Faded stars get new role: Hubble takes a long look. (This week).

By setting their sights on the galaxy's faintest stars, scientists have calculated the universe's age in an entirely new way.

Assuming that the age of the dimmest, oldest stars would give a minimum age for the universe, astronomers including Harvey Richer of the University of British Columbia and his colleagues set out to find some of these old-timers (SN: 9/2/95, p. 151; 2/10/01, p. 85). Richer's team has now pushed the Hubble Space Telescope's vision to the limit by looking at some of the Milky Way's least-luminous white dwarfs--embers of stars that depleted their hydrogen fuel long ago.

The scientists found ancient white dwarfs inside a globular star duster 7,000 light-years away. Globular dusters are among the oldest groups of stars in the universe, and some of the stars are only one-billionth as bright as the faintest ones visible to the naked eye. It took 8 days of exposure for Hubble cameras to detect the dim white dwarfs.

White dwarfs start out hot but cool in a highly predictable manner. The coolest, dimmest dwarfs observed by Hubble are the oldest. Richer and his team calculated that these stars are 12 to 13 billion years old. Previous observations suggest that the very first stars formed less than a billion years after the Big Bang. So, the combined data yield 13 to 14 billion years as the age of the universe. The findings were announced at a NASA press briefing in Washington, D.C., on April 24 and are slated for publication in Astrophysical Journal Letters.

Over the years, estimates of the universe's age have varied between 8 billion and 20 billion years. One of the most recent and best guesses also used data collected by Hubble to arrive at an age of 13 to 14 billion years, Richer says. That estimate required measuring the universe's rate of expansion. This was determined by Wendy Freedman of the Carnegie Observatories in Pasadena, Calif., and her colleagues. With a complex calculation that incorporates the effects of gravity, dark energy, and other exotica, these researchers modeled cosmic expansion in reverse, so that it runs from the present back to the moment of the Big Bang (SN: 5/29/99, p. 340).

"Some scientists think it's pretty audacious that astronomers can claim to measure the age of the universe," says Bruce Margon of the Space Telescope Science Institute in Baltimore, who was not a member of either research team. "To suddenly find some totally independent way of estimating this same vital quantity and to get the same answer is really a fantastic advance" he adds.

Astrophysicist Brian Chaboyer of Dartmouth College in Hanover, N.H., cautions that the newly estimated age could be low since the white dwarfs observed by Richer's team were probably not among the universe's very first stars. Even so, he adds, the fact that the independently produced figures fall in the same range as prior estimates "gives us a lot of confidence that we really are getting the right answer."

Richer and his colleagues now plan to search for other dim stars with Hubble's new Advanced Camera for Surveys (see story, p. 278).