Research Lends Insights on Neutron Star Dance of Death

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Imagine a star so dense that it contained all the mass of our sun squeezed into a ball the size of Guelph.

Then imagine a second super-dense star no farther than Toronto from the first one. Now set the two stars whizzing around each other about one-third as fast as the speed of light.

Detecting that kind of neutron star tango and its finale — a cosmic collision as the bodies spiral together – provides clues that may tell us more about what these bodies are made of, according to University of Guelph professor Eric Poisson.

In a new paper published this month in Physical Review D, he and master’s student Eamonn Corrigan describe the effects that scientists seeking these elusive binary systems should expect to “see” with super-sensitive instruments here on Earth.

By learning more about the makeup of neutron stars, astronomers may also gain clues to the workings of the universe and the origins of matter, said Poisson.

“I think there’s endless fascination with the universe. What’s our place, but also what’s in the universe — how big is it?”

Prof. Eric Poisson

Paired neutron stars – basically the smallest, densest stars in the universe – and binary black holes generate gravitational waves as they spiral together and collide.

Those oscillations in space-time ripple across vast distances like waves on a lake or sound through air, and can be detected by instruments on Earth, said Poisson.

The new paper describes the forces at play and what astronomers should expect to see before such collisions.

As these super-dense bodies perform their death dance, they produce tidal deformations in each other, not unlike how the moon’s gravity causes tides in Earth’s oceans.

Poisson said those tidal forces should affect the bodies’ orbital motion and resultant gravitational waves that may be picked up by detectors on Earth.

Tidal deformations in neutron stars depend on what those bodies are made of, he said. “We can use gravitational waves to study that deformation and learn what’s inside. We hope to learn about the nature of matter at extreme densities. Right now, it’s a mystery.”

Gravitational waves were predicted by Albert Einstein a century ago.

In fall 2015, the Laser Interferometer Gravitational Wave Observatory (LIGO) in the United States detected its first-ever gravitational waves caused by a collision between two black holes.

Later that year, LIGO picked up the signal of gravitational waves from another pair of black holes that merged about 1.4 billion light years from Earth.

Last fall, LIGO and Virgo, a similar detector in Europe, “saw” two neutron stars merge in a massive collision.

That cataclysm generated gravitational waves as well as gamma rays and visible light. It also spewed elements created in the smashup into space.

Hundreds of binary systems exist in the Milky Way, said Poisson. But we need to look far beyond our own galaxy to find any neutron star or black hole pairs orbiting closely enough to generate gravitational waves that could be detected on Earth.