Younger stars and an older bigger cosmos.

The oldest stars in the Milky Way may be considerably younger than astronomers had thought, and the universe about 1 billion years older, according to a new analysis of stellar data from the Hipparcos satellite. The calculations also increase the estimated size of the cosmos by about 10 percent.

Scientists announced these findings Feb. 14 at a meeting of the Royal Astronomical Society in London. Hipparcos has fixed the positions of 120,000 nearby stars 100 times more precisely than is possible from the ground. These measurements also enable astronomers to deduce stellar distances from the tiny seasonal shifts in position caused by Earth's motion around the sun.

The new findings put globular clusters, the oldest groupings of stars in the Milky Way, at 11 billion years old, thus helping to resolve a cosmological controversy. Several previous studies indicated that globular clusters are, on average, 14.6 billion years old-or several billion years older than the universe, according to a recent estimate (SN: 10/8/94, p. 146).

An analysis of the Hipparcos data, collected from 1989 through 1993, eliminates this discrepancy by refining a yardstick often used to measure cosmic distances, says Robin M. Catchpole of the Royal Greenwich Observatory in Cambridge, England. That yardstick relies on the luminosity of variable stars called Cepheids, which wax and wane in brightness over periods of days to weeks.

The rate at which a Cepheid's brightness varies is closely tied to its luminosity: Those with longer periods are intrinsically brighter. By measuring the period of a Cepheid and its apparent brightness, astronomers can deduce the distance to the star and to the galaxy in which it resides.

Catchpole and Michael W. Feast of the University of Cape Town in South Africa reexamined the period-luminosity relationship for Cepheids, using Hipparcos to measure the distances to 26 of the nearest such stars. Even those Cepheids are too far for Hipparcos to obtain accurate, individual distances, but by averaging results, the researchers found that the stars reside farther away than previously estimated.

This revision, in turn, increases the distances to nearby galaxies containing Cepheids by about 10 percent, Feast reported at the London meeting. For example, the Large Magellanic Cloud galaxy would lie about 179,000 light-years from Earth rather than the standard value of about 163,000.

Because calculated distances to more remote galaxies often depend indirectly on the Cepheid yardstick, the new calibration may increase the estimated size of the entire universe, Feast and Catchpole speculate. That would make the universe about 10 percent older.

Applying the Cepheid results to our own galaxy, the astronomers recalibrated the brightness of RR Lyrae stars, another type of variable star. They deduced that RR Lyrae stars in several globular clusters are brighter and younger than had been assumed. Since all stars in a globular cluster are presumed to have formed at the same time, this gives a more youthful age for the clusters.

The finding agrees with the results of a recent report that arrived at a younger age for globular clusters by a different means (SN: 12/14/96, p. 374).

Floor van Leeuwen of the Royal Greenwich Observatory cautions that the Cepheid measurements represent the very limits of what Hipparcos can do.

In his own study, also based on Hipparcos data, van Leeuwen and his collaborators calibrated the distance to the Large Magellanic Cloud using the positions of Mira variables, yet another class of variable star. Depending on his team's method of calculation, Van Leeuwen finds that the galaxy resides at either 166,000 or 171,000 light-years away, farther than the standard estimate but not as far as Catchpole and Feast calculate.