Vasily Belokurov is one in all three winners of the 2026 Kavli Prize in Astrophysics. The award is for uncovering fossil proof of previous galactic mergers that show how the Milky Means advanced.
Irrespective of the time or vantage level, from a pre-Neolithic cave to a post-lockdown London high-rise, the predictability of the evening sky has at all times been humanity’s image of permanence and reassuring stability.
But this obvious calm is misleading. Our galaxy, the Milky Means, emerged from chaos and turbulence, and its constellations are stuffed with migrants, exiles and survivors. Proper now, it has begun to stretch and warp once more, pulled by an enormous companion and heading for an inevitable collision.
How can I be so certain? As a galactic archaeologist, my job is to reconstruct the previous of our galaxy and skim the indicators of its future.
As a substitute of digging by way of soil, I take advantage of the legal guidelines of dynamics and stellar evolution to sift by way of tons of of hundreds of thousands of stars—trying to find essentially the most historic and chemically peculiar amongst them, deciphering their orbits and piecing collectively the occasions that formed the Milky Means. One historic encounter left scars so deep that, billions of years later, they nonetheless outline the galaxy round us.
I need to perceive what governs the lives of those large cosmic techniques: which modifications are nature—the sluggish inside evolution of a galaxy disk—and that are nurture, imposed by collisions and mergers.
Questions concerning the supply of darkish matter underpin all of it. That is the invisible substance whose gravity holds galaxies collectively, however whose true id stays one of many biggest unsolved puzzles in astrophysics.
The Milky Means is the one galaxy the place stellar motions might be measured in extraordinary element. This enables cosmologists together with myself to assemble our most exact map but of darkish matter: how far it reaches, how dense it’s across the solar, what form it has, and how easy or lumpy it could be. If we will construct this map in sufficient element, we could start to grasp not simply the place darkish matter is, however what it’s.
A Cataclysmic Collision
Our work has been reworked by a revolution in open sky surveys. From 2000, the Sloan Digital Sky Survey confirmed what turns into potential when huge astronomical datasets are made public, enabling discoveries far past the objectives for which the survey was first constructed.
And since 2014, Gaia, the European house telescope, has taken this transformation to a different degree by mapping the positions and motions of almost 2 billion stars, turning the galaxy into an enormous archaeological report. No ruins, no shards, and no bones—solely stars that maintain the clues.
The clearest giveaway that one thing cataclysmic occurred way back in our galaxy is the migrants we observe: stars that weren’t born within the Milky Means.
Whereas native stars principally journey collectively, circling the galactic heart within the nice rotating move of the disk, migrants reduce throughout that order. They slide previous the locals, plunge into the internal galaxy, then fly again out to its outskirts, many times.
These uncommon orbits go hand-in-hand with uncommon chemistry. A lot of the migrant stars are much less enriched in heavier parts than the regionally born inhabitants. Their chemical composition is an indication of a slower fee of evolution that’s typical of a dwarf galaxy.
This makes the migrants doubly priceless. They’re each fossils of the Milky Means’s violent previous and probes of its outer areas, touring the place the native stars hardly ever go.
How the Milky Means Was Rewired
One of many central concepts within the idea of cosmic construction formation is that galaxies develop hierarchically. Smaller galaxies fall into bigger ones and are torn aside, leaving their stars behind as migrants.
Within the Milky Means, the most important historic construction of this type is called Gaia-Sausage-Enceladus. It’s the stays of a vanished galaxy that collided with our personal between 8 and 11 billion years in the past (the “sausage” refers to a sample in its stars’ motions).
The Milky Means additionally didn’t undergo that crash unscathed. The collision rewired and reshaped it.
A few of these modifications are simply seen within the knowledge. Stars from the previous disk had been splashed into our galaxy’s halo, changing into exiles within the place the place they had been born. A new posse of star clusters had been additionally acquired.
On the similar time, we predict one thing much more momentous was going down. The encounter modified the orientation of the Milky Means’s disk, and its alignment with the darkish matter halo.
Whereas darkish matter is too diffuse to dominate our photo voltaic system, within the outer galaxy it’s the major gravitating mass—shifting, streaming, and in the usual image, clumping right into a hierarchy of lumps.
Across the Milky Means, this darkish matter types an enormous halo, a lot bigger than the luminous a part of our galaxy. We regularly think about this halo as a sparse, spherical cloud, however Gaia has helped present this image is just too easy.
The darkish halo might be stretched out of practice by a significant encounter. Like a ship starting to listing, the Milky Means began to lean—not immediately, not visibly, however over billions of years.
A New Galactic Dance
Unusually, in contrast with many galaxies of comparable mass, the Milky Means was allowed ample time to recuperate from the shock of the “sausage merger.” No different cosmic cataclysm seems to have shaken our galaxy since, letting it settle right into a quiet, uneventful life. That’s, till now.
The Massive Magellanic Cloud (LMC), at present our galaxy’s most large companion, is already pulling on the Milky Means, disturbing its halo once more. In an echo of what occurred some 10 billion years in the past, the Milky Means is being drawn into an accelerating dance with this neighboring dwarf galaxy, recoiling in response to the LMC’s strategy.
This can be 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 fantastic thing about the evening sky—it deepens it. The calm band of sunshine above us shouldn’t be a logo of permanence, however the seen reminder of a protracted survival.
The Milky Means has been damaged, rebuilt, and is now being disturbed once more. Its stars bear in mind the previous; their motions reveal the longer term. What seems to be everlasting is, in fact, a second in a for much longer story.
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