  The Milky Way galaxy is observed to have multiple components with distinct
properties, such as the bulge, disk, and halo. Unraveling the assembly history
of these populations provides a powerful test to the theory of galaxy formation
and evolution, but is often restricted due to difficulties in measuring
accurate stellar ages for low mass, hydrogen-burning stars. Unlike these
progenitors, the "cinders" of stellar evolution, white dwarf stars, are
remarkably simple objects and their fundamental properties can be measured with
little ambiguity from spectroscopy. Here I report observations and analysis of
newly formed white dwarf stars in the halo of the Milky Way, and a comparison
to published analysis of white dwarfs in the well-studied 12.5 billion-year-old
globular cluster Messier 4. From this, I measure the mass distribution of the
remnants and invert the stellar evolution process to develop a new relation
that links this final stellar mass to the mass of their immediate progenitors,
and therefore to the age of the parent population. By applying this technique
to a small sample of four nearby and kinematically-confirmed halo white dwarfs,
I measure the age of local field halo stars to be 11.4 +/- 0.7 billion years.
This age is directly tied to the globular cluster age scale, on which the
oldest clusters formed 13.5 billion years ago. Future (spectroscopic)
observations of newly formed white dwarfs in the Milky Way halo can be used to
reduce the present uncertainty, and to probe relative differences between the
formation time of the last clusters and the inner halo.
