April 25, 2002
Burned-Out Stars Help Experts Reaffirm Universe's Age
By DENNIS OVERBYE
Using the cooling cinders of burned-out stars as a kind of cosmic clock, astronomers said today that they had arrived at a new measure of the age of the universe: the cosmos, they said, is at least 13 billion years old.
This conclusion is happily -- and uncharacteristically for the fractious field of cosmology -- consistent with recent estimates based on measurements of the expansion of the universe and other lines of study. And it reassures cosmologists that the universe is at least as old as its component parts, something that earlier measurements had occasionally seemed to challenge.
In a feat of observational power, an international team of astronomers used the Hubble Space Telescope to search a cluster of ancient stars for so-called white dwarf stars less than a billionth as bright as the faintest stars visible to the naked eye. The oldest and faintest white dwarf, they concluded from comparing its temperature to theoretical calculations, was just under 13 billion years old. Since it is thought that the first stars formed about a billion years after the Big Bang, that put the age of the universe right in the ball park of the 13 to 16 billion years indicated by various recent measurements of the rate of expansion of the universe.
The star cluster, known as M4, is about 7,000 light years away in the constellation Scorpius of the Milky Way galaxy, home of the solar system. "We're not peering at the edge of the universe," said Dr. Harvey Richer, an astronomer from the University of British Columbia. "We're a peeping tom looking at our neighbor." Dr. Richer was coleader, along with Dr. R. Michael Rich of UCLA, of the 12-member international team that did the work.
The results were announced at a press conference at NASA headquarters in Washington, DC today and have been submitted as a pair of papers for publication in The Astrophysical Journal Letters.
Previous calculations of the universe's age, based on competing measurements of the rate the universe is expanding, have careered as high as 20 billion years and as low as 8 billion years -- less than the age of many stars. Much of the variance is due to the great difficulty of measuring cosmic distances.
"This new observation short-circuits getting to the age question, and offers a completely independent way of pinning down that value," Dr. Richer said.
Once they exhaust their thermonuclear fuel, most stars end up as white dwarfs, typically dense, Earth-sized balls containing mass about two-thirdsthat of the Sun. Their attraction as cosmological tools is their simplicity. "It's like watching a cinder burn out," said Dr. Bruce Margon, an astronomer at the Space Telescope Science Institute in Baltimore who was not part of the team. The longer a white dwarf sits the cooler and fainter it gets. In an observation that totaled almost 8 days spread over two months in 2001, the Hubble team recorded some 600 white dwarfs in the M4 cluster. The faintest and coolest, and thus the oldest still had a temperature of about 2500 degrees Celsius. "They're still pretty warm at 13 billion years," Dr. Richer said.
Dr. Brad Hansen, an astronomer at UCLA and Princeton, and team theorist, said that the age of the cluster was uncertain by about half a billion years, because the compositions of the white dwarfs could vary depending on their previous histories.
Dr. Wendy Freedman of the Carnegie Observatories,in Pasadena, Calif., who headed a space telescope team that reported an age of 13-15 billion years three years ago, said it was "very exciting" to have this complementary independent technique come out with same result as her team.
Dr. Michael Turner, a cosmologist at the University of Chicago, called the results "very cool," pointing out that the new age was consistent not just with Dr. Freedman's measurements, but with more uncertain calculations based on the oldest regular stars and on the cosmic radiation presumably left over from the Big Bang. "No age crisis here; no things in the universe older than the Universe," he said in an email, "A beautifully consistent picture!"
Dr. Margon compared the result to opening a drawer and finding your birth certificate.
But not all controversy has cooled like the white dwarfs. While terming the observation "a very fine one," Dr. Allan Sandage, an astronomer at the Carnegie Observatories, who favors a lifetime of 16 to 17 billion years, pointed out that what Dr. Richer's team had done was to age-date the cluster and the Milky Way, not the universe.
"The next step is considerably less certain," said Dr. Sandage, referring to the lag time between the birth of the universe and the formation of M4. Dr. Hansen said that it was possible that there was a generation of stars before the globular clusters, but that so far there was no evidence of it. "The globular clusters may very well be the first generation."
Dr. Freedman pointed out that this one of the rare instances in which the results were consistent with both the measurements of her team and those of Dr. Sandage.
"That's progress," she said.