Some metal-poor stars are believed to have formed at a location very close to where an early generation star exploded in a violent supernova explosion. This is sort of illustrated in the figure below. The ESO folks just forgot to include that supernova! 🙂
Anyway, the neat thing here is that such a supernova must have been of some special type. The special thing is that during this explosion a particular nucleosynthesis process took place which facilitated the production of the heaviest elements known in the universe. During the explosions all those elements were then released in the interstellar medium. That means that the star we are observing now inherited all those heavy elements at the time of its birth. And luckily enough the star preserved this “birth mark” until today so we can observed directly some 13 billion years later.
This special nucleosynthesis process is called the “r-process”. “r” stands for “rapid” nucleosynthesis which refers to the build-up time of the newly created nuclei via bombardment of neutrons onto a seed nucleus (such as carbon or iron) compared with their decay time (neutron-capture process). In the rapid case, the heavy nuclei are build on time scales shorter than the beta-decay. This leads to a rapid build up of the heaviest elements in the periodic table (see below), including the radioactive elements thorium and uranium.
In the same way that archaeological funds on earth are age-dated with the radio-carbon method, we can then go about age dating stars based on the amounts of thorium and uranium left in their atmospheres since they were born.
In 2006, I discovered a metal-poor star that shows this particular chemical fingerprint of an r-process supernova explosion. This star actually offers the best empirical data for theoretical explorations of the r-process since it has the largest overabundance of those r-elements compared to iron found in any of the r-process metal-poor stars. They are only about 15-20 such stars known since they are extremely rare! This is understandable since being born in close proximity to a special supernova is obviously not happening very often even though the universe is a fairly big place.
We then went ahead and measured the age of this star, HE 1523-0901, to be around 13 billion years. This is pretty old, given that we think the universe is 13.7 billion years old. But it is not surprising given that the deficiency in elements such as iron is pretty significant. This means the star must have formed at a time when the universe was not much enriched in elements heavier than hydrogen and helium.
A review on r-process enhanced stars such as HE 1523-0901 that allow the detailed study of neutron-capture processes in the cosmos can be found here. It will be published in the proceedings of the “Nuclei in the Cosmos X” symposium, 2008.
At the moment, I am working on measuring the lead (Pb) abundance in the star from extremely high-quality data collected with the Very Large Telescope and the UVES spectrograph. This measurement will allow a self-consistent check with theoretical models for the r-process, since thorium and uranium both decay into lead. And if our models are good enough they should predict just the right amounts of thorium, uranium AND lead.