The gravitational field of neutron stars is so strong that so-called mountains poking out from their surfaces only grow to a fraction of a millimetre in height in simulations.

When certain massive stars finish burning all their fuel and go supernova, the leftover core matter collapses in on itself to form a neutron star. These bodies are compressed to such a degree that their electrons and protons combine into neutrons. Their mass – typically about 1.4 times the mass of our Sun – is squeezed into a sphere just 20km or so across. Our star has a diameter of 1.4 million km, for comparison.

Neutron stars are thus among the densest objects in the known universe, and have extreme gravitational fields, so much so that mountains on their surfaces may only be a fraction of a millimetre tall. This would make their surface smoother and more uniform than previously thought, according to Nils Andersson, professor of applied mathematics at England's University of Southampton. These conclusions were presented this week at 2021's [1]National Astronomy Meeting hosted by the Royal Astronomical Society.

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“Colloquially, 'mountain' is taken to mean 'quadrupole deformation,' basically stretching a spinning star in such a way that it becomes optimal at emitting gravitational waves,” Prof Andersson explained to The Register . “Perhaps the word is also ironic given that these 'mountains’ are tiny.'

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The gravitational wave aspect is interesting. Spinning neutron stars should produce these waves, which are basically ripples in the fabric of spacetime, from their surface deformations. If neutron stars' mountains, if you will, truly are so small, it may be more difficult than some anticipate to detect their gravitational waves.

Unlike mountains on Earth, these minuscule structures on neutron stars aren’t formed by geological processes. Instead, mountains on these dead stars are forged by how much material is pulled outward when they spin. It doesn’t stretch by much since the star's gravity crushes it inwards.

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“First of all, one would never expect these mountains to be huge, given that the gravity on a neutron star is so strong," the professor added. "But if you compare the relative change in the gravitational potential then the predicted neutron star mountains would no longer be so puny compared to, say, Mount Everest."

[6]You wait ages for a neutron star and black hole to collide, then two pairs come along at once

[7]Years after we detected two neutron stars crashing into each other, we're still picking up X-rays. We don't know why

[8]Grav wave boffins are unsure if they just spotted the smallest black hole or the biggest neutron star seen yet

[9]We spent billions building atom smashers – and now boffins think nature's doing the same thing for free?

Since the 2000s, at least, scientists have been trying to figure out what's going on at the surface of neutron stars. Prof Andersson and his colleagues emitted [10]two [11]papers this year and last describing computer models that aim to predict neutron stars' mountain heights; these simulate the objects as bodies of dense fluid contained in an elastic crust.

“These results show how neutron stars truly are remarkably spherical objects,” [12]said co-author Fabian Gittins, a PhD student in theoretical astrophysics at Southampton. “Additionally, they suggest that observing gravitational waves from rotating neutron stars may be even more challenging than previously thought.”

Gravitational-wave detectors like LIGO and Virgo have spotted ripples of spacetime caused by pairs of neutron stars smashing into one another; we've yet to see waves from a lone spinning neutron star. And if these surface simulations are correct, that may not be a surprise. ®

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[1] https://nam2021.org/

[2] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=2&c=2YPfwRsfRr8Bsu6r-CGBcpwAAANQ&t=ct%3Dns%26unitnum%3D2%26raptor%3Dcondor%26pos%3Dtop%26test%3D0

[3] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44YPfwRsfRr8Bsu6r-CGBcpwAAANQ&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0

[4] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=3&c=33YPfwRsfRr8Bsu6r-CGBcpwAAANQ&t=ct%3Dns%26unitnum%3D3%26raptor%3Deagle%26pos%3Dmid%26test%3D0

[5] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44YPfwRsfRr8Bsu6r-CGBcpwAAANQ&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0

[6] https://www.theregister.com/2021/06/30/gravitational_waves_neutron/

[7] https://www.theregister.com/2020/10/14/binary_star_xray/

[8] https://www.theregister.com/2020/06/24/grav_wave_boffins/

[9] https://www.theregister.com/2020/06/03/quark_star_formation/

[10] https://arxiv.org/abs/2009.12794

[11] https://arxiv.org/abs/2105.06493

[12] https://ras.ac.uk/news-and-press/research-highlights/bugs-life-millimetre-tall-mountains-neutron-stars

[13] https://whitepapers.theregister.com/