Millimetre-mountains on neutron stars

Computational modelling suggests that neutron stars are even weirder than we thought. According to researchers from the University of Southampton in the UK, the tallest mountains on these stars could be just fractions of millimetres high.

Neutron stars are among the densest objects in the universe. They are one potential end point in the process of stellar evolution: when a massive star exhausts its fuel source and fusion stops, there is no longer enough outward pressure to stop the star from collapsing. Gravity drags the star inwards under its own weight, squeezing it so tightly that electrons get pulled into the nucleus of atoms, protons turn into neutrons, and soon the star is just an ultra-dense ball of neutrons.

This stellar corpse is several times the mass of our own Sun but compressed to about the size of a city (around 20km in diameter). Because it’s so dense, a neutron star has an enormous gravitational pull around a billion times stronger than the Earth’s.

This means any bumps or features on its surface have miniscule dimensions, according to these new models.

“These results show how neutron stars truly are remarkably spherical objects,” says Fabian Gittins at the University of Southampton, who led the research.

Gittins and team built realistic neutron stars in computational models, then subjected them to a range of forces to see how the “mountains” on their surfaces are created.

They found that the largest mountains were just a fraction of a millimetre tall. This is around a hundred times smaller than previous estimates, which had suggested that such mountains could be as large as a few centimetres.

“For the past two decades, there has been much interest in understanding how large these mountains can be before the crust of the neutron star breaks, and the mountain can no longer be supported,” Gittins explains.

Previous research assumed that the crust of neutron stars was close to breaking point at every part of the surface, but the new models indicate that this isn’t realistic.

Gittins adds that his team’s results “suggest that observing gravitational waves from rotating neutron stars may be even more challenging than previously thought”.

Astronomers suspect that because neutron stars have bumps on their surface, as they rotate they should produce continuous gravitational waves. But if these stars are really as smooth as Gittins and team have calculated, these ripples in spacetime could be difficult to detect.

The research was presented at the National Astronomy Meeting 2021, run by the UK’s Royal Astronomical Society.

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