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 National Astronomy Meeting hosted by the Royal Astronomical Society.

“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.’

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…