Context : 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 .

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