Photography reveals the Milky Way in detail with nebulae and the Antares star in Potirendaba (SP) Jefferson Renee Benatti Mazzoni/Personal archive Vasily Belokurov is one of the three winners of the 2026 Kavli Prize in Astrophysics. The award was granted for discovering fossil evidence of past galactic mergers that prove how the Milky Way evolved. Regardless of the era or the 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, emerged from chaos and turbulence, and its constellations are full of migrants, exiles, and survivors. At this moment, it has begun to stretch and distort again, pulled by a massive companion and headed toward 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 into the soil, I use the laws of dynamics and stellar evolution to sift through hundreds of millions of stars looking for the oldest and chemically peculiar ones among them, interpreting their orbits and reconstructing the events that shaped the Milky Way.
Among these events is an ancient collision that left marks so deep 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 are at the base of all 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 with extraordinary detail.
This allows us cosmologists to build our most precise map so far of dark matter: how far it extends, how dense it is around the Sun, its shape, and how uniform or irregular it may be. If we can build this map with enough detail, we can begin to understand not only where dark matter is, but what it is. A cataclysmic collision Our work has been transformed by a revolution in sky surveys with open data.
Since 2000, the Sloan Digital Sky Survey (SDSS) has shown what is possible when vast astronomical datasets are made public, enabling discoveries far beyond the objectives 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 nearly 2 billion stars, turning the galaxy into a vast archaeological record. No ruins, no fragments, and no bones – only stars that hold the clues about the past. The clearest evidence that something cataclysmic occurred a long time ago in our galaxy are the migrants we observe: stars that were not born in the Milky Way. While native Milky Way stars travel mainly together, orbiting the galactic center in the large rotational flow of the disk, migrant stars break this order.
They glide across the orbits of local stars, dive into the inner regions of the galaxy, and then fly back to their periphery, repeatedly. These unusual orbits go hand in hand with unusual chemistry. Most migrant stars are less rich in heavier elements than the population born locally in the Milky Way.
Their chemical composition is a sign of a slower evolution rate, typical of a dwarf galaxy. This makes migrants doubly valuable. They are both fossils of the Milky Way's violent past and probes of its outer regions, traveling to 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 this type is known as Gaia-Sausage-Enceladus. It is the remains of a long-gone galaxy that collided with ours between 8 and 11 billion years ago (the “sausage” in the English name of the structure 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 our galaxy's halo, “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 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, clustering in 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 often imagine this halo as a sparse and round cloud, but Gaia has helped show that this image is too simplistic. The dark halo can be deformed by a large encounter. Like a ship that begins to tilt, 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 shock of the “sausage-shaped merger.”
No other cosmic cataclysm seems to have shaken our galaxy since then, allowing it to establish itself in a peaceful life without major events. That is, until now. The Large Magellanic Cloud (LMC), currently the most massive companion of our galaxy, is already pulling on the Milky Way, disturbing its halo again.
In an echo of what happened about 10 billion years ago, the Milky Way is being dragged into an accelerated dance with this neighboring dwarf galaxy, retreating in response to the LMC's approach. This is a dance from which probably only one galaxy will 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 a long survival. The Milky Way was broken, rebuilt, and is now being disturbed again. Its stars remember the past; its movements reveal the future.
What seems eternal is, in fact, just a moment in a much longer story. *Vasily Belokurov is Professor of Astronomy at the Institute of Astronomy, University of Cambridge. **This text was originally published on The Conversation Brasil website.
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