Unusual triple star system may test some of Einstein's theories

Bill Saxton/NRAO/AUI/NSF

While we are most accustomed to our single star system, it is not uncommon to hear about binary systems where two stars orbit one another. Multiple star systems are made of three or more stars, and scientists have just discovered one of the most unusual triple star systems yet: a millisecond pulsar and two white dwarfs. The systemcould help test some portions of Albert Einstein’s Theory of General Relativity that have troubled scientists over the years. The research was carried out by an international team of scientists and the results were published in Nature.

The star system PSR B1620-26 is about 12,400 light years away in the M4 globular cluster in the Scorpius constellation. In an area that is smaller than Earth’s orbit around the sun, the system contains two white dwarf stars and a millisecond pulsar. Astronomers have never before seen a millisecond pulsar in a triple star system. The system is set up with the pulsar in the middle, surrounded by the other two stellar remnants. The innermost white dwarf completes an orbit in under 48 hours, while the outer white dwarf takes nearly an Earth year to complete the same task.

White dwarf stars are formed after the death of a low to medium mass star, which is anything smaller than eight times the mass of our sun, Sol. The resultant body is very bright and dense, as it is about the same size as Earth, but with the mass of the sun. This equals roughly 1 billion kilograms per cubic meter. While Sol has another billion years in its current state, it will eventually expand into a red giant before collapsing into a white dwarf.

Pulsars are a variety of neutron star that emits a constant spinning burst of light and radiation, kind of like a lighthouse. They rotate very quickly, and those that rotate in 1-10 milliseconds are known as “millisecond pulsars.” By measuring the radio wave beam that is cast out of the pulsar, astronomers are able to explore how gravitational waves in the galaxy.

Astronomers will be able to use this system as a laboratory in which to study theoretical concepts of gravity that only occur under such extreme circumstances. With a few different gravitational theories out there, this will allow scientists to start reevaluating some or possibly rejecting some outright. There is much to debate about how three stellar remnants formed such a system, especially given its small orbital area. 

The part of Einstein’s theory that is going to be most scrutinized is the Equivalence Principle. This states that the force of gravity has more to do with position and velocity than composition. This is very commonly demonstrated in science classrooms by showing that a feather and a coin will fall at the same rate in a vacuum, as there is no air resistance to interfere.

The results will be presented this week at the 223rd Meeting of the American Astronomical Society in Washington D.C.


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