A Michigan State University astronomer is a member of an international team that has measured the amount of the radioactive element uranium in a star in the Milky Way Galaxy, a first-of-a-kind discovery that will help scientists more accurately determine the age of the universe.
Timothy Beers, an MSU professor of physics and astronomy, is a member of the team that reported the findings in the Feb. 7 edition of the journal Nature. He also is presenting a paper on the subject at an international conference in Hawaii at which astronomers are considering the latest techniques for assessing age estimates in astronomy.
Until now, the estimated age of the universe — the time elapsed since the "Big Bang" — has ranged from 10 billion to 18 billion years.
But by using the information gathered from this star, along with nuclear physics calculations, Beers and colleagues have determined the age of this star to be approximately 12.5 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."
The technique is similar to the carbon-14 dating method that has been successfully used in dating archaeological artifacts over time scales of tens of thousands of years.
The star that Beers and colleagues focused on is known as CS 31082-001. Located in the constellation Cetus, the star is not visible to the naked eye, but can be seen with a small telescope.
Also significant is the fact that 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."
For nearly 20 years Beers and his colleagues around the world have carried out extensive surveys to discover these "metal-poor" stars. To date they have discovered thousands of stars with low metal content, some as low as 1/10,000 that of the Sun.
These particular stars, said Beers, were obviously 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."
Determining the age of the universe has long been considered the so-called "holy grail of cosmology."
"It 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."
Beers and his colleagues made their recent discovery using a powerful high-resolution spectrograph on the European Southern Observatory’s 8-meter Very Large Telescope in Chile, where the international team of astronomers are presently 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.
Other members of Beers’ team include astronomers from the Observatoire de Paris, France, the European Southern Observatory in Munich, Germany, the Copenhagen and Lund Observatories of Denmark and Sweden, as well as astronomers from Italy and Brazil.