[♩INTRO].

There are a lot of incredible things in space, but neutron stars are some of the most mind-blowing. They pack the mass of a whole Sun into a ball just kilometers across.

And when you squeeze that much mass into a space that small, things start to get strange. From liquid plasma oceans on the surface, to a possible neutron superfluid in the core as you go deeper in a neutron star, the physics gets truly wild. Now, as tiny as these stars are, they actually form from supergiant stars.

As a supergiant dies, its core collapses into a hot, dense sphere, where the pressure is so extreme that atoms themselves cave in. All the empty space between the nucleus and electrons disappears, and many protons and electrons merge to form neutrons. So where there were atoms, there are now just nuclei.

These particles are called nuclear matter, and this stuff is so dense that a teaspoon of it would weigh at least millions of tons. And that turns out to have some pretty strange consequences. Research suggests that, throughout these stars, nuclear matter takes on many different, and bizarre, forms.

Starting at the surface, where the pressure is the least extreme, some neutron stars have a sort of “ocean” made of liquid plasma. That’s weird enough by itself, because we normally encounter plasma as a sort of hot, electrically-charged gas. But to have a gas, your atoms need to be spaced far apart.

And here, the pressure is too high for that so the nuclear matter seems to be kind of melted and liquidy. That changes quickly, though. Because below this ocean, neutron stars actually have a solid crust more like rocky planets than other stars.

It’s just… made of nuclear matter instead of rock. Simulations suggest that this crust forms as pressure increases and the nuclei get even closer together getting so close that the nuclear material becomes a solid and forms crystal-like structures. At this point, the atomic nuclei are still pretty much spherical, as they usually are.

But as you go deeper, even that starts to change. One level down, in what’s called the inner crust, the pressure becomes so intense that nuclei may begin to touch. Now, normally, nuclei repel each other because they’re all positively-charged.

It’s like how the north poles of two magnets will repel each other, and it happens because the electromagnetic force is pushing them apart. But under this much pressure, models suggest this force can’t do that. It’s just not strong enough.

So instead, the nuclei start to interact through what’s called the nuclear force. This is the force that holds particles in the nucleus together. And it’s not just found in neutron stars it also holds every nucleus in every atom in your body together.

We just don’t think about it often because this force is only relevant over really short distances. So normally, it doesn’t affect interactions between nuclei. But at this distance, the nuclear force begins to pull these nuclei together.

That deforms and merges them, squashing and stretching them into spaghetti-like shapes. In fact, although there are some more precise, technical names, scientists often call this nuclear pasta. And it gets more extreme as you go deeper in the crust.

Like, at the top of the inner crust, nuclei are still more or less spherical, so they’re called nuclear gnocchi. But, farther down, the increasing pressure squishes those nuclei together, combining them into long strands known as nuclear spaghetti. Deeper down, these strands merge into sheets of nuclei or, naturally, nuclear lasagna.

And after that, things just keep collapsing. At even higher densities, these sheets themselves start to merge together, forming a mostly-solid mass with some tube-like voids — a.k.a. anti-spaghetti. And eventually, even those tubes begin collapsing, leaving behind bubbles of less-dense material inside the denser mass.

This one is either called the Swiss cheese phase or the anti-gnocchi phase because if you’re going to make pasta analogies, you might as well commit. The theme, though, is that when the nuclear force dominates and nuclei start squishing together, matter gets weird. But what might be weirdest is that scientists believe that if you slice through a neutron star, the nuclear pasta phases only make up about 1% of the star’s radius.

Then, 10% of the total radius is crust, and a bit under 90% is actually the core. And the core is still a pretty big mystery to us. Research suggests that, here, the nuclear material isn’t really arranged in distinct nuclei at all the pressure is so high that it’s just a uniform fluid of neutrons and protons.

Some scientists speculate that it might even be a superfluid, meaning it moves with zero friction under weird quantum rules. Overall, though, there’s nothing like any of this on Earth. And that’s exactly what makes neutron stars so valuable.

Even though they’re dim and hard to spot, astronomers can learn a lot about them by observing their light and the gravitational waves they create. Sometimes, they can even infer what’s going on inside them by observing seismic events called starquakes that cause wobbles in their light. And all of this insight helps us understand what physics is like at its limits and gives us a close-up look at some of the strangest places in the universe.

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