In 1937  Pyotr Kapitsa and John F. Allen discovered a strange behavior of ultracold liquids known as superfluidity. A superfluid is a fluid with no viscosity, basically a frictionless liquid. Without viscosity, the fluid has no way to dampen its motion. Because of this, superfluids have some pretty unusual behaviors. If a bit of superfluid is suspended in an open container, it will creep up along the walls, then drip down to a lower container. It can flow through tiny pores that regular liquids can’t, and can create fountains that could flow forever. This seeming defiance of gravity and common sense is due to the fact that its behavior is rooted in quantum physics. Though it is not a truly quantum state such as a Bose-Einstein condensate, it shares some commonality with it. In the lab, superfluids are only seen at temperatures barely above absolute zero. The most common example, helium-4, becomes superfluid when cooled below 2.17 K. So it might seem odd that superfluids are also found in the hot interiors of neutron stars. 

A neutron star is a stellar remnant formed with a star runs out of hydrogen and heavier elements to fuse. After a star explodes as a supernova, the remaining core of the star collapses under its own weight to the point that only the pressure of nuclei can counter the force of gravity. A neutron star has a mass of about two Suns, but are only about 20 kilometers in diameter. They have a dense atmosphere of carbon only a few centimeters thick, and a thin crust of iron nuclei. In the interior of a neutron star, nuclei are pushed together ever more tightly, and reach a point where the nuclei can’t hold themselves together. As a result, individual neutrons “drip” out, and sink into the star’s core, forming a neutron fluid. As a neutron star cools, this neutron fluid transitions to a superfluid state. This happens not at a few degrees Kelvin, but at 500 to 800 million Kelvin. The interior of a neutron star is a hot superfluid sea.

When the interior transitions to a superfluid, the temperature of the star’s surface can drop dramatically, as has been seen in the neutron star of Cassiopeia A. Once the interior is superfluid, it can significantly affect the star’s behavior. Neutron stars have strong magnetic fields, and coupled with their fast rotation they radiate intense radio energy (which we can sometimes see as pulsars). Because the crust of a neutron star is magnetically pinned, its rotation gradually slows down, but the superfluid interior has no viscosity, and doesn’t slow down.  Over time this means the interior can rotate much faster than the outer crust. Eventually the difference between the two becomes so great that some of the rotation is transferred to the crust, causing it to speed up quickly. In pulsars this is seen as a glitch in the rate of its radio pulses. These glitches can be used to determine the mass of a neutron star.

Perhaps what’s most amazing about all this is how two radically different materials can share such similar properties. Ultracold helium and superhot neutrons both acting as a strange quantum fluid.

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Yesterday I talked about millisecond pulsars, and the way in which they might gain such rapid rotation. Another property of millisecond pulsars is that they demonstrate very clearly that pulsars are neutron stars. It all has to do with their rapid rotation and the physics of centripetal (or centrifugal) force.

When you stand on the Earth, there are two basic forces acting on you. The first is gravity, where the attraction of Earth’s mass tries to pull you to the center of our planet. The second is the ground, which prevents you from moving through it by pushing up on you. This is often called the normal force. Standing on the ground, these two forces are in balance. The weight of gravity is countered by the normal force of the ground, which is why you can stay put.

If the Earth weren’t rotating, then these two forces would be equal in magnitude. But since the Earth rotates, the normal force is very slightly smaller than the attraction of gravity. Just how much smaller depends on where you are on the Earth, but it is smallest if you are standing on the equator. You can see why that is if you’ve ever been in a car going around a corner. As the car turns, it feels like you are being pulled slightly outward, away from the turning car. This is sometimes called a centrifugal force, and it has to do with the fact that your body (like any object with mass) would like to keep moving in a straight line at a constant speed. To change your direction (and keep you in your seat) the car has to push you in the direction of the turn. Whenever you are pushed in some direction, it feels like you are being pulled in the opposite direction, so it feels like you are being pulled outward. A similar thing occurs when your car accelerates, and it feels like you are being pulled back into your seat.

As you stand upon our rotating planet, a similar thing occurs. You are moving around in a circle, once per day, so your direction is always changing. Your body would like to keep moving in a straight line, but Earth’s gravity keeps changing your direction. Earth’s gravity is more than strong enough to overcome your centrifugal force, so the normal force of the ground also acts to keep you on the ground. But the normal force doesn’t have to counteract all of gravity, just the extra gravity beyond the centrifugal force.

Diagram of a pulsar Source: NRAO

Now suppose we could spin our spherical world faster and faster. As we are standing on its surface, we would have a tendency to fly off the world due to its rotation. The gravity is stronger than the centrifugal force, so we stay put. But if we could spin the world faster and faster, we would reach a point where the centrifugal force is equal to the force of gravity. Spin the world any faster, and we would fly off. Earth’s gravity wouldn’t be strong enough to hold us. The surface gravity of a planet or star depends upon its density, so far a given density for a planet/star, there is a maximum rate of rotation. Any faster and it would fly apart. Put another way, if we measure the rotation of a planet or star, we know its minimum density.

When we do the math, we find we can calculate that minimum density in grams per cubic centimeter by taking 140 million and dividing it by the square of an object’s period in seconds. So for the Earth, the period is 24 hours or 86,400 seconds. Plug that into our equation and we get a minimum density of about 0.02 g/cc. The Earth’s real density is about 5 g/cc, well above that minimum.

Pulsars are kind of like cosmic light houses. As they rotate they sweep out a beam of radio waves from their magnetic poles. If those poles are pointing in our direction, we can hear each rotation as a pulse. The fastest millisecond pulsar has a rotation of 716 times a second, or a rotational period of 1.4 milliseconds. Plug that into our equation, and you get a minimum density of about 7 x 10¹³ g/cc. Thats that’s about 7 billion kilograms per sugar cube volume, which is an unimaginably high density. It also happens to be a bit less than the density of atomic nuclei.

So pulsars are as dense as atomic nuclei, only several kilometers in diameter. We call them neutron stars.

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In the 1990s the ROSAT x-ray observatory made an all-sky survey. By 2001, seven soft x-ray sources were found from the survey data, and shown to be neutron stars. They came to be known as the magnificent seven.

The magnificent seven differ from most neutron stars in the way they appear to us. Most of the neutron stars we observe are either young and hot, meaning they are sources of hard x-rays, or they are pulsars, meaning that their magnetic fields are aligned in such a way that it sweeps a beam of intense energy in our direction like a lighthouse.

The seven are cooler than other neutron stars. While young neutron stars can have surface temperatures of more than a million Kelvin, the seven have temperatures of 500,000 to 800,000 Kelvin, and they lack the hard x-ray spectrum of other neutron stars. Neutron stars cool over time, so by their temperatures they are middle-aged, being about 100,000 years old or so.

The light they give off follows a blackbody pattern, which is how we know their temperatures. This also means we can calculate something called the flux density. The flux density is a measure of the amount of light an object gives off per unit area. If you know the flux density and size of an object, then you can calculate its brightness, or absolute magnitude. This is useful because we can also measure how bright an object appears (apparent magnitude), and by comparing the two we can calculate the object’s distance.

The difficulty is that we don’t know the precise size of these neutron stars. From what we know of neutron stars in general they are likely between 12 and 20 kilometers in diameter, but it’s hard to be more precise. From their apparent brightness, this means they are between 500 to 1500 light years away, which is fairly close.

Ideally it would be nice to have an alternative distance measurement to compare with. For stars in this distance range we could use the method of parallax, which is similar to the depth perception of human vision. By measuring the position of a star at different times of the year (when the Earth has moved from one side of its orbit to the other), we can observe the apparent shift of the star relative to more distant stars. With a bit of trigonometry we can accurately determine a star’s distance. But these neutron stars don’t give off much visible light, so that trick doesn’t work with them.

Still, we do have some indirect evidence that our distance estimates are reasonable. Stars are not fixed in space, but instead are moving relative to each other. For distant stars this motion is too small to observe directly, but close stars exhibit this as proper motion, or the gradual changing of position relative to other stars. Large proper motion is an indication that a star is relatively close. We’ve observed such a proper motion with some of these neutron stars, as seen in the image above. This shift in only a few years is a good indication of the closeness of the neutron stars.

Because of their closeness and age, the magnificent seven provide an opportunity to learn more about the structure of neutron stars. In particular, further observation should provide insight on the equation of state of a neutron star’s interior, which would help us understand the interactions of dense, high-temperature nucleons.

Learning more about that would be pretty magnificent indeed.

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A neutron star is the remnant of a large supernova. When a large star explodes, a remnant of its core is compressed so tightly that the electrons are squeezed into protons, resulting in a mass of neutrons. A neutron star typically has a mass of about 2 solar masses, but it is only about 12 kilometers in diameter. Imagine taking two suns and squeeze it into the size of a small city, and you get the idea of how incredibly dense these objects are.

Neutron stars are often represented as a simple mass of neutrons, but we know that they actually have a complex structure. Just as the Earth has a crust, mantle and core, a neutron star has an iron crust, neutron mantle and core. We know from variations the rotation rate of pulsars that neutron stars undergo “starquakes”, that they likely have mountains (assuming you can call a rift a couple meters high a mountain range). They are geologically active. We also know they have atmospheres.

Just how we observe the atmosphere of a 12 kilometer wide neutron star light years away is pretty interesting. The surface temperature of a young neutron star is about a million Kelvin. This means it radiates a significant amount of light in the x-ray spectrum. If a neutron star had no atmosphere, then light should follow a distribution known as the blackbody curve. In other words, with no atmosphere the x-ray light should depend only on the surface temperature of the neutron star. But if it has an atmosphere, then the atmosphere will absorb some of the light and emit light at a different wavelength. So the x-ray spectrum would measurably differ from a blackbody spectrum. Just how it differs would depend upon the makeup and thickness of the atmosphere.

In 2009, Wynn Ho and Craig Heinke published a paper analyzing the x-ray spectrum of a neutron star known as Cassiopeia A. (Actually, Cassiopeia A is the name of the supernova remnant where the neutron star resides.) This is a young neutron star about 11,000 light years away. They found that the x-ray emission did not match a blackbody spectrum well, so they compared the spectrum to various model atmospheres such as pure hydrogen, helium, carbon, nitrogen, and oxygen. They found the best match to be a carbon atmosphere.

This carbon atmosphere isn’t like anything we’ve experienced. It’s only about 4 centimeters thick, and while it is “gaseous”, its density is about the same as diamond (3.5 grams/cc). Over time this atmosphere is expected to change. The gravity is so intense that carbon would eventually settle out of the atmosphere while lighter elements such as hydrogen and helium accumulate. Eventually even the lighter elements would settle to the surface.

But Cassiopeia A is still young, so for now it can enjoy its diamond sky.

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Yesterday I talked about quark stars. These are hypothetical stars similar to neutron stars, but smaller and a bit more massive. Their gravity and density would be enough that instead of being largely made of neutrons (which are made of up and down quarks), they would be made of free up, down and strange quarks. If such quark stars exist, they would lie between neutron stars and solar mass black holes.

But how can we tell a quark star from a neutron star? If it has reached a stable state, then a quark star should be smaller than a neutron star, say 10 – 12 miles in diameter versus 15 – 20 miles in diameter, as you can see in the figure below. This is a difference of only a few miles. Given that these stars are light years away, making measurement of a star’s size precise enough to tell them apart is a huge challenge. It would help if we had a complete equation of state for a quark star so that we could compare things like size and temperature, but we don’t. So it can be very difficult to distinguish a quark star from a neutron star. That’s assuming quark stars exist, which we aren’t sure is the case.

Credit: CXC/M. Weiss

In 1992 a possible quark star was discovered, named RX J1856.5-3754 (great name, eh?) and it’s measured diameter was about 12 miles, which would make it too small to be a neutron star. But this was assuming it was about 200 light years away, and it was later determined to be about 400 light years away. As a result it is closer to 17 miles in diameter, which makes it too big to be a quark star. Measurements of its temperature also confirm it is a neutron star. This happens to be one of the closest neutron stars to Earth, and even then it took a great deal of effort to pin down its size. So it isn’t very likely that we will be able to distinguish quark stars from neutron stars by their size. At least not with our current instruments.

There may be another way to distinguish them. Neutron stars are quite hot when they form, with surface temperatures of several hundred thousand Kelvin. Since they don’t have any way to generate heat, they gradually cool over time. A neutron star massing 2 – 3 solar masses will eventually cool and shrink slightly until it reaches a stable state where the neutron pressure balances its gravity. But a more massive neutron star would cool and shrink until it reaches a critical point. If it is too massive to stabilize as a neutron star, it will reach a point where the up and down quarks can collide hard enough to become strange quarks. This would induce a phase change from neutron matter to strange matter (a mixture of up, down and strange quarks) within a few milliseconds.

This phase change would also produce large quantities of neutrinos. The quark star would be so dense that the neutrinos couldn’t escape all at once. But they could escape faster than photons. This means that after the phase change a quark star would cool faster than a neutron star. Interestingly, we’ve seen just this effect in a star known as 3C 58. This object appears to be a neutron star, but is cooling faster than expected. It’s possible then that 3C 58 is a quark star.

Of course to know for sure, we need to pin down its size and other properties. It might just be an unusual neutron star. So 3C 58 is a candidate quark star, not a confirmed one. There are a handful of other candidates as well.

At the moment, though, there is no confirmed quark star. They remain a hypothetical possibility.

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Imagine a star twice as massive as the Sun, compressed to the size of a city. All that matter squeezed into a sphere about 15 miles wide. Such an object is known as a neutron star. The matter of a neutron star is so dense and its gravity is so strong that atoms cannot support themselves. Instead they collapse, with the electrons being squeezed into the nuclei until what remains is a mass of neutrons. Hence the name.

The exact structure of a neutron star is still a bit of a mystery. At a broad level, they are just large balls of neutrons, but they have strong gravitational and magnetic fields that can affect their structure, and then there are the neutrons themselves. How do we understand “stuff” made out of neutrons, when what we see around us are made of atoms? The key is to determine its equation of state.

The equation of state of a material determines its bulk properties. If you’ve had a chemistry class you are likely familiar with the ideal gas law, which describes how the pressure, temperature and volume of a gas are related. The ideal gas law is an equation of state for a simple gas.

We have observed the gas law experimentally, but we can also derive its properties mathematically using kinetic theory. Since a gas tends to be relatively low density, we can treat the atoms or molecules almost as billiard balls that bounce against each other. Modelling a gas in this way gives us the ideal gas law.

Equations of state are the key to understanding stars and the like, because stars are a balance between the weight of the star’s matter and its pressure. Knowing the equation of state for a star’s material means we can calculate things such as size and temperature. Likewise if we know a stars size and temperature we can gain an understanding of the equation of state.

Neutrons (and electrons) obey a different equation of state from that of a simple gas. It is derived from Fermi-Dirac statistics, which takes into account what is known as quantum degeneracy pressure. If you are familiar with the Pauli exclusion principle, which states that no two electrons can occupy the same state, then you are familiar with one example of degeneracy pressure. The basic idea of degeneracy pressure is that there is a minimum volume into which you can squeeze a neutron. Beyond that limit the degeneracy pressure gets too large for you to overcome.

This doesn’t mean you can never overcome that pressure. You can overcome the degeneracy pressure if you have enough mass. Based on the neutron equation of state that limit is a bit more than 3 solar masses. Anything bigger than that, and the neutron star is so massive it collapses into a black hole. A similar thing occurs with white dwarfs. In a white dwarf star the weight of its mass is balanced by the degeneracy pressure of the electrons. As long as the white dwarf is less than 1.4 solar masses (the Chandrasekhar limit), then everything is fine. But if the white dwarf has more mass, then it will collapse into a neutron star.

There is, however, one important difference. Electrons are elementary particles, which means they aren’t made of anything smaller. So when a white dwarf is more massive than the Chandrasekhar limit it can’t do anything but collapse into a neutron star. But neutrons aren’t elementary particles. They are made up of a trio of quarks. So when if the mass of a neutron star is close to the neutron limit, then it might not collapse into a black hole. It might collapse into a quark star.

In a quark star, instead of being clumped into neutrons the quarks would move freely, making the star a mass of quarks, as you can see in the figure above. Of course this means that a quark star needs an entirely different equation of state. This is further complicated by the fact that the interior of quark stars would have so much heat and energy that some of the neutron quarks (up and down quarks) could transform into strange quarks. (Not strange in that they are weird, but strange because they are named strange quarks). The interior of a quark star could then be strange matter, and we don’t know exactly what its equation of state would be.

We don’t have a solid theoretical understanding of quark matter, but most models predict that quark stars would be slightly smaller than neutron stars. The one catch is that the largest quark stars could be as large as the smallest neutron stars, so it might be difficult to distinguish them. There have been a few observations of stars that might be quark stars, but so far there are no confirmed quark stars.

Then again it might be the case that quark stars are unstable. This would mean that large neutron stars might enter a short quark star period before collapsing into a black hole, but there would never be a long-lived quark star. We aren’t really sure at this point.

In the end it will all come down to their equation of state.

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Neutron stars typically form when a large star dies in a supernova explosion. The outer layers of the star are cast outward to form a supernova remnant, while the core of the star collapses into a dense neutron star.  What keeps a neutron star from collapsing under its own weight is the pressure of the neutrons pushing against each other, similar to the way electron pressure works in a white dwarf star. But there is a limit to how much weight the neutrons can counter, known as the Tolman-Oppenheimer-Volkoff (TOV) limit.  This limit means that a neutron star can’t be more massive than about three solar masses.  More than that, and it would collapse into a black hole.  

Because of this limit, it’s generally thought that the supernovae of most large stars produce neutron stars, but the supernovae of really large stars can produce black holes.  But a recent paper in Astronomy and Astrophysics proposes that some really large stars might produce a neutron star that is more massive than the TOV limit, which only collapses into a black hole later.

In the paper, the authors propose that if the progenitor star is rapidly rotating, the resulting neutron star it produces could also be spinning rapidly. Since the TOV limit is for a non-rotating (or slowly rotating) neutron star, a fast-rotating neutron star could be over the limit. Basically, the rapid rotation would cause the neutron star to bulge out a bit, preventing it from collapsing into a black hole.  Of course neutron stars have strong magnetic fields, and this means that they radiate electromagnetic energy as they rotate, which causes them slow down over time.  So eventually these supermassive neutron stars will slow down enough that they collapse into a black hole.

The Lorimer burst, first observed FRB.

What’s interesting about this idea is that it could explain a mysterious phenomena known as fast radio bursts, or FRBs. These are bursts of radio energy that last for a fraction of a second, but are extraordinarily bright.  They don’t repeat like radio pulsars, but instead just occur as a single burst. They appear similar to gamma ray bursts, but don’t seem to have a corresponding burst of gamma rays, or even x-rays.

Neutron stars have a strong magnetic field, but uncharged black holes don’t have magnetic fields (known as the no-hair theorem). So when a supermassive neutron star (with a strong magnetic field) slows enough to collapse into a black hole (which can’t have a magnetic field) the magnetic field must snap free.  This would produce a large burst of radio energy without the burst of x-rays and gamma rays that occur when a supernova-produced black hole forms.  The authors have named such objects blitzars.

So blitzars could explain these FRBs.  To be sure, we’ll need to analyze the spectra in more detail. Right now we only have a handful of FRB observations, which isn’t enough to confirm them as FRBs. There are also other possible solutions to the mystery, such as massive stellar flares, and binary white dwarf or neutron star mergers.

But it could be that these strange radio burst are simply the result a neutron star that has been over the limit.

Paper: Heino Falcke1 and Luciano Rezzolla. Fast radio bursts: the last sign of supramassive neutron stars. A&A 562, A137 (2014).

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A magnetar is a neutron star with an extremely strong magnetic field, a billion times stronger than the strongest fields we can create on Earth.  As a neutron star, magnetars also have very strong gravitational fields, with a surface gravity a hundred billion times that of Earth.  Such a high gravity would seem to ensure that a magnetar is spherical, but a magnetar’s strong gravitational field could distort the star, making it more of an oblate spheroid.  We’ve suspected that such a magnetic distortion could occur with magnetars, but now a research team seems to have found an example of this phenomenon.

The result was published in Physical Review Letters, and analyzes data from a magnetar known as 4u 0142+61, which is also a pulsar.  As you might remember, pulsars are neutron stars that emit strong energy pulses in our direction as they rotate. Because these pulses are of very short duration, we can make very accurate measurements of their timing.  This allows us to measure the rotation of a pulsar with great precision.  The measurement is so precise that we can observe their how rotational periods gradually lengthen as the neutron star loses rotational energy.  We can even observe small glitches in their rotation due to starquakes.

The precession of the magnetar due to its oblate shape. Credit: K. Makishima, et al.

Normally the rotational period of a pulsar will gradually lengthen, with the occasional glitch that causes a sudden shortening of its period, only to continue its gradual lengthening.  But in the case of 4u 0142+6, the authors discovered something rather odd. They measured the pulses of x-rays from the magnetar, and found that sometimes the pulses arrived a bit earlier than expected, and at other times a bit later.  It appeared that the rotational period had a slight oscillation to it. This shouldn’t be possible for a spherical neutron star, but it is possible for an oblate neutron star. That’s because an oblate spheroid precesses as it rotates.

Precession is the effect you see with a spinning top, where it wobbles as it spins on its axis.  We see the effect with the Earth because it is an oblate spheroid, which is why the north star of today is not the same as the north star thousands of years ago. The magnetar 4u 0142+6 seems to wobble in a similar way, which is why its pulses are sometimes sooner or later than expected.  For the Earth, the precessional period is about 26,000 years.  For  4u 0142+6, it is about 15 hours.  This means the magnetar differs from a sphere by about 1 part in 5000. When the authors calculated the magnetic field strength necessary for such a deviation, they found that it was about a trillion tesla.  That is higher than most magnetars, but within a reasonable range.

So it seems that this particular magnetar is indeed warped by its own magnetic field, which gives it is weeble wobble motion.

Paper: K. Makishima, et al. Possible Evidence for Free Precession of a Strongly Magnetized Neutron Star in the Magnetar 4U 0142+61. Phys. Rev. Lett. 112, 171102 (2014).

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This post originally appeared at Universe Today.

You may have heard that CERN announced the discovery (confirmation, actually. See addendum below.) of a strange particle known as Z(4430).  A paper summarizing the results has been published on the physics arxiv, which is a repository for preprint (not yet peer reviewed) physics papers.  The new particle is about 4 times more massive than a proton, has a negative charge, and appears to be a theoretical particle known as a tetraquark.  The results are still young, but if this discovery holds up it could have implications for our understanding of neutron stars.

A periodic table of elementary particles.
Credit: Wikipedia.

The building blocks of matter are made of leptons (such as the electron and neutrinos) and quarks (which make up protons, neutrons, and other particles).  Quarks are very different from other particles in that they have an electric charge that is 1/3 or 2/3 that of the electron and proton.  They also possess a different kind of “charge” known as color.  Just as electric charges interact through an electromagnetic force, color charges interact through the strong nuclear force.  It is the color charge of quarks that works to hold the nuclei of atoms together. Color charge is much more complex than electric charge.  With electric charge there is simply positive (+) and its opposite, negative (-).  With color, there are three types (red, green, and blue) and their opposites (anti-red, anti-green, and anti-blue).

Because of the way the strong force works, we can never observe a free quark.  The strong force requires that quarks always group together to form a particle that is color neutral. For example, a proton consists of three quarks (two up and one down), where each quark is a different color.  With visible light, adding red, green and blue light gives you white light, which is colorless. In the same way, combining a red, green and blue quark gives you a particle which is color neutral.  This similarity to the color properties of light is why quark charge is named after colors.

Combining a quark of each color into groups of three is one way to create a color neutral particle, and these are known as baryons.  Protons and neutrons are the most common baryons.  Another way to combine quarks is to pair a quark of a particular color with a quark of its anti-color.  For example, a green quark and an anti-green quark could combine to form a color neutral particle.  These two-quark particles are known as mesons, and were first discovered in 1947.  For example, the positively charged pion consists of an up quark and an antiparticle down quark.

Under the rules of the strong force, there are other ways quarks could combine to form a neutral particle.  One of these, the tetraquark, combines four quarks, where two particles have a particular color and the other two have the corresponding anti-colors.  Others, such as the pentaquark (3 colors + a color anti-color pair) and the hexaquark (3 colors + 3 anti-colors) have been proposed.  But so far all of these have been hypothetical.  While such particles would be color neutral, it is also possible that they aren’t stable and would simply decay into baryons and mesons.

There has been some experimental hints of tetraquarks, but this latest result is the strongest evidence of 4 quarks forming a color neutral particle.  This means that quarks can combine in much more complex ways than we originally expected, and this has implications for the internal structure of neutron stars.

Very simply, the traditional model of a neutron star is that it is made of neutrons.  Neutrons consist of three quarks (two down and one up), but it is generally thought that particle interactions within a neutron star are interactions between neutrons.  With the existence of tetraquarks, it is possible for neutrons within the core to interact strongly enough to create tetraquarks.  This could even lead to the production of pentaquarks and hexaquarks, or even that quarks could interact individually without being bound into color neutral particles.  This would produce a hypothetical object known as a quark star.

This is all hypothetical at this point, but verified evidence of tetraquarks will force astrophysicists to reexamine some the assumptions we have about the interiors of neutron stars.

Addendum: It has been pointed out that CERN’s results are not an original discovery, but rather a confirmation of earlier results by the Belle Collaboration.  The Belle results can be found in a 2008 paper in Physical Review Letters, as well as a 2013 paper in Physical Review D.  So credit where credit is due.

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A neutron star is the remnant of a large supernova.  When a large star explodes, a remnant of its core is compressed so tightly that the electrons are squeezed into protons, resulting in a mass of neutrons.  A neutron star typically has a mass of about 2 solar masses, but it is only about 12 kilometers in diameter.  Imagine taking two suns and squeeze it into the size of a small city, and you get the idea of how incredibly dense these objects are.  Given their extraordinarily high density and gravity, you might think there is no way a neutron star can be geologically active.  But in fact we know that they are active, and even prone to “starquakes”.

We know that starquakes occur by observing a type of neutron star known as a pulsar.  Since neutron stars have strong magnetic fields, they generate intense x-ray beams that stream from the magnetic polar regions.  If this beam is aligned so that it can point in our direction we see it flash regularly, with a flash for every rotation of the pulsar.  You can think of a pulsar as a kind of cosmic lighthouse, if you will.  Since the rate at which a pulsar flashes is determined by the rate at which it rotates, if you see a pulsar flash 10 times a second, you know it rotates once every tenth of a second.  (It can be a bit more complicated than that, but you get the idea).

The rotation of a pulsar is usually pretty regular.  Pulsars can gradually slow down over thousands or millions of years due to its radiated energy, but this is a gradual process. Occasionally, however, a pulsar will speed up just a bit in a very short time (on the order of minutes).  This rapid speed-up is known as a glitch.  After a glitch the pulsar will return to its previous speed within weeks or months, and then continue with its gradual, thousands-year slowdown.

It is generally thought that these glitches are due to changes in the shape of the neutron star.  Because of their rotation, neutron stars should bulge a bit at their equator.  The faster their rotation, the greater the bulge.  As a neutron star gradually slows down, its equatorial bulge would tend to decrease.  But it’s thought that the neutron matter in the crust of the star is fairly rigid.  This means that as the pulsar gradually slows down, stresses would build up in the pulsar’s crust.  This would eventually reach a breaking point.  The crust would then collapse to form a more stable, less bulgy shape.  Because of this new shape, the pulsar would speed up a bit, just like a spinning figure skater who spins faster when she pulls her arms inward.

Smaller glitches can also occur when the rotation of a pulsar shifts between layers.  A neutron star has an outer crust about a kilometer thick, with a superfluid neutron interior.  As the neutron star slows down, its crust slows at a higher rate than its core. When the rotations of the crust and interior differ by too much, some of the rotational momentum of the core is transferred to the crust, causing it to speed up slightly.  The result is a minor glitch in a pulsar’s period.

Because we can measure a pulsar’s rotation is so precisely, we can actually see the effects of interior and crustal activity.  Which is pretty cool when you realize that it is an object light years away and about the size of Deimos, the smaller moon of Mars.

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If you’ve ever played Minecraft, you know that most things are made out of blocks that are supposedly a cubic meter each. Steve, the protagonist of the game, has 36 holding spaces, each of which can hold 64 blocks. So apparently Steve can hold 2,300 cubic meters of material while running, jumping and climbing around the world. Basically that’s a cube a bit more than 13 meters on a side. Steve doesn’t even carry a backpack, so somehow he must be carrying it in a special pocket which can compress matter to a small volume.

But it gets better. The heaviest block in Minecraft would be one of solid gold. Since gold has a density of 19,300 kg per cubic meter, that means he can carry more than 44 million kilograms in his pocket. If you figure his pocket is only about 10 cm on a side, that means the density of Steve’s magic pocket is about 44 billion kilograms per cubic meter. That’s roughly as dense as the upper layers of a neutron star.

Something to think about when you are deep in a mine and have to drop something to pick up that diamond you find.

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