previous measurements of the distance to the Hyades gave incompatible results. Estimates relying on the motions of the cluster stars exaggerated the distance, because of small systematic errors in the ground-based reference system used in assessing the motions. When astronomers tried to measure the distances of the stars directly by parallax (shifts in apparent positions as the Earth orbited the Sun) small systematic differences in the ground-based determinations led typically to a smaller estimated distance. Hipparcos, from its vantage point in space, gives much better parallaxes and stellar motions, and these fit together in a perfectly consistent and comprehensive description.
The distance to the Hyades is also the starting point for astronomical distance measurements which extend throughout the Milky Way Galaxy and beyond. Its accurate measurement will therefore impact upon the overall distance scale and the age of the Universe, which have already emerged as salient areas of research where Hipparcos results are making historic contributions.