A team led by Indian astronomers has discovered that a celestial object previously identified as a single star cluster is actually composed of two distinct star clusters that experienced a remarkably close encounter millions of years ago.
The discovery concerns UBC 63, a Galactic open cluster located approximately 4,892 light-years from Earth. Researchers have identified two separate stellar systems within the region, now designated UBC 63A and UBC 63B.
Editorial Insight
Key Highlights
Important points readers should notice.
Issue/Event: Indian astronomers identify two distinct star clusters within the previously classified UBC 63 system.
Location: India.
Authority/Organisation: Gauhati University, Indian Institute of Astrophysics and Department of Science and Technology.
Action Taken: Researchers analysed Gaia observations using machine learning and advanced dynamical simulations.
- Impact: The discovery provides new evidence about close stellar encounters and the evolution of star clusters in the Milky Way.
The findings provide a rare opportunity to study how close encounters between star clusters influence their structure, movement and long-term evolution within the Milky Way.
The research was announced by the Department of Science and Technology (DST) on November 5, 2026. The study was published in The Astronomical Journal.
How scientists identified two clusters
Editorial Analysis
Why This Matters
The discovery is significant because it provides scientists with an opportunity to examine how gravitational encounters influence the evolution of star clusters. - Astronomy: Helps improve understanding of the structure and evolution of the Milky Way. - Scientific research: Demonstrates the value of combining space observations with computational techniques. - Machine learning: Shows how advanced data analysis can reveal previously overlooked astronomical structures. - Indian research: Highlights contributions from Indian universities and scientific institutions. - Future studies: Provides an opportunity to investigate how stellar systems move and interact over millions of years.
The research team initially investigated UBC 63 to understand star formation dynamics within the region. Earlier observations had classified it as a single, relatively sparsely populated open cluster.
Scientists used data from the European Space Agency's Gaia mission, which provides highly precise measurements of the positions, distances and movements of stars across the Milky Way.
During the investigation, researchers noticed that the brightness and colours of stars within UBC 63 formed two distinct sequences in a scientific diagram known as the colour-magnitude diagram.
This suggested that the region might contain two separate stellar populations rather than one cluster.
The team then examined stellar movements and other characteristics using machine-learning techniques and advanced computer simulations. The analysis confirmed two distinct populations, leading to the identification of UBC 63A and UBC 63B.
Two clusters with dramatically different ages
One of the most significant findings is the substantial difference in the estimated ages of the two clusters.
UBC 63A is approximately 21 million years old, making it the younger system. UBC 63B is estimated to be around 562 million years old.
The age difference indicates that the two clusters did not originate together as a single stellar system.
Researchers identified 98 members in UBC 63A and 148 members in UBC 63B through their analysis.
The discovery demonstrates how two stellar systems with different evolutionary histories can appear together in observations of the same region of the sky.
A close encounter around six million years ago
The researchers reconstructed the movement of both clusters through the Milky Way using orbital calculations and N-body simulations.
Their analysis indicates that the clusters came remarkably close to one another approximately six million years ago.
At their closest approach, the distance between them was estimated at just seven parsecs, equivalent to approximately 23 light-years.
At present, the two clusters are separated by around 26 parsecs, or approximately 85 light-years.
Importantly, the researchers found that the two systems are not gravitationally bound to each other. Their relative motion indicates that they are moving apart following the encounter.
The study therefore offers a valuable example of a close stellar interaction that did not result in the two clusters becoming a permanently bound system.
Indian research institutions contribute to the discovery
The research was led by Samrat Biswas, an INSPIRE Senior Research Fellow at Gauhati University, along with Biman J. Medhi of Gauhati University.
The wider international research team included scientists from the Indian Institute of Astrophysics, Cotton University, Assam University, INAF-Catania Astrophysical Observatory, Shanghai Normal University, Shanghai Astronomical Observatory and the University of Tokyo.
The study demonstrates the contribution of Indian research institutions to international astronomy and the use of modern astronomical datasets.
Why this discovery matters
Star clusters are important building blocks of galaxies. They help scientists understand how stars form, how stellar systems evolve and how gravitational interactions shape the structure of galaxies.
The UBC 63 discovery highlights the importance of combining precise space-based observations with machine learning and computer simulations.
It also shows how objects that appear to be a single astronomical structure can contain multiple systems with different histories.
By studying such encounters, astronomers can improve their understanding of the dynamic environment of the Milky Way and the evolution of stellar populations.
