If you look up on a dark night, the Milky Way appears as a quiet band of stars in the sky. But our galaxy has a violent past. For more than 13 billion years it has grown through star formation and mergers with other galaxies. We already know that several important galaxies helped build the young Milky Way. A new study, published in Nature Astronomy, now brings one of the first of these events into much closer focus. Led by astronomer Davide Massari of the Observatory for Astrophysics and Space Sciences in Bologna, Italy, the team used exceptionally precise ages of very dense star clusters to better determine when this ancient merger occurred, how massive the incoming galaxy was, and how it evolved before becoming part of the Milky Way. This brings us to one of the most formative (and most difficult to reconstruct) periods in our galaxy’s history: its first billion years. Galaxies grow through mergers In our current picture of galaxy formation, large galaxies gradually grow as smaller systems fall together under gravity and merge. Gravity separates an incoming galaxy and spreads its stars across the larger system. Billions of years later, those stars may still hold clues to their origins. Therefore, the Milky Way contains a fossil record of the galaxies it has absorbed. The importance of this idea was highlighted this year when the 2026 Kavli Prize in Astrophysics was awarded to Vasily Belokurov, Amina Helmi and Rodrigo Ibata for their pioneering work that uncovered fossil evidence of past mergers and showed how the Milky Way grew through this process. One of the clearest chapters in that story is Gaia-Sausage-Enceladus, or GSE: a substantial galaxy that merged with the Milky Way about 10 billion years ago. But going further back becomes much more difficult. In earlier times, the Milky Way itself was still small, and the galaxies that merged with it were substantial building blocks rather than minor satellites falling into a mature galaxy. Previous studies had already established evidence for another major merger event before the GSE, associated with populations of stars known as “Kraken”, “Heracles” and a group of “low energy” globular clusters. The new study sharpens our view of this event considerably. Globular clusters sharpen the image Massari and his colleagues used globular clusters: dense groups containing hundreds of thousands of stars that formed around the same time. This makes them unusually precise cosmic clocks. Dating individual ancient stars is difficult, but globular clusters allow astronomers to compare many stars that share the same age. Using exquisite observations from the Hubble Space Telescope and sophisticated modeling, researchers determined very precise relative ages for globular clusters in the Milky Way. They then compared these ages to each group’s metallicity: the abundance of elements heavier than the two lightest elements: hydrogen and helium. Galaxies become chemically enriched over time. As generations of stars form and die, they produce new elements that are incorporated into subsequent generations. By comparing the ages and metallicities of these globular clusters, they were able to see the history of how they formed. The surprising result is that the globular clusters of the Milky Way trace three distinct sequences of age and metallicity. One is associated with the early Milky Way itself. Another belongs to Gaia-Sausage-Enceladus. Among them is a third sequence associated with the previous fusion. The authors estimate that this event occurred about 1.8 billion years before the GSE and involved a galaxy containing approximately 500 million stars from the Sun, similar in stellar mass to the GSE. Much of its material was deposited in the innermost areas of the Milky Way. Connecting several structures named above, the authors call their progenitor low-energy Kraken-Heracles or LKH. From knowledge of what happened to the reconstruction of what happened. This is what makes the result particularly interesting. The breakthrough is not simply another branch of the Milky Way family tree. Previous studies had already revealed this early accretion event. The new precision allows us to put a much better date and description on that branch. The three age-metallicity sequences begin to separate the chemical histories of the early Milky Way, LKH, and GSE. Therefore, we can ask not only when these galaxies collided, but also what they were like and how they had evolved before. This is especially valuable in these early times. The first billion years of the Milky Way were remarkably eventful, with major stages of its growth occurring in rapid succession. There are still limitations. Globular clusters provide an incomplete record: some early galaxies may have formed few clusters, while others may have lost them. Reconstruction of merger properties is also inevitably model-dependent. But the approach offers something unique. Telescopes like the James Webb Space Telescope can now observe distant galaxies as they appeared more than 12 billion years ago. Those observations show us galaxies directly during the same era that the young Milky Way was being assembled, but generally without resolving their individual stars. The Milky Way provides a complementary view. Here we can examine in extraordinary detail the surviving stars and clusters from that same era. Distant astronomy offers us snapshots of young galaxies. Galactic archeology gives us its fossils. Together, they are turning the once-hazy infancy of the Milky Way into an increasingly detailed story of how galaxies formed in the young universe.
