Lightest known neutron star pair tests Einstein's relativity to within 1%

Astronomers using China's FAST telescope found pulsar PSR J1856-0039 paired with a neutron star in the lightest known double neutron star system. The 2.36-hour orbit shows three relativistic effects that match general relativity to about 1%, with total mass around 2.49 solar masses.

• Pulsar spins 43 times per second; orbit shrinks 2.7 m per year
• Orbit's periastron shifts 17.6 degrees per year
• Masses are 1.3 and 1.19 solar; merger in ~82 million years
• FAST data: 17 observations between 2020 and 2025