Reviews of Modern Physics / American Physical Society

Neutron stars exist under unfathomable conditions. For one to form, another star must grow old and die. When a giant star has burned through the fuel supply that makes it shine, its core collapses under its own weight, mashing electrons and protons together to produce neutrons. The force of the breakdown unleashes a shock wave that floods the rest of the star and shatters it. A spectacular glow envelops the shards, briefly outshining the rest of the stars in the galaxy. The light show eventually fades, but the compressed core—now a newly formed neutron star, shining brightly on its own—remains.

And what a strange creature it is. A neutron star is about the size of a major city, but a teaspoon of the material inside has the mass of a small mountain on Earth. Its violent creation can leave it spinning rapidly, flashing beams of light across space like a lighthouse. When two neutron stars collide, the crash unleashes ripples that stretch the very fabric of the universe.

For all astrophysicists have learned about these bizarre objects since they were first predicted in the 1930s, they still don’t know exactly what neutron stars look like on the inside. For now, the study of their interiors relies on theoretical models, and astrophysicists employ a very specific framework that helps make the unfathomable realities of neutron stars delightfully relatable: They compare them to pasta.

“I guess physicists just love their food,” says Matt Caplan, an astrophysicist at McGill University who studies neutron stars.

It’s not just that, of course. The carb-heavy framework, first proposed in the early 1980s, may sound silly, but it makes a lot of sense. The structures that astrophysicists predict exist inside neutron stars really do resemble some classic pasta shapes.

Let’s take a very brief tour of the complicated layers of neutron stars, as predicted by theoretical models. (Before we dive in, an important note: Despite their name, neutron stars are not entirely composed of neutrons. They also contain a small fraction of protons and electrons.)

First, there’s a thin, rigid crust of mostly iron. Here, nuclei—the protons and neutrons at the center of an atom—are spherical in shape: gnocchi. As you descend deeper, the pressure of gravity increases. Under its crush, the competing forces of neutrons and protons bend the material into other shapes. The nuclei are squeezed together into long tubes: spaghetti. Go deeper, and they flatten out into sheets: lasagna. Farther down, neutrons escape from the nuclei, producing cylindrical voids in the material: antispaghetti. Even farther, and the voids morph into blob-like shapes: antignocchi.

These layers eventually give way to the very core of the neutron star, where not even pasta comparisons can help describe the scene. Astrophysicists suspect that this innermost layer contains nuclear material in a bizarre state of matter that can’t be re-created in laboratories on Earth. “We have some crazy hypothetical ideas of what might be in the core, but nothing that’s really established science yet,” Caplan says.

According to Caplan, the nuclear pasta between a neutron star’s crust and core may be one of the strongest known materials in the universe. He and a team of researchers from Indiana University and the California Institute of Technology recently ran scores of simulations that stretched and distorted the nuclear material in all kinds of ways. They found that the combined effects of an extremely dense environment and unusual subatomic shapes produce very stiff material.

Caplan says that studying the secret interiors of neutron stars could help scientists better understand some of their dramatic behavior, which they’ve only recently started to observe. Last year, astronomers detected two exciting signatures from the same source in a galaxy about 130 million light-years from Earth: gravitational waves and a burst of gamma rays, the most energetic form of radiation. Both came from the merger of two neutron stars. The discovery, the first of its kind, produced an unprecedented amount of data for a single astronomical event.

Caplan says the pasta paradigm is not exclusive to physics. He points out that pasta shapes abound in the natural world, including in the membranes of our cells. The flattened sacs and noodly tubules of the endoplasmic reticulum—the organelle that produces, folds, and transports proteins—resemble a bowl of lasagna and rigatoni.

“You can find weird correspondences between unrelated fields because self-assembly is, in some way, universal,” Caplan says.

This is a comforting thought for Caplan. Neutron stars and endoplasmic reticula have very little in common. But they, like everything else, are made of intricate arrangements of atoms, all subject to the forces of the natural world that push them into certain, common shapes. Some of these forces scientists understand quite well. Others, they may never be able to crack. If they can’t solve all the puzzles of the universe, they can at least find comfort in carbs like the rest of us.

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