Vol. 533
In section 5. Galactic structure, stellar clusters and populations 26 August 2011

The age of the Milky Way halo stars from the Sloan Digital Sky Survey

The authors used a sample of more than 100 000 halo stars selected from SDSS imaging to determine the ages of the stellar population. The sample was chosen to have low-resolution spectra from which the temperature and metallicity could be estimated. The turn-off temperatures showed no age gradient as a function of metallicity for the bulk of the population. This means that the Galactic inner halo most likely formed rapidly, probably during the collapse of the proto-Galactic gas. Significant numbers of stars hotter than the turnoff temperature of the dominant population were also found. Some of these could be younger turnoff stars that formed in external galaxies and were later accreted by our Milky Way. The authors were also able to make a strong argument for including gravitational settling in stellar models, because the age of the stellar population of the halo ends up older than the age of the Universe if it is not.