Indian Astronomers Find Star Cluster UBC 63 Is Really Two

A group of stars long catalogued as a single galactic open cluster has turned out to be two. UBC 63, a thinly populated and loosely bound group lying about 4892 light years from Earth, is actually made up of two separate clusters that recently drifted past one another, a team led by Indian astronomers has found. The chance finding gives researchers a rare look at what happens soon after two stellar systems brush past each other, and adds to what is known about how clusters change over time in the Milky Way.

Who did the work

The team included a Department of Science and Technology INSPIRE Senior Research Fellow and researchers from Gauhati University, together with scientists of the Indian Institute of Astrophysics, an autonomous institution of the department. They had set out only to study how stars form inside UBC 63, using measurements from the European Space Agency's Gaia mission, which maps the positions and motions of stars across the Galaxy with great precision. The paper appeared in the Astronomical Journal, with Samrat Biswas of Gauhati University as lead author and Maheswar Gopinathan and Ram Sagar of the Indian Institute of Astrophysics among the co-authors.

How two clusters were spotted

Studying the brightness and colours of member stars, the researchers saw two distinct sequences in the colour magnitude diagram, the standard tool for reading the evolutionary stage of a cluster. That hinted at two systems rather than one. Machine learning methods and dynamical simulations run on high-performance computers then confirmed two separate populations, now named UBC 63A and UBC 63B.

Two very different ages

The ages settled the question. UBC 63A is young, roughly 21 million years old, while UBC 63B is far older at about 562 million years. Such a gap rules out a common origin, so the two could not have begun life as one cluster.

Drifting apart

Simulating their paths through the Milky Way showed that the pair came remarkably close some 6 million years ago and are now moving away from each other. Their motions show they are not gravitationally bound, so this was an encounter and not a marriage. Encounters between clusters are uncommon, and catching one so soon afterwards helps astronomers judge how gravity reshapes a cluster's structure and motion. The study also shows how surveys such as Gaia can reveal structures that stay hidden when stars are seen only as a flat pattern projected on the sky.