Despite great efforts during recent years, the various estimates of this basic number have resulted in rather diverse values. When derived from current cosmological models, it depends on a number of theoretical assumptions that are not very well constrained by the incomplete available observational data.

At present, a value in the range of 10-16 billion years is considered most likely.

But now, an international team of astronomers has used the powerful European Southern Observatory (ESO) Very Large Telescope (VLT) and its very efficient spectrograph (UVES) to perform a unique measurement that paves the way for a new and more accurate determination of the age of the Universe.

They measured for the first time the amount of the radioactive isotope Uranium-238 in a star that was born when the Milky Way, the galaxy in which we live, was still forming.

This is the first measurement ever of uranium outside our Solar System.

The method works in a way similar to the well-known Carbon-14 dating in archaeology -- but over much longer times. Ever since the star was born, the Uranium "clock" has ticked away over the eons, unaffected by the turbulent history of the Milky Way.

It now reads 12.5 billion years. Since the star obviously cannot be older than the Universe, it means that the Universe must be older than that.

Although the stated uncertainty is still about 25%, or about +/-3 billion years, this is only to a minor extent due to the astronomical observation. The main problem is the current absence of accurate knowledge of some of the basic atomic and nuclear properties of the elements involved.

However, further laboratory work will greatly improve this situation and a more accurate value for the age of the star and, implicitly, of the Universe, should be forthcoming before long.

This research is reported in a research article, "Measurement of stellar age from uranium decay", that appears in the international research journal Nature today.

08-Feb-2001