Cosmic Clock

The Star, HE 1523-0901. Credit ESO Click Image to Enlarge

Using ESO's VLT, astronomers recently measured the age of a star located in our Galaxy.

"It is very hard to pin down the age of a star", the lead author of the paper reporting the results, Anna Frebel, explains. "This requires measuring very precisely the abundance of the radioactive elements thorium or uranium, a feat only the largest telescopes such as ESO's VLT can achieve."

The technique is analogous to the carbon-14 dating method that has been so successful in archaeology over time spans of up to a few tens of thousands of years. In astronomy, however, this technique must obviously be applied to vastly longer timescales.

For the method to work well, the right choice of radioactive isotope is critical. Unlike other, stable elements that formed at the same time, the abundance of a radioactive (unstable) isotope decreases all the time. The faster the decay, the less there will be left of the radioactive isotope after a certain time, so the greater will be the abundance difference when compared to a stable isotope, and the more accurate is the resulting age.

Yet, for the clock to remain useful, the radioactive element must not decay too fast - there must still be enough left of it to allow an accurate measurement, even after several billion years.
Large amounts of these elements have been found in the star HE 1523-0901, an old, relatively bright star that was discovered within the Hamburg/ESO survey. The star was then observed with UVES on the Very Large Telescope (VLT) for a total of 7.5 hours.

For the first time, the age dating involved both radioactive elements in combination with the three other neutron-capture elements europium, osmium, and iridium. "Until now, it has not been possible to measure more than a single cosmic clock for a star. Now, however, we have managed to make six measurements in this one star"," says Frebel.

HE 1523-0901, estimated to be 13.2 billion years old was clearly formed very early in the life of our own Galaxy. Born at the dawn of time in the observable Universe, estimated to be 13.7 billion years old.

Nearby Star A Galactic Fossil ESO Press Release

This research is reported in a paper published in the 10 May issue of the Astrophysical Journal "Discovery of HE 1523-0901, a Strongly r-Process Enhanced Metal-Poor Star with Detected Uranium", by A. Frebel et al.
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