Astronomers from India and abroad found that UBC 63 is actually two distinct star clusters, not one. Using data from the Gaia mission, the team showed that UBC 63A and UBC 63B recently passed close to each other. This rare discovery offers new insights into how star clusters evolve together.
A team of astronomers from India and abroad have found that UBC 63, a poorly populated group of stars which was previously identified as a single star cluster, has now been found to be composed of two distinct star clusters.
UBC 63 is bound loosely by gravity (galactic open cluster) about 4,892 light years away from the Earth.
Rare opportunity
This chance discovery by a Department of Science and Technology (DST) INSPIRE Senior Research Fellow (SRF) at Gauhati University, along with astronomers from the Indian Institute of Astrophysics (IIA) and abroad, has provided a rare opportunity to study the aftermath of a close flyby between two stellar systems and offers new insights into the dynamical evolution of star clusters in the Milky Way.
“Star clusters are important building blocks of galaxies. They are groups of stars that form within the same large reservoirs of gas and dust and subsequently evolve under the influence of their own gravity as well as the broader galactic environment. Among them, historically, UBC 63 has remained largely overlooked, with limited literature providing only preliminary parameter estimates that presumed it to be a single cluster,” DST said.
The astronomers, who set out to investigate star formation dynamics within the UBC 63 cluster using data from the European Space Agency’s (ESA) Gaia mission, discovered to their surprise that UBC 63 actually consists of two clusters, UBC 63A and UBC 63B, that recently passed remarkably close to each other.
DST said that encounters between star clusters are relatively uncommon. Detecting a system soon after such an encounter can therefore provide valuable clues about how gravitational interactions influence cluster structure, motion, and long-term evolution.
What Gaia showed
Gaia, which provides exceptionally precise measurements of the positions and motions of stars across the Milky Way, showed that the two clusters are of starkly different ages.
The researchers studied the brightness and colours of the member stars of UBC 63 and found that the stars formed two distinct sequences in the colour-magnitude diagram, a key astronomical tool used to investigate the evolutionary stages of star clusters. This indicated that UBC 63 might actually consist of two separate stellar systems.
“A detailed analysis of the stellar motions and other properties, using robust machine learning techniques and advanced dynamical simulations powered by high-performance computing, confirmed the presence of two distinct populations, now designated UBC 63A and UBC 63B,” DST said.
It added that the two clusters have markedly different ages: UBC 63A is a young cluster, approximately 21 million years old, whereas UBC 63B is much older, with an estimated age of about 562 million years. “This substantial age difference demonstrates that the two systems did not originate as a single cluster,” it added.
Close location
The researchers then investigated how two clusters with such different histories came to be located so close to each other. By simulating their motions through the Milky Way, the study found that, at the time when the younger cluster formed, the two systems were separated by about 60 parsecs (approximately 196 light years). The two clusters came remarkably close to one another about 6 million years ago, reaching a separation of only about 7 parsecs (approximately 23 light years).
Today, the two clusters are approximately 26 parsecs (approximately 85 light years) apart. Their motion shows that they are not gravitationally bound to each other. Instead, they are moving apart after a close encounter.