Neutron stars are so dense that their surfaces may only vary in height by up to 0.1 millimetres, thanks to their extreme gravity.

The remnant cores of stars that have gone supernova, neutron stars are among the densest known objects in the universe. They can contain up to twice the mass of our sun packed into an incredibly small space just two dozen kilometres across, the size of a city.

The intense gravitational pull of neutron stars means that their surfaces, a thin crust of hydrogen and helium, are flattened to an extremely smooth degree, but there can be small bump-like deformations resulting from the star’s activity. Now new modelling has shown that these deformations are probably at least 100 times smaller than previously thought.

“Neutron stars are just incredibly spherical objects,” says Fabian Gittins at the University of Southampton, UK. “It’s really quite remarkable.”

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Gittins and his colleagues modelled different mathematical forces acting on neutron stars and found that any deviations in the surface could reach only 0.1 millimetres high before the crust fractured.

“We found a number of assumptions were made that were incorrect,” says Gittins. “Previous work forced the stars into a shape that isn’t physically possible.” The causes of the deformations could include the cooling of the star, its spin rate changing or the accretion of material from another star, the team found.

Astronomers had thought that variations in a neutron star’s surface might deform space-time enough to produce gravitational waves that we could detect, but this latest work suggests that they might be harder to spot than hoped.

“We would only be able to do that with third-generational gravitational wave detectors”, such as the proposed underground Einstein Telescope in Europe, says Gittins.