Galactic Archaeologists Win 2026 Kavli Prize for Reconstructing the Milky Way’s Violent Past
Before the 1990s, the Milky Way was thought to have formed the way a chef builds a sauce, smooth, gradual, continuous. Gas drifted together, cooled, and condensed into stars over billions of years. The galaxy grew from within.
The discovery that this picture was almost entirely wrong, that the Milky Way was built through repeated, violent collisions with smaller galaxies, is the achievement recognized by the 2026 Kavli Prize in Astrophysics, awarded jointly to three researchers who pioneered the field of galactic archaeology.
Vasily Belokurov (University of Cambridge), Amina Helmi (Kapteyn Astronomical Institute, University of Groningen), and Rodrigo Ibata (CNRS and University of Strasbourg) share the $1 million prize, which will be presented in Oslo in September. The Norwegian Academy of Science and Letters cited the trio “for uncovering the fossil evidence of past mergers proving that the Milky Way galaxy was built through hierarchical accretion.”
The field’s origin can be traced to a specific discovery by Rodrigo Ibata in 1994. While working at Cambridge, Ibata identified a small, dim dwarf galaxy on the opposite side of the Milky Way’s center, the Sagittarius Dwarf Spheroidal Galaxy. It had escaped detection for centuries because it was hidden behind the dense stellar bulge at the Galactic center, and because its stars were so thinly spread that they blended into the Milky Way’s own halo.
The Sagittarius Dwarf was, at the time, the closest known satellite galaxy to the Milky Way. More importantly, it was in the process of being torn apart. Ibata traced a long ribbon of stars, a tidal stream, stretching from the dwarf and wrapping around the Milky Way like a scarf. This was direct, visible evidence that the Milky Way was still consuming its neighbors.
“The discovery of the Sagittarius stream was the first clear observational proof that the Milky Way is actively accreting smaller galaxies,” the Kavli committee noted. “It opened a new window into galaxy formation.”
The Chemo-Dynamics Revolution
Amina Helmi, then a PhD student, developed the methods to find the fossil remnants of older, more ancient mergers, accretions that happened billions of years ago, whose dwarf galaxies had been fully digested.
Using the kinematics of halo stars (their positions and velocities measured from Earth), Helmi identified the “Helmi streams”, groups of stars sharing similar orbits, indicating they originated from the same disrupted dwarf galaxy. These streams contribute roughly 15% of the stars in the Milky Way’s stellar halo. Her approach combined stellar motions with chemical abundances, chemo-dynamics, to distinguish stars born inside the Milky Way from those accreted from outside.
Later, working independently with Belokurov, Helmi co-identified the Gaia-Enceladus Sausage, the remnant of the last major merger the Milky Way experienced, when a dwarf galaxy roughly one-quarter the size of the Milky Way collided with it 8 to 11 billion years ago. The “sausage” name comes from the distinctive shape of the merger remnant in velocity space: the stars follow highly radial, elongated orbits. This single event contributed a large fraction of the inner stellar halo and likely heated the early Galactic disk, triggering a burst of star formation.
The Field of Streams
Vasily Belokurov, working with the Sloan Digital Sky Survey and later ESA’s Gaia mission, produced the most complete map of stellar streams in the Galactic halo, a work known as the “field of streams.” The map reveals dozens of streams crisscrossing the halo, each the remnant of a different accretion event, some going back to the earliest epochs of galaxy formation.
Belokurov also demonstrated the importance of ultra-faint dwarf galaxies, the smallest, dimmest galaxies known, as building blocks in the Milky Way’s assembly. These tiny systems, each containing only a few hundred to a few thousand stars, are thought to be the most common type of galaxy in the universe, and their disruption contributed countless stars to the Milky Way’s halo.
The Observational Backbone
The work of all three laureates was enabled by the same technological revolution: large digital sky surveys. The Sloan Digital Sky Survey provided the first uniform, deep, multi-color imaging of one-third of the sky. ESA’s Gaia mission, launched in 2013, measured positions, distances, and motions for nearly two billion stars, providing the three-dimensional kinematic data that made it possible to trace stellar orbits back to their origins.
“The combination of Gaia and SDSS was the technological enabler,” the Kavli committee stated. “The laureates developed the analytical methods and simulations to mine these enormous datasets.”
What It Means
The picture that emerged from this work is one of a galaxy built through hierarchical merging, the same process predicted by the ΛCDM cosmological model, which holds that dark matter structures grow by consuming smaller structures. The Milky Way, far from being a serene spiral, has a history of violent collisions. The last major merger (Gaia-Enceladus) was 8 to 11 billion years ago. Smaller accretions have continued to the present day, the Sagittarius Dwarf is being consumed right now.
Stellar streams have become powerful tools beyond Milky Way history. Because they are exquisitely sensitive to the gravitational potential they move through, they serve as probes of dark matter distribution in the Galactic halo. And the methods developed by the laureates are now being applied to galaxies beyond the Milky Way, including the Andromeda Galaxy, whose outer halo Ibata and his colleagues have mapped, revealing a rich substructure of accretion remnants.
“The Milky Way,” the citation concludes, “is a living laboratory for hierarchical galaxy formation.”
Source: Physics World (2026). “‘Galactic archaeologists’ share the 2026 Kavli Prize in Astrophysics.” June 11, 2026. https://physicsworld.com/a/galactic-archaeologists-share-the-2026-kavli-prize-in-astrophysics/