https://phys.org/news/2018-10-astronomers-giant-early-days-milky.html

The recent second data release from the Gaia satellite mission last April provided Professor Helmi with data on around 1.7 billion stars. Helmi has been involved in the development of the Gaia mission for some 20 years, and was part of the data validation team on the second data release. She has now used the data to look for traces of mergers in the halo: "We expected stars from fused satellites in the halo. What we didn't expect to find was that most halo stars actually have a shared origin in one very large merger."

    
        
         
    
        The movie shows an N-body simulation of the merger of a Milky Way-like galaxy (with its stars in blue) and a smaller disky galaxy resembling the Small Magellanic Cloud in mass (with it stars in red). At the beginning, the two galaxies are clearly separated, but gravity pulls them together and this leads to the full accretion of the smaller one. Distinguishing the accreted stars from the rest is not easy by the final stage, but it is possible using the motions of the stars and their chemical composition. Credit: H.H. Koppelman, A. Villalobos, A. Helmi (University of Groningen)