“Our home is the Milky Way galaxy, but we do not know how our house was built,” says Davide Massari, an astrophysicist in Bologna, Italy. That sentence captures something genuinely strange about astronomy, we can photograph galaxies billions of light-years away in exquisite detail, yet piecing together our own galaxy’s construction history has remained frustratingly difficult. Hubble just filled in the oldest chapter of that story yet found.
Reading a House’s Foundation From Its Bricks
The Milky Way didn’t arrive at its current size, hundreds of billions of stars strong, all at once. It grew the same way most big things do, partly by building new material internally, and partly by absorbing smaller neighbors that wandered too close. Astronomers already knew about two major mergers, an ongoing collision with the Sagittarius dwarf galaxy that started 6 billion years ago, and a bigger, older one with a galaxy called Gaia-Sausage-Enceladus roughly 10 billion years ago, which reshaped the structure of our galaxy’s entire disk. Evidence had long hinted at something even older lurking further back, but nobody could pin the details down.
A team led by Massari turned to globular clusters, dense, spherical swarms of tens of thousands to millions of ancient stars, to hunt for it. These clusters function almost like archaeological sediment layers, preserving stars from galaxies the Milky Way consumed long ago, largely undisturbed by time.
A Third Population, Hiding in Plain Sight
Researchers analyzed Hubble observations of 39 globular clusters sitting in the Milky Way’s inner 20,000 light-years, precisely measuring each one’s age and metal content, its abundance of elements heavier than helium. Combined with additional data from the European Space Agency’s Gaia spacecraft, the team expected to find two familiar groups, clusters born in the young Milky Way itself, and clusters inherited from the Gaia-Sausage-Enceladus merger. Instead, they found three.
“Coupled with measurements from Gaia, this made it possible to distinguish a population of globular clusters that are different from the others,” said co-author Chiara Zerbinati of the University of Bologna. This third group was older than the Gaia-Sausage-Enceladus clusters but younger than the Milky Way’s own homegrown stars, a clear chemical fingerprint pointing to a separate, earlier collision entirely, one that occurred roughly 11.8 billion years ago, just 2 billion years after the Big Bang. If you’re curious about a much more recent example of galactic evolution playing out next door, our piece on Andromeda’s slowing star formation covers our other large neighbor further along in its own life story.
Named for a Mythical Strongman
The team calculated that the dwarf galaxy responsible carried roughly 500 million times the Sun’s mass in stars, a genuinely substantial fraction of the young Milky Way’s total mass at the time. They named it Low-energy-Kraken-Heracles, or LKH, a nod to three separate earlier research papers that had each championed pieces of the idea that an early merger like this one existed, long before anyone could actually prove it. “In this paper we discover where the first significant batch of bricks came from,” Massari said.
The discovery pushes back what’s directly known about our galaxy’s construction by 1.8 billion years, and challenges an older assumption. “Some past studies have argued that the earliest phases of our galaxy’s evolution were defined by stars born only in our galaxy,” Massari noted. “Here, we have shown that stars born in external galaxies also need to be considered.”
The Foundation Is Still Being Excavated
Massari’s team isn’t finished. They plan to keep studying additional globular clusters, hoping to map out the complete sequence of mergers that shaped the Milky Way across its full history. As co-author Fernando Aguado-Agelet of the University of Vigo put it, Hubble is now examining globular clusters that have simply never been studied this closely before, work that keeps pushing the known edges of our galaxy’s biography further into its own deep, chaotic past.
For more on the research, check out the full announcement from NASA and the published study in Nature Astronomy.