Astronomers have pushed the recorded history of the Milky Way roughly 1.8 billion years farther into the past, uncovering evidence of a dwarf galaxy that collided with the young Milky Way in the earliest phases of its formation. The finding, drawn from Hubble Space Telescope observations of ancient star clusters, points to a major merger that took place about 11.8 billion years ago, just 2 billion years after the Big Bang.
How a galaxy grows by swallowing others
The Milky Way today is a sprawling spiral home to hundreds of billions of stars, but it did not start out that way. Galaxies of its size assemble in part by forming stars from their own gas and in part by absorbing smaller neighbors, pulling in their stars, gas and dark matter through repeated collisions. Each merger leaves an imprint, though the oldest events are the hardest to reconstruct because billions of years of churning can scramble the evidence.
Before this study, the clearest chapters of that story reached back to a handful of known mergers. The most recent large one involves the Sagittarius dwarf galaxy, a collision that began more than 6 billion years ago and continues today. Deeper in the past, the Milky Way consumed a dwarf galaxy known as Gaia-Sausage-Enceladus about 10 billion years ago, an event that reshaped the galaxy’s disk of stars, as detailed by the European Space Agency’s Gaia mission team.
Reading the Milky Way’s oldest fossils
To probe still earlier times, the research team turned to globular clusters, dense and roughly spherical swarms of tens of thousands to millions of stars. These clusters hold some of the oldest stars in the galaxy and behave like cosmic archaeological sites, preserving stellar populations that a galaxy inherited when it devoured a smaller companion. Sorting clusters by age and chemistry can therefore expose where each group originally formed.
Researchers analyzed Hubble observations of 39 globular clusters packed within the inner 20,000 light-years of the galaxy, the region where the most ancient merger debris should linger. Using Hubble’s resolution to pin down each cluster’s precise age and its metallicity, meaning the abundance of elements heavier than helium, the team distinguished the clusters born inside the young Milky Way from those it collected during later collisions.
The dwarf galaxy named LKH
The analysis surfaced a third population that did not match either expected group. These clusters were older than those tied to the Gaia-Sausage-Enceladus merger yet younger than the ones native to the Milky Way, regardless of their metal content. That combination marked them as survivors of a separate and even earlier collision.
The team concluded that the Milky Way absorbed a dwarf galaxy holding roughly 500 million times the mass of the Sun in stars, a substantial share of the growing galaxy’s mass at the time. They named the vanished galaxy Low-energy-Kraken-Heracles, shortened to LKH, honoring three earlier research efforts that had argued for just such an ancient merger. The results were published in the journal Nature Astronomy.
Why an early merger changes the picture
A collision this large, occurring so early, carries weight for models of how the Milky Way came together. Some prior studies argued that the galaxy’s first building phase was dominated by stars born in place. The LKH discovery indicates that stars formed in an outside galaxy also contributed significantly to the early Milky Way, meaning imported material helped lay the foundation rather than home-grown stars alone.
The timing is what makes the result striking. Because the collision happened only about 2 billion years after the Big Bang, it captures the Milky Way at a moment when it was still small, chaotic and rapidly assembling. Absorbing a companion holding a meaningful share of its mass at that stage would have influenced the shape of the young disk and the mix of stars that later generations inherited, effectively setting some of the initial conditions for the galaxy that exists today.
Lead author Davide Massari of the Astrophysics and Space Science Observatory of Bologna framed the result as identifying the source of the galaxy’s earliest components, describing the discovery of where the first significant batch of building blocks originated. The work draws on both the depth of Hubble imaging and precision measurements from the Gaia mission, a pairing that allowed the team to separate cluster populations that would otherwise blur together.
A method built to keep digging
The approach that revealed LKH is designed to scale. By continuing to date and chemically fingerprint globular clusters, astronomers hope to catalog the full sequence of major mergers the Milky Way experienced across cosmic time, including events buried so far in the past that they have never been characterized. NASA’s summary of the study, published on the agency’s Hubble mission site, notes that the telescope is now observing clusters that had never been studied in this way.
More than three decades after it launched, Hubble continues to contribute to questions about the galaxy’s origins, working alongside newer facilities and archival data. Each cluster it dissects adds another data point to a reconstruction of how the Milky Way assembled itself, one absorbed galaxy at a time, from an era when the cosmos itself was still young.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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