By measuring the age of one of the oldest stars in our galaxy, astronomers have determined that the Universe is 12.5 billion years old.
The researchers scrutinised the light from the ancient star designated CS31082-001 and found a feature in its spectrum attributed to the element uranium.
Because the particular form of uranium responsible for the spectral line decays at a known rate over many billions of years, it can be used to provide a rough estimate of the age of the star.
Combined with similar observations of the element thorium-232, it leads to an improved value for the age of the oldest stars. The Universe must therefore be a little older, say astronomers.
Scientists have used the presence of thorium-232 to measure the age of a star before. It is radioactive with a half-life of 14 billion years, meaning that less than half of the original material has so far decayed.
Radioactive decay
But thorium-based measurements are not particularly accurate. If uranium-238 could be detected in the spectrum of a star then it would provide a more precise measure, as its half-life is a handy 4.5 billion years. But until now no sign of it has been detected in stellar spectra.
Using sensitive instruments connected to the European Southern Observatory's (Eso) Very Large Telescope at Paranal in Chile, Roger Cayrel of the Observatoire de Paris-Meudon, France, and colleagues have succeeded in measuring this spectral line in a metal-poor star.
By comparing the present day abundances of the two elements, a value of 12.5 bn years is obtained for the age of the Cosmos, with an uncertainty of about 3 bn years.
"We may expect to find more examples of such stars, as our surveys with the new generation of very large telescopes are just beginning. With new discoveries, more age estimates will be found, further nailing down the exact age of the Universe," says Christopher Sneden of the University of Texas at Austin, US.
The research is published in the journal Nature.