Astronomers report that the Milky Way swallowed a dwarf galaxy about 11.8 billion years ago, an event the team names Low-energy-Kraken-Heracles or LKH. The researchers used Hubble and Gaia observations of stellar clusters within the Milky Way's innermost 20,000 light-years to measure the clusters' ages, chemistry and orbits and link them to a single merger. They estimate LKH contained about 500 million solar masses in stars, roughly one quarter of the Milky Way's stellar mass at the time. The collision occurred about 2 billion years after the Big Bang and represents the earliest-known major merger in the Milky Way's history, according to lead author Davide Massari of INAF in Bologna. Massari says the merger was peaceful for individual stars but explosive for gas, with collisions between LKH and Milky Way gas likely triggering bursts of star formation. The study concludes the merger delivered stars, interstellar gas and dark matter that helped build the Milky Way, and that many LKH-born stars remain buried in the Galaxy's inner regions. The team places this event before other major mergers, including Gaia-Sausage-Enceladus about 1.8 billion years later and the ongoing assimilation of the Sagittarius dwarf over the past roughly 6 billion years.
![Fig 2 (left) and 5 (right) : Fig. 2 : Toomre diagram for the MSTO sample colour-coded by metallicity. The candidate star is highlighted with a star symbol. The dashed curves indicate the kinematic loci commonly used to distinguish thin-disc, thick-disc, and halo populations based on total velocity with respect to the LSR. Fig. 5: Distribution of the MSTO sample in the [Al/Fe]-[Mg/Mn] plane, colour-coded by metallicity. The black solid lines indicate the empirical separation between Galactic components proposed by Alinder et al. (2026). The candidate is marked with a star symbol.](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:cugc4wn2mruqtvx7lm2kztiy/bafkreih7rxafw72ko5t7tjfvzslwc4radgnzxpp5hcnaszjttv3the2yoq)
