Thursday 15 May 1997

Star wars in Venice: Hipparcos astronomers disagree about the ages

Differences of opinion about the scale and age of the Universe came to the surface in Venice today (15 May) in discussions of the results from ESA's star-fixing satellite Hipparcos. In February, Michael Feast of Cape Town and Robin Catchpole of Cambridge, England, announced a Hipparcos Cepheid Scale which increased all distances in the Universe by about 10 per cent. At the same time Feast and Catchpole reduced the ages of the oldest stars to 11-12 billion years. This result eliminated a contradiction in which the oldest stars appeared to be older than the Universe -- a philosophical impossibility. Since then, Hipparcos astronomers have divided into two camps. While some groups (in Italy and the USA for example) broadly support the bold new result, others in Switzerland, France, Spain, Hungary, Japan and the USA interpret the Hipparcos data differently, to insist on ages for the oldest stars of the order of 14-15 billion years. Among the speakers in Venice today, Tom Barnes of Austin, Texas, said that Hipparcos results on a class of variable stars called RR Lyraes could not be reconciled with the distance scale derived by Feast and Catchpole from the Cepheid variable stars.

Indirect support for that point of view came from Frédéric Pont of the Geneva Observatory, reporting on a study by a Swiss-French group which combined Hipparcos distances to nearly 1000 elderly stars with detailed analyses of their light from the ground, using a high-precision spectrometer. The results were applied to some of the beautiful globular clusters of stars, which follow independent orbits in the Milky Way Galaxy and contain the oldest lights in the Universe. Estimates of their ages depend critically on their distances, but these are too great even for Hipparcos to measure directly. Instead, astronomers study solitary stars that are both very old and close enough for rangefinding by Hipparcos, and compare them with similar stars in the globular clusters. By this method an Italian team led by Fluvio Fusi Pecci of Bologna Observatory obtained, for example, a distance of 32,000 light-years for the globular cluster M92 in the constellation Hercules. Pont reported that the Swiss-French group preferred a distance of 27,000 light-years, closer to the pre-Hipparcos estimates. This shorter distance to M92, Pont said, added an extra 3-4 billion years to the stars' ages.

Michael Feast and other supporters of the lesser ages were unrepentant, insisting that they were using the Hipparcos data in an unbiassed, statistically appropriate way. The arguments became increasingly technical, because the interpretation of results depends on theories of how stars burn and evolve. Nobody expected a resolution of the differences of opinion in the course of the Venice meeting, but a leading cosmologist, Gustav Tammann of Basel, Switzerland, offered a positive comment on the debate.
"For me," Tammann said, "the most dramatic result from Hipparcos is that the distances of the globular clusters increase. So they are automatically younger, no matter what precise ideas people have about the evolution of stars. The remaining differences of opinion about the age of the oldest clusters then depend on the exact models that people use. As a cosmologist I am happy that the time crisis -- where stars seemed to be older than the expansion age of the Universe -- seems to disappear, thanks to Hipparcos."

Hipparcos star of the day: Geminga

One of the strongest sources of gamma-rays in the sky is a collapsed star, an isolated neutron star akin to the pulsating radio sources or pulsars, but in this case silent at radio wavelengths. It lies in the constellation Gemini, and the group of astronomers in Milan who have made it a subject of special study for more than 20 years named it Geminga. The neutron star is far too faint, by visible light, for Hipparcos to have observed it directly. Neverthless Hipparcos positions of other stars, close to the line of sight to Geminga, have enabled the Milanese team, working with astronomers in Turin, Copenhagen, Garching (Germany) and Noordwijk (Netherlands), to pinpoint the position of Geminga to within ten millionths of a degree.

Patrizia Caraveo from Milan explained to the Venice symposium how she and her colleagues applied star positions in the Hipparcos and Tycho Catalogues to calibrate a series of sky images on progressively larger scales. These culminated in an image from the Planetary Camera in the Hubble Space Telescope which showed Geminga itself. Never before was the position of so faint an object so precisely fixed.
"Our aim," Caraveo commented, "was not to break records in astrometry but to improve our knowledge of the physics of this remarkable neutron star. Geminga has been observed pulsating for more than 20 years, but the position of the star in relation to the Earth's motion in orbit alters the apparent pulsation rate. Knowing Geminga's precise position, we can correct for that error and measure very accurately a gradual reduction in Geminga's rate of spin."

Background summary of Hipparcos and the Venice symposium

(repeated from previous releases)

Built for ESA by Europe's aerospace industry, and sent into orbit by an Ariane 4 launcher, the Hipparcos satellite scanned the sky for nearly four years, 1989-93, measuring angles between stars. Protracted calculations by multinational teams then produced the Hipparcos Catalogue giving the positions and motions of 118,000 stars with 100 times the precision of previous surveys. The accompanying Tycho Catalogue contains a million stars charted with lesser but still unprecedented accuracy.

ESA will release the catalogues to the world's astronomers in June. Meanwhile astronomers who helped in the mission have had privileged access to the data. At the Hipparcos Symposium in Venice 13-16 May they give their early conclusions in 190 reports, in the form of talks or scientific posters. The results affect many branches of astronomy from asteroids to cosmology, but especially the theories of stellar physics and evolution, because Hipparcos provides, for the first time, accurate distances to many thousands of stars.