Chinese telescope finds lightest known double neutron star system

By the Everything Space news desk · 02 Oct 2026 · written from the sources below

Image: Phys.org

The compact binary's unusually low mass and shrinking orbit give astronomers a rare new tool to test general relativity and probe how dense matter behaves.

Astronomers using China's FAST radio telescope have identified a binary neutron star system with the lowest combined mass ever measured for such a pair. The system, designated PSR J1856−0039, consists of two neutron stars orbiting each other every 2.36 hours, making it one of the most compact known binaries of its kind.

The pulsar was first spotted in 2020 during an ongoing FAST survey that has catalogued hundreds of pulsars. Researchers tracked it over seventeen observing sessions through 2025, collecting hundreds of precise timing measurements of its radio pulses to study how its orbit evolves.

By analyzing shifts in pulse arrival times, the team measured three effects predicted by general relativity: the gradual shrinking of the orbit, the slow rotation of its closest approach point, and a subtle timing distortion caused by gravitational effects on the pulsar's clock. These measurements let them calculate the mass of each neutron star and confirm that the orbit's decay matches what Einstein's theory predicts.

Because the combined mass of the pair is unusually low, the researchers suggest that when the two stars eventually merge, the result may be a surviving massive neutron star rather than a black hole. This outcome would offer insight into the extreme physics of ultradense matter inside neutron stars.

The team estimates the stars will spiral together and merge in roughly 82 million years. Depending on how fast the merged object spins, it could remain a stable neutron star or later collapse into a black hole.

The system also shows strong relativistic effects, ranking second among known double neutron star pairs in this regard, and its orbital geometry and timing precision make it a strong candidate for detecting a subtler relativistic effect tied to the way spinning mass drags spacetime around it. Researchers plan to continue monitoring the pulsar for up to a decade to pursue that measurement, which could eventually help reveal the internal structure of neutron stars.

Written in Everything Space’s own words by our news-desk model from the sources above, and checked against them in code: every number in this report appears in its sources, and no sentence is copied from them.