INFO 04-1998: Hipparcos makes an accurate 3-D chart of an important star cluster
19 February 1998A landmark result in the science of the stars comes with a complete and accurate description of the Hyades cluster of more than 200 stars, from measurements by the European Space Agency's star-mapping satellite Hipparcos. With the distance to this historically important tribe of stars now known to better than 1 per cent, theories of the evolution of stars are put on a secure basis at last.
Star clusters are crucial for understanding the lives of the stars everywhere, because all the members of a cluster formed at the same time from the same raw materials. Astrophysicists can see how the evolution of each star depends on its mass and chemical composition. The heavier a star is, the more intensely it burns and the faster it consumes its thermonuclear fuel. But the accuracy of the theories has been limited hitherto by inaccuracies in the observations.
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 Côte 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.
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.
Cleaning a Rosetta Stone
"The Hyades cluster has almost assumed the role of the Rosetta Stone of astronomy," comments Michael Perryman of ESA's Astrophysics Division in Noordwijk, the Netherlands, who is the lead author of the Hyades study. "It allows us to decipher many of the mysteries of stars. But until now, uncertainties in the observations left it muddy and hard to read with confidence. Let's say we've now cleaned this Rosetta Stone to the point of complete legibility."
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.