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Discovery Helps Pinpoint Age of Universe

Michigan State University astronomer, Timothy Beers, is part of an international team that has measured the amount of the radioactive element uranium in a star in the Milky Way Galaxy. This discovery will help scientists determine the age of the universe more precisely.

By using the information gathered from this star, along with nuclear physics calculations, Beers and colleagues determined the age of this star to be approximately 12.5 billion years. The previous estimated age of the universe was 10 to 18 billion years.

"Since this star cannot be older than the universe, it means that the universe must be older than that," he said.

The presence of uranium in the star, along with that of an additional radioactive isotope, thorium, is significant, Beers said. Uranium has a relatively short half-life—about 4.5 billion years—compared to thorium's 14 billion years.

"We can take the presently measured abundances of uranium and thorium in this star, the known half-lives of these elements, and the theoretically predicted ratio of uranium to thorium when they were formed," Beers said, "then use straightforward nuclear physics calculations to provide a relatively precise 'chronometer' that measures the time that has passed since these elements were created."

This technique has been used to date archaeological artifacts over time scales of tens of thousands of years.

Beers and colleagues focused on the star known as CS 31082-001 located in the constellation Cetus. Though it is not visible to the naked eye, it can be seen with a small telescope. The team used a powerful high-resolution spectrograph on the European Southern Observatory's 8-meter Very Large Telescope in Chile.

This star has very low levels of other metals such as calcium, magnesium and iron. The older a star is, Beers said, the lower its content of heavy elements.

"Hydrogen, helium, and lithium were produced during the Big Bang. But all of the so-called heavier elements have resulted from nuclear reactions in the interiors of stars and at the end of stellar lifetimes," he said. "When stars 'die,' often as the result of energetic supernova explosions, heavy-element enriched matter is dispersed into space and is later incorporated into the next generation of stars."

Beers and his colleagues have carried out extensive surveys to discover these "metal-poor" stars for nearly 20 years, discovering thousands of stars with low metal content, some as low as 1/10,000 that of the Sun.

According to Beers, these stars were formed during the infancy of the Milky Way Galaxy.

"I'm confident we'll have even more exciting discoveries as we begin to zero in on these stars," he said. "We are already planning new surveys that are directed at the discovery of many additional metal-poor stars in which we can measure the abundances of uranium and thorium. In the next few years, we expect to find perhaps 10 or 20 of these stars."

Learning the age of our universe "is one of those fundamental pieces of knowledge which we astronomers require as inputs to models of the formation and evolution of the universe," Beers said. "It is of primary importance to measure this number, or to set limits on it that are as accurate as possible."

The international team of astronomers are now in Chile carrying out an extensive spectroscopic study of the most metal-poor stars discovered during the past two decades as part of the previous survey work led by Beers.

The team reported their findings in the February 7 edition of the journal Nature. Beers will present a paper on the subject at an international conference in Hawaii at which astronomers are considering the latest techniques for determining age in astronomy.