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

Age dating of an early Milky Way merger via asteroseismology of the naked-eye star $\nu$ Indi