The gravitational field of neutron stars is so strong that so-called mountains protruding from their surface only grow to a fraction of a millimeter in simulations.
When certain massive stars burn all of their fuel and become a supernova, the remaining nuclear matter collapses on itself and forms a neutron star. These bodies are compressed so much that their electrons and protons combine to form neutrons. Their mass – typically around 1.4 times the mass of our sun – is squeezed into a sphere only 20 km in diameter. For comparison, our star has a diameter of 1.4 million km.
Neutron stars are among the densest objects in the known universe and have extreme gravitational fields, so that mountains on their surface are possibly only a fraction of a millimeter high. This would make their surface smoother and more even than previously thought, according to Nils Andersson, professor of applied mathematics at the University of Southampton, England. These conclusions were presented this week around 2021 National Astronomy Meeting organized by the Royal Astronomical Society.
“Colloquially, a ‘mountain’ is understood to mean a ‘quadrupole deformation’ in which a rotating star is essentially stretched in such a way that it optimally emits gravitational waves,” explained Prof. Andersson The registry. “Maybe the word is ironic considering that these ‘mountains’ are tiny.”
The gravitational wave aspect is interesting. Rotating neutron stars should generate these waves, which are basically waves in the fabric of spacetime, from their surface deformations. If the piles of neutron stars really are that small, discovering their gravitational waves might be more difficult than some expect.
Unlike mountains on Earth, these tiny structures on neutron stars are not formed by geological processes. Instead, mountains are forged on these dead stars by how much material is pulled outward when they are turned. It doesn’t expand much as the star’s gravity pushes it inward.
“First, you would never expect these mountains to be huge, considering the gravity on a neutron star is so strong,” added the professor. “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 Mount Everest.”
Scientists have been trying to find out what is happening on the surface of neutron stars at least since the 2000s. Prof. Andersson and his colleagues issued two papers this year and last year describe computer models aimed at predicting the mountain heights of neutron stars; these simulate the objects as bodies of dense liquid contained in an elastic crust.
“These results show that neutron stars are really remarkably spherical objects.” said Co-author Fabian Gittins, PhD student in Theoretical Astrophysics in Southampton. “They also suggest that observing gravitational waves from rotating neutron stars could be even more difficult than previously thought.”
Gravitational wave detectors like LIGO and Virgo have discovered waves in spacetime caused by the collision of pairs of neutron stars; We have not yet seen waves from a lonely spinning neutron star. And if these surface simulations are correct, it may not come as a surprise. ®