Last Updated on June 12, 2026 by Staff
The Milky Way galaxy was not formed alone. It took billions of years for the Milky Way to grow. The Milky Way grew by taking in galaxies through collisions and mergers. These big events left behind streams and clusters of stars that still orbit the Milky Way galaxy today.
The Gaia-Sausage/Enceladus structure, also known as the GSE is one of the important leftovers from the Milky Way’s ancient history. For a time astronomers thought the GSE was formed from one big collision that happened between 10 and 13 billion years ago. They believed this event was the big merger for the Milky Way and a key moment in shaping the inner part of the galaxy.
A new study says this story might not be entirely correct. Now researchers think the GSE could be the result of separate collisions, not just one big event.
New Research
To learn more about the GSE astronomers looked at data from the Dark Energy Spectroscopic Instrument, an advanced tool for studying stars and galaxies. The team of researchers studied 87,000 stars and used a new computer algorithm called GS³ Hunter. This tool helps find groups of stars with chemical makeup and movement patterns.
The study found 17 streams and substructures of stars. It included the Sequoia stream, which was already known and several new groups that were just discovered.
Importantly the researchers found four distinct substructures within the area that is normally associated with the Gaia-Sausage/Enceladus. These groups were named GSE-GSH1, GSE-GSH2, GSE-GSH3 and GSE-GSH4.
This discovery made the researchers question whether the GSE was really formed from one collision.
Chemical Signs
One of the clues came from the chemical makeup of the stars.
The scientists studied elements like magnesium, calcium, titanium, aluminum and carbon in the stars. While some patterns looked the same across all four groups others were very different.
These differences suggest that the stars did not all form in the way. Some groups showed signs of forming stars intensely early on while others formed stars more slowly over a longer time.
One group, GSE-GSH2 stood out more. Its chemical signs showed peaks and unusual patterns, which means it had a more complicated history than the other substructures.
These differences mean that the stars might have come from galaxies with their own unique star formation histories before they became part of the Milky Way galaxy.
Different Paths
The way the stars moved through space also provided clues.
All four substructures followed elongated paths, which is often seen in stars left behind after a galaxy merger. When a smaller galaxy is torn apart by the Milky Way’s gravity its stars are stretched into elliptical paths.
Although the four groups shared this behavior they occupied slightly different areas of space. This means they might have been stripped from their galaxies at different times or might have come from entirely separate galaxies.
The researchers believe these differences in movement are consistent with accretion events happening over billions of years rather than one big collision.
The stars seem to keep a record of ancient encounters that helped the Milky Way grow.
A New View
Maybe the convincing evidence comes from the ages of the stars themselves.
The researchers found that the stars in the four substructures are between 7 billion and 12 billion years old. This big age gap is hard to explain if all the stars came from one merger event.
One collision would likely produce stars with age. Instead the wide age range suggests that different galaxies were absorbed at times throughout the Milky Way’s history.
The findings point to a more complex scenario, where the Gaia-Sausage/Enceladus represents the result of many mergers and accretion episodes.
If future studies confirm these results astronomers might need to rethink one of the important parts of the Milky Ways formation story. Of being the remnant of one ancient collision the GSE could represent a collection of several galactic encounters that helped build the Milky Way galaxy we see today.
This new view shows just how dynamic and chaotic the early universe was and it makes the Milky Ways history more interesting than we thought.