Hubble Finds Evidence of the Milky Way’s Earliest Major Merger

NASA’s Hubble Space Telescope has provided evidence that the young Milky Way absorbed a dwarf galaxy about 11.8 billion years ago. The event, identified with the progenitor name Low-energy-Kraken-Heracles (LKH), pushes the documented history of the Milky Way’s assembly roughly 1.8 billion years farther back than before.

The key clue was not a direct image of the collision. Instead, researchers examined the Milky Way’s globular clusters—dense groups of stars that formed at roughly the same time—and found a third age–metallicity sequence between clusters associated with the original Milky Way and those linked to the Gaia-Sausage-Enceladus merger.

What Hubble’s evidence shows

Globular clusters act as archaeological records. Their ages, chemical composition, and present-day locations can preserve information about the environments in which they formed, even after their parent galaxy has been disrupted.

Hubble’s resolved-star observations supplied the data needed to study these clusters in detail. NASA describes the resulting evidence as definitive for a dwarf galaxy, LKH, colliding with and merging into the young Milky Way approximately 12 billion years ago.

The associated research identifies three distinct cluster sequences: one connected to the Milky Way’s original population, one associated with Gaia-Enceladus, and an intermediate sequence associated with at least one earlier merger. That intermediate population is the central evidence for an additional major event in the Galaxy’s early history.

When did the merger happen?

The event is estimated to have occurred about 11.8 billion years ago, or approximately 1.5 billion years after the Big Bang according to the supplied research reporting.

NASA’s public summary rounds the date to about 12 billion years ago and says the finding extends knowledge of the Milky Way’s history by 1.8 billion years. These figures are consistent at the level of precision presented by the sources: 11.8 billion years is the more specific estimate, while NASA gives the rounded value.

The LKH merger therefore predates the better-known Gaia-Sausage-Enceladus event. One supplied report places it roughly 1.8 billion years earlier than Gaia-Sausage-Enceladus.

What was LKH?

The sources identify LKH as a dwarf galaxy whose stars and globular clusters were incorporated into the inner Milky Way. Its estimated stellar mass was approximately 500 million solar masses, comparable to that of Gaia-Sausage-Enceladus.

The name combines previously discussed structures and labels: Low-energy, Kraken, and Heracles. The supplied reporting says the name connects these structures to a single progenitor system rather than treating them as wholly separate events.

That naming also reflects the history of the hypothesis. Earlier work had associated low-energy globular clusters with the proposed Kraken system, while other research identified Heracles-like structure. The new interpretation links them as evidence of the same early accretion event. The available sources support that connection, but do not provide a detailed account of every earlier point of disagreement.

How this changes the Milky Way’s assembly timeline

The result adds a major stage to the Galaxy’s formative history:

1. The primordial Milky Way began assembling.
2. LKH merged with it about 11.8 billion years ago.
3. Gaia-Sausage-Enceladus merged later, roughly 1.8 billion years after the LKH event according to the supplied reporting.

This makes the Milky Way’s growth look less like a single continuous formation process and more like a sequence of mergers that added stars and stellar systems at different times. LKH’s material was deposited mainly in the inner regions of the Galaxy, according to the supplied research summary.

The finding does not mean Hubble watched the collision happen. The merger occurred billions of years ago; Hubble instead helped identify surviving stellar fossils whose ages, metallicities, and groupings retain evidence of the event.

What remains uncertain

The strongest supported conclusion is that globular-cluster observations reveal an early merger between the Milky Way and LKH. The supplied sources do not independently document every detail in the original question, including a complete description of a 39-cluster sample within 20,000 light-years or a precise explanation of which historical hypothesis was contested.

Nor do they establish how many additional ancient mergers astronomers will find. NASA’s Hubble Missing Globular Clusters Survey is designed to complete observations of the Milky Way’s globular-cluster system and investigate still-unexplored clusters, which could improve the reconstruction of the Galaxy’s past.

For now, the significance is clear: Hubble-backed globular-cluster archaeology has moved the known merger history of the Milky Way back to its earliest major chapter—an event about 11.8 billion years ago involving a dwarf galaxy with roughly half a billion Suns’ worth of stars.