Super-Dense Stars May Squash Neutrons Into Cubes

Deep inside the super-dense hearts of exploding stars, gravity may squash neutron particles from spheres into cubes.

The idea could mean that neutron stars, as researchers call the stellar corpses, are denser than anyone expected. It could also question what stops them from collapsing into black holes and out of existence.

“If you take this result purely at face value, it means neutron stars are in trouble. They should collapse into black holes at lower masses,” said theoretical physicist Felipe Jose Llanes-Estrada of Complutense University of Madrid, co-author of a study published Aug. 9 on the prepublication server arXiv.

“But that’s not what we observe. It’s possible there’s an additional repulsive interaction [between neutrons] to counter a collapse that we just haven’t thought of yet.”

A star between nine and 20 times the sun’s mass detonates as a supernova toward the end of its life. At that weight, a star isn’t heavy enough to create a critical, ultra-dense state and shrink into a black hole. Instead, it collapses into a sphere no bigger than 15 miles wide and so dense that a single teaspoon of it weighs as much as everyone on Earth, multiplied by 18.

Late last year, astronomers discovered the biggest-ever neutron star, called J1614-2230, that weighed in at 1.97 times the sun’s mass.  Prior to its discovery, the most massive neutron weighed 1.67 solar masses.

The find left more than a few astrophysicists scratching their heads. Its existence ruled out some models of neutron stars that relied on exotic forms of matter to explain why they didn’t collapse farther, and instead supported models of neutron stars as containing only neutrons and protons.

When Llanes-Estrad and his university colleague Gaspar Moreno Navarro heard of J1614-2230, they wanted to know what might be happening inside of it.

The duo knew of a model from the 1970s suggesting pure neutrons could form a crystal lattice under incredible pressure (similar to how carbon forms diamonds in the bowels of the Earth). When they tweaked a familiar computer model to incorporate the idea, they discovered that — at the pressures anticipated deep in neutron stars — neutrons deformed from spheres into cubes.

“There’s an optimum packing density with spheres, including neutrons. It’s about 74 percent. No matter how efficiently you arrange them, like oranges on display at a supermarket, there’s always space in between,” Llanes-Estrada said. “If you want to be most efficient, you distort the oranges. Pack them a mile high and squish the ones on the bottom.”

Gravity shapes aggregate particles of matter into the simplest, most efficiently-packed object possible, normally a sphere like the Earth. The particles themselves, though, remain individually unaffected; gravity is too weak to overcome the strong interactions that hold neutrons and other particles together. But if gravity becomes intense enough, it might overpower the interactions.

So deep within a neutron star, a neutron’s most efficient shape may be a cube. “They’ll be flattened on all sides, like dice,” Llanes-Estrada said.

So far, the response to their study has proven lukewarm.

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