Millisecond pulsar, left foreground, is orbited by a hot white dwarf star, center, both of which are orbited by another, more-distant and cooler white dwarf, top right Bill Saxton; NRAO/AUI/NSF

A triple-star system discovered by astronomers could help resolve problems with Einstein's theory of General Relativity and shed light on the true nature of gravity.

The unique system is comprised of two white dwarf stars and a superdense neutron star (pulsar) packed into a space smaller than the Earth's orbit around the Sun. The constellation allows scientists to measure the complex gravitational interactions between the stars in a way that hasn't been possible before to help reveal the true nature of gravity.


The pulsar -- a neutron star that emits lighthouse-like beams of radio waves through space as the object spins -- was first discovered by a graduate at West Virginia University called Jason Boyles using the National Science Foundation's Green Bank Telescope (GBT). It is 4,200 light-years away from Earth and spins 366 times every second. These sorts of rapidly-spinning pulsars (known as millisecond pulsars) are very useful to astronomers, who can use them to study gravitation.

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After spotting the pulsar, astronomers noticed two white dwarf stars -- one in close orbit with the pulsar and another more distant one. It is the first time a millisecond pulsar has been found in such a system, which provides, in the words of Scott Ransom, "tremendous opportunity to study the effects and nature of gravity".

Ransom is an astronomer at the National Radio Astronomy Observatory. He says: "This triple system gives us a natural cosmic laboratory far better than anything found before for learning exactly how such three-body systems work and potentially for detecting problems with General Relativity that physicists expect to see under extreme conditions".


Having identified the system, scientists observed it intensively with the GBT, the Arecibo radio telescope in Puerto Rico and the Westerbork Synthesis Radio Telescope in the Netherlands, incorporating additional data from other satellites and telescopes. "The gravitational perturbations imposed on each member of this system by the others are incredibly pure and strong. The millisecond pulsar serves as an extremely powerful tool for measuring those perturbations incredibly well," says Ransom.

By recording the lighthouse-like beams of radio waves with accuracy, the team was able to calculate the geometry of the system and the masses of the stars with 'unparalleled precision' -- some of them are accurate to hundreds of metres, according to Anne Archibald from the Netherlands Institute for Radio Astronomy.

Archibald used the measurements gleaned from the system to build up a simulation.

The research combines techniques pioneered by Isaac Newton to study Earth's gravity with the alternate theory of gravitation proposed by Albert Einstein. It is hoped that it could pave the way for the next theory of gravity.

The team is particularly interested in looking for a deviation from the Equivalence Principle, which states that the effect of gravity on a body doesn't depend on the size of material of that body. It explains why on the Moon, where there is no air resistance, a feather and a hammer will fall at the same speed. It also explains why balls of different materials will roll down a slope at the same time -- as first shown by Galileo.

Ransom explains: "While Einstein's Theory of General Relativity has so far been confirmed by every experiment, it is not compatible with quantum theory. Because of that, physicists expect that it will break down under extreme conditions. This triple system of compact stars gives us a great opportunity to look for a violation of a specific form of the equivalence principle called the Strong Equivalence Principle."


When a star explodes and then collapses into a superdense neutron star, some of its mass is converted into gravitational binding energy that holds the star together. According to the Strong Equivalence Principle, the binding energy will still react gravitationally as if it were mass. Most of the alternative theories to General Relativity suggest that it will not.

The triple star system should allow astronomers to know which is the case through high-precision observation of the pulsar's "lighthouse" flashes. If the Strong Equivalence Principle holds, then the gravitational effect of the outer white dwarf would be identical for both the inner white dwarf and the pulsar. If it doesn't hold, then the outer star's gravitational effect on the inner white dwarf and pulsar will be slightly different. "Finding a deviation from the Strong Equivalence Principle would indicate a breakdown of General Relativity and would point us toward a new, correct theory of gravity," said Ingrid Stairs of the University of British Columbia

The findings are published in Nature and will be presented at the 223rd American Astronomical Society meeting.