Lower panel: effective temperature of a SDSS field star population with metallicities in the range of  [Fe/H]  = −1.70  ±  0.15. Upper panel: 11 Gyr isochrone with metal fraction Z = 2  ×  10^-4, which corresponds to the metallicity of the field population. The turn-off point of the isochrone is represented by the vertical red line, which agrees with the hottest stars of the field population. This agreement suggests an age 11 Gyr for the field population.