Age dating of an early Milky Way merger via asteroseismology of the
  naked-eye star $\nu$ Indi
  Over the course of its history, the Milky Way has ingested multiple smaller
satellite galaxies. While these accreted stellar populations can be
forensically identified as kinematically distinct structures within the Galaxy,
it is difficult in general to precisely date the age at which any one merger
occurred. Recent results have revealed a population of stars that were accreted
via the collision of a dwarf galaxy, called \textit{Gaia}-Enceladus, leading to
a substantial pollution of the chemical and dynamical properties of the Milky
Way. Here, we identify the very bright, naked-eye star $\nu$\,Indi as a probe
of the age of the early in situ population of the Galaxy. We combine
asteroseismic, spectroscopic, astrometric, and kinematic observations to show
that this metal-poor, alpha-element-rich star was an indigenous member of the
halo, and we measure its age to be $11.0 \pm 0.7$ (stat) $\pm 0.8$ (sys)$\,\rm
Gyr$. The star bears hallmarks consistent with it having been kinematically
heated by the \textit{Gaia}-Enceladus collision. Its age implies that the
earliest the merger could have begun was 11.6 and 13.2 Gyr ago at 68 and 95%
confidence, respectively. Input from computations based on hierarchical
cosmological models tightens (i.e. reduces) slightly the above limits.
