A lower limit of 9.5 Gyr on the age of the Galactic disk from the oldest white dwarf stars

WHITE dwarf stars represent the final evolutionary state for most main-sequence stars. They cool slowly enough that even the oldest white dwarfs are still observable in sufficiently deep surveys and they therefore provide a record of the age and star-formation history of the Local disk of the Milky Way(1-7)-and hence a useful constraint on the age of the Galaxy itself. Here we report the initial results of a very deep survey of white dwarfs, that avoids many of the problems associated with the incompleteness of earlier surveys. We use model age-luminosity relations to interpret the luminosity function of our sample of stars, and thus obtain a minimum age for the local Galactic disk of similar to 9.5 Gyr, Our results lend weight to an emerging picture of the evolutionary history of the Milky Way, in which the halo formed similar to 14-17 Gyr ago(8,9), followed by the bulge globular dusters similar to 12-14 Gyr ago(10), with a modest hiatus before the onset of star formation in the local disk similar to 10 Gyr ago.