The space-time warp caused by the extreme gravity of a binary star system has been measured to determine the mass of a constituent neutron star, just before it spun out of visibility.

The young neutron star, a pulsar here, located about 25,000 light years from Earth, is expected to wobble back into view but not before 160 years.

It orbits what could be another neutron star or a white dwarf in a little under four hours.

The international team measured the masses of both stars in binary pulsar system J1906.

"By precisely tracking the motion of the pulsar, we were able to measure the gravitational interaction between the two highly compact stars with extreme precision," says Ingrid Stairs, professor of physics and astronomy at University of British Columbia.

The two stars in the binary system both weigh more than the Sun but are 100 times closer than the Earth is to the Sun. The resulting gravity causes a warp in the space-time fabric, as predicted by the relativity theory.

As the fast spinning neutron star (pulsar) moves through this curved space-time, it wobbles like a spinning top. As its spin axis gets tilted more and more, the beams it emits no more reach Earth and it becomes invisible, even to the largest telescopes.

The team measured this geodetic precession in J1906 to arrive at the masses of the binary stars.

The curved space time means 1 part in about a million of the pulsar's orbit is "missing", compared to a flat space time. In a year of observations, this adds up to a change of 2.2 degrees in the orientation of the pulsar rotation axis.

"Our result is important because weighing stars while they freely float through space is exceedingly difficult," said Joeri van Leeuwen, an astrophysicist at The Netherlands Institute for Radio Astronomy ASTRON, and University of Amsterdam, The Netherlands, who led the study.

The pulsar in J1906 spins and emits a beam of radio waves every 144 milliseconds.

This is a relatively young binary system as the supernova explosion that formed it occurred only 100,000 years ago.

Normal pulsars live to be around 10 million years old after which they can be recycled by binary companion to live for yet another 1 billion years.

If the companion to the pulsar in J1906 is a neutron star, it is likely recycled.

The mass of only a handful of double neutron stars have ever been measured. While all pulsars are neutron stars, not all neutron stars are pulsars.

Pulsars are neutron stars emitting bursts of radio waves, x-rays or gamma rays. Neutron stars are made of neutrons and result from the supernova explosion at the end of a star's life. These stars are massive but not massive enough to form black holes.

The pulsar in J1906 was discovered in 2004 with the Arecibo radio observatory, following which the team monitored it almost daily with the five largest radio telescopes on Earth: the Arecibo Telescope (USA), the Green Bank Telescope (USA), Nançay Telescope (France), the Lovell Telescope (UK) and the Westerbork Synthesis Radio Telescope (The Netherlands).

Over five years, it has recorded one billion rotations of the pulsar.

The results are published in The Astrophysical Journal.