Cracking Crusts Might Set a Neutron Star Speed Limit

Neutron star crust is the strongest material in the universe, but it’s not infinitely strong. New research explores how the cracking of a neutron star’s crust might determine how fast these extreme stellar remnants can spin.

Stellar Speed Limit

Neutron stars are the remnant cores of massive stars that have expired as core-collapse supernovae. These stars are extraordinarily compact, typically packing more than the mass of the Sun into a sphere with a volume about a billion times smaller than Earth’s volume.

Neutron stars also rotate extremely quickly, with the fastest-spinning neutron stars zipping around hundreds of times each second. That’s fast — but it turns out to be only about half as fast as a neutron star could hypothetically spin before being ripped apart by its rotation, a speed known as the breakup rate. What’s stopping neutron stars from spinning even faster?

Crust-Covered Pasta

Jorge Morales and Charles Horowitz (Indiana University) have explored one possibility: that the cracking of a neutron star’s crust occurs at roughly half the star’s breakup rate, and that once the crust cracks, the star can spin no faster.

Neutron stars are composed of a strange substance called nuclear pasta, which is enclosed by a shell of atomic nuclei and electrons. Both of these layers are reportedly billions of times stronger than steel.

Morales and Horowitz modeled the strain that a neutron star’s crust encounters under different spin rates and neutron star masses. They showed that as a neutron star spins, the material around its equator bulges outward while the material near its poles draws inward. When the strain of this deformation surpasses the crust’s strength, the crust splits along the star’s equator, at the base of the crust where the crust meets the interior nuclear pasta. For reasonable estimates of the crust’s breaking strength, this happens at 58% of the star’s breakup rate — roughly the maximum spin speed drawn from observations.

Continuous Gravitational Waves

Why does the breaking of a neutron star’s crust inhibit faster rotation? This is especially relevant for neutron stars that are expected to “spin up” to faster speeds over time as they accrete matter from their companions; even these appear to obey the spin speed limit.

After the crust breaks, the pieces of the crust can shift around. This may transform the neutron star from one that is symmetric around its spin axis to one that is asymmetric. These objects might produce a continuous gravitational wave signal, and Morales and Horowitz proposed that any angular momentum added to the star — through accretion, for example — is carried away in the form of gravitational waves, preventing the star from spinning faster.

Morales and Horowitz noted that there are more aspects of this scenario to be explored, such as the impact of crustal magnetic fields, the importance of relativistic effects, and the prospects of detecting gravitational waves from these sources.

Citation

“Limiting Rotation Rate of Neutron Stars from Crust Breaking and Gravitational Waves,” J. A. Morales and C. J. Horowitz 2025 ApJL 978 L8. doi:10.3847/2041-8213/ad9ea7