The Milky Way’s Stolen Globular Clusters

By Ehtesham

Published on:

In the vast cosmos, where galaxies collide and stars dance in the void, secrets are concealed within the Milky Way’s embrace. It’s a story of grand theft, cosmic larceny, and celestial pilferage as we explore the enigmatic origins of our galaxy’s globular clusters.

These stellar treasures, which have enriched our night skies for eons, hold a profound secret—they may have been stolen, or at least borrowed, from other galaxies.

Cosmic Puzzle

Modern astronomy’s prevailing wisdom tells us that galaxies, including our own Milky Way, are the result of numerous celestial mergers. Therefore, it’s no surprise that some of the globular clusters within our galaxy may have extragalactic origins.

This revelation began to take shape in the 1990s, but recent research is offering a more comprehensive understanding of which clusters were pilfered and from where.

Galactic Satellites

The Milky Way plays host to a diverse ensemble of satellite galaxies. While the Magellanic Clouds are among the famous members of this celestial menagerie, there are lesser-known entities such as the Fornax dwarf and the Antlia II dwarf.

These galactic companions orbit our Milky Way, adding to its grandeur. However, not all of these companions are entirely independent. Some, like the Sagittarius dwarf galaxy discovered in 1994, appear elongated and distorted, resembling streams of stars rather than organized galaxies.

This peculiar appearance hints that they are in the process of merging with the Milky Way, gradually succumbing to tidal forces.

Dwarf Galaxies

The curious case of our globular clusters began with an observation—many of these clusters share a similar age, while others are relatively young. Astronomers were perplexed by this dichotomy and postulated that the younger clusters may have formed within the dwarf galaxies being absorbed by the Milky Way.

Slowly but surely, the pieces of this cosmic puzzle started to fall into place. In 2002, astronomers studying the globular cluster NGC 5634 made a remarkable connection.

This cluster’s position was within the tidal stream of the Sagittarius dwarf galaxy. Moreover, its motion and metal-poor composition aligned perfectly with the rest of the dwarf galaxy.

Since then, astronomers have unearthed compelling evidence connecting several more globular clusters with the tumultuous fate of the Sagittarius dwarf galaxy. This list includes AM 4, Arp 2, Pal 12, NGC 2419, NGC 4147, Terzan 7, Terzan 8, and Whiting 1.

Cosmic Streams

As the cosmic story unfolded, additional dwarf galaxies experiencing a tidal breakdown came to light, including the Helmi stream, the Gaia-Enceladus Sausage, and the Sequoia galaxy. With these discoveries came more associations with globular clusters.

Cosmic Swap Shop

In a recent series of papers published by astronomers at the Institute for Advanced Studies in Basic Sciences in Iran, a different angle was explored.

These researchers delved into how galaxies surrounding the Milky Way, whether they were merging or peacefully orbiting, might engage in a celestial game of globular cluster exchange.

In the first paper, published in the Monthly Notices of the Royal Astronomical Society, the scientists devised hypothetical dwarf galaxies populated with globular clusters, placing them in various orbits around the Milky Way. The goal was to understand how readily these clusters could be disentangled from their host galaxies.

Their findings revealed that the percentage of clusters liberated varied, ranging from 12% to a staggering 93%. Clusters with elliptical orbits that extended to the outskirts of their host galaxies were more likely to be set free. However, the gravitational might of the host galaxy played a pivotal role in retaining its clusters.

Based on this simulation, the paper suggested that at least two globular clusters may have originated from the Fornax galaxy, four from the Large Magellanic Cloud, two from the Small Magellanic Cloud, and an astonishing 14 from the Sagittarius dwarf galaxy.

Globular Cluster Associations

In their second paper, also featured in the Monthly Notices of the Royal Astronomical Society, the researchers took the opposite approach.

They scrutinized the orbital properties of 154 globular clusters, comparing them with the properties of 41 known dwarf galaxies orbiting the Milky Way, as well as some tidally disrupted systems.

Based solely on these kinematic properties, the scientists identified 14 globular clusters that had a strong connection with the Sagittarius dwarf galaxy, aligning with their earlier findings. However, the identity of these clusters didn’t entirely match with other studies.

Interestingly, they found five clusters previously associated with the Sagittarius dwarf galaxy that didn’t share enough parameters to be considered likely candidates for theft.

In a twist of cosmic intrigue, the researchers identified four new globular clusters not previously linked to the galaxy but with a high likelihood of association, as well as six others with a lower likelihood. Furthermore, six clusters were deemed likely transplants from the Large Magellanic Cloud.


As this cosmic detective story unfolded, the researchers arrived at a fascinating conclusion. They found that 29% of the known globular clusters shared properties similar enough to be associated with the reviewed dwarf galaxies.

However, the researchers acknowledge that their models are somewhat simplified. They didn’t fully map the intricate 3-dimensional structure of the dwarf galaxies, a complex task that awaits future research.

With this revelation, the cosmic heist that is the Milky Way’s stolen globular clusters continues to be a captivating astronomical mystery. Further exploration and in-depth investigations will undoubtedly uncover more clues and unravel more celestial connections.


The Milky Way’s stolen globular clusters are a testament to the cosmic drama that unfolds in the vast expanse of space. These celestial jewels, some potentially borrowed, provide a window into the intricate dance of galaxies.

The universe’s secrets are boundless, and as we peel back the layers, we draw closer to understanding the complex tapestry of our cosmic neighborhood.


What are globular clusters, and why are they significant in astronomy?

Globular clusters are dense groups of stars that orbit galaxies. They are essential in astronomy because they offer insights into the history and formation of galaxies, including our Milky Way.

What is the connection between dwarf galaxies and globular clusters in the Milky Way?

Recent research suggests that some of the globular clusters in the Milky Way may have been acquired from dwarf galaxies that were merged or cannibalized by our galaxy.

What is the role of tidal forces in the formation of streams of stars in dwarf galaxies?

Tidal forces exerted by larger galaxies, such as the Milky Way, can distort and elongate dwarf galaxies, creating streams of stars. These streams are a key part of the puzzle in understanding the origin of globular clusters.

How do scientists determine the likely origin of globular clusters in the Milky Way?

Scientists use kinematic properties, such as the motion and composition of globular clusters, to identify their likely origin. These properties can reveal associations with specific dwarf galaxies or other celestial structures.

What’s next in the study of the Milky Way’s stolen globular clusters?

Future research will involve more detailed modeling of the 3-dimensional structures of dwarf galaxies and further investigation into the newly identified associations. This ongoing exploration promises to uncover more cosmic connections and unveil additional secrets of our galaxy.