Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies¹. Although these accreted stellar populations can be forensically identified as kinematically distinct structures within the Galaxy, it is difficult in general to date precisely 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 Gaia–Enceladus¹, leading to substantial pollution of the chemical and dynamical properties of the Milky Way. Here we identify the very bright, naked-eye star ν Indi as an indicator 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) billion years. The star bears hallmarks consistent with having been kinematically heated by the Gaia–Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 billion years ago, at 68% and 95% confidence, respectively. Computations based on hierarchical cosmological models slightly reduce the above limits.

在其历史进程中，银河系吞噬了多个较小的卫星星系¹。尽管这些吸积的恒星群体可以通过法医鉴定为银河系中运动学上不同的结构，但通常很难准确确定任何一次合并发生的年龄。最近的结果揭示了一群恒星是通过矮星系的碰撞吸积而成的，称为盖亚-土卫二¹，导致银河系的化学和动力学特性受到严重污染。在这里，我们将非常明亮的肉眼恒星ν Indi 确定为银河系早期原地人口年龄的指标。我们结合了星震、光谱、天体测量和运动学观测，表明这颗金属贫乏、富含 α 元素的恒星是光晕的土著成员，我们测量它的年龄为\(11.0\pm 0.7\) (stat) \(\pm 0.8\)(sys) 亿年。这颗恒星具有与盖亚-土卫二碰撞运动学加热一致的特征。它的年龄意味着合并最早可能开始于 11.6 和 132 亿年前，置信度分别为 68% 和 95%。基于分层宇宙学模型的计算略微降低了上述限制。