The standard cosmological model predicts that galaxies are built through hierarchical assembly on cosmological timescales^1,2. The Milky Way, like other disk galaxies, underwent violent mergers and accretion of small satellite galaxies in its early history. Owing to Gaia Data Release 2³ and spectroscopic surveys⁴, the stellar remnants of such mergers have been identified^5,6,7. 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 due 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 (~11%) for a sample of stars observed by the Kepler space mission⁸. 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 disk) before the infall of the satellite galaxy Gaia-Enceladus/Sausage^5,6 around 10 billion years ago.

标准的宇宙学模型预测，星系是通过宇宙学时间尺度^1,2的分层组装而构建的。与其他盘状星系一样，银河系在其早期历史中经历了小型卫星星系的剧烈合并和吸积。由于盖亚数据发布 2 ³和光谱调查⁴，此类合并的恒星残余已被确定^5,6,7. 此类事件的年代年代测定对于揭示高红移星系的形成和演化至关重要，但由于难以获得这些最古老恒星的精确年龄，迄今为止一直具有挑战性。^{在这里，我们将星震学（对恒星振荡的研究）与运动学和化学丰度相结合，以估计开普勒太空任务8}观测到的恒星样本的精确恒星年龄（~11%）. 至关重要的是，这个样本不仅包括一些在银河系内部形成的最古老的恒星，还包括在外部形成并随后吸积到银河系中的恒星。^{利用这一年龄分辨率，我们提供了令人信服的证据，支持在卫星星系 Gaia-Enceladus/Sausage 5}坠落之前银河已经形成大量恒星（现在主要位于厚盘中）的模型^,6大约 100 亿年前。