By Paul Rincon
BBC News Online science staff
Astronomers have used measurements from two distant stars to come up with an age for our galaxy, the Milky Way.
The Milky Way is one of many spiral galaxies in the Universe
A team working with the Very Large Telescope (VLT) in Chile report that our galaxy is 13.6 billion years old, give or take 800 million years.
This was determined by measuring the amount of the element beryllium in two stars in a so-called globular cluster.
The beryllium content of stars rises with time, so it can be used as a "cosmic clock" to calculate their ages.
"This is the first time we have obtained an independent determination of this fundamental value," said team member Daniele Galli of INAF-Observatorio di Arcetri in Florence, Italy.
The researchers studied two stars called A0228 and A2111 in the globular cluster NGC 6397.
Some of the oldest stars in the Milky Way are found in large stellar clusters, particularly globular clusters.
Stars in these clusters were born from the same cloud and at the same time. But they were not the first stars to be formed in the Milky Way.
This is known because they contain small amounts of certain chemical elements which must have been synthesised in an earlier generation of massive stars that exploded as supernovae after a short and energetic life.
Astronomers have not been able to find any less massive stars from this first generation that are still shining today, preventing them from determining when these first generation stars formed.
Stars in globular clusters like NGC 6397 form at the same time
But by measuring the amount of beryllium in two faint stars within the NGC 6397 cluster, an international team determined the time interval between the formation of the Milky Way's first generation of stars and those in the cluster.
They arrived at a value of 200-300 million years. Stellar evolution models suggest the stars in NGC 6397 are 13.4 billion years old, give or take 800 million years.
Adding on the time interval gives an age of 13.6 billion years for the Milky Way.
However, the measurements were not straightforward. Beryllium is destroyed at temperatures above a few million degrees.
When a star evolves toward a giant phase, convection brings hot gas from the interior into contact with gas from the star's upper atmosphere.
This dilutes the initial beryllium content in the upper stellar atmosphere. For the beryllium clock to be of any use, beryllium content needs to be measured in less massive and less evolved stars known as "turn-off stars". But these stars are very faint.
By using the UV-Visual Echelle Spectrograph (UVES) instrument on the 8.2m Kuyen telescope that is part of the VLT, the astronomers say they were able to obtain beryllium measurements from A0228 and A2111.
The work is due to be published in an upcoming issue of Astronomy & Astrophysics.