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

This image shows the Vela pulsar wind nebula. Pulsar wind nebulae are powered by pulsars, which are a type of neutron star. [X-ray: (IXPE) NASA/MSFC/Fei Xie & (Chandra) NASA/CXC/SAO; Optical: NASA/STScI Hubble/Chandra processing by Judy Schmidt; Hubble/Chandra/IXPE processing & compositing by NASA/CXC/SAO/Kimberly Arcand & Nancy Wolk]
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.

The strain experienced by a neutron star’s crust as a function of the rotation rate divided by the breakup rate. [Morales & Horowitz 2025]
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