Title image: ‘Neutron star’ by Kevin M. Gill. Licensed under CC.

## Stellar Evolution

Figure 1: A flow chart of the life cycle of a star [1]

### Birth and Aging of Main Sequence Stars:

Stellar evolution (the life cycle of a star) begins with the gravitational collapse of a stellar nebula (giant molecular cloud). Eventually, this stellar nebula forms a main sequence star. These main sequence stars exist because of a fragile balance. The force of gravity pulls in trillions of tons of hot plasma inwards, squeezing material together with so much force that hydrogen fuses into helium. This process of fusion releases energy which counteracts gravity and tries to escape. Stars remain stable as long as an equilibrium exists between these two fundamental forces. [2][3] In the end though, the hydrogen is exhausted. Lower-mass stars, like our sun, turn into red giants. They burn helium into carbon and oxygen before gradually turning into white dwarfs after becoming a planetary nebula. [4]

Figures 2 and 3: Balance of fusion and gravity in a star; Our Sun [5]

### Star Death-Supernovae:

On the other hand, when stars are many times the mass of our sun, things get interesting when the fuel runs out. Depending on their size, they either end up as neutron stars or black holes. Before this though, the stars go through a supernova. Once the primary fuel in the star has been consumed, gravity temporarily overpowers fusion and squeezes in on the star. As a result, the core burns faster and hotter, making the outer layers of the star swell by orders of magnitude. This fuses heavier and heavier elements. [6][7] Carbon burns to neon in centuries, neon to oxygen in a year, oxygen to silicon in months, and silicon to iron in a day. Once all the material has turned into iron, fusion suddenly stops: iron cannot be fused and so produces no further energy. [8][9] Suddenly, without the balancing force of fusion, the core of the star implodes in on itself as it’s crushed by the enormous weight of the star above it. The pressure of the star is so powerful that a lot of electrons and protons fuse into neutrons. The resulting nucleons get squeezed as tightly as in atomic nuclei. To visualise this, picture an iron ball, the size of the earth, being squeezed into an extraordinarily dense ball the size of a city. [10][11] Meanwhile, while this is happening, the outer layers of the star implode, with gravity pulling them in at 25% the speed of light. This implosion rushes inwards until it meets the tremendously dense iron core. A shock wave is produced that explodes outwards and catapults the rest of the star into space. This is what we call a supernova explosion. Such explosions outshine entire galaxies and are one of the most violent things that happen in nature. [12][13]

Figure 4: Image obtained by Hubble Telescope shows a supernova at the centre of a galaxy [14]

### Star Remnants:

If a red supergiant is big enough, it turns into a black hole. Black holes are regions of space-time where gravity is so powerful that not even light can escape once it passes the event horizon, the point of no return, so to speak. [15]

On the other hand, if a red supergiant doesn’t reach the critical mass to form a black hole, it instead turns into a neutron star. Black holes may be extreme, but neutron stars are one of the most unusual things in the universe. They’re giant atomic nuclei, a few kilometres in diameter, but they contain enough material to form stars (which, for context, can have diameters in the millions of kilometres). They have really strange properties and come in a few different variations. [16]

Figures 5 and 6: First ever black hole captured on image [17]; Simulated view of a neutron star [18]

## Neutron Star Properties and Structure

### Neutron Star Properties:

A neutron star’s mass is about a million times greater than the mass of the earth and yet it’s compressed to an object about 20 kilometres in diameter. To give a sense of its density, imagine one billion tons of mass, the mass of Mount Everest, being packed into one cubic centimetre, i.e. the size of a sugar cube. A sugar-cube-sized pellet of mass from a neutron star would contain more mass than 7 billion people. [19]

Its incredible density gives it exceptional properties. If its gravity were any stronger, it would become a black hole. [20] Light bends around a neutron star, meaning you can see the front and parts of the back. An object released 1 metre above the surface of a neutron star would hit the star in $10^{-6}$ seconds, accelerating to around 7.2 million km/h. [21] The star is so tightly compressed that irregularities/bumps on its surface are a maximum of 5 millimetres. The star does, in fact, contain an atmosphere, albeit a really thin one consisting of hot plasma. The surface of a neutron star can reach 1 million degrees Celsius, compared with a meagre 6,000 degrees for the surface of the Sun. [22]

Figures 7 and 8: Neutron Star Size compared to Earth and a White Dwarf [23]; Neutron Star Size compared to Vancouver [24]

### Neutron Star Structure:

Scientists don’t know much about neutron stars and a lot of what’s discussed in this essay is theoretical physics. Despite this, they’re getting better at analysing these unique objects by replicating their conditions and it’s possible that if we ever got to come close to one in the future, it could reveal the nature of existence and display clues about the origin of the universe.

Neutron stars aren’t just a really dense sphere of matter: they’re made up of layers as gravity gets stronger and their properties change. In this way, they’re like planets, with solid crusts over a liquid core. [25] Their crust is remarkably hard and most likely made of an iron atom nuclei lattice with a sea of electrons flowing through them. The closer we get to the core, the fewer protons and electrons we see until it’s just a dense soup of nucleons (protons and neutrons). [26]

Figure 9: Cross-Section of a Neutron Star [27]

As we reach the base of the crust, nuclei are squeezed so hard together that they start to touch, rearranging and making long sheets or enormous nuclei consisting of millions and millions of nucleons. [28] These sheets are called nuclear pasta, due to their resemblance to various types of pasta like lasagna or spaghetti. Nuclear pasta is really dense and most likely the strongest material in the universe, essentially unbreakable. Lumps of pasta inside a neutron star that are a few centimetres high contain more material than the entire Himalayas. [29]

Figure 10: Simulations of Nuclei inside a Neutron Star resemble pasta, hence the name ‘nuclear pasta’ [30]

Eventually, beyond the pasta, we finally arrive at the core. This is where it starts to get tricky: scientists don’t know what the properties of matter are when it’s squeezed this hard. [31] Perhaps nucleons retain their form, simply getting squeezed really tightly together. Another theory is that nucleons dissolve into an ocean of quarks, a so-called quark-gluon plasma. Strange quarks may then arise from this matter, creating strange matter. This strange matter is purely theoretical, but it has the most extreme properties in the universe and its existence has really interesting implications. [32]

## Strange Matter

### Quantum Physics in Neutron Stars:

Strange matter is so bizarre that it bends the rules of the universe. It could destroy anything it comes in contact with, while simultaneously being able to teach us about the origin of the universe.

Nucleons are made up of smaller particles called quarks. Quarks are confined, meaning that they don’t want to be alone. If you try to separate quarks from each other, pulling them apart, they’ll just pull themselves back together with more and more energy. Think of a rubber band-it gets harder and harder to pull it apart as it stretches out more. This analogy isn’t fully correct though: if you pull hard enough, a rubber band snaps. On the other hand, if you reach the tipping point and invest enough energy in trying to separate a quark, it’ll just use this extra energy to create new quarks. Scientists have never observed quarks alone – they only exist together, forming subatomic particles. [33]

There are many types of quarks, but only two types make stable matter: the ‘up’ and ‘down’ quarks found in nucleons. All other quarks decay away quickly into these stable quarks. [34] However, these ‘laws’ of quantum physics might break down inside neutrons stars. Neutron star cores are so extreme that it turns out they’re pretty similar to the state of the universe not long after the Big Bang. [35] This means that observing or replicating the conditions inside a neutron star would allow us to peer back in time. Learning how quarks behave inside a neutrons star could teach us how the universe began. [36]

Figures 11 and 12: Chart of different kinds of Quarks [37]; Nucleons are composed of up and down quarks [38]

### How is strange matter formed?

Some scientists theorize that inside the core of a neutron star, neutrons and protons deconfine, meaning they ‘dissolve’ into a sea of quarks. [39] An indistinguishable soup of matter emerges called quark matter. [40] A star containing such matter would be called a quark star, a kind of neutron star. Subsequently, if pressure were powerful enough, strange matter could be formed from this sea of quarks. Strange quarks are heavier and ‘stronger’ than normal quarks. Strange matter formed from strange quarks is an ‘ideal’ state of matter: it’s immensely dense, infinitely stable and indestructible. [41][42]

### The most dangerous substance in the universe:

Strange matter is so stable that it can actually exist outside neutron stars. [43] Such matter would be indestructible and, at the same time, infectious. Every piece of matter it touches would immediately turn into strange matter too. The up and down quarks in nucleons (neutrons and protons) would lose their bonds and become part of the quark bath, releasing energy and creating more strange matter. [44] In fact, the only possible way to eliminate such matter would be to eject it over the event horizon of a black hole.

### Strangelets:

If such strange matter exists, it could only be found inside neutron stars, except when neutron stars collide with other neutron stars or black holes. These collisions make neutron stars spew out huge amounts of themselves. Included in this debris could be small amounts of strange matter, named strangelets. [45] Strangelets are strange matter that has retained its properties – they’re still as dense as a neutron star core without being in one. Their size can range from subatomic to the size of a house. These strangelets would theoretically drift for billions of years throughout the galaxy after a kilonova explosion (when two neutron stars, or a neutron star and a black hole, collide) until they came into contact with a planet or a star. If a strangelet came into contact with the Earth, it would start converting matter on Earth into strange matter, growing bigger and rapidly consuming the entire planet, until Earth became the size of an asteroid. [46]

Figure 13: Quarks in a nucleus as opposed to a strangelet [47]

If a strangelet hit the sun, it would start consuming it like a fire raging through a dry forest, resulting in the sun collapsing into a strange star. This would make the Sun shrink, becoming less bright and meaning life on Earth would freeze to death. If a strangelet was on a collision course with our Solar System, we’d have no way of seeing it coming.

### Dark Matter:

Some theories hypothesise that strangelets could be really common, outnumbering all the stars in the galaxy: the majority of these strangelets would’ve formed just after the Big Bang, during the beginning of the universe, when it was as hot and dense as a neutron star care everywhere. [48][49] Therefore, strangelets could potentially be the dark matter we suspect holds galaxies together. [50][51] However, this is just a theory, and an improbable one at that: our solar system hasn’t been consumed by strangelets for billions of years, suggesting there probably aren’t that many around.

Figure 14: A simulation of dark matter in the universe 13.6 billion years ago [52]

## Types of Neutron Stars

### Magnetars:

When red supergiants first collapse to form neutron stars, they spin exceedingly quickly, often several times a second. [53] This spinning creates pulses: a neutron star’s magnetic field creates radio wave beams, which are emitted every full spin cycle. These are called radio pulsars and they’re the most common type of neutron star in the universe. We know of at least 2,000 that exist in the Milky Way alone. [54] [55] These neutron stars’ magnetic fields are the strongest in the universe, a quadrillion time stronger than Earth’s after they’re born – so strong that atoms get bent when they enter their influence. [56] While in this state, neutron stars are called magnetars. If there’s a nearby star to feed the neutron star, it can rotate up to several hundred times per second. The neutron star PSRJ1748-2446ad spins at approximately 252 million km/h. [57]

Figures 15 and 16: Artist’s impression of a magnetar [58]; Magnetar found at the centre of the Milky Way Galaxy [59]

### Binary Systems:

Some neutron stars exist in binary systems with other neutron stars. In such binary systems, the neutron stars’ orbits gradually decay as they radiate away energy in the form of gravitational waves, causing ripples in space-time. Eventually, the neutron stars crash into each other, resulting in a kilonova explosion that spews out their insides. [60] In this explosion, conditions become so extreme that, for a brief moment, heavy nuclei are made again: this happens as nucleon-rich matter falls apart and instantaneously reassembles. This process is most likely the origin of most of the heavy elements in the universe such as gold, platinum, uranium and dozens more. [61] This means that for elements like gold to be formed, stars have to die twice. Over millions of years, the material spewed out from a kilonova explosion mixes back into the galaxy: some of it ends up in a giant molecular cloud, forming new stars and new planets while some of it gets deposited in an asteroid, thus repeating the stellar life cycle. This process is limited though: as the universe goes towards a higher state of entropy, molecular clouds will form fewer and fewer stars in the future. Furthermore, most of the matter in a kilonova explosion gets sucked into a black hole that’s created from the two neutron stars. [62]

In reality, our solar system consists of the remains of those neutron stars that came before us. Our entire technological modern world was built out of the elements that neutron stars made billions of years ago, which then came together to make us and our world.

Figures 17: Artist’s impression of a kilonova explosion [63]

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