Stellar Triple System May Help Reveal The ‘True Nature’ Of Gravity

Researchers have discovered a rare stellar system containing two white dwarf stars and a neutron star located 4,200 light-years from Earth. This stellar triple system could lead to new insights on gravitational interactions in stellar systems and maybe even the “true nature of gravity” itself.

Stellar Triple System The stellar triple system features a neutron star (left), a hot white dwarf (center) and a cooler white dwarf that is farther away (right).  Bill Saxton; NRAO/AUI/NSF

The neutron star in the stellar system is a pulsar, a highly magnetized, spinning neutron star that emits electromagnetic radiation. Pulsars act as lighthouses and have precise pulses which researchers can use to measure distance or the effects of gravity.

Jason Boyles was a graduate student at West Virginia University, currently at Western Kentucky University, discovered the pulsar using the National Science Foundation's Green Bank Telescope, or GBT. The three stars are sharing a space that is “smaller than Earth's orbit around the Sun.” The research was published in the journal Nature.

The pulsar discovered by Boyles is classified as a “millisecond pulsar” due to how fast the neutron star is spinning, at a rate of 366 times per second. According to the astronomers, millisecond pulsars could be used to detect gravitational waves, “ripples in space-time” that may be caused by the two black holes orbiting each other or through the merger of two galaxies.

Researchers could, in theory, use millisecond pulsars to indirectly observe gravitational waves by measuring any change in the precise pulsing intervals. The stellar triple system could also serve as a test to Albert Einstein’s theory of General Relativity.

Scott Ransom, from the NRAO, said in a statement, “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.”

The researchers, using subsequent data collected from the Arecibo radio telescope, located in Puerto Rico, and the Westerbork Synthesis Radio Telescope, located in the Netherlands, measured the pulsar’s pulses to identify the shape of the stellar system as well as the mass of each star. One white dwarf and the neutron star are in close orbit with the second white dwarf is farther away. “The gravitational perturbations imposed on each member of this system by the others are incredibly pure and strong,” said Ransom.

For the astronomers the dense neutron star, the collapsed remains of a star that went supernova, can be used to test the Strong Equivalence Principle. According to the researchers, some of the neutron star’s mass is changed to gravitational binding energy and this energy will “react gravitationally as if it were mass.” The principle states “that the effect of gravity on a body does not depend on the nature or internal structure of that body” and is part of Einstein’s General Relativity, notes the University of British Columbia. Researchers hope to test the principle using the timed pulses of the pulsar.

According to General Relativity, “the gravitational effect of the outer white dwarf would be identical for both the inner white dwarf and the neutron star.” If the pulsar timing is different that would indicate the Strong Equivalence Principle is incorrect which would help refine the current theory of gravity.

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