Why in news?
A team including Indian astronomers has used Gaia mission data to find evidence that UBC 63 contains two star clusters seen after a close encounter. The Department of Science and Technology highlighted the research on 5 November 2026. The study identifies two distinct stellar populations, UBC 63A and UBC 63B, and interprets their motion as a flyby. It shows how a space survey’s measurements can reveal structure within an apparently single group of stars.
What Gaia measured
The European Space Agency launched Gaia in 2013 to measure the positions, distances, motions and properties of stars. Its core technique was astrometry, the precise measurement of positions and changes in position on the sky. Repeated observations helped separate different components of the apparent movement.
Parallax is the apparent shift in a star’s position as the observer views it from different points along its orbit around the Sun. It provides information about distance. Proper motion is the star’s angular change in position over time arising from its motion through space. Radial velocity describes movement towards or away from the observer.
Gaia also recorded information about stellar brightness and colour. These measurements help astronomers study stars’ physical properties and evolutionary stages. Taken together, position, motion and brightness data provide several ways to examine whether stars belong to the same population.
Finding two populations
An open cluster is a group of stars formed from a large reservoir of gas and dust. UBC 63 had previously been treated as one cluster. The researchers found two distinct sequences in its colour–magnitude diagram, which plots stellar colour against brightness.
They used statistical modelling of Gaia’s position, parallax and proper-motion measurements to separate two populations. They then compared dust-corrected brightness and colour measurements with models of stellar evolution. The estimated ages were approximately 21 million years for UBC 63A and 562 million years for UBC 63B.
Reconstructing an encounter
To investigate the clusters’ history, the team combined their estimated positions and motions with additional radial-velocity information. A model of the Milky Way’s gravitational field allowed them to calculate possible past paths. Simulations of the stars’ gravitational interactions provided another way to examine the encounter.
The calculations suggest a close approach about 6 million years ago, with an estimated separation of 7 ± 2 parsecs. The estimated present separation is 26 ± 8 parsecs. A parsec is a unit of distance equal to approximately 3.26 light-years. The accompanying figure displays the reported separation estimates and their uncertainties.
The researchers interpret the pair as gravitationally unbound: their relative motion allows them to move apart after the encounter. Comparing their relative velocity with the estimated escape velocity helped establish this interpretation. The paper describes the system as a compelling candidate for a double cluster caught after a flyby.
Research from a completed observing mission
ESA switched off Gaia’s spacecraft systems on 27 March 2025 and moved it into a retirement orbit around the Sun. Astronomers continue analysing its archived observations. The UBC 63 study used Gaia Data Release 3, demonstrating how a shared catalogue can support new questions after spacecraft operations have ended.
Conclusion
Gaia’s measurements connect the appearance of stars on the sky with their distances, motions and evolution. In UBC 63, combining those clues with models has revealed evidence of two populations and a past gravitational encounter.
Sources
- Department of Science and Technology: UBC 63 research highlighted on 5 November 2026
- Biswas and colleagues: original study, The Astronomical Journal 172, 151, published 13 August 2026
- ESA: Gaia’s astrometric, photometric and velocity measurements
- ESA: proper motion and motion through space
- ESA: parallax and the mapping of stellar distances
- ESA: spacecraft operations ended on 27 March 2025