Did Milky Way secretly swallow another galaxy? Ancient stars hint at lost star system

The original size and mass of the Milky Way galaxy have always driven scientists to search for evidence of the galaxies it consumed to determine its history and evolution. To determine those puzzles of the galaxy, astronomers have identified a group of metal-poor stars located unusually close to the galactic disk, according to a study published in May in the journal Monthly Notices of the Royal Astronomical Society.

Researchers are studying these stars because they are located near the Milky Way’s galactic disk, a huge rotating region that contains many of the Milky Way’s stars, as the earliest stars in the universe were mainly made of hydrogen and helium. Over time, they created heavier elements in their cores before exploding and unleashing, helping in enriching future generations of stars.

Stars with very low amounts of heavy elements, known as metal-poor stars, are usually linked to ancient dwarf galaxies. Scientists believe the Milky Way may have absorbed many such smaller galaxies over billions of years as it grew larger. The discovery of these ancient metal-poor stars near the galactic disk suggests that the Milky Way may have swallowed a large galaxy early in its history, leaving behind hidden traces inside our galaxy today.

Cara Battersby, an associate professor of physics at the University of Connecticut who was not involved in the study, said astronomers work like detectives trying to uncover the origins of the universe. She added that very metal-poor stars are an important clue in understanding how galaxies formed and evolved. “VMP stars have been around for billions of years, holding within them clues to the formation of the Universe’s earliest generations of stars,” Battersby wrote in an email. She added that studying the metal-poor stars’ composition and motion can open up the details about the conditions and dynamics of the early universe.

Inside the disk of the Milky Way

Researchers' search for metal-poor stars in the Milky Way has always centred around the galaxy’s stellar halo, which is quite large and has a round diffuse cloud that surrounds the galactic disk. Several scientists think that evidence of more ancient mergers could be discovered deeper inside the disk of the Milky Way.

Meanwhile, an author, Dr Federico Sestito, a postdoctoral fellow at the University of Hertfordshire’s Centre for Astrophysics Research in England, said that a huge amount of young, metal-rich stars, as well as a plethora of dust, crowded within the galactic disk, has made it very hard to track metal-poor stars.

Davide Sestito and his team discovered 20 metal-poor stars located unexpectedly close to the galactic disk using data from the European Space Agency’s Gaia telescope. The observatory tracked the movement and composition of nearly 2 billion stars in the Milky Way between July 2014 and January 2025. The researchers later used a high-resolution spectrograph at the Canada-France-Hawaii Telescope on Maunakea in Hawaii to study these stars in greater detail.

Although scientists have not been able to determine the stars’ exact ages, their chemical makeup indicates they are more than 10 billion years old, according to Sestito. All 20 stars are located around 7,000 light-years from our solar system and share similar chemical properties, suggesting they likely originated from the same metal-poor dwarf galaxy, the study said.

The Milky Way stretches nearly 100,000 light-years and contains an estimated 100 to 400 billion stars, according to NASA. Scientists believe the galaxy gradually expanded over the past 12 billion years by merging with smaller dwarf galaxies. However, its original size and mass remain unknown, prompting researchers to search for traces of ancient galaxies absorbed during its evolution.