Ages of oldest stars won't fall below 15 billion years

17 Jun 2002

It's a curious cosmic conundrum: The most ancient stars appear older than the universe itself.

Many astronomers favor a universe between 8 and 12 billion years old, based on Hubble Space Telescope data and other measurements of how quickly space expands. Stellar modelers, however, point to stars in our Milky Way that don't look an aeon younger than 13 billion years, and likely are a few billion years older than that. There's the merest hint of potential overlap at the extremes of those ranges, but not enough to make anyone comfortable--especially cosmologists, whose careful theories of how the universe has grown can't account for the eldest galactic geezers.Something has to give, but it probably won't be the stellar models, Michael Bolte believes. He presented his case at the American Association for the Advancement of Science conference this week.

"I think there's no question that our models for stars are extraordinarily good," says Bolte, an assistant professor of astronomy and astrophysics at the University of California, Santa Cruz. "All the physics of this century goes into stellar modeling. Every time we have a chance to test the models, they give us the right answers." This concept of when stars drift off the main sequence, it turns out, is critical for gauging the ages of stars elsewhere. Big stars live fast and die young; small stars conserve their energy and linger on the main sequence. Bolte compares this process to a candle burning down: As a cluster of stars ages, its main sequence steadily erodes as more and more stars wax into red giants, then wane. Bolte and others have scrutinized the stars in many globular clusters to pinpoint how much of each cluster's main sequence has eroded. When they convert those data into ages, the results are consistent: between 13 and 18 billion years.

Bolte notes that the ages of extremely old stars may deflate the most popular versions of a theory known as cosmic inflation. Inflation holds that the infant universe experienced an unimaginably rapid burst of growth in its first fraction of a second. It also holds that the universe contains just enough matter to perpetually slow down its expansion via gravity-- the so-called "critical density." Stars as old as 15 billion years, says Bolte, virtually rule out that scenario. But if the universe contained some fraction of that mass, say 20 to 30 percent, astronomers might have some hope of bridging the gap between the expansion age and the stellar ages, he said.