![The the merger of Gaia-Enceladus with the Milky Way progenitor. Upper panel: artistic view of the merger of Gaia-Enceladus with the Milky Way progenitor, and the CMD inferred for their stars 10 billion years ago. Lower panel: artistic view of the current Milky Way and the CMD of the stars in the halo near the Sun, as observed by the Gaia satellite.](/sites/default/files/styles/crop_square_2_2_to_320px/public/images/news/Figura_IAC_Gallart.jpg?h=685da992&itok=6U5pDMCn)
Among the myriad discoveries presented in the second data release of the Gaia mission, there was an enigmatic color-magnitude diagram (CMD) of Milky Way halo stars, showing a striking double (blue/red) sequence. The blue sequence was linked to a major merger that our Galaxy experienced early in its history (Gaia-Enceladus). The origin of the red sequence was unclear, and it was generally associated, because of its chemical composition, with the Milky Way thick disk. However, the lack of accurate ages precluded a clear understanding of its nature. We compared this double-sequenced observed
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