Photo credit: ESO / L. Calçada

New models of neutron stars show that their tallest mountains may only be fractions of a millimeter high due to the enormous force of gravity on the ultra-dense objects. The research will be presented today at the 2021 National Astronomy Meeting.

Neutron stars are among the densest objects in the universe: They weigh about as much as the sun, but are only about 10 km in diameter, which is the size of a large city.

Due to their compactness, neutron stars have an enormous pull, which is about a billion times stronger than that of the earth. This crushes every feature on the surface to tiny dimensions and means that the stellar remnant is an almost perfect sphere.

Although they are billions of times smaller than on Earth, these deformations from a perfect sphere are still called mountains. The team behind the work, led by PhD student Fabian Gittins of the University of Southampton, used computer modeling to build realistic neutron stars and subject them to a range of mathematical forces to identify how the mountains are formed.

The team also examined the role of ultra-dense nuclear matter in supporting the mountains, finding that the largest mountains produced were only a fraction of a millimeter high, a hundred times smaller than previous estimates.

Fabian comments: “In the last two decades there has been a great interest in understanding how big these mountains can be before the crust of the neutron star breaks and the mountain can no longer be supported.”

Previous work has shown that neutron stars can tolerate deviations of up to a few parts in a million from a perfect sphere, suggesting that the mountains could be several inches tall. These calculations assumed that the neutron star was so stressed that the crust was on the verge of breaking at any point. However, the new models show that such conditions are not physically realistic.

Fabian adds: “These results show that neutron stars are really remarkably spherical objects. In addition, they suggest that observing gravitational waves from rotating neutron stars could be even more difficult than previously thought. “

Although they are single objects, spinning neutron stars with slight deformations due to their strong gravitation should create waves in the fabric of spacetime known as gravitational waves. Gravitational waves from rotations of individual neutron stars have yet to be observed, although future advances in extremely sensitive detectors such as the advanced LIGO and Virgo could be key to probing these unique objects.


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