The age of the Milky Way halo stars from the Sloan Digital Sky Survey
  We determined the age of the stellar content of the Galactic halo by
considering main-sequence turn-off stars. From the large number of halo stars
provided by Sloan Digital Sky Survey, we could accurately detect the turn-off
as a function of metallicity of the youngest dominant population, which was
done by looking at the hottest (bluest) stars of a population. Using the
turn-off temperature of a population of a given metallicity, we looked for the
isochrones with that turn-off temperature and metallicity and found no age
gradient as a function of metallicity. This would mean that this dominating
population of the Galactic halo formed rapidly, probably during the collapse of
the proto-Galactic gas. Moreover, we could find a significant number of stars
with hotter temperatures than the turn-off, which might be blue horizontal
branch (BHB) stars, blue stragglers, or main sequence stars that are younger
than the dominant population and were probably formed in external galaxies and
accreted later on to our Milky Way. Motivated by the current debate about the
efficiency of gravitational settling (atomic diffusion) in the interior of old
solar-type stars, we used isochrones with and without settling to determine the
ages. When ignoring diffusion in the isochrones we obtained ages of 14-16 Gyr.
This result is a strong argument against inhibited diffusion in old halo field
stars, since it results in a serious conflict with the age of the Universe of
13.7 Gyr. The age obtained including diffusion in the isochrones was 10-12 Gyr,
which agrees with the absolute age of the old globular clusters in the inner
halo.
