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Southampton astrophysicists find neutron stars to be almost perfect spheres

Published: 26 May 2021
Fabian Gittins
Fabian Gittins

University of Southampton researchers, Fabian Gittins, Nils Andersson and Ian Jones, have shown that deformations on neutron stars – relevant for gravitational waves – cannot be as large as previously predicted.

Neutron stars constitute the densest state of matter in the Universe. They compactify approximately the same mass of the Sun into a volume that is twenty kilometres in diameter. For this reason, neutron stars have incredibly strong gravitational fields – rivalled only by black holes.

According to Einstein’s general theory of relativity, when masses are accelerated, they induce small ripples in the fabric of spacetime known as gravitational waves. Since 2015, sensitive gravitational-wave instruments (advanced LIGO and Virgo) have been able to observe these minute oscillations from the collisions and mergers of black-hole and neutron-star pairs. However, although isolated, rotating neutron stars hosting deformations – called mountains – are also potential sources of gravitational waves, they have yet to be seen with these detectors.

For the past two decades, there has been interest in calculating the largest possible mountains that neutron stars can support before their crust fractures. Past analyses suggested that neutron stars can sustain departures from perfect sphericity of up to a few parts in one million. However, this recent study, conducted by the STAG scientists, has shown that the problem is more complicated than expected.

“Previous work forced the stars into a shape that isn’t physically possible,” said Gittins. “Our insight was to explicitly consider the way in which the mountains were formed.”

The team used computational modelling to construct realistic neutron stars and subjected the stars to a variety of mathematical forces to produce the mountains. They also considered, for the first time, the role of nuclear-matter equation of state in supporting the mountains. They found that the maximum induced deformations deviated from a sphere by approximately one part in one-hundred million – one-hundred times smaller than previous estimates.

“Neutron stars are just incredibly spherical objects,” explained Gittins. “It’s really quite remarkable.”

This result suggests that observing gravitational waves from rotating neutron stars may be even more challenging than hoped.

The research has recently caught public attention in New Scientist and part of this work has been published in Monthly Notices of the Royal Astronomical Society.



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