NASA reports that observations from the Hubble Space Telescope have supplied definitive evidence that the young Milky Way absorbed a dwarf galaxy during the earliest stages of its growth. The event, involving a system the researchers call Low-energy-Kraken-Heracles, or LKH, occurred about 11.8 billion years ago. The finding gives astronomers a sharper record of how our galaxy assembled from stars, gas and dark matter over cosmic time.
The result matters because the merger had been suggested by earlier observations and simulations but its details were disputed. In its official account of the Hubble study, NASA says the new evidence pushes the known timeline of the Milky Way’s evolution about 1.8 billion years farther back. The claim is about a reconstruction from stellar populations, not a direct image of a collision that happened in the distant past.
What the new evidence shows
The team examined 39 globular clusters in the inner 20,000 light-years of the Milky Way. Globular clusters are dense, roughly spherical groups containing tens of thousands to millions of stars. Many contain some of the oldest stars in the galaxy, so their ages, chemical composition and motion can preserve traces of galaxies that the Milky Way later absorbed.
Hubble’s high-resolution observations helped researchers measure the clusters’ ages and metallicity, the abundance of elements heavier than helium. When those measurements were combined with data from the European Space Agency’s Gaia mission, the researchers identified a third population of clusters in the Milky Way’s inner regions. They were older than the clusters associated with the Gaia-Sausage-Enceladus merger, but younger than clusters that formed inside the early Milky Way itself.
That intermediate pattern is the key to the result. The clusters appear to have formed in another galaxy and then become part of ours through an earlier merger. NASA’s report describes that population as the archaeological trace of LKH, a dwarf galaxy that contained roughly 500 million times the mass of the Sun in stars. That estimate refers to the stellar mass inferred for the dwarf galaxy; it is not a measurement of the total mass of all its dark matter.
How astronomers reconstructed the collision
There is no surviving photograph of LKH approaching the Milky Way. Instead, astronomers work backward from the evidence that remains. A cluster’s age indicates when its stars formed, while its metallicity helps distinguish stars born in different chemical environments. Comparing those properties across many clusters allows researchers to separate groups that originated in the Milky Way from groups that arrived from elsewhere.
Gaia’s measurements add information about how stars move through the galaxy. Hubble contributes the detailed observations needed to characterise faint, crowded globular clusters near the galactic centre. Together, the datasets make it possible to test whether a group of clusters shares a common external origin rather than being a random mixture of Milky Way stars.
The conclusion is therefore stronger than a visual resemblance or a single unusual cluster, but it still describes a scientific inference from surviving stellar evidence. The NASA report explains the measurements and the researchers’ interpretation while keeping the focus on what the observations can establish: a distinct population linked to a major early merger.
A longer, more crowded origin story
The Milky Way is not a finished structure that formed in one event. It grew through star formation within its own gas and through the capture of material from smaller galaxies. NASA identifies the Sagittarius dwarf galaxy as the most recent massive merger, beginning more than 6 billion years ago and continuing today. It also places the Gaia-Sausage-Enceladus merger roughly 10 billion years ago, when that dwarf galaxy changed the structure of the Milky Way’s stellar disk.
LKH would precede both events, arriving when the Milky Way was only about 2 billion years old. At that stage, the galaxy was smaller and closer in size to the galaxies around it. A merger with a dwarf containing hundreds of millions of solar masses in stars would have altered the young galaxy’s contents and added a population that could survive in globular clusters for billions of years.
The broader implication is that the Milky Way’s earliest development was not driven solely by stars born within its own boundaries. External galaxies contributed material even during the first major phases of its assembly. That changes the question astronomers ask about the galaxy’s beginnings: instead of tracing one uninterrupted birth, they must identify the sequence of mergers, star formation episodes and structural changes that built the present-day spiral.
What comes next
The Hubble team plans to study globular clusters that have not yet been examined in the same detail. More observations could reveal additional merger events and help determine how many distinct stellar populations are hidden in the inner Milky Way. Each population would offer another time capsule from a different stage of galactic assembly.
Hubble has been operating for more than three decades, and this result shows why its ability to resolve crowded stellar fields remains scientifically valuable. The telescope did not witness the Milky Way’s first mergers, but its measurements are helping researchers read the evidence those events left behind. For now, the LKH result adds a well-defined early chapter to the galaxy’s history while leaving the full chain of its formation open to further observation.
