Photography reveals Milky Way in detail with nebulae and star Antares in Potirendaba (SP) Jefferson Renee Benatti Mazzoni/Personal archive Vasily Belokurov is one of three winners of the 2026 Kavli Prize for Astrophysics. The prize was awarded for discovering fossil evidence of past galactic mergers that prove how the Milky Way evolved. Regardless of era or viewpoint, from a pre-Neolithic cave to a post-lockdown London skyscraper, the predictability of the night sky has always been a symbol of permanence and reassuring stability for Humanity. But this apparent calm is deceptive. Our galaxy, the Milky Way, arose from chaos and turmoil, and its constellations are filled with migrants, exiles, and survivors. At this moment, it began to stretch and distort again, pulled by a massive companion and heading towards an inevitable collision. How can I be so sure? As a galactic archaeologist, my job is to reconstruct our galaxy's past and read the signs of its future. Instead of digging in the ground, I use the laws of stellar dynamics and evolution to comb through hundreds of millions of stars looking for the oldest and most chemically peculiar among them, interpreting their orbits and reconstructing the events that shaped the Milky Way. Among these events is an ancient collision that left such profound marks that, billions of years later, it still defines the galaxy around us. Now on g1 I want to understand what governs the life of these enormous cosmic systems: which changes are innate – the slow internal evolution of a galactic disk – and which are acquired, imposed by collisions and mergers. Questions about the origin of dark matter underlie all of this. It is the invisible substance whose gravity holds galaxies together, but whose true identity remains one of the greatest unsolved enigmas in astrophysics. The Milky Way is the only galaxy where stellar movements can be measured in extraordinary detail. This allows us cosmologists to build our most accurate map yet of dark matter: how far it extends, how dense it is around the Sun, what its shape is, and how uniform or irregular it can be. If we can build this map in enough detail, we can begin to understand not just where dark matter is, but what it is. A cataclysmic collision Our work has been transformed by a revolution in open data sky surveys. Since 2000, the Sloan Digital Sky Survey (SDSS) has shown what is possible when vast sets of astronomical data are made public, enabling discoveries far beyond the purposes for which the survey was initially created. And since 2014, Gaia, a European space telescope, has taken this transformation to another level by mapping the positions and movements of almost 2 billion stars, transforming the galaxy into a vast archaeological record. No ruins, no fragments and no bones – just stars that hold clues to the past. The clearest evidence that something cataclysmic occurred long ago in our galaxy are the migrants we observe: stars that were not born in the Milky Way. While the Milky Way's native stars travel mostly together, circling the galactic center in the great rotating flow of the disk, migrating stars disrupt this order. They glide across the orbits of local stars, dive into the galaxy's inner regions, and then fly back toward its outskirts, again and again. These unusual orbits go hand in hand with unusual chemistry. Most migrating stars are less rich in heavier elements than the locally born population in the Milky Way. Its chemical composition is a sign of a slower rate of evolution, typical of a dwarf galaxy. This makes female migrants doubly valuable. They are both fossils of the Milky Way's violent past and probes of its outer regions, traveling where local stars rarely go. How the Milky Way was restructured One of the central ideas of the theory of cosmic structure formation is that galaxies grow hierarchically. Smaller galaxies fall into larger galaxies and are torn apart, leaving their stars behind as migrants. In the Milky Way, the largest ancient structure of its kind is known as Gaia-Sausage-Enceladus. It is the remains of a long-vanished galaxy that collided with ours between 8 and 11 billion years ago (the “sausage” in the structure's name in English is a reference to a pattern in the movements of its stars). The Milky Way also did not emerge unscathed from this collision. The shock reconfigured and reshaped it. Some of these changes are easily visible in the data. Stars from the ancient disk were scattered into the halo of our galaxy, “exiled” from the place where they were born. A new group of star clusters was also acquired. At the same time, we believe that something even more significant has occurred. The encounter altered the orientation of the Milky Way's disk and its alignment with the dark matter halo. Although dark matter is too diffuse to dominate our Solar System, in the outer part of the galaxy it is the main gravitational mass – moving, flowing and, in the standard model, assembling into a hierarchy of clusters. Around the Milky Way, this dark matter forms a vast halo, much larger than the luminous part of our galaxy. We usually imagine this halo as a sparse, round cloud, but Gaia helped show that this image is too simplistic. The dark halo can be deformed by a large encounter. Like a ship starting to list, the Milky Way began to tilt – not suddenly, not visibly, but over billions of years. A new galactic dance Unusually compared to many galaxies of similar mass, the Milky Way had enough time to recover from the “sausage-shaped merger” shock. No other cosmic cataclysm appears to have shaken our galaxy since then, allowing it to settle into a quiet, uneventful life. That is, until now. The Large Magellanic Cloud (LMC), currently our galaxy's most massive companion, is already tugging at the Milky Way, disturbing its halo again. In an echo of what happened about 10 billion years ago, the Milky Way is being drawn into an accelerated dance with this neighboring dwarf galaxy, retreating in response to the LMC's approach. This is a dance from which only one galaxy will likely emerge intact. A new chapter of migration, survival and adaptation has begun. None of this spoils the beauty of the night sky – on the contrary, it deepens it. The serene band of light above us is not a symbol of permanence, but the visible reminder of long survival. The Milky Way has been shattered, rebuilt, and is now being disrupted again. Its stars remember the past; their movements reveal the future. What seems eternal is actually just a moment in a much longer story. *Vasily Belokurov is Professor of Astronomy at the Institute of Astronomy at the University of Cambridge. **This text was originally published on The Conversation Brasil website.