Milky Way Stars Traced Back to Ancient Galaxy Merger 12 Billion Years Ago

Galactic archaeologists analyzing the chemical compositions and ages of stellar formations have discovered that stars in the Milky Way came from another galaxy 12 billion years ago, reshaping our understanding of galactic evolution during the universe’s formative epochs.

Decoding the Fossil Record of Ancient Mergers

When gazing upward on a pitch-black evening, the Milky Way presents itself as a peaceful ribbon of stars stretching across the firmament. Underneath that calm exterior lies a violent past. For more than 13 billion years, our galaxy has grown through star formation and mergers with other galaxies. Astronomers already know that several substantial galaxies helped build the young Milky Way. A study published in Nature Astronomy brings one of these foundational events into sharper focus, offering clarity on the early cosmos.

Led by astronomer Davide Massari from the Astrophysics and Space Science Observatory of Bologna, Italy, a research team utilized exceptionally precise ages of very dense clusters of stars. This data allowed scientists to better determine when an ancient merger occurred, how massive the incoming galaxy was, and how it evolved before becoming part of the Milky Way. This research targets a difficult epoch to reconstruct: the first few billion years of galactic history.

Cosmic Clocks and Chemical Signatures

Galaxies grow through mergers. Smaller systems fall together under gravity, pulling an incoming galaxy apart and spreading its stars through the larger system. Billions of years later, those stars can still preserve clues to where they came from. The Milky Way therefore contains a fossil record of galaxies it has absorbed.

Vasily Belokurov, Amina Helmi, and Rodrigo Ibata were honored with the 2026 Kavli Prize in Astrophysics this year for this overarching mechanism, bringing it widespread distinction. Their pioneering work uncovered the fossil evidence of past mergers, showing how the Milky Way grew through this process. One of the clearest chapters in this timeline is the Gaia–Sausage–Enceladus (GSE) system, a substantial galaxy that merged with the Milky Way around 10 billion years ago.

Photo: inkl.com

Pushing further back into antiquity, however, becomes much harder. Early on, the Milky Way itself was still small. Incoming systems were substantial building blocks rather than minor satellites falling into a mature galaxy. Earlier investigations had already uncovered traces indicating another significant collision prior to GSE, linked to stellar groupings identified as “Kraken,” “Heracles,” and a “low-energy” collection of globular clusters. Massari’s team utilized globular clusters—dense groups containing hundreds of thousands of stars that formed at roughly the same time—to act as precise cosmic clocks.

Dating individual ancient stars is difficult. Globular clusters allow astronomers to compare many stars sharing the same age. Leveraging observations from the Hubble Space Telescope alongside sophisticated modelling, the researchers determined very precise relative ages for globular clusters across the Milky Way. They paired these age metrics with metallicity data, measuring the abundance of elements heavier than the two lightest elements of hydrogen and helium.

The Discovery of Low-Energy–Kraken–Heracles

Galaxies chemically enrich themselves over time. As generations of stars form and die, they produce new elements that become incorporated into later generations. By comparing the ages and metallicities of globular clusters, Massari and his colleagues were able to see the histories of how they formed.

The resulting data revealed three distinct age–metallicity sequences within the Milky Way:

• Sequence One: Associated with the early Milky Way itself.
• Sequence Two: Belongs to Gaia–Sausage–Enceladus.
• Sequence Three: A sequence associated with the earlier merger, occurring about 1.8 billion years before GSE.

This progenitor galaxy contained roughly 500 million Suns’ worth of stars, similar in stellar mass to GSE. Much of its material was deposited in the innermost parts of the Milky Way. By connecting several previously named structures, the authors call its progenitor Low-energy–Kraken–Heracles, or LKH.

Moving from knowing it happened to reconstructing what happened represents a leap in galactic archaeology.