The distance scale of the universe before and after Hipparcos


Wilhelm Seggewiss, Daun



The wealth of precise astrometric data obtained by the HIPPARCOS mission should have placed the various distance ladders of the Universe on sound ground.

An early press release in January 1997, stating that »ESA's Hipparcos satellite has revised the scale of the cosmos« alarmed the scientific community and excited the public. After one year of subsequent work it seems clear now that, contrary to that press release, HIPPARCOS has fully confirmed the previously known period-luminosity relation for Cepheids.

Much more complicated, however, is the situation for Population I open clusters in our Galactic neighbourhood. On the one hand, HIPPARCOS has confirmed the ground-based distance to the Hyades (and a group of apparently related clusters) but, on the other hand, has reduced the distance to the Pleiades (and another group of clusters) by about 15%.

Even more delicate is the situation for Population II objects: red subdwarfs and RR Lyrae stars. Different groups of researchers have arrived at two basically different brightness scales which have been haunting astronomers for years. Related to this are the »long« and »short« distance scales of the Large Magellanic Cloud (LMC). All the various distance indicators calibrated by HIPPARCOS and applied to LMC objects should converge to a single, precise distance of the Large Cloud. Instead, however, values between 40 kpc and 55 kpc have been found.

HIPPARCOS demands new reflection on all the usual assumptions in theory and observation relating to the cosmic distance scale. Of particular importance would be a careful discussion of intrinsic differences of seemingly similar objects, whether in the field or in clusters, the Galaxy or the LMC.

Bibliographical details:

Wilhelm Seggewiss: The distance scale of the universe before and after Hipparcos. In: Peter Brosche, Wolfgang R. Dick, Oliver Schwarz, Roland Wielen (Eds.): The Message of the Angles - Astrometry from 1798 to 1998. Proceedings of the International Spring Meeting of the Astronomische Gesellschaft, Gotha, May 11-15, 1998. (Acta Historica Astronomiae ; 3). Thun ; Frankfurt am Main : Deutsch, 1998, p. 150-170.