Vasily Belokurov is one among three winners of the 2026 Kavli Prize in Astrophysics. The award is for uncovering fossil evidence of past galactic mergers that prove how the Milky Way evolved.
Regardless of the time or vantage point, from a pre-Neolithic cave to a post-lockdown London high-rise, the predictability of the night sky has all the time been humanity’s symbol of permanence and reassuring stability.
Yet this apparent calm is deceptive. Our galaxy, the Milky Way, emerged from chaos and turbulence, and its constellations are filled with migrants, exiles and survivors. At once, it has begun to stretch and warp again, pulled by an enormous companion and heading for an inevitable collision.
How can I be so sure? As a galactic archaeologist, my job is to reconstruct the past of our galaxy and browse the signs of its future.
As an alternative of digging through soil, I exploit the laws of dynamics and stellar evolution to sift through a whole lot of hundreds of thousands of stars—looking for essentially the most ancient and chemically peculiar amongst them, interpreting their orbits and piecing together the events that shaped the Milky Way. One ancient encounter left scars so deep that, billions of years later, they still define the galaxy around us.
I need to know what governs the lives of those massive cosmic systems: which changes are nature—the slow internal evolution of a galaxy disk—and that are nurture, imposed by collisions and mergers.
Questions on the source of dark matter underpin all of it. That is the invisible substance whose gravity holds galaxies together, but whose true identity stays one among the best unsolved puzzles in astrophysics.
The Milky Way is the one galaxy where stellar motions will be measured in extraordinary detail. This enables cosmologists including myself to construct our most precise map yet of dark matter: how far it reaches, how dense it’s across the sun, what shape it has, and how smooth or lumpy it could be. If we are able to construct this map in enough detail, we may begin to know not only where dark matter is, but what it’s.
A Cataclysmic Collision
Our work has been transformed by a revolution in open sky surveys. From 2000, the Sloan Digital Sky Survey showed what becomes possible when vast astronomical datasets are made public, enabling discoveries far beyond the goals for which the survey was first built.
And since 2014, Gaia, the European space telescope, has taken this transformation to a different level by mapping the positions and motions of nearly 2 billion stars, turning the galaxy into an unlimited archaeological record. No ruins, no shards, and no bones—only stars that hold the clues.
The clearest giveaway that something cataclysmic took place way back in our galaxy is the migrants we observe: stars that weren’t born within the Milky Way.
While native stars mostly travel together, circling the galactic center in the good rotating flow of the disk, migrants cut across that order. They slide past the locals, plunge into the inner galaxy, then fly back out to its outskirts, repeatedly.
These unusual orbits go hand-in-hand with unusual chemistry. Many of the migrant stars are less enriched in heavier elements than the locally born population. Their chemical composition is an indication of a slower rate of evolution that’s typical of a dwarf galaxy.
This makes the migrants doubly invaluable. They’re each fossils of the Milky Way’s violent past and probes of its outer regions, traveling where the local stars rarely go.
How the Milky Way Was Rewired
Considered one of the central ideas in the speculation of cosmic structure formation is that galaxies grow hierarchically. Smaller galaxies fall into larger ones and are torn apart, leaving their stars behind as migrants.
Within the Milky Way, the most important ancient structure of this sort is referred to as Gaia-Sausage-Enceladus. It’s the stays of a vanished galaxy that collided with our own between 8 and 11 billion years ago (the “sausage” refers to a pattern in its stars’ motions).
The Milky Way also didn’t undergo that crash unscathed. The collision rewired and reshaped it.
A few of these changes are easily visible in the info. Stars from the old disk were splashed into our galaxy’s halo, becoming exiles within the place where they were born. A latest posse of star clusters were also acquired.
At the identical time, we predict something much more momentous was going down. The encounter modified the orientation of the Milky Way’s disk, and its alignment with the dark matter halo.
While dark matter is too diffuse to dominate our solar system, within the outer galaxy it’s the primary gravitating mass—moving, streaming, and in the usual picture, clumping right into a hierarchy of lumps.
Across the Milky Way, this dark matter forms an unlimited halo, much larger than the luminous a part of our galaxy. We frequently imagine this halo as a sparse, round cloud, but Gaia has helped show this picture is just too easy.
The dark halo will be stretched off form by a serious encounter. Like a ship starting to list, the Milky Way began to lean—not suddenly, not visibly, but over billions of years.
A Latest Galactic Dance
Unusually, compared with many galaxies of comparable mass, the Milky Way was allowed ample time to get well from the shock of the “sausage merger.” No other cosmic cataclysm appears to have shaken our galaxy since, letting it settle right into a quiet, uneventful life. That’s, until now.
The Large Magellanic Cloud (LMC), currently our galaxy’s most massive companion, is already pulling on the Milky Way, disturbing its halo again. In an echo of what happened some 10 billion years ago, the Milky Way is being drawn into an accelerating dance with this neighboring dwarf galaxy, recoiling in response to the LMC’s approach.
It is a dance that just one galaxy is prone to survive intact. A brand new chapter of migration, survival and adaptation has begun.
None of this spoils the great thing about the night sky—it deepens it. The calm band of sunshine above us will not be an emblem of permanence, however the visible reminder of an extended survival.
The Milky Way has been broken, rebuilt, and is now being disturbed again. Its stars remember the past; their motions reveal the longer term. What looks everlasting is, in fact, a moment in a for much longer story.