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Neutron stars, as many of you probably know they are dead cores of Stars between 8-25 Solar masses mainly. We'll talk about that, too, why these mass ranges contain so many varieties. The neutron Star is constructed by the ejected neutrons of atoms or molecules.

Here are the Terms and Conditions to call stellar Cores neutron Stars:

1) Core's mass should be exceeded than 1.44 Solar masses (Chandrasekhar limit). This is not possible unless Star is more than 8 Solar Masses.

2) On the upper end, the core's mass should be under 2.9 Solar masses (Tolman-Oppenheimer-Volkoff Limit). Otherwise, crossing beyond this limit would turn the core into a black hole.

3) The outer layers explode into a Supernova while the Core compresses into a sphere almost entirely made of Neutrons due to extreme gravity.

4) Its Radius is about 10 -20 Kilometers while its Mass is still comparable to the sun.


There are some neutron stars which are know for planetary system. You can see them in SpaceEngine Some Pulsars like PSR B1257+12 (lich) and 1620+26


A view from PSR B1257+12's planets

So those are some main characteristics of neutron stars. Let's explore them in depth.

Since Most of us know that a Neutron Star is the dead core of a star with 8-25 Solar masses. If this star is a Population II Star, then it will need around 8-20 Solar masses due to its low metallicity. In this section, you'll know about what happens to the core and how gravity turns it into Neutronium Matter.

The Star with 8-25 Solar masses creates around 5 to 6 fusion zones around its core. Here's the quick Description for such stars.

The star will first fuse Hydrogen, but this hydrogen fusion is not enough to support the star against gravity's compression so it will need to find another quickly. Hydrogen Fusion creates Helium and when this helium becomes abundant in the central part of the core, it can be used as a fuel source to fight against gravity; otherwise, the core will shrink to the singularity, resulting in a black hole.

The higher the mass, the faster things happen, because this core is facing the compression of the Upper layers, which is already too massive, and where's the mass, there's the gravity. Hydrogen fusion starts very aggressively, many times higher than the sun's fusion mechanisms. The energy released from fusion tries to expand the core, and hence, gravity can't crush it easily. Hydrogen fusion creates helium, which soon after starts to burn and is used as a fuel as soon as it becomes abundant or gets enough temperature and density. It also depends on the star's mass due to its rate of gravitational compression.


Gravity working on Neutron stars around 1/1000 than on black hole, but both can bend the light

Now here comes the twist. Hydrogen fusion requires less energy and gives enormous energy output around 26 MeV (million electron volts), while Helium fusion gives around 7 MeV

Another twist is that stars with higher masses show a higher rate of particle ejections or stellar winds; they expel streams of Neutrinos, protons, electrons, and other subatomic particles in the universe. This could lower their mass and disqualify them from supernova candidates, and they lose their outer layers, which become a planetary nebula while the core compresses into a White dwarf rather than a neutron star.

So let's assume that in most cases Population I (current generation stars with higher metallicities) show more particle ejections or stronger stellar winds and lose more mass than Population II (older generation with less metallicities). So here 8 solar mass Pop I star can't become a supernova, mainly if it has optimum rotation, mass loss mechanisms, and conditions, then it will leave a neutron star. Otherwise, its core shrinks into a White dwarf size but with more mass than regular white dwarfs.

That's why it's uncertain that every star with 8 solar masses can explode into a supernova and leave a neutron star. So we start with 10 solar mass stars because it's not so weak, and if it loses mass through winds, then it will remain in the supernova candidate boundary.

a massive Star's fusion layers, just before Supernova

As you now know that a certain kind of stars can become supernova, it mustn't lose the mass less than 8 Solar masses otherwise it would do explode like a supernova. Still, it can't release enough energy and If it loses more mass before explosion, it won't blast and lose outer layers like Less massive stars and core would also become a White dwarf instead of a Neutron star.

Let's quickly see how such a star will enter such a violent condition and what happens to the core. It's all reason lies in the mass, which triggers an aggressive fusion due to brutal compression of gravity. Imagine a sun-like star mainly produces energy through PP chain, which takes almost 9 billion years to fuse two protons (ionized hydrogen) and convert into helium, while the CNO cycle, which powers the massive stars with more than 1.5 Solar masses. You can read about them in their links, so I'm not going into details.

Hydrogen Fusion creates Helium and gives the star almost 26 MeV, by either of those methods. Helium Fusion provides the star almost 7 MeV and creates Carbon and Oxygen. When appropriate conditions are met Star will get the following energy amounts from fusion reactions and Mind it this is true for a 12 solar Mass Star and if this mass or metallicity changes, then it may also change. Here's the list.



As you can see that the Iron Fusion is impossible and it doesn't give energy to the star instead it absorbs the energy, making the Star powerless and gravity wins, since it has no opposite force and Millions of years of battle between Gravity and Nuclear force end this hour as soon as Iron accumulates in the core.

Imagine Our sun like Stars and all other Main sequence Stars are supported by one Fusion zone while Massive Stars ignite different kinds of fusion layers one by one when they achieve appropriate conditions. in final time they burn 6 Fusion layers together but still struggle against gravity because all those elements were requiring densities and masses, which makes gravity unstoppable no matter how efficient those outer fusion layers are.

When a Massive Star with more than 8 (10 Ms is certain) Solar Masses collapses due to its own gravity and it releases tremendous energy is called a supernova. Our sun would release such energy in 100 years, while a supernova can do it in just one second. So they are one of the most energetic events in the cosmos. It's mostly related to the death of Massive Stars, whereas some other types of supernovae do not denote a stellar death, but that's out of the topic of this post.

So you know that the Star is drawing energy from 6 Fusion layers and trying to save itself from gravity, while Iron is impossible to fuse, so it has no choice but to yield to gravity. Now, the Star has enough mass and density to die.The futile iron core is ready as well. Now begins the Dance of Death.

All outer layers begin to collapse nearly at the speed of light. They become dense and hot enough. As the gravity squeezes the Star, the hydrogen around the core in outer regions begins to fuse but is immediately crushed by gravity. Since Star is not small either, it's a blue Giant or supergiant, so gravity could take almost 8 - 10 hours or more. depending on the star's size. The entire system becomes so small.

The core is also crushed by gravity it's all the fusion layers become ineffective and the core continues to shrink.

When the entire compressing system reaches near a critical point, the iron core is reached at a point called the Chandrasekhar Limit, about 1.4 solar mass. The entire outer layer's bottom part touches this iron core and this core rebounds outwards, creating a shockwave. This pierces the star's outer layers and causes a powerful explosion called a supernova.


A Supernova Remnant Nebula - Neutron Star is hidden in its core (Made in Blender)

The entire System explodes with incredible energy; it's all the outer layers, including the photosphere, the Radiative layers, Corona, torn apart. This remnant expands to several light-years and creates a nebula called the supernova remnant nebula.

When the Core is getting compressed, its upper fusion layers get destroyed, and all the atoms are mixed. Separated atoms once freely move but gravity compresses them brutally, and their free roaming space gets smaller every second. Now the electrons and protons merge and form a common Neutron due to their quantum level interactions. This releases a massive amount of Neutrinos, which escape from this compression zone and travel in space. When these Neutrinos move through Earth and interact with Neutrino detectors, our scientists can estimate that a star is dying and becoming a supernova, such detectors can alert 7-8 hours before the Star dies.

Now, the core, which was several million km large, shrinks to around 10-20 km. The result is ultra-high speed spinning, a dense sphere called a Neutron Star. Let's see first why Gravity can't compress them beyond this point?

Just as Gravity can't compress us due to our insufficient mass, our body is supported by chemical composition so unless we get enough mass, gravity can't compress us.

In Movies, you may have seen giant humans almost the size of a planet, which is impossible to get such a size for humans in real life. When we achieve around 600 km in size, Gravity will show its effects, and we'll become spheres like asteroids.

Even though the Neutron Star is safe from gravity's compression, gravity is not done yet. Instead, it is constantly trying to shrink but can't due to a pressure called Degeneracy pressure, which means 99% neutrons are left in the neutron star, and they have formed an ocean on he surface and their combined movement is exerting pressure outwards and this is called degeneracy pressure. It is degenerate because it is not gonna fuse or compress further. Like in a white dwarf, the electrons were the main ingredient of degenerate matter, but here, Neutrons are.

Both are called Degenerate Stars, and as of now, there are two real entities, White dwarf and Neutron star, while in theory, there are many degenerate matter ingredients like Degenerate Electrons, Protons, Neutrons, quarks, etc.

The degenerate Neutron ocean is mainly a kind of sea or pool of neutrons, hydrogen and other products of the CNO cycle. Let's see some of its properties, and we'll try to understand this matter.


Degenerate Matter, composed of neutrons, is a super smooth, frictionless, and it is quantum fluid. Meaning If you put this liquid in your glass, it will seep through the glass's wall and go deep down into the ground. It is super compressible and has zero viscosity. For those who were not science students, viscosity means that when any liquid or gas's particles move together, they rub against each other or experience a kind of friction, which slows down the overall fluid's flow.

Whereas, that Degenerate matter on earth is just a theory because if you go near a neutron star, it will immediately pull you with great speeds, and before hitting its 20 km pool, your body would disintegrate into many pieces, and not even a single disturbance would emerge due to extreme gravity, unlike if you throw a stone on a lake.

To see it, try to spill a teaspoon of water and honey or milk on a gently sloped surface. If your water and other fluids are clean and pure, they will flow at different speeds; it's due to viscosity. The more Viscous the liquid, the slower the flow. In degenerate matter of Neutron, viscosity is zero. (It might not be exactly zero, but very close to it; Neutron stars are nature's smoothest objects)

This Matter can only exist in neutron stars; otherwise, It will explode like a nuke, because it needs crushing gravity and without it, it would blow up.

However, earlier I said there are some other elements in this pool, so it might not be a perfect Superfluid. While the majority of Degenerate Neutron makes it super smooth and super fluid. It's not sparsely packed like earthly fluids; instead, this is super dense, it's one teaspoon that weighs about a billion tons.


When a Neutron star forms, some lucky protons make a special pair called Cooper pairs and this is the secret of the Neutron star's superconductivity. Mainly, the Protons form a conductive fluid, and the neutrons form the zero-viscosity fluid that we've discussed in the previous subsection.

In Magnetars, this conductivity is specialized, causing several failures and refinements of magnetic field lines. This leads to X-ray and gamma-ray bursts occasionally. while Pulsars have less specialized conductive proton pairings or magnetization than this one.

In the next sections, we'll see the two kinds of Neutron Stars.

A pulsar with its magnetic field

A pulsar is usually a rapidly spinning neutron star, which transforms rotational energy into Radiation energy through its intense magnetic field and super-fast spin.

This is a very common group of Neutron Stars, they are also among the fastest spinning bodies in our universe. The young or newly formed pulsar glows due to its heat, much like White dwarfs, which should decrease over time but its magnetic properties can't let it cool. Let's see why.

In this section, we're not going into their types; instead, we'll see some of their basic properties, which make a pulsar a pulsar.


Inner structures of a Pulsar


The spheroid region around a Neutron star where its Magnetic lines show activities. its magnetic lines have two types, open and close (in the animation, there is only closed (looping) lines have shown). The open lines direct the particle outflow from the star, while closed lines go through the core and can extend to several km in a closed loop. Its main job is to accelerate particles and produce Gamma rays.

The magnetosphere helps to produce powerful electromagnetic radiation by accelerating particles, and it can trigger a spin-down glitch by altering the particle behavior in the Neutron star or its parts.


This is a solid lattice of Heavy nuclei like Iron, surrounded by electron degenerate matter. There are structures like coulomb crystal, which is a lattice of Ions, it is highly electrically and thermally conductive, thanks to the degenerate electrons.

This layer controls the magnetic field decay and local heating. Magnetic lines can extract the electrons and ions to power the magnetic field from this layer. Since this region hosts powerful heat-conductive properties, it regulates the Surface temperature and emits thermal X-rays.


This is represented as a sea of free-floating neutrons and electrons. Around its boundary of inner crust-outer core, the nuclear matter can create various shapes like pasta, rod, slab, tubes, etc. This is a Superfluid that can carry angular momentum and speeds up the Neutron Star called spin-up glitch

This region also regulates the temperature by cooling down the local area.


This is also a liquid composed of Superfluid Neutrons, Degenerate electrons, and superconducting Protons. This layer mediates the overall spin and after-glitch relaxation processes.


This is a mystery in current neutron star physics. Many theories suggest it could be made of degenerate quarks, hyperons, pions or kaons but it's still uncertain now.

Magnetar - Simplified magnetic lines show its field deformity

This is another major group of Neutron Stars. Typically, a Magnetar has magnetic fields approximately 1000 times more powerful than those of Common Pulsars, about 10¹⁵ Gauss. They rotate slower, too.

In the animation above, you see a magnetar with simplified and clean yet little bit distorted lines while in reality they tend to be much more twisted. it shows multiple anomalies, tensions and interaction which is rapidly recovered.

Magnetars emit light due to the magnetic line reconnection process, in which, if any irregularity occurs in the magnetic field, other lines can merge, repair, or join each other. In magnetars, Such an anomaly in the magnetic field is far more frequent and its rapid arrangement of magnetic lines allows it to emit Gamma rays and X-rays through its surface fractures.


The magnetosphere around magnetar is a structure made with extremely twisted and clustered magnetic lines. These magnetic lines are imaginary but it help us to estimate their strength and magnetic configuration. When these lines rearrange, they emit massive amount of radiation since everything is linked together so Neutron star's inner structure drives magnetosphere, magnetosphere shapes the Neutron Star.


A Magnetar with internal structures, Compare it with Pulsar


This is similar to pulsars, a magnetar's surface also consists of degenerate electrons but they feel heavy magnetic tension and sometimes their surface cracks or bursts. Hence, their surface is more distorted. If any point of the surface cracks or distorts, it would be a plastic distortion, meaning it can keep deforming to an extreme and can't be restored. All these distortions are a result of an extremely complex magnetic field and its interaction. These fractures emit massive flares and energy bursts.



This layer is a liquid mix of Degenerate Electron and neutron; it is more magnetically distorted than the pulsar counterpart. The Base of this layer contains Nuclear pasta which is a phase of quantum particles. Magnetic deformations are stored in the inner crust, which sometimes transfers it to upper layer then Radiation flare occurs when Magnetic lines reconnect. The fractures, cracks, flares and Magnetic lines phenomena are connected. like Solar flares and their magnetic line Relationship.


this zone is dominated by Superfluid Neutrons, Superconducting Protons and some electrons. The interactions of proton and neutrons causes the changes and stresses in the magnetosphere. This layer is also responsible for Flares and long term emissions.


This is hypothesized to be made of exotic matter consisting Hyperons, mesons or quarks etc. Its main functions are uncertain as of now