Indian astronomers, working with international collaborators, have made a rare breakthrough in stellar astronomy by directly observing a Blue Straggler Star (BSS) in the process of formation, providing some of the strongest evidence yet for how these mysterious stars are created.
The discovery, published in The Astrophysical Journal Letters, offers direct observational support for the long-standing theory that Blue Straggler Stars are formed through mass transfer in binary star systems.
Blue Straggler Stars are an unusual class of stars found in star clusters that appear hotter, brighter and more massive than other stars of the same age. Their existence has puzzled astronomers for decades because they appear much younger than neighbouring stars despite belonging to ancient star clusters.
The international research team, led by scientists from the Indian Institute of Astrophysics (IIA), an autonomous institute under the Department of Science and Technology (DST), studied the binary star system TIC 327546480 in the old open star cluster Collinder 261, located about 9,500 light-years from Earth.
The system was selected for detailed investigation because earlier observations suggested that the two stars were actively interacting. Researchers combined high-precision light curves from NASA’s Transiting Exoplanet Survey Satellite (TESS) with radial velocity measurements obtained using the European Southern Observatory’s Very Large Telescope in Chile.
The study found that the binary system is semi-detached, meaning one of the stars has expanded beyond its gravitational boundary, known as the Roche lobe, allowing matter to flow onto its companion. This ongoing transfer of material is causing the receiving star to gain mass and effectively rejuvenate itself, transforming it into a Blue Straggler Star.
According to the researchers, the observations provide a rare opportunity to witness a Blue Straggler Star while it is still forming rather than after the process has already been completed.
The team also detected strong X-ray emission from the system, an important indicator that material from the companion star is actively being accreted by the Blue Straggler. The X-ray evidence independently confirms that the mass-transfer process is currently underway.
Detailed analysis revealed that the Blue Straggler has a mass about 1.67 times that of the Sun, while its companion has a mass of around 0.32 solar masses. The two stars orbit each other every 2.11 days, and the Blue Straggler rotates rapidly at approximately 69.55 kilometres per second, a speed researchers believe is the result of material accumulated from its companion.
Evolutionary modelling suggests that the mass-transfer process began about 5.46 billion years ago and continues today. The host star cluster, Collinder 261, is estimated to be around 7 billion years old.
The researchers said the discovery represents one of the clearest examples yet of a Blue Straggler forming through mass transfer in a binary system. Besides helping resolve a decades-old mystery in stellar astrophysics, the system provides astronomers with a unique natural laboratory to test theories of binary star evolution and stellar rejuvenation, with broader implications for understanding the evolution of star clusters and interacting stars across the universe.
The study was led by Ali Hasan Sheikh, a Ph.D. student at Gauhati University and an INSPIRE Fellow under the Department of Science and Technology. The research involved scientists from the Indian Institute of Astrophysics, Gauhati University, IUCAA Pune, INAF Italy, LUX Observatoire de Paris, and the University of North Florida, USA.
