The Milky Way Swallowed a Lost Galaxy First
Hubble found a third family of ancient star clusters inside the Milky Way. They are the surviving fingerprints of a major merger 11.8 billion years ago.
The Discovery
Before the Milky Way devoured the Gaia-Sausage-Enceladus galaxy, it appears to have survived another major collision. Astronomers have identified a distinct family of ancient globular clusters that arrived in a merger about 11.8 billion years ago.
The Nature Astronomy study combined precise Hubble measurements of cluster ages and chemical composition with orbital information. A Bayesian analysis favoured three cluster populations: stars born in the early Milky Way, debris from Gaia-Sausage-Enceladus and a separate older group from a lost dwarf galaxy.
The first wreck left no visible galaxy, only a trail of ancient star cities.
What Makes It Strange
The inner Milky Way is a terrible archive. Billions of years of mixing, chaotic orbits and the galaxy's rotating bar have erased much of the motion that once separated native stars from captured ones. Globular clusters kept a different kind of record.
- The analysis used homogeneous age estimates for 39 globular clusters.
- Twelve clusters form the newly isolated intermediate population.
- Most sit within six kiloparsecs of the galactic centre.
- The merger happened roughly 1.8 billion years before Gaia-Sausage-Enceladus.
- Its progenitor had a stellar mass near 500 million Suns.
The researchers named the vanished system Low-energy-Kraken-Heracles, or LKH, combining names used in earlier competing reconstructions. The new age sequence links those disputed clues to one merger history.
The Implications
According to NASA's Hubble announcement, the result extends the firmly reconstructed history of our galaxy by about 1.8 billion years. It also changes the portrait of the infant Milky Way: stars formed elsewhere were already becoming part of it only two billion years after the Big Bang.
That conclusion comes from statistical grouping and stellar models, not a direct image of LKH. Future measurements can test membership cluster by cluster and reveal whether the event was one collision or a more complicated early assembly.
What Happens Next
Hubble is observing additional globular clusters that have not received the same precision treatment. Expanding the sample will either sharpen LKH's outline or expose more ancient contributors hiding in the crowded centre.
The galaxy beneath our feet was assembled from galaxies that no longer exist.