The first measurement of uranium outside our solar system suggests that the universe is
at least 12.5 billion years old. Just as the radioactive decay of carbon-14 is used for dating
archaeological remains, astronomers are using the decay of uranium-238, which has a
half-life of 4.5 billion years, to estimate the age of the universe.
By measuring the
uranium line in the spectrum of a star and comparing it to the amount of other stable
elements, it is possible to calculate the age of the star. This benchmark is interesting to
astronomers because it is independent of models of stellar evolution.
In the past,
this method has not been possible because the amount of uranium in stars is tiny and its
spectral lines are hidden by emission from more abundant elements.
Now, ESO’s
Very Large Telescope in Chile has sufficient collecting power and resolution to measure
the uranium abundance in an old Milky Way star. The results give an age of 12.5 billion
years and, because the universe must be older than the star, a constraint on the age of the
universe.
Unusually, the error in the measurement (±1.5 billion years) does not
have astronomical origins. The largest contribution comes from errors in the knowledge
of how strengths of spectral emission lines are related to element abundances.
Astronomers are therefore confident that in a few years a much more accurate result will
be possible.
