Chronologically dating the early assembly of the Milky Way
  The standard cosmological model ($\Lambda$-CDM) predicts that galaxies are
built through hierarchical assembly on cosmological timescales$^{1,2}$. The
Milky Way, like other disc galaxies, underwent violent mergers and accretion of
small satellite galaxies in its early history. Thanks to Gaia-DR2$^3$ and
spectroscopic surveys$^4$, the stellar remnants of such mergers have been
identified$^{5-7}$. The chronological dating of such events is crucial to
uncover the formation and evolution of the Galaxy at high redshift, but it has
so far been challenging owing to difficulties in obtaining precise ages for
these oldest stars. Here we combine asteroseismology -- the study of stellar
oscillations -- with kinematics and chemical abundances, to estimate precise
stellar ages ($\sim$ 11%) for a sample of stars observed by the
$\mathit{Kepler}$ space mission$^8$. Crucially, this sample includes not only
some of the oldest stars that were formed inside the Galaxy, but also stars
formed externally and subsequently accreted onto the Milky Way. Leveraging this
resolution in age, we provide compelling evidence in favour of models in which
the Galaxy had already formed a substantial population of its stars (which now
reside mainly in its thick disc) before the in-fall of the satellite galaxy
Gaia-Enceladus/Sausage$^{5,6}$ around 10 billions years ago
