The Physics of Stars Made Simple Plasma and Degenerate Matter

S

tars are oceans of plasma, much like Earth's seas. Two main types of matter rarely found on Earth—Plasma and Degenerate matter—make up stars. Today, we'll explore the materials stars are made of. Welcome to our blog discussing cosmic objects with minimal math.

Introduction:

AI Generated Thumbnail

As children, we heard the sun described as a giant fireball. Scientists call it a nuclear furnace made of plasma. While star discussions often dive into the core, we'll focus on stellar surface activities involving plasma and degenerate matter. Imagine us standing on a beach, talking about the sea and its waves.

Before diving into plasma, the fourth state of matter, let's review how atoms behave in different states. On Earth, matter exists as solid, liquid, and gas, shaped by heat energy.

Molecules form when atoms bond, each atom having a nucleus orbited by electrons. The nucleus contains protons and neutrons, made of quarks. Stability depends on balanced numbers of these particles.

In solids, molecules form a lattice and vibrate slightly. Increasing heat raises vibration, called temperature.

More heat melts solids into liquids, where molecules collide and move freely. Further heating boils liquids into gases, with molecules flying randomly and exerting pressure if confined.

Extreme heat breaks molecular bonds, freeing electrons and nucleons, creating plasma—a condensed gas flowing like a liquid—usually above 4000-5000 ℃.

Almost all elements can transition through these states under the right conditions. For example, water freezes at 0℃ and boils at 100℃, breaking into various components at higher temperatures.

Hydrogen transitions to plasma around 4000-5000℃, whereas 10,000℃ around Stars. while oxygen requires even higher heat. Now, you have a solid grasp of the star's surface ingredients. Next, we'll explore plasma and degenerate matter.

Plasma

Animation of Plasma Stream

Now, let's discuss the Plasma in detail. In this state, matter behaves like a fluid. Electrons leave the orbits around nuclei (Ionization) and freely move. Plasma is a fundamental material of Stars, lightning, neon glow, Accretion disks and Jets of many celestial bodies, like an ocean contains water.

Since electrons are moving, this generates a tiny magnetic field, and the electric potential difference creates the acceleration on a large group of Plasma. To understand their movement, you need to go through this basic idea about electric and magnetic fields.

Electric Field

In our childhood, we used to play a game in which we rubbed our hair with a plastic comb and hover it around pieces of paper; the paper pieces attracted and stuck to the comb. This is called Static electricity.

When you rub hair with a comb, the electrons of your hair shift to another material (the comb), or the comb's electrons join the party of electrons in hair. Either way, it creates a number difference between positively charged particles (the proton) and negatively charged particles (the electron). If this difference is sufficient, then it will show an effect on the macro level.

The same charges (Positive-Positive) or (Negative-Negative) repel each other, while Opposite charges (Positive-Negative) attract, in an electric field.

With such an environment where the number of charged particles differs, it naturally tends to fill that gap by losing or gaining charges or ejecting/capturing protons or electrons. This zone of influence is called an electric field.

Since this type of charge difference is localized, meaning where this difference occurs, it will remain in its place; that's why it's called Static electricity.

We have another type of electricity, which we call Dynamic electricity, where charges move from one to another. This one is used in our daily life.

Electrons must move; when they move, they feel a kind of push, which is called Electric Potential in physics. It's measured in Volts whereas the actual flow of electrons is called current and measured in Amperes.

Remember, electrons naturally tend to move from high to low potentials, higher potential difference will push the electrons aggressively, like rolling balls on a sharp slope. Now you have the idea about how Plasma can move from one place to another.

Let's talk about where or how to move.

Magnetic Field

In the last sub-section, we learned about how an electric field generates, you know that electrons freely float in Plasma but their movement in loops or a coordinated manner generates the Magnetic field. In this field, other particles may trap or join the movement of electrons. These moving particles follow a strict path called a magnetic field line.

The movement of electrons, electric and magnetic fields are interconnected; So, if any one of these three starts, then the other two will be active sooner or later.

Cyclotron Motion

The Cyclotron Animation of Electrons and Ions

The electrons in plasma always keep moving at high speeds spirally around imaginary magnetic field lines, while the ions choose a different direction around the same imaginary line, but in a diffused way instead of a tighter formation than electrons. This motion is called the cyclotron motion.

It generates energy and their movement stores this energy. Some amount of energy is also released in forms of Radiation like Infrared, visible, or Radio waves. We find this kind of motion and radiation in Aurorae, Solar plasma, and accretion disks. It occurs in less energetic events.

Synchrotron Motion

In addition to the cyclotron, there is a stronger version, which is called the synchrotron motion. This is the same cyclotron manner, but those particles move at nearly the speed of light. Therefore, it is far more energetic. It usually happens in Solar flares and spots, Quasars, Pulsars, Magnetars, Jets of Black holes, white dwarfs, Neutron stars, and Protostars.

The Strength of a Magnetic field is measured in Tesla (SI unit) and Gauss (CGS). 10,000 Gauss is equal to one Tesla. Let's quickly see how they are intertwined.

Plasma dynamics

Sun like Star showing Plasma Eruption

Now we have the idea of moving electrons, Electric and Magnetic fields. Let's put them together. The root is electron here; they move in a coordinated manner and generate a magnetic field, hence their paths, but they will keep oscillating around their chosen lines. They need something that pushes them, which is why electrical potential differences and charges matter.

Suppose in our sun, there are two points named A and B. Plasma-Point B loses some electrons, hence it becomes positively charged. Indeed, it can summon more electrons from the neighborhood to reach a neutral state, but for some reason, this gap is too large.

At the same time, Plasma point A has the opposite condition, and an electric field will be created between these points.

If we remove the magnetic field, then the plasma will easily diffuse to the appropriate site, but a stronger magnetic field forces the electron more tighter cyclotron motion, hence they slowly move to the targeted area.

However, this happens in theory, but in Sun, it's mainly a Plasma and almost like a superconductor. A stronger magnetic field causes groups of plasma electrons to move easily.

Now, the Magnetic lines are also connected well, and the electron-ion group is ready to move; a slight difference in potential will give them swift acceleration between these two sites, but with high to low potential differences. That's how Plasma behaves in Stars. While in accretion disks, the gravity's pull and turbulence are much stronger, so we rarely see the Plasma eruption on these disks.

Now you know the dynamics of plasma. If we calculate things here, if a sun-like star's two points' field lines and potential are formed in that way, then:

• Plasma Stream whose radius is 10 meters and 100 meters long, if it flies at a speed of 10 m/s then, then it will generate 9.3 million amps caused by only 190 Volts potential.

• If its speed is 20 m/s, it will generate a similar 9.3 million Ampere electricity but with 374 Volts.

• 5 m/s will also give the same electricity, but with potentials around 93 volts. That's how such a low potential can push the streams of electrons and ions easily.

Keep in mind: don't be confused by the collective movement of electrons or ions and the microscopic one. Just as Water particles in Brownian motion, even water stays in a container, the plasma particles move in a cyclotron or Synchrotron manner depending on the context.

While movement by electric field and magnetic field, or overall, a collective particles movement as a Stream of Plasma, would be like Water flowing in a river or Waves in the sea.

Cosmological Importance

Plasma is one of the most important materials in our universe. A protostar first converts the hydrogen gas and dust into Plasma then assimilates through an accretion disk, when it crosses the Eddington Luminosity, then releases Jets. They gather mass this way, once prepared for the main sequence phase, then they keep their plasma until the final moments. When a star dies, whether its Supernova or shell depletion, the outer plasma scatters in shapes of nebulae. 

Black holes, White dwarfs, and Neutron stars can accrete matter in the form of Plasma and gather more mass. Stellar winds are simply ejected particles from Stellar Plasma. If they reach outer space or the interstellar medium, we call them cosmic rays.

Degenerate Matter

Structure of Degenerate Matter

All the previous types of matters we've discussed; they were supported by Heat energy. They were coping with the gravity's compression by the movement of particles or thermal pressure, and they resisted the compression. The Degenerate state is very different, and it occurs in more extreme events.

In the degenerate state of matter, electrons are forced to come closer and share the same quantum state, but they belong from Fermion family. No two fermions (particles that create physical matter) can share the same quantum state. But gravity crushes them so brutally that they arrange in lattice-like structure, as we see in solids or crystals. However, normal solids form in a specific thermodynamic state, while a degenerate-like lattice arrangement forms due to Gravity.

The nuclei are surrounded by electrons; this is their final compressed phase. They can't be crushed any longer; this arrangement exerts an outward pressure called Degeneracy pressure. Since it comes from electrons, we call it electron degeneracy pressure.

Such a matter flows at almost zero viscosity and it's highly conductive, meaning it creates very efficient magnetic fields, whose effects are easily observable in Neutron stars.

In principle, all the elements can enter a degenerate state if conditions allow. In such a case, the nuclei or ions of the element will be surrounded by electrons. As we said earlier, this needs very strong gravity. Let's explore some degenerate matters.

1) Helium Degenerate Matter

It occurs when a Star generates energy due to Hydrogen fusion in the core, it makes an "ash" in the form of helium. This kind of byproduct can't be used for fuel by the star because it doesn't have the capacity. In young stars like our sun, this type of helium usually circulates in the inner cores and prevents to accumulate, but in the older stars that have little to no hydrogen, their cores will be dominated by Degenerate helium.

In this type of matter, electrons will be forced to form a lattice-like structure surrounding the helium ions; this way, they don't even have a source of energy, but the outward pressure or degeneracy pressure fights the gravity.

This type of matter's 1 cm square 3D block would weigh about 1000 kg. If such a core reaches a temperature of about 100 million Kelvins, then Helium's degeneracy lifts, and it's ready to fuse like hydrogen. It causes a lot of damage to low-mass stars, but eventually they manage it. This happens when stars become stable in the red giant phase. Helium Fusion creates Carbon-oxygen mixed "ash", which would be degenerate, obviously. 

2) Carbon-Oxygen Degenerate Matter

When low-mass stars, in red giant phases, produce much Ca-O, it also accumulates like Degenerate Helium. Its structure is similar to that of degenerate helium, but Oxygen or Carbon ions are trapped among the electrons. This matter forms in much more brutal compression by gravity than the previous matter.

Its one cm block would weigh about 10,000 kgs. Mostly low-mass stars kneel before gravity at this point because to ignite Carbon-Oxygen, the core must be 500 million Kelvin hot, and the star must be more massive than 4 Solar masses. Therefore, low-mass stars become White dwarfs.

3) White dwarf's Matter

This is a common and most recognized degenerate matter. As we said, electrons trap the ions or nuclei; that's a common feature in degeneracy. In white dwarfs, whatever element or ion is dominant, it would be called X-degenerate matter, and hence the X-white dwarf.

For example, A helium white dwarf material's common structure would be He2+ surrounded by the electron lattices. A Carbon-Oxygen WD would consist of the C⁶⁺ and O⁸⁺ ions among electrons, and Oxygen-Neon-Magnesium WD's electrons would trap ions like O⁸⁺, Ne¹⁰⁺, and Mg¹²⁺

Their densities and progenitor stars' masses would be different accordingly because not every star can create those elements at the fusion level.

4) Iron Degenerate matter

When the massive stars create Iron in the core after Silicone fusion, it would be degenerate, of course. Structure is the same as before, with iron ions trapped among dense electron lattices. At this point, no massive Star can overcome the Iron degeneracy and gravity wins. This material's one cm block would weigh 100 million kgs.

5) Neutron Degenerate Matter

This is the common material in Neutron stars and is known as one of the densest matters. In this matter, Gravity doesn't stop even electron arrange in a lattice formation. When the Iron degeneracy fails the Star, gravity pushes the electrons closer enough to ions and it triggers an event called inverse beta decay in which electrons are absorbed by protons and form Neutrons. In this matter, Neutrons are themselves arranged in a lattice formation.

It's one cm 3D block would weigh about 1 trillion Kg with densities about 10¹⁴ grams/cc. If the star is more massive than merely 8 to 25 Solar masses, it will compress the core even further. No voice of degeneracy pressure would be raised against gravity; it will erase the concept of density and create a Black hole.

Note that there are several types of Degeneracies that have proposed, like Quark Degenerate matter. But we only focus on observed instead of theoretical ones.

So that's all for today. Stay tuned for the next awesome post.

Thanks for reading

Have a nice day