Astronomers have uncovered more than 400 pairs of star clusters in our Milky Way—cosmic siblings that were born together or later became partners as they moved through space.
This major discovery, made using highly accurate measurements from the European Space Agency’s Gaia satellite, gives scientists a clearer picture of how stars form, evolve, and interact over millions of years.
Stars don’t form alone. They are usually born in large groups inside giant molecular clouds—enormous regions of gas and dust.
These stars often form clusters, and sometimes two clusters are so close in space and motion that they form what astronomers call a binary cluster. Studying these cluster pairs helps scientists understand the earliest stages of star formation and how these systems change over time.
A research team led by Ph.D. student Liu Guimei and her supervisor, Professor Zhang Yu from the Xinjiang Astronomical Observatory, analyzed nearly 4,000 well-studied open clusters in the Milky Way.
Open clusters are loose groups of young stars that formed together and continue to travel through the galaxy as a family. Using the latest Gaia DR3 data, the team examined the positions and motions of these clusters with high precision.
The researchers developed a strict, uniform method to decide whether two clusters should be considered a real pair.
They compared the distances between clusters and how fast and in what direction they move through space. They also tested their method against thousands of random cluster pairings to make sure the results were reliable.
With this careful approach, they identified 400 binary cluster candidates—an impressive number, considering that 268 of them were previously unknown.
The clusters were sorted into three main groups: clusters that were born together from the same cloud, clusters that became partners later due to gravitational interactions, and cluster pairs that only look close together from our point of view but are not actually connected.
The team found that most of the discovered pairs share similar ages and motions, which strongly suggests they formed together like twin stars in the same cosmic nursery.
They also discovered that about 83% of the pairs show signs of tidal interaction, meaning they subtly pull on each other through gravity. The closer the clusters are, the stronger this mutual tug becomes.
The study shows that around 17% of all open clusters in the Milky Way exist in binary or multi-cluster systems.
About 10% probably formed as true primordial pairs from the same cloud. These numbers match well with earlier predictions but now come from one of the most thorough analyses ever done.
This research provides strong evidence that star formation happens in a hierarchical way, meaning stars and clusters form in multiple layers and structures within giant clouds.
The discovery of so many cluster siblings strengthens our understanding of how the Milky Way built its stellar populations—and how star families continue to shape our galaxy today.
Source: Chinese Academy of Sciences.
