Astronomers have discovered the lightest known pair of neutron stars, designated PSR J1856-0039, which circle each other every 2.36 hours. Using China’s FAST radio telescope, researchers measured three subtle relativistic effects, proving Albert Einstein’s general theory of relativity correct to within about 1 percent.
FAST Telescope Detects PSR J1856-0039
Situated roughly 4,200 to 6,500 light-years from Earth, the system consists of two ultradense stellar remnants packed into an exceptionally compact orbit. The binary system turns up in a 2020 survey of the Milky Way’s disk conducted by China’s Five-hundred-meter Aperture Spherical Radio Telescope (FAST), recognized as the world’s largest single-dish radio telescope.
The system features an orbital period of 2.36 hours. It stands as the second-shortest known orbital period among binary pulsar systems.
The visible pulsar rotates about 43 times per second. It utilizes its steady radio pulses to function as an extraordinarily precise cosmic clock.
Researchers Measure Masses of Neutron Stars
The visible pulsar weighs about 1.30 solar masses. Its companion accounts for approximately 1.19 solar masses.
Researchers determined the component masses by tracking pulse arrival times across 17 observations conducted between 2020 and 2025.
Their combined mass of 2.488 solar masses makes it the lightest binary neutron star system confirmed to date. Simulations suggest that supernova explosions cannot forge neutron stars much lighter than these limits. This places both components close to the theoretical minimum mass.
Scientists Test General Relativity With Relativistic Effects
Separated by roughly twice the distance between Earth and the moon, the two objects travel in such a tight orbit. Because they do, the system provides an intense laboratory for testing gravity.
Researchers detected three distinct effects predicted by Albert Einstein’s theory.
First, the system loses orbital energy through the emission of gravitational waves. This causes the orbit to shrink by about 2.7 meters each year.
Second, the oval orbit rotates by 17.6 degrees per year.
Third, time dilation from gravity and speed alters the pulsar’s clock along its path. This shifts pulse arrival times by up to half a millisecond.
We found that general relativity is needed to explain the data, and the consistency is about 1%.
JinLin Han, National Astronomical Observatories of the Chinese Academy of Sciences
Any two of these three effects are sufficient to calculate both stellar masses independently. The third effect acts as a check. It verifies that all calculations point to the same mass values.
Two Stars Will Collide in 82 Million Years
Gravitational wave emission will eventually force the two stars to collide. Researchers estimate that the merger will occur in approximately 82 million years.
The event will leave behind either a heavier neutron star of about 2.26 solar masses or a black hole. The result depends on how much the remnant’s rotation slows down.
Scientists plan to continue monitoring the pulsar over the coming decades. Their primary long-term objective is to detect the Lense-Thirring effect, or frame dragging. In this process, the pulsar’s rapid spin twists surrounding spacetime and causes neighboring orbits to wobble, as noted when scientists plan to continue monitoring the pulsar over the coming decades.