A team of astronomers from Pune's National Centre for Radio Astrophysics-Tata Institute of Fundamental Research (NCRA-TIFR) has discovered the first pulsar in the ancient star cluster Messier 80 (M80) using India's upgraded Giant Metrewave Radio Telescope (uGMRT) near Pune. The star cluster is also called NGC 6093. The pulsar, named PSR J1617-2258A, spins 232 times every second and orbits with a small companion star.

The discovery is part of new Globular Clusters GMRT Pulsar Search (GCGPS) project involving scientists from NCRA-TIFR (India), the Max Planck Institute for Radio Astronomy (Germany), the National Radio Astronomy Observatory (USA), and the University of Oxford (UK).

The result was published in the Astrophysical Journal on July 22.

According to the astronomers, pulsars are dense, city-sized remnants of stars that sweep radio beams across space like cosmic lighthouses. The newly discovered pulsar orbits its companion once every 19 hours, but instead of a smooth circle, the orbit is stretched into a long oval. Its unusual orbit makes the system very rare.

Lead researcher Jyotirmoy Das, a graduate student at NCRA-TIFR, said, "This precision is about half a degree per year. To put that in perspective, this pulsar's orbit shifts in a single day by roughly as much as Mercury's perihelion shifts in an entire decade."

The shifting of the orbit, called the advance of periastron, was predicted by Albert Einstein's theory of general relativity. By observing it, scientists could measure the combined mass of the pulsar and its companion, which is about 1.67 times the mass of the Sun.

Jayanta Roy, NCRA-TIFR scientist and co-author of the study, said, "The mix of a tight orbit, a highly stretched path, and a lightweight companion, makes this system a rarity. Among known binary millisecond pulsars, only a few are both more compact and more eccentric, and those all have heavier partners. PSR J1617-2258A sits in a nearly empty corner of the millisecond-pulsar family portrait, hinting at an unusual evolutionary history,"

The discovery also provides an opportunity to test Einstein's ideas in a new environment. Bhaswati Bhattacharyya, another NCRA-TIFR scientist and member of the project, said, "Continued monitoring with the uGMRT can sharpen the mass estimates and may spot additional relativistic quirks. This offers fresh tests of Einstein's theory and new clues to how close stellar encounters shape pulsars in globular clusters."

uGMRT, the world's largest radio telescope, is located at Khodad in Narayangaon, around 80km from Pune.