An international research team, led by scientists from the Chinese Academy of Sciences (CAS), has discovered an isolated pulsar located within the tidal tail of NGC 6791, one of the oldest and most massive open clusters in the Milky Way. By combining observations from FAST, MeerKAT and the Canada-France-Hawaii Telescope (CFHT) with direct N-body simulations, the researchers provide the first direct observational evidence of a neutron star in the tidal tail of an open cluster—revealing a key stage in the journey of neutron stars from their birth clusters into the Galactic field.

In 1934, astronomers Walter Baade and Fritz Zwicky proposed that supernova explosions create neutron stars. A decade later, Baade introduced the concept of stellar populations, characterizing the distinct groups of stars that make up the Milky Way. However, a critical link between these two foundational ideas remained missing: observational proof of how a neutron star born in a dense star cluster eventually becomes part of the thousands scattered across the Galactic field.

The research team targeted NGC 6791, an 8-billion-year-old open cluster, to search for this missing evolutionary stage. The discovery of the pulsar, designated PSR J1921+3745, effectively provides a "birth certificate" that traces the object's journey from the cluster to the Galactic tide.

The identification of this unique object was achieved through a global scientific relay combining multi-telescope observations with advanced simulations. China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST) detected the 1.9-second isolated pulsar PSR J1921+3745. The MeerKAT radio telescope in South Africa determined the pulsar's position with arcsecond-scale precision. Deep observations from the Canada-France-Hawaii Telescope (CFHT) mapped the surrounding stellar field.

The combined observations place PSR J1921+3745 squarely within a prominent tidal tail of NGC 6791—a stream of stars gradually being stripped from the cluster by the Milky Way’s gravity. The team confirmed the physical association based on the pulsar's estimated distance, the low probability of chance alignment, and the absence of other young stellar populations in the vicinity.

To understand how the pulsar arrived at this location, the team utilized direct N-body simulations tailored to the specific dynamics of NGC 6791. Simulations show that more than 95% of neutron stars are rapidly expelled from clusters by powerful supernova kicks immediately after formation. The team found that a neutron star formed via a relatively gentle "electron-capture supernova"—characterized by a small kick velocity—could remain bound to the cluster for billions of years. Over time, stellar encounters and Galactic tides gradually carried the neutron star into the cluster’s tidal tails.

This discovery provides the first direct observational evidence of a neutron star in the tidal tail of an open cluster, revealing a previously missing stage between a neutron star’s birth in a stellar group and its later life in the Galactic field. It also opens a new avenue at the intersection of stellar evolution, supernova explosion, pulsar astronomy, star-cluster dynamics and the structure of the Milky Way.

Nearly a century after Baade connected supernovae, neutron stars, and stellar populations, this discovery now provides a traceable route from a neutron star's birth cluster to its future in the wider Milky Way.

The study was led by ZHANG Lei (National Astronomical Observatories, CAS), FU Xiaoting (Purple Mountain Observatory, CAS), and LI Di (Tsinghua University), with collaborators from Australia, Canada, China, Germany, Italy, South Africa and the United States.

Artist’s impression of the pulsar PSR J1921+3745 within the tidal tail of the 8-billion-year-old open cluster NGC 6791. Three telescopes contributed to the study: FAST detected the pulsar, MeerKAT pinpointed its position, and CFHT mapped the surrounding stellar structure. (Image by FU Xiaoting)

Paper link: https://doi.org/10.1016/j.scib.2026.07.006