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Physicists are still arguing over whether black holes have “hair,” but we’re pretty sure neutron stars have mountains. These dead stars are extreme in every respect, from their magnetic field to gravitational influence, but the only thing extreme about the mountains is how small they are. A new analysis of neutron star physics predicts the “mountains” on the surface are less than a millimeter tall. 

A neutron star is what’s left of a medium-large star (10-25 solar masses) after it exhausts its nuclear fuel and explodes in a supernova. Stars that are a bit larger can collapse into black holes after expiring, and smaller ones like the sun will become white dwarfs. That means neutron stars are the densest objects known aside from black holes. They’re more massive than the sun even after blowing their top but are only a few miles in diameter. A spoonful of neutron star matter would weigh billions of tons. 

Neutron stars are compressed into an almost perfect sphere by their incredible gravitational pull — “almost” is the keyword here. In the past, scientists have estimated that mountains rising up on the surface of a neutron star could be up to a centimeter tall. However, that assumption relies on an understanding of neutron star structure that is not born out by the new research, which was conducted at the University of Southampton by Ph.D. student Fabian Gittins. 

Your average neutron star compared to New York City.

The researchers devised a new model that allowed them to subject a realistic virtual neutron star to various forces. It all comes down to how the star’s layers behave — something we cannot know for sure until we see a neutron star up close. However, this work may get us closer to the truth. The previous estimates of centimeter-scale mountains relied on the assumption that the elastic crust of a neutron star was near the breaking point across the entire surface. Instead, the new model predicts that breaks in the crust will cover a larger surface area at a lower altitude. Thus, mountains that are only a fraction of a millimeter tall. 

The extreme physics at work inside a neutron star are still unclear, but this work could get us closer to the truth. It’s possible scientists will be able to measure these mountains in the future. Theoretically, deformations in the surface of a spinning neutron star should be visible in gravitational waves. However, we have yet to see waves from a single neutron star. So far, we can only detect cataclysmic events like two neutron stars colliding. As more sensitive detectors come online, we might be able to size up the mountains in real life.

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