Lightest neutron star pair discovered by FAST

China's Five-hundred-meter Aperture Spherical radio Telescope, or FAST, the world's most sensitive single-dish radio telescope, has discovered a binary neutron star system with an extremely short orbital period and the lowest total mass ever recorded, providing a new laboratory for studying matter under extreme conditions, testing theories of gravity and exploring the origins of heavy elements in the universe.

The system, which contains a pulsar and is named PSR J1856-0039, completes one orbit every 2.36 hours — the second-shortest orbital period ever recorded for a binary neutron star system — and has a combined mass of 2.488 times that of the sun. It is the lightest binary neutron star system confirmed to date, according to findings recently published in the journal Physical Review Letters, which selected the study as an editors' highlight.

Neutron stars are the extremely dense remains of massive stars that have exploded as supernovae. Although they are only about 20 kilometers across, they can have more mass than the sun and rotate rapidly, with pulsars emitting regular radio signals that can serve as highly precise cosmic clocks. When two neutron stars orbit each other, they form an exceptionally rare type of system. Only about 30 such systems have been firmly identified so far, making them valuable natural laboratories for studying extreme physics and gravity.