The universe, like so many fading stars, does not readily give up its age. Indeed, the very best guesses cosmologists have made range anywhere from 10 billion to 18 billion years old. So how long ago was it that a colossal explosion known as the big bang birthed our world and others? A new report published today in Nature helps resolve the mystery. Timothy Beers of Michigan State University and an international set of colleagues have raised the lower limit on all estimates by dating what appears to be an ancient star.

Using a high-resolution spectrograph on the European Southern Observatory's eight-meter Very Large Telescope in Chile, the researchers are studying so-called metal-poor stars. Because heavier elements such as metals were not produced during the big bang, stars lacking them are thought to be among the very oldest. In particular, they recently analyzed a star called CS 31082-001 in the constellation Cetus, measuring the amounts of uranium and another radioactive isotope, thorium, that it contained.

"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 explains, "then use straightforward nuclear physics calculations to provide a relatively precise 'chronometer' that measures the time that has passed since these elements were created." In this way they calculated the age of CS 31082-001 to be approximately 12.5 billion years old. "Since this star cannot be older than the universe," Beers says, "it means that the universe must be older than that."