Dating the cosmos: Hubble eyes aging stars.

Everyone knows you can't be older than your mother. But over the past year, observations with the Hubble Space Telescope and several other instruments seem to have contradicted this cardinal rule. On the one hand, measurements of the speed at which the most distant galaxies are moving from Earth suggest that the universe may be no older than 8 billion to 12 billion years (SN: 10/8/94, p.232). On the other hand, astronomers estimate the ages of our galaxy's oldest stars at 13 billion to 16 billion years.

Now, new findings from Hubble may provide a step toward resolving this cosmic conundrum.

In viewing the globular cluster M4, the dense grouping of elderly stars nearest to Earth, Hubble has detected more than 200 white dwarfs--the largest community of these old, shrunken stars ever identified. Because they all reside at the same distance and were spawned from stars that formed at the same time, dwarfs in a cluster provide a unique opportunity to compare theoretical predictions with the actual evolution of these geriatric objects.

In particular, notes Hubble investigator Harvey B. Richer of the University of British Columbia in Vancouver, the dwarfs discovered in M4 should help fine-tune predictions about how rapidly these objects cool--a critical factor in attempts to infer the age of the universe from the temperature of white dwarfs. Richer and a team of U.S. and Canadian colleagues detail their work in the Sept. 20 Astrophysical Journal Letters.

Formed by the gravitational collapse of old, sun-like stars that have run out of nuclear fuel, white dwarfs represent the last and by far the longest phase in the life cycle of these stars. Dwarfs start out hot but gradually cool and fade in a highly predictable manner. Indeed, a dwarf cools at such a predictable rate that its surface temperature indicates its age. The coolest, dimmest dwarfs--those that have lived the longest--thus provide an estimate of the age of the cosmos, independent of such parameters as the expansion rate of the universe and the distance to far-off galaxies.

Richer emphasizes that none of the M4 dwarfs detected by Hubble are faint enough, or old enough, to serve as cosmic clocks. But, he adds, "we can use these dwarfs to refine our theories and make sure we understand in exquisite detail white dwarf cooling." Richer estimates that the oldest objects detected during Hubble's 30-hour survey have been white dwarfs for about 5 billion years.

The team's ultimate goal--to detect a large group of white dwarfs old enough to date the universe--looms ahead. However, notes study coauthor Roger A. Bell of the University of Maryland in College Park, Hubble will have to detect dwarfs with one-fortieth the brightness of those seen in M4 in order to find the oldest stars.

To pick out such faint stars, Richer recently proposed that Hubble take a much longer exposure--about 100 hours--of a single region in another globular cluster, NGC 6752. This cluster, about 12,000 light-years from Earth, lies nearly twice as far away as M4. However, its position in the sky enables Hubble to stare at the cluster almost continuously as the telescope orbits Earth. In contrast, Earth often blocks the telescope's view of M4.

If administrators at the Space Telescope Science Institute in Baltimore approve the 100-hour survey, Hubble might make the observations a year from now, Richer says, perhaps yielding results by early 1997.