Astronomers Identify 20 Stars as Possible Remnants of Ancient Galaxy Merger

A research team has identified 20 stars inside the Milky Way whose chemical signatures and orbital paths suggest they originated in a small dwarf galaxy that merged with the Milky Way about 10 billion years ago. The findings were published this month in the Monthly Notices of the Royal Astronomical Society. The team named the hypothetical parent galaxy Loki.

The stars are described as very metal-poor, containing almost none of the heavier elements found in younger stars. Their composition indicates they formed within the first two to three billion years after the Big Bang. Eleven of the stars follow prograde orbits aligned with the Milky Way's disc, while nine move in retrograde orbits.

Despite the opposing directions, the stars share nearly identical chemical fingerprints.

Observations came from the European Space Agency's Gaia telescope, which recorded positions, velocities, and compositions of two billion stars between July 2014 and January 2025. Follow-up spectra were obtained with the Canada-France-Hawaii Telescope.

Dr Federico Sestito of the University of Hertfordshire led the study. He said the naming choice reflected the challenge of reconciling the stars' mixed orbital directions with a single origin.

Ji of the University of Chicago said that if confirmed, the merger would indicate a previously unrecognized event in the Milky Way's assembly. He added that further work is needed to distinguish it from extensions of known merger remnants such as Gaia-Sausage-Enceladus.