Astronomers have identified evidence of a long-lost dwarf galaxy that once merged with the Milky Way, offering new insight into how the galaxy formed.
The study, published in the Monthly Notices of the Royal Astronomical Society, focuses on a group of 20 ancient stars that researchers say originated in a small galaxy they call “Loki.” The system is believed to have merged with the Milky Way during its early growth.
Unusual stars reveal hidden past
The stars are metal-poor, meaning they contain very low amounts of heavy elements. Such stars are among the oldest in the universe and often preserve records of early cosmic events.
What makes this group unusual is its location. Most metal-poor stars are found in the Milky Way’s outer halo. These stars, however, lie in the galactic plane.
Researchers also found that the stars move along highly eccentric orbits. Some travel in the same direction as the galaxy’s rotation, while others move in the opposite direction.
Chemical clues point to violent origins
Scientists analyzed the chemical composition of the stars to trace their origin. The results revealed a distinct pattern not seen in typical halo stars.
The stars show signs of enrichment from high-energy supernovae, hypernovae, fast-rotating massive stars, and neutron star mergers. However, there is no evidence of material from white dwarf explosions.
Astronomers have uncovered traces of a lost dwarf galaxy, “Loki,” hidden inside the Milky Way.
A group of 20 ancient, metal-poor stars reveals its existence, showing signs of violent cosmic events and a shared origin.#dwarfgalaxy #Loki #milkyway pic.twitter.com/ToP1mdQxYz
— Tom Marvolo Riddle (@tom_riddle2025) May 1, 2026
Researchers say this combination points to a short-lived and energetic environment. This type of activity is common in small dwarf galaxies that form stars rapidly before fading out.
Evidence supports a single origin
Despite differences in their motion, all 20 stars share nearly identical chemical signatures. The study found very little variation in their composition.
This consistency suggests the stars formed in the same system. Researchers say they likely originated in a single dwarf galaxy rather than multiple sources.
Models of the system’s mass also support this conclusion. The total mass of stars and gas matches that of a small galaxy. A scenario involving two separate systems would require nearly double the mass and identical chemical histories, which is unlikely.
Distinct from other ancient stars
The Loki group stands apart from other metal-poor stars in the Milky Way. Stars in the halo usually show a wide range of chemical properties, reflecting different origins.
In contrast, this group shows a narrow chemical spread. Researchers point to these points for the formation of a closed system, where material evolved in a more uniform way.
Future surveys to refine the picture
The researchers note that their sample size is small. Larger surveys will be needed to confirm the findings.
Upcoming projects such as WEAVE and 4MOST are expected to collect data on many more stars across the Milky Way.
These observations may reveal whether other hidden galaxies, like Loki, contributed to the formation of the Milky Way. Scientists say each discovery helps reconstruct the galaxy’s complex past.
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