Astronomers have discovered an extraordinary pair of neutron stars locked in an extremely tight orbit—and together, they have the lowest combined mass ever measured for a confirmed double neutron star system.
The system, known as PSR J1856−0039, was discovered using China's massive Five-hundred-meter Aperture Spherical radio Telescope (FAST). Its compact 2.36-hour orbit also makes it an exceptional laboratory for testing Einstein's theory of general relativity and studying some of the densest matter in the universe.
The findings, published in Physical Review Letters, could provide new clues about how neutron stars form, how binary systems evolve and what happens when these extraordinary objects eventually collide.
A remarkable pair of dead stars
Neutron stars are the incredibly dense remnants left behind after some massive stars explode as supernovae. Although they are typically only about the size of a city, they can contain more mass than the Sun.
Some neutron stars are pulsars. These objects rotate rapidly and emit beams of radio waves from their magnetic poles. When one of these beams sweeps across Earth, astronomers detect a regular pulse of radio emission—much like the beam from a cosmic lighthouse.
PSR J1856−0039 contains a pulsar orbiting another neutron star. The two objects are separated by a relatively small distance and complete one orbit in just 2.36 hours.
That makes the system particularly interesting because its strong gravitational effects are measurable with extraordinary precision.
Einstein's prediction put to the test
According to Einstein's general theory of relativity, massive objects accelerating through space should produce tiny disturbances in spacetime called gravitational waves.
When two neutron stars orbit each other, they lose energy through gravitational-wave emission. As energy leaves the system, the stars gradually spiral inward.
This means their orbital period should become shorter over time.
PSR J1856−0039 provides astronomers with an opportunity to observe this effect directly. By repeatedly measuring the arrival time of the pulsar's radio pulses, researchers can track tiny changes in the system's orbit.
The Chinese research team monitored the system with FAST between 2020 and 2025. During 17 observing sessions, they collected 253 measurements of pulse arrival times.
Those measurements revealed several subtle effects predicted by general relativity.
FAST provides an incredibly precise view
FAST, located in a natural basin in southwestern China, is the world's largest single-dish radio telescope. Its enormous collecting area gives astronomers exceptional sensitivity when detecting faint radio sources.
PSR J1856−0039 is relatively faint, with an average radio brightness of only about 0.1 millijansky, although its brightness varies between observations.
FAST's sensitivity allowed researchers to measure its pulses accurately enough to detect tiny changes in their arrival times.
The team used specialized pulsar-timing software to analyze the observations and construct a highly precise model of the binary system.
Three important relativistic effects emerged from the data.
The first was the gradual shortening of the orbital period, consistent with energy being carried away by gravitational waves.
The second was periastron advance. This refers to the gradual rotation of the point in the orbit where the two stars come closest to each other.
The third was the Einstein delay, a timing effect caused by the combination of gravitational time dilation and the changing speed of the pulsar as it moves through its orbit.
Together, these effects allowed the researchers to determine the masses of the two neutron stars.
The lowest combined mass yet measured
The most surprising result was the system's total mass.
According to the researchers, the combined mass of the two neutron stars is the lowest measured so far for a confirmed double neutron star system.
This is important because the masses of neutron stars contain information about how their original stars lived, exploded and formed binary systems.
The researchers found that PSR J1856−0039 also has unusually strong relativistic effects. Among confirmed double neutron star systems, it ranks second in the strength of these effects, according to the research team.
Its compact orbit, favorable geometry and extremely precise timing make it particularly valuable for future observations.
What will happen when the stars collide?
The two neutron stars are not going to remain separate forever.
Because the system continuously loses orbital energy through gravitational radiation, the stars are slowly moving toward each other.
The researchers estimate that they will eventually merge in approximately 82 million years.
That collision could produce an extremely energetic event and create a massive remnant.
Because the system has such a low total mass, the researchers suggest that its merger could potentially leave behind a massive neutron star rather than immediately forming a black hole.
However, the ultimate fate of the remnant would depend on its physical properties. It could remain stable for some time, while a later loss of rotational support could cause it to collapse into a black hole.
Such mergers are particularly interesting because they provide a natural laboratory for studying matter under pressures and densities that cannot be reproduced on Earth.
A future test of frame-dragging
The researchers believe PSR J1856−0039 could reveal another fascinating prediction of Einstein's theory: Lense–Thirring precession, commonly known as frame dragging.
According to general relativity, a rotating massive object can slightly drag the surrounding spacetime along with its rotation.
In this binary system, the researchers hope that continued observations will eventually allow them to detect this effect.
If successful, the measurements could reveal something even more fundamental—the moment of inertia of a neutron star.
The moment of inertia describes how an object's mass is distributed relative to its rotation. Measuring it for a neutron star could provide important information about the material inside it.
Neutron-star interiors remain one of the biggest mysteries in modern astrophysics. At their enormous densities, matter behaves in ways that are still not completely understood.
A decade-long cosmic experiment
The discovery is therefore much more than simply finding an unusually lightweight pair of neutron stars.
PSR J1856−0039 could become a long-term natural laboratory for testing general relativity, studying gravitational-wave-driven orbital decay and investigating the extreme physics inside neutron stars.
The researchers plan to continue monitoring the system for many years. Detecting the Lense–Thirring effect and measuring the neutron star's moment of inertia will require extremely precise timing over a long period.
The work could take around a decade.
For now, PSR J1856−0039 offers astronomers something rare: a nearby cosmic system where Einstein's predictions, neutron-star physics and the evolution of stellar remnants can all be studied together.
And although the two stars will not merge for roughly 82 million years, every precisely measured pulse from this distant cosmic lighthouse is already revealing what happens as these two extraordinary objects slowly spiral toward their ultimate encounter.
Reference: Z. L. Yang et al., Relativistic Effects of PSR J1856–0039 Double Neutron Star System in a 2.36-Hour Compact Orbit, Physical Review Letters (2026). DOI: 10.1103/hmjp-htd1.
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