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 that the Milky Way devoured about 10 billion years ago. Astronomers have dubbed the ancient galaxy Loki, after the Norse god of mischief. The finding could change the current understanding of how the Milky Way evolved in the distant past.

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

Our home galaxy wasn’t always such a cosmic giant. It grew over time starting about 12 billion years ago by merging with a multitude of dwarf galaxies. But the original size and mass of the Milky Way remain an open question — driving scientists to search for evidence of the galaxies it consumed to determine its history and evolution.

To identify those missing puzzle pieces, astronomers have now zeroed in on a cluster of metal-lacking stars detected oddly close to the galactic disk, according to a study published in May in the journal Monthly Notices of the Royal Astronomical Society.

The astronomers are interested in these stars near the disk — a massive rotating pancake-like region containing much of the Milky Way’s stars — because the first stars in the universe were comprised of hydrogen and helium, which fused heavier elements together in their cores before exploding and unleashing the heavy elements that enriched future generations of stars.

Metal-poor stars are often associated with ancient dwarf galaxies, which the Milky Way might have consumed over time to grow to its current massive state — and remnants of these cosmic meals might be hiding deep within the galaxy.

The metal-poor composition of such ancient stars close to the galactic disk suggests that the Milky Way once made a rather large meal of another galaxy early in its history — and it could represent a critical, previously overlooked building block of our galaxy.

The search for metal-poor stars

Astronomers are like the detectives of the universe, searching the cosmos for clues of its origins, and very-metal-poor, or VMP, stars are a powerful tool in that quest, said Dr. Cara Battersby, associate professor of physics at the University of Connecticut, who did not participate in the study.

“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. Studying the metal-poor stars’ composition and motion can unlock details about the conditions and dynamics of the early universe, she added.

The search for metal-poor stars in the Milky Way has largely centered on the plentiful range of old stars in the galaxy’s stellar halo, so named because it’s a large, round diffuse cloud that surrounds the galactic disk.

Some astronomers believe evidence of more ancient mergers could be found deeper inside the Milky Way, such as in its disk.

An abundance of young, metal-rich stars, as well as a plethora of dust, crowded within the galactic disk has made it hard to spot metal-poor stars there, said lead study author Dr. Federico Sestito, a postdoctoral fellow at the University of Hertfordshire’s Centre for Astrophysics Research in England.

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

The exact age of the stars is hard to pin down, but their chemical composition suggests they are older than 10 billion years, Sestito said, and all of them are located roughly 7,000 light-years from our solar system. The stars also have similar chemical compositions, suggesting they all came from the same metal-poor dwarf galaxy, according to the study.

Eleven of the stars were in a prograde orbit, or moving in the same direction of the galactic disk, while nine were on a retrograde orbit, or moving in the opposite direction — possible remnants of a dwarf galaxy gobbled up by the Milky Way just a few billion years after the big bang that created the universe about 13.8 billion years ago.

The study authors believe the accreted, or hijacked, stars, simply remained as part of our galaxy, getting knocked around and ending up in different orbital patterns, Battersby said.

“If the Loki scenario is correct, then a system merged with our galaxy could deposit its stars into both prograde and in the opposite direction,” Sestito wrote in an email. “This can be allowed only if the merger event happened when our Milky Way was still infant/smaller and its gravitational potential was weaker than nowadays. Cosmological simulations suggest that this could have happened no later than 3 or 4 billion years from the Big Bang.”

Dr. Hans-Walter Rix, director of the department of galaxies and cosmology at the Max Planck Institute for Astronomy in Germany, said what was most impressive about the study was “how they use the detailed chemical element abundances as a fingerprint to identify a common birth origin of these stars in a now-shredded satellite galaxy, even though some of the stars are going the right way around and some the wrong way.” Rix was not involved with the research.

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

“Similarly, our accreted stars gave us some hard time in understanding their origin,” Sestito said. “At first it was not easy to reconcile the fact that an accreted system can disperse its stars in both prograde and opposite orbits.”

Another explanation for the stars could be that they stem from more than one merger event with the Milky Way, he said.

But the idea of a single galaxy’s stars being enfolded into the Milky Way is intriguing and worthy of further study, Battersby said.

Tracking the Milky Way’s meals

The Milky Way has grown through galactic cannibalism, or when a large galaxy eats a small galaxy and uses immense gravitational force to absorb its stars and gas. The leftover shreds of such meals enable astronomers to assemble the galaxy’s “eating history,” said Dr. Alexander Ji, assistant professor in the department of astronomy and astrophysics at the University of Chicago. He was not involved with the new research.

“There are lots of little mergers happening all the time, but the really big meals can change the growth history of the Milky Way,” Ji wrote in an email.

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

“We think it helped ‘reset’ the Milky Way from its early turbulent phase to the more stable growing disk that it has today,” Ji said.

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

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

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

“It’s an interesting new possibility worth pursuing, and I expect there will be people looking to test whether Loki is real with larger datasets,” Ji said.

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