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Dim stars shed light on age of galaxy

Beryllium count gives the game away

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A group of astronomers working on the Very Large Telescope (VLT) in Chile have determined that the galaxy is 13.6bn years old.

Its calculation of the age of the Milky Way is based on observations of two very faint stars in a local globular cluster, the BBC reports. Globular clusters are very concentrated clusters of stars which orbit galaxies, and contain some of the oldest stars still burning.

By measuring the amount of beryllium in each of the stars, the astronomers could calculate their age, since the element forms at a constant rate as stars burn. The team made its observations ib the 8.2m Kuyen telescope that is part of the VLT, using the UV-Visual Echelle Spectrograph.

Incidentally, some scientists think globular clusters may actually be remnants of smaller "dwarf" galaxies, caught in the gravitational maw of their larger counterparts. The implications this theory may have for these calculations is unclear.

The team carefully selected the stars they would measure, known as A0228 and A2111 in the NGC 6397 globular cluster. Star selection is vital because beryllium will burn up if the star has become too hot, making it useless as a benchmark.

Smaller stars are best, but are difficult to observe because they are so faint. Many of the larger stars, which burn faster and hotter, have already reached the giant phase of their evolution.

It is well known that the stars in globular clusters were not the first stars that ever burned in our galaxy: they contain elements that need supernovae to form. However, this new research has allowed scientists to calculate the interval between the first (now extinct) generation of stars and the ones we can still observe today.

By adding this number to the age of the stars, as determined by stellar evolution models, the team determined that the galaxy is 13.6bn years old, give or take 800m years.

Daniele Galli of INAF-Observatorio di Arcetri in Florence, Italy, told the BBC: "This is the first time we have obtained an independent determination of this fundamental value." ®

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