Pasta helps marathon runners keep the pace – and maybe some spinning stars too. The key to neutron stars’ steady rotation may be spaghetti-shaped groupings of atomic nuclei that form lumps in the stellar crust.

A neutron star is the ultra-dense remnant of a stellar explosion, made up of a solid crust of atomic nuclei and a liquid core of free neutrons. These stars are born spinning rapidly, sometimes making multiple rotations per second. Some neutron stars also emit beams of radiation from their magnetic poles. If the beams sweep past Earth, we can detect the regular pulses of light and time the star’s spin.

Without any outside influences, a neutron star will slow down over time as it radiates away energy. Curiously, X-ray pulsars, which are brighter and easier to observe than other types, appear to stop slowing down when they reach a rate of about 12 seconds per rotation.

The explanation may lie in the star’s surface. José Pons at the University of Alicante in Spain and his colleagues ran computer simulations of neutron stars with different crust configurations. After just 100,000 years, the spins of stars with lumpy crusts slowed to between 10 and 20 seconds and then stayed steady for the next million years or so, roughly the amount of time it takes for the stars to cool so much they stop emitting X-rays. Stars with smoother crusts kept slowing down over time, getting as slow as 100 seconds per rotation.


Lumpy crust

This makes sense, the team says, because the lumpier the crust, the worse it is at conducting the electrical currents that maintain the star’s magnetic field. With a lower magnetic field, the star radiates less energy into space and its spin remains stable for longer.

So what can make neutron stars lumpy? Pons and colleagues think that their crusts might be full of “nuclear pasta“. Because neutron stars are so dense, atomic nuclei are packed together tightly in the crust. The particles in these compacted nuclei could be forced into exotic groupings that resemble spaghetti, macaroni and layers of lasagna. Mixing these shapes together in the crust would make it bumpier than one that only contains regular nuclei arranged in orderly crystals.

But those hoping for a taste of pulsar pasta are out of luck. “There is probably no other place in the universe where such conditions are reached,” says Pons. Instead, more sensitive X-ray telescopes could increase the number of pulsars known and check whether some manage to get slower than the 12-second spins. If so, the star crusts may be more orderly after all, potentially putting the pasta theory down the drain.

Journal reference: Nature Physics, DOI: 10.1038/nphys2640