How old are the oldest stars ?

An international team of astronomers have been able to give a precise age to an old metal-poor star of the halo of our Galaxy, HE 1523-0901. They used a high quality spectrum taken with VLT-UVES, that allowed them to measure the abundance ratio of some radioactive isotopes of uranium and thorium. They have inferred an age of 13.2 billions years for this star born in the early time of the Universe.