What’s inside a neutron star? What strange states of matter do we encounter? And what mysteries will we find deep in the core? I discuss these questions and more in today’s Ask a Spaceman!

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Hosted by Paul M. Sutter.

 

EPISODE TRANSCRIPTION (AUTO GENERATED)

We are currently in orbit around the neutron star. The star itself is large for its kind, roughly twice the mass of the sun. However, all that mass is compressed into a volume no wider than 20 kilometers across. That's about 12 miles. That's two suns worth of material crammed into an area roughly the size of the Island Of Manhattan.

Our orbit here is fast. Achieving speeds of 25% the speed of light is easy, considering the incredible gravitational strengths of the neutron star. In some rare cases, the gravity around a neutron star is so intense that it can bend light in the path of a circle. That's right. Light can orbit a neutron star.

In fact, when we look at the star, it appears larger than life. Light from the opposite side curves around to reach us, inflating its dimensions beyond all reasonable proportions. The temperatures and radiation here are, in a word, intense. The surface temperature of the star is around 1,000,000 Kelvin, comparable to the core temperatures of a hydrogen burning star like the sun. However, neutron stars do not generate their own heat.

What we see is the leftover heat from their formation, a process of violence unleashed. When a giant star, at least eight times the mass of the sun, reached temperatures and pressures in its core to fuse iron. But it could not release energy from the fusion of iron, and so it collapsed. And in that process of collapse, it literally shoved electrons into protons, turning them into neutrons and creating the neutron star. So initially, neutron stars have a temperature of well over 10,000,000 Kelvin.

But within less than a million years, they're only a tenth of that temperature. We estimate there are around 1,000,000,000 neutron stars in the Milky Way. Most are far too cool for us to detect. At these insane temperatures, the environment around the stars is soaked in deadly x rays, gamma rays, and ultraviolet radiation. Not to mention the strong magnetic fields that in some cases are powerful enough to literally dissolve molecular bonds and tear apart any microscopic object.

It's not pleasant. So let's get closer. It will not be difficult. The surface gravity of a neutron star is over 10,000,000,000 times stronger than the Earth. If we let go and fall freely, by the time we reach the surface, we'll be traveling at a sizable fraction of the speed of light.

The first part of the neutron star we encounter is the atmosphere. Even though gravity is incredibly strong here, the temperatures are also incredibly high. High enough for a thin layer of gas to exist just above the surface. The interactions among the atoms frequent enough to keep them mobile despite the crush of gravity. The atmosphere of a neutron star is mostly carbon and possibly some heavier elements, compressed into a thin layer somewhere between a few millimeters and possibly a few centimeters thick.

Don't let the name atmosphere fool you. It's technically an atmosphere because it's technically a gas because technically, the atoms here are mobile. But the density of the material here is greater than diamond, and it's still much much less dense than what awaits us below. Beneath the atmosphere is an ocean of charged particles, a layer barely a millimeter thick, if that, a liquid of ions and electrons. They carry waves, tides, and everything else all squeezed to microscopic proportions.

We plunge through the ocean, its incredible depth of the width of a human hair, and reach the real surface of the neutron star. This is the outer crust, perhaps the smoothest entity in the entire universe. Despite stretching for over tens of kilometers, the tallest mountains on the surface are no more than a few millimeters high. Anything higher just gets squashed down because of the extreme gravity. But cracks can appear in this crust.

Flaws and defects due to the incredible pressures and tensions and forces at work here. And sometimes those cracks lead to starquakes on the surface that rattle the entire interior of the star and lead to glitches or sudden changes in the neutron star's rotation. This is all powered by extreme gravity and that's going to be a common phrase throughout this journey. Folks, we are on the doorstep of a black hole. The densest thing in the universe before oblivion itself.

This is matter in its most extreme state and perhaps even more extreme and exotic than a black hole. Once you tip over the edge and reach too high densities, too high pressures, gravity takes over. Black holes are objects of pure gravity in a way that makes them simple. Neutron stars are just above that, just below the threshold of turning into a black hole, which means they are objects of more than gravity. The other forces of nature, electromagnetism, weak nuclear, strong nuclear, get to play a role here in a way that they can't inside and around a black hole.

And that means the deeper we go into a neutron star, the more it's going to get weird. The outer crust of the neutron star extends to a depth of around half a kilometer. And already, we are encountering states of matter that we cannot replicate in the laboratory. Here, you'll find the usual constituents of the material universe. Atomic nuclei of various masses in their normal configurations.

Iron, oxygen, carbon, nothing unfamiliar. But they are packed so tightly together that they form a lattice work, like a gigantic crystal structure. Swimming through this lattice work are free electrons, like rivers of charge. The density of this outer core region, this gigantic crystal structure made of iron, oxygen, carbon, and more, is around a billion times greater than air. That's 44,000 times denser than osmium, the densest known naturally occurring mineral.

We must go deeper. As we plunge further into the neutron star, what we thought was strange becomes normal and what we thought was impossible becomes commonplace. The densities and pressures here are hard to describe. The gravitational pull brings those atomic nuclei closer and closer together. It breaks down that lattice work because even the crystal structure can't support itself against the extreme pressures at these depths.

So, not only do the atomic nuclei become closer together, they also become larger and heavier, swelling and bursting with hundreds of neutrons crammed into them. Outside of the star in the laboratory, these nuclei would be unstable, instantly dissolving, ejecting their neutrons and decaying into lighter, more stable versions of themselves. But the extreme gravity and the extreme pressure keeps them contained creating isotopes of the elements beyond what we can even hope to recreate here on Earth. But even that has a limit. Once we cross a depth of roughly one kilometer not even a fraction into the full depths of the neutron star we reach the inner crust.

The boundary of this region is known as the drip line. The atomic nuclei becomes so full of neutrons that even under these extreme conditions, they can't possibly hold on to anymore. The extreme gravity, the intense pressures, force neutrons to squeeze or drip out of the nuclei. In this region, which extends for roughly another kilometer, the remaining nuclei, the ones that are able to survive, are surrounded by a sea of free neutrons. Outside of an atomic nucleus, these are the only known places in the entire universe where neutrons can roam free.

That's because neutrons usually have a short lifetime. They have a half life of around fifteen minutes. If you pluck a neutron out of an atomic nucleus and let it roam free, in fifteen minutes, it's likely to be gone. But here, the strength of gravity is so intense that it keeps the free neutrons bound and stable. They can live forever inside of a neutron star.

There are electrons surviving down here too. But at these depths, they have gone relativistic. At the outer edges of the neutron star, at the surface, the ocean, the outer crust, the electrons are mobile. They they move like like water currents through the lattice work. But down here, their movements are confined.

They try to roam freely, but then they hit a neutron. They recoil off of a nuclei. They can't travel very far, and so they bounce around again and again and again and again. They try to move, but they can't. This ramps up their velocities like trapping a bee in a small cage and making that cage smaller and smaller and smaller, and the bee gets angrier and angrier and angrier.

It vibrates against the walls of its cage, eventually reaching relativistic speeds, and this vibration, this refusal to be crammed down into a small volume is what's responsible for what we call degeneracy pressure. The electrons themselves in this region of the neutron star support some of the weight of the star against further gravitational collapse, and that's because the electrons refuse to go easily. They refuse to be crammed down into incredibly small volumes easily. They vibrate against that. They resist that, and that creates a pressure.

This is the same kind of pressure that supports the cousins of the neutron stars, the white dwarfs. But in the white dwarfs, almost all the support comes from electrons. In neutron stars, the electrons only supply a portion of the pressure. But to get to the rest of the support of the neutron star, to get the rest of the pressure against gravity, the only thing holding up this neutron star against total oblivion and collapse into a black hole, well, that comes a little deeper. We're not there yet.

The bottom boundary of the inner crust, the transition to the core region, contains perhaps the strongest material in the known and probably unknown universe. We are already at a density a thousand times greater than the upper crust, and we've only traveled somewhere between one and two kilometers. It's already a thousand times denser. At these extremes, atomic nuclei themselves become distorted. In the upper levels, the outermost levels of the neutron star atomic nuclei were normal.

They were just close together. Then at deeper regions, they became crammed with neutrons, but they were still recognizably atomic nuclei. Then the neutrons started to be squeezed out of the nuclei, but still the nuclei remained. But at these depths, they are completely unlike anything else we could ever possibly observe anywhere else in the universe or recreate in the laboratory. At these pressures, the nuclei themselves are swollen with up to thousands of neutrons.

And because of that, they aren't able to maintain their usual spherical nature. There are many forces at work here that distort their shapes. There's the crush of gravity. There's the extreme pressure of their neighbors. There's also the repulsion of the strong nuclear force between individual protons and neutrons.

There's also an intense electric force between the protons and any free floating electrons. All of these forces compete to distort the shape of nuclei. We have entered a region called the nuclear pasta. It's a transition region. It's about 100 meters thick, but it contains a mass equivalent to over 3,000 Earths.

In the outermost layers of the pasta, the beginning of the transition region, the nuclei become lumpy and irregular. Small collections of thousands of protons and neutrons glued together into odd random shapes embedded in this sea of free floating electrons and neutrons. We call these the gnocchi. Even deeper, the individual lumps bond together, but they can more easily bond together along one direction than any of the others. So they form long strands, long here in a certain sense.

They're only up to a few thousand nucleons wide. These are still sub microscopic but they are still not exactly atomic nuclei anymore. These are different kinds of structures. These strands can no longer support a spherical shape because of the extreme and exotic forces this becomes the spaghetti layer. Even deeper the strands of nuclear spaghetti bond together forming sheets.

You guessed it, it's here we'll find the lasagna. And then finally when pressed together the lasagna become largely homogeneous but small defects open up. Some of these defects are just tiny little holes called the anti gnocchi. Sometimes, long tubes open up, the anti spaghetti or the bucatini. Finally, finally, at the end of this transition region at the bottom of the nuclear pasta region after we've traveled all but a hundred meters, all the atomic nuclei are fully crushed.

And we have finally entered the core we have only traveled to a depth of around two kilometers into the neutron star two kilometers you could walk that in less than a half hour And we have already encountered states of matter that are simply impossible to exist in any other environment. Only the extreme conditions of a neutron star support this. Only when we are on the precipice of a black hole itself do we get to experience states of matter like this. The lattice work of ions, the atmosphere and ocean of heavy elements, the nuclear pasta. None of that exists outside of a neutron star.

None of it. But at least at those layers when it comes to the atmosphere, the crust, the ocean, we have at least a somewhat solid understanding of what goes on. Don't get me wrong. There are plenty of mysteries here, but at least we understand the scope of the problem. And we have the tools, which is our understanding of nuclear matter, to at least attempt solutions and paint a hazy portrait, but at least it's something.

But once we've reached a depth of two kilometers and entered the core, once we've left the nuclear pasta behind, it's only guesswork. We do know that eventually, at extreme enough conditions, atomic nuclei themselves dissolve. The pressures are simply too great. They can't hold on to their individual protons and neutrons because they are squeezed so tightly against each other. What we have in the outermost core is an ocean of free floating neutrons and any remaining protons and electrons.

Most of the protons and electrons disappeared long ago when the star first formed. Most of the ones that survive live in the outer layers. A few manage to survive here in the core. But in the core, it is mostly neutrons. These free neutrons liberated from an atomic nuclei, but thanks to the extreme conditions able to live forever and stabilized, form a soup, a sea that is now a superfluid.

The neutrons flow with zero resistance, zero friction, and zero viscosity. Strange quantum forces enabling this seemingly paradoxical state. There are no impediments to their movements. And the protons, the ones that do remain down here in the core, are now superconducting. There is no electrical resistance here in the core.

And it's here in the core, that we have finally reached atomic densities. Protons and neutrons here are squeezed as tightly as they are in the core of an atom itself, except that instead of the strong nuclear force doing the work of binding them together, it is gravity itself that binds them together in this macroscopic structure that stretches for kilometers upon kilometers. It is for all intents and purposes a gigantic atomic nucleus. It's here where the neutrons themselves also contribute to supporting the star against collapse. They supply their own source of degeneracy pressure.

They resist further collapse because if you try to squeeze neutrons together in these kinds of extreme conditions, they resist that. They can always be squeezed so tight. They start to vibrate on their own. They create energies of their own. They have a pressure of their own.

This aids in the work of supporting a neutron star against collapse. And because neutrons are so much more massive than electrons, it takes a lot more work to get them up to relativistic speeds. So you can compress neutrons much much more tightly than electrons, which is why neutron stars are so much smaller than a white dwarf. A typical white dwarf will be less massive than a neutron star, but roughly the size of a planet. A neutron star will outweigh a white dwarf and yet be orders of magnitude smaller.

And that's all thanks to the incredible mass of neutrons compared to electrons. But it's not just degeneracy pressure that holds up a neutron star. There's also the strong force itself. The strong force is attractive at certain distances, say, a femtometer, 10 to the minus 15 meters. The width of an atomic nucleus, the strong force there is attractive, but at smaller scales, it can be repulsive.

This is what prevents atomic nuclei from just collapsing in on themselves. The strong force manifests itself in different ways at different scales. And here, if we're thinking of a neutron star as a gigantic atomic nucleus, it's bound together with gravity, but then it holds itself up through the natural repulsion of the neutrons. But this is only a guess. It's our best guess based on our current understanding of nuclear physics about what happens in the cores of these exotic stars, but it's still a guess.

Even deeper, the innermost core with a radius of around three kilometers, the nature of that is a complete mystery. One of the biggest problems here is that we do not contribute to Patreon. That's patreon.com/pmsutter. Maybe if you contributed to Patreon more, we would understand the core of a neutron star. I'm not guaranteeing it, but, you know, it's worth a shot.

And I truly do appreciate all of your contributions. That's patreon.com/pmsutter. Now the real biggest problem here is that we don't know the equation of state. The equation of state is a very special kind of equation that appears in physics, chemistry, materials engineering, all sorts of applications. And it's a general description of the relationship between the material properties of an object, like the relationship between temperature and pressure.

So if you took high school physics or chemistry, you probably learned about the ideal gas law or Boyle's law. This is an equation of state. It tells you that if you change one thing about an object, if I have a big chunk of air and I change its volume, I also change its temperature and I also change its pressure. It tells you how those quantities relate. And in many cases in astrophysics, we know the equation of state, or at least we have a good handle on the equation of state, where we understand how the pressure, temperature, density, all these properties of a star relate to each other depending on the complex physics.

But the physics in a neutron star is too complex. It's too far beyond what we can recreate in the laboratory. It's too far beyond what we can grapple with theoretically. And so the equation of state of a neutron star is one of the biggest outstanding mysteries in astrophysics. At the core of a neutron star, we do not know how pressure, density, and temperature relate to each other and in what ways.

And because of this, because of that fundamental lack of understanding of the equation of state, we don't know how matter behaves. Because the equation of state is the summary of how matter behaves, and without even that summary, we don't know the details. The densities of the outer core of a neutron star, where neutrons first began to roam free, are comparable to an atomic nucleus. Here in the inner core, it's higher than an atomic nucleus. That's right.

The density of the core of a neutron star is greater than the density of an atomic nucleus. And we don't know how that behaves. We don't know how that acts. We don't know what matter does in these kinds of conditions. It's too strange.

We can't even describe the inner core of a neutron star as a gigantic atomic nucleus anymore because it's weirder than that. It's stranger than that. We don't know what goes on here. It could be that the neutrons and what remaining protons there are smash together, spill out their guts. And what are protons and neutrons made of?

They're made of quarks. It could be that quarks recombine in interesting and exotic ways forming new kinds of particles. Particles named hyperons and deltas and boson condensates. These are purely hypothetical particles that can only exist in the conditions of the inner core of a neutron star, where things are so strange there aren't even neutrons anymore. There are new forms of matter, new collections of quarks.

It could transform into kaons. Kaons are a form of matter made of a pair of quark and antiquark. These might glue themselves together to make a Bose Einstein condensate. Usually, kaons are unstable, but as is a common refrain when it comes to neutron stars, in these conditions, they might be stable. It could be that neutrons themselves break down into their quarks, that protons break down into their quarks, and nothing emerges out of that.

That there are no more larger structures. There are no neutrons. There are no protons. There are no hyperons, deltas, kaons. Nothing except quarks.

The most fundamental constituent of matter. Outside of a neutron star, quarks only exist in atomic nuclei. They only exist in very brief products out of our most powerful particle colliders. We can't see quarks individually ever. We can only see them bound up together to form larger collections.

Maybe here in the core of a neutron star, they're free to roam. Conditions in the universe not seen since the earliest moments of the big bang itself. The interior core of a neutron star may be a soup called a quark gluon plasma. Free quarks swimming around. And gluons, the carrier of the strong nuclear force, unfettered from the chains of an atomic nucleus, free to govern reactions across kilometers and kilometers of the inner core.

A state of matter so strange, so alien, sewn foreign to us, we don't even have the mathematical language to describe it, let alone the experimental expertise. This is beyond what we could even dream of recreating. It could be at these depths that different kinds of quarks become involved. Protons and neutrons are just made of up and down quarks. But there are six kinds.

There are four others top and bottom strange and charm. Usually these do not participate in common reactions, but it could be that strange quarks dominate the interior of a neutron star. There can even be collections of hundreds of strange quarks bound together called strangelets roaming around in this alien quark gluon plasma. The truth is we don't know what goes on in these strange alien depths. And there's only one way to find out.

So who's with me? Thanks to Lorenzo b, Tripp b, Paul w, and at 865 for the questions that led to today's episode. Thank you so much for all the questions. Keep them coming. Ask a spaceman at g mail dot com or go to the website askaspaceman.com.

You can ask questions through there. Thank you for all of your support, your good vibes, your your positive feedback on your favorite podcasting platform that really helps the show. Get out there and let other people know about it so that there are more questions, and then there are more episodes, and then more questions, and then more episodes, and we never stop doing this. And, of course, thank you to all the people who contribute on Patreon. That's patreon.com/pmsutter.

I'm grateful to every single one of you that fork over your hard earned money so that we can keep doing this show. I can't believe it. I'm very, very luckiest astrophysicist in the world. That's for sure. And I'd like to thank the top contributors this month.

They're Justin g, Chris l, Alberto m, Duncan m, Corey d, Michael p, Nyla, Sam r, John s, Joshua, Scott m, Rob h, Scott m, Louis m, John w, Alexis, Gilbert m, Rob w, Jessica m, Jules r, Jim l, David, s, Scott, r, Heather, Mike, s, Pete, h, Steve, s, Wot Wotberg, Lisa, R, Cousy, Kevin, b, Michael, b, Eileen, g, Dante, Steven, w, and Brian o. Keep those questions coming, and we'll see you next time for more complete knowledge of time and space.

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