Unusual stars may be the leftovers of ancient galaxy gobbled up by the Milky Way

An unusual collection of stars may represent the remnants of a dwarf galaxy. Milky Way It was destroyed about 10 billion years ago. Astronomers named the ancient galaxy Loki, after the Norse god of mischief. This finding could change current understanding of how the Milky Way evolved in the distant past.

The vast Milky Way spans approximately 100,000 light-years and contains between 100 and 400 billion stars. According to NASA. A. light year It is the distance light travels in one year, that is, 5.88 trillion miles (9.46 trillion kilometers).

Our galaxy wasn’t always such a cosmic giant. It grew over time by merging with many dwarf galaxies about 12 billion years ago. However, the original size and mass of the Milky Way remains an open question; This leads scientists to look for evidence of the galaxies the Milky Way has consumed to determine its history and evolution.

To identify the missing pieces of the puzzle, astronomers focused on a metal-free star cluster detected oddly close to the galactic disk, according to a study published in May in the journal. Monthly Notices of the Royal Astronomical Society.

Astronomers are interested in these stars near the disk — a huge, spinning pancake-like region that contains most of the Milky Way’s stars — because the first stars in the universe were composed of hydrogen and helium, which combined heavier elements in their cores and then exploded, releasing heavy elements that enriched future generations of stars.

Metal-poor stars are often associated with ancient dwarf galaxies that the Milky Way has consumed over time to reach its current massive state, and remnants of these cosmic nutrients may be hiding deep within the galaxy.

The metal-poor composition of such ancient stars near the galactic disk suggests that the Milky Way formed a sizable meal from another galaxy early in its history and may represent a previously overlooked critical building block of our galaxy.

Searching for metal-poor stars

University of Connecticut physics professor Dr., who did not participate in the research. Cara Battersby said astronomers are like detectives of the universe, searching for clues to the origins of the cosmos, and very metal-poor, or VMP, stars are a powerful tool in that search.

“VMP stars have been around for billions of years and hold within them clues to the formation of the Universe’s earliest generations of stars,” Battersby wrote in an email. He added that studying the composition and motion of metal-poor stars could reveal details about the conditions and dynamics of the early universe.

The search for metal-poor stars in the Milky Way has largely focused on the large numbers of older stars in the galaxy’s stellar halo; It is so named because it is a large, circular diffuse cloud surrounding the galactic disk.

Some astronomers believe that evidence of older mergers may be found deeper in the Milky Way, such as in its disk.

Lead study author Dr. D., a postdoctoral researcher at the Astrophysics Research Center at the University of Hertfordshire in England. The abundance of young, metal-rich stars crowding the galactic disk, as well as the abundance of dust, makes it difficult to detect metal-poor stars there, Federico Sestito said.

Using observations from the European Space Agency’s Gaia telescope, Sestito and colleagues identified 20 metal-poor stars surprisingly close to the disk. The space observatory mapped the movements and compositions of 2 billion stars in the Milky Way between July 2014 and January 2025. Sestito and his team then used the high-resolution spectrograph instrument on the Canada-France-Hawaii Telescope near the summit of Maunakea in Hawaii to observe the stars.

It’s difficult to determine the exact age of the stars, Sestito said, but their chemical composition suggests they are more than 10 billion years old, and they are all located about 7,000 light-years from our solar system. According to the research, the chemical compositions of stars are also similar; This suggests that they all come from the same metal-poor dwarf galaxy.

Eleven of the stars were in a prograde orbit, or moving in the same direction as the galactic disk, while nine were in a retrograde orbit, or moving in the opposite direction; Possible remnants of a dwarf galaxy that was swallowed by the Milky Way just a few billion years after the big bang that created the universe approximately 13.8 billion years ago.

Battersby said the study’s authors believe stars that accrete or are missed remain part of our galaxy, being blown around and evolving into different orbital patterns.

“If the Loki scenario is true, a system merging with our galaxy could accumulate its stars in both the forward and reverse directions,” Sestito wrote in an email. “This would only be allowed if the merger event occurred at a time when the Milky Way was still infant/smaller and its gravitational potential was weaker than today. Cosmological simulations suggest that this could have occurred no later than 3 or 4 billion years after the Big Bang.”

Director of the galaxies and cosmology department at the Max Planck Institute for Astronomy in Germany, Dr. Hans-Walter Rix said the most fascinating thing about the study was “how they used detailed chemical element abundances as fingerprints to determine the common birth origin of these stars in a now fragmented satellite galaxy, even though some stars were going in the right direction and some in the wrong direction.” Rix was not involved in the research.

He said Sestito was inspired to name the ancient dwarf galaxy Loki because the trickster god’s intentions are difficult to decipher in mythological stories.

“Similarly, our accumulating stars have given us a bit of a hard time understanding their origins,” Sestito said. “At first, it was not easy to reconcile the fact that an accreting system could disperse its stars in both prograde and anticorrosive orbits.”

Another explanation for the stars could be that they result from multiple merger events with the Milky Way, he said.

But Battersby said the idea of ​​a single galaxy’s stars being in the Milky Way was intriguing and worthy of further study.

Tracking the Milky Way’s meals

The Milky Way grew through galactic cannibalism, or when a large galaxy ate a smaller galaxy and used an enormous gravitational force to absorb its stars and gas. Dr. is an assistant professor in the department of astronomy and astrophysics at the University of Chicago. Pieces left over from such meals allow astronomers to “piece together the eating history” of the galaxy, Alexander Ji said. He was not involved in the new research.

“There are always lots of small mergers happening, but really big meals could change the growth history of the Milky Way,” Ji wrote in an email.

One such transformative event occurred 8 to 10 billion years ago when the Milky Way merged with the Gaia-Sausage-Enceladus galaxy.

“We think this helped ‘reset’ the Milky Way from its initial turbulent phase to its more steadily growing disk today,” Ji said.

The new study suggests that the Milky Way’s merger with the Loki galaxy is almost on the scale of the Gaia-Sausage-Enceladus event. But Ji added that the evidence was largely hidden because Loki’s remains were difficult to find near the disk of the Milky Way.

“If this is real, it would indicate that we have missed a large part of the Milky Way’s formation history, and we may need to revisit our current picture to see the impact of such an event,” Ji said.

Ji doubts that Loki is a previously unknown galaxy, given that possible discoveries related to merger events often turn out to be extensions of known systems, but noted that the study authors included appropriate caveats in their work.

“This is an interesting new possibility worth pursuing, and I hope there will be people who want to test with larger data sets whether Loki is real,” Ji said.

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