Why and how stars lose their vitality

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ou all know that the sun is the most powerful energy source nearby to Earth, but have you ever thought, how powerful it is compared to other stars? In this post, we'll decide the parameters that make a star weak or powerful and give you a general idea to imagine their strength. Let's begin.

Massive Star Showing Flares and CMEs

Introduction:

It is often shouted in the media and scientific community that our sun is a giant nuclear furnace with immense energy output. When we think of powerful things, they directly jump to black holes, supernovae, or quasars. They are indeed powerful, but we've left behind many stars and structures, and it might feel like there is a large gap between our sun and Betelgeuse. It's time to look at those rarely discussed stars. Welcome to our blog, CosmicWisdom, where we discuss... (Just look at the header of your screen :)

Since most of us know that Stars, in their core, fuse hydrogen nuclei and convert them into helium, which is called Nuclear fusion, and it releases a lot of energy. Stars are mainly byproducts of Gravity and Fusion in the interstellar medium. Let's see how stars are powered and conduct their energy flow. Please note that the Stellar maintenance part is described in this post. Here, we'll focus on Stellar energy output and its strength, outcome, and side effects.

Before we dive in, let's see some points that will help us understand their power.

1) Gravity:

As always, it tries to compress the star and makes the stellar core enormously violent. Stars form when any piece of a nebula violates the Jeans mass, and gravity can compress it. The massive cloud, powerful compression or gravity, and its reverse happen when there is less mass.

2) Fusion: 

The central part of a collapsing hydrogen cloud or star constantly faces inward force from the surrounding medium. If this crosses a certain temperature and density limit, the process of fusion begins, where light nuclei merge and produce heavier nuclei.

3) Energy transport:

Normally, every star has its own sufficient energy transport system; without it, the immense energy coming from Fusion would be trapped in the core, and the star would explode. There is a balance needed between how much energy should be transported or captured.

4) Energy output:

This star's total energy output is often defined by Luminosity and measured in watts. If gravity's compression is high, the star must release much energy to survive. If compression is low, then it should release less energy.

These four will give us enough idea about what makes them powerful, why or how they shine, and how long they will sustain, because gravity will win in the last, but they won't easily kneel either. We're skipping the Gravity because it's a long topic to discuss; instead, we'll start from the Fusion.

I- Fusion

When lighter nuclei than iron merge and produce heavier elements, it's called fusion. In this process, enormous energy is released. Talking about main-sequence stars, meaning these stars burn hydrogen as their main energy source. However, in later stages, stars may be supported by other elements' fusion processes. Main-sequence stars are the most stable stars, and they spend almost 90% of their lives in this stage.

Mass is the major player that defines whether a star will be weak or strong, because gravity tries to compress massive stars more aggressively. The star must pull enormous energy from fusion to prevent itself from gravity's compression otherwise, a star will become a black hole if it fails this battle. This also means massive stars have higher temperatures and densities in their core.

We have two kinds of distinct fusion methods of hydrogen, which power weak or massive stars depending on the core's temperature and density. But all these properties are mainly dependent on mass, which gives an estimation of how brutally gravity is compressing them. Let's see those two kinds of hydrogen fusion methods in the stellar core.

a) PP chain

It may sound like old anime character names, but its expanded form is the Proton-Proton chain. In this method, a proton or ionized hydrogen, which doesn't carry electrons, meets with other protons or elements and releases energy in different steps. This is the main Hydrogen fusion method of low-mass stars.

Minimum Stellar requirements:

Stellar Mass - 0.08 M⊙ (Solar mass)

Core's Temperature - 13 million Kelvin

Core's Density - 100 Gram/cm³ (if you make a 5 cm cubic box of this material, it will weigh around half a kg)

*If any star fails these minimum requirements to fulfill, then it will be called a Brown Dwarf or failed star, meaning hydrogen fusion would not be possible.

Let's see how this happens. Protons merge and take some time to fuse, release energy, and transform into a new species. If protons want to merge due to high temperature and density, they can also repel each other because the same charge causes repulsion, but if with a certain angle, speed, or quantum tunnelling is involved, they can overcome this repulsive behavior. The stellar core is a very messy place for subatomic particles. Imagine being told to cross a football field crowded with 10 million people who are colliding with each other; it's a similar situation, so collisions are not rare.

Here's the step-by-step description:

Step I :)

Two Hydrogen nuclei (Protons) smash into each other, transforming into Deuterium, and a positron and neutrino are released in this process. It takes around a billion years and gives 1.44 MeV energy.

Normal hydrogen has one proton and one electron, but this hydrogen has only a proton, so call it what you will. This step is the longest process. Two protons have merged, but the product has one proton and one neutron. To form a stable combination, either proton must lose its positive charge. So, one of the protons loses a lepton called a positron and an electron neutrino to form a neutron. Because it's a process mediated by the weak force. Therefore, the product has one proton and one neutron; it's called Hydrogen-2 or Deuterium. If you want to know about subatomic particle interactions, stay tuned because we'll discuss them in another post.

Step II :)

The Deuterium nuclei or Deuteron capture a proton, and it makes Helium-3.Gamma rays are released. This step takes a few seconds, gives 5.49 MeV energy.

Since a deuteron has one proton and one neutron, it gains another proton, hence 2p + 1n, and a gamma ray is released. The product is known as Helium-3. It's not common helium we know because Normal helium has 2 Neutrons and protons, but it has only one.

Step III :)

Two Helium-3 nuclei merge and make helium-4; two protons are released. This process takes around a million years and gives 12.86 MeV energy.

Now, assume Steps I and II happened somewhere else and formed a Helium-3. Both Helium-3 particles merge, and the product should have with 4p + 2n configuration. But that's not possible due to charge imbalance. That's why 2 extra protons have escaped to participate in other reactions or just to wander in the star's core.

In these steps, if you calculate total output energies, you will get around 19.79 MeV energy, but we said this entire process gives 26.7 MeV. Keep reading this post to know what happened to the stolen energy.

b) CNO Cycle

This is the main fusion method of massive stars' energy production. It gives a similar amount of energy as the PP chain, but works better with Higher temperatures, hence more energy output. The CNO cycle means Carbon, Nitrogen, Oxygen cycle because these elements work as Catalysts in this fusion process. Here are the Minimum and recommended requirements.

Minimum Requirements:

*Stellar mass = 1.3 M⊙

*Core's Temperature = 15 million Kelvin

Core's Density = 150 Gram/cm³

Recommended Requirements:

Stellar mass = 1.5 M⊙ or more

Core's Temperature = 15-18 million Kelvin or more

Core's Density = 150 Gram/cm³ or more

* These Requirements have no clear boundaries; This process can start even when these properties are still lower but near.

This process also gives the 26.7 MeV. Unlike millions of years lasting PP chain steps, it takes a few minutes. That's a great advantage and a flaw in massive stars.

Explanation:

Look at the Monster above, you see 6 steps are arranged clockwise. Here's its brief breakdown.

Step - I:

Carbon-12 captures a proton, it transforms into Nitrogen-13 and releases a Gamma ray.

Those numbers show the combined numbers of protons and neutrons, which is why we use Atomic mass to denote an element's species.

Step II:

Nitrogen-13 is unstable; it emits an electron neutrino and a positron due to a process called Beta decay, transforming into stable Carbon-13.

When an unstable atomic nucleus transforms by converting a proton into a neutron or a neutron into a proton, it releases a beta particle (electron or positron) with a neutrino or an antineutrino. This kind of radioactive decay is called Beta Decay; it's basically a kind of Transmutation.

Note: All the further steps until VI are similar.

Step - IV:

Nitrogen-15 captures a proton, and instead of becoming Something-16, it loses 4 bundled particles labeled as Helium-4. Now the entire process can repeat since Carbon-12 has returned.

Fate of Ejected Particles:

These steps generate immense energies in the form of light and heat, often in an explosive way. Stars are powered by several such reactions, often millions of such fusing couples in one cubic cm. That's why, in just a cubic cm, 1000 hydrogen bombs explode in stars like the Sun's core. The Gamma ray goes through a long cycle of Radiative and convective transports to reach the photosphere of Stars, which often takes thousands to millions of years. Each cycle and reaction weakens it. When it reaches to photosphere, we see that light escapes from here and starts its journey in space

Remember, these steps were leaving some uninvited guests like Neutrinos? We discussed on emitted Gamma rays. Neutrinos leak easily through the stars and carry some amount of energy with them, which is why the calculated energy is less than 26.7 MeV.

Also note that the PP chain has 3 main variations named Branch I, Branch II, and Branch III; the IV Branch is rare but exists. The CNO cycle also has some variations; by combining all of them, stars generate energy to fight against gravity.

PP chain vs CNO Powered Stars:

Since everything is dependent on stellar mass, there is a transition point where a PP chain-powered star can become a CNO-powered star. Its lower limit starts when stars get 0.8 Solar masses. At this point, the CNO cycle will appear to be almost nothing.

As the Star gets more mass, the PP chain drops and CNO increases. That's why our Sun's Core temperature is 15 million Kelvin, and its density is enough to show CNO cycle signs. When a star reaches 1.3 solar masses, the CNO cycle starts to dominate and lowers the PP chain fusion reaction. Stars with 1.5 Solar masses are mainly powered by CNO cycles and a very small amount of the PP chain. While more massive stars like more than 5 solar masses, there is no sign of the PP chain.

You also know that the PP chain takes almost a billion years. That's the secret of the long-lasting youth of our sun, it burns 4 billion kg of Hydrogen every second in the core, the process is slow, and even after 10 billion years, it would have lost only 0.07% mass into energy.

But in massive stars, such a slow process can't help in their survival, which is why they need a fast method like the CNO cycle. This was why they burn fuel faster and incompletely. The PP chain gives the star a sustained, long life but less luminosity, while CNO is an ultra-fast method to fight against gravity's brutal compression. We'll see further how the star can save itself just by shining in space from gravity.

II- Energy Transport:

Sun-like Star showing Plasma flares and winds

Stars are densely packed objects whose density distribution is not uniform, as well as their temperature. Stars' outer layers could be 3000-30,000 K hot while their core touches a few million to billions of Kelvins. This temperature difference is mainly due to density, which is a result of gravity's compression.

In the last section, you read that Stars generate energy in the form of Gamma rays and heat from fusion reactions, whereas neutrinos don't contribute to the fight against gravity. In this section, we'll know how their energy flows in the star and how it helps in survival.

The generated energy in the core must travel through the star's inner layers and reach to outer zones and finally be dumped into space. Immense Density and pressure block the energy, which is why light or heat can't directly reach space and have to travel in the interiors. There are groups of carriers and reservoirs to transport or hold energy.

The energy from fusion carries an outward pressure that counteracts the gravity. It also means that if the energy doesn't move or leaks too much, then stars can either collapse or explode; therefore, this process must be balanced. There are two main methods called Radiative and Convective Transport. Let's see them briefly.

Radiative transport:

When energy (light, radiation, or heat) is captured by subatomic particles like a proton, neutron, ion, or molecule, it becomes excited, and after some time, it reemits the radiation in a random direction but with slightly less energy and elsewhere. Since Stellar interiors are a chaotic place even for subatomic particles, it's pretty common to have such interactions. Despite the fact, photons (particles of light) can move 300,000 km/s, in the core of a Sun-like star, they will have less than a cm to move freely, which is why the sun's luminosity comes millions of years after it is created in fusion. In the radiative method, energy diffuses outward slowly.

Convective transport:

Unlike the previous method, this collects the group of photons in a hot gas reservoir, which itself is moving upward or downward. The convective process transports the energy efficiently than the Radiative method.

To understand it, just hover your left palm over your right palm, and you will feel slight warmth. Because the heat from your right palm is being carried away by relatively cold air particles. A similar process happens in the star's interior, too. The hot reservoirs carry heat and rise and dump the heat in cooler zones and fall back while collecting energy or heat again, once hot enough, then rise again. This way, heat is delivered to gradually cooler regions. When it reaches the photosphere, then emitted into space, and we see the star giving us light and heat.

Let's think what if a star refuses to shine or emit heat. Pretty simple, the core will overheat, and gravity will crush the star into a black hole if that tremendous energy doesn't create any outward pressure by moving in stellar interiors.

Here's one thing to remember that the intermediate stars like our sun have Radiative cores and Convective outer layers, while Massive Stars with more than 5-8 Solar masses have Convective cores and Radiative outer layers. Low mass M-type stars have fully convective layers.

III- Stellar Comparison:

You now have read about the interiors of stars, energy production, and their transportation methods. It's said that everything has advantages and disadvantages. Stars are not exceptions; being weak doesn't mean being really disadvantaged, or strong doesn't mean being advantageous.

We'll talk about them in 3 different sub-sections for ease. We'll also give you its type according to the MK System. Remember, all these boundaries have no clear line; they intermix properties and values around transition points.

A => Massive stars:

Structure of a massive star

These stars are powered entirely by the CNO cycle because of gravity's brutal compression. Higher temperatures and pressures are used to keep its fire sustained. Since this process can be done in a few minutes, energy generation is most important to cope with the gravity.

It releases immense energy due to the vast temperature difference between the Core's inner and outer layers. Energy here transfers with the Convective method and delivers it to the outer radiative layers of the envelope. Immense energy output is used to battle against gravity and Star survives by throwing a tremendous amount of energy, which heats its outer layers and glows bluish white.

These stars are denoted by O, B and A, where O is the most massive and A is the least massive Stellar group. Type-A's less massive stars also share the properties of intermediate stars.

You can't imagine how powerful and fierce a massive star fights for its survival. They burn billions to trillions of kg of hydrogen every second. The Brighter the main-sequence star, the more fuel it burns. This is mainly dependent on Gravity's compression.

Such an intense energy generation causes immense mass loss in the outer envelope because of Radiation pressure. If our sun had enough radiation pressure, and you walked in sunlight, you'd feel a kind of gentle or aggressive push like high-speed wind does; this is called radiation pressure. That's why powerful luminosity pushes outward the envelope's gas atoms, and we see it as stellar wind. This way, Star is being consumed by the inside and outside.

By the time its hydrogen is not exhausted completely, the gravity starts to overtake the system, star then finds a solution. It desperately starts to burn helium; thus two fusion zones form in the core. When this renders futile after thousands of years, Gravity grows stronger because the Combined energies of Both Fusion processes are not enough for gravity, hence Stars ignite Carbon fusion.

The cycle continues if the star is massive enough. There will be another post where we'll discuss what happens to such stars in detail; until then, let our massive star burn with 3 fusion shells.

B => Intermediate Stars:

Internal Structure and Stellar winds of an Intermediate star

Stars that are powered by both the PP chain and the CNO cycle can be classified as intermediate. Our Sun falls in this category; it's mainly powered by the PP chain, while the CNO has very little dominance. The cores of such stars have the Radiative type of energy transport, and the convective outer layers. Because the temperature around the core doesn't have that difference, as massive stars do. Instead, these stars have such a temperature gradient around the outer layers.

Radiation pressure is strong enough to blow out the photospheric layers, but not as strong as massive stars; hence, intermediate stars show a moderate mass loss and don't significantly contribute to the life cycle.

Since these stars' fusion is not as aggressive of massive stars so they burn fuel at a moderate speed and live longer. They don't shine that dim; instead, they are the perfect spot for life. not too hot, not too cold.

Such stars are assigned to F, G and K groups. However, K is the group containing low and intermediate-mass stars. Massive F-type members can also reveal the properties of weak A-types.

These stars live long enough to form planetary systems and sometimes nourish life, which is why these are among the most balanced stars in our universe.

C => Low mass stars:

Interiors of a Low mass star

Now, let's see the stars powered by the PP chain. The immense energy of these stars is not that powerful, but more than enough for stars' survival because being lighter means gravity won't compress them aggressively. These stars have only the convective zone, which mixes the helium and other fusion products more efficiently than previously mentioned stars. This is the secret of their longest life span. They do glow Red-orange but also live longer.

A vast number of Exoplanets have also been found around these kinds of stars. These stars are mainly assigned to letter M.

Remember, these assignment works well if a star is fusing hydrogen as its main energy source, aka main main-sequence star. in later stages of stars, they might move to other categories due to fusion anomalies like an O-type or Blue star. If it starts to lose the battle against gravity, it may look Red but as its fusion starts to win, the color moves toward blue.

Also MK system doesn't guarantee that if a Star appeared Blue white but thousands of years later appeared orange, then it will be classified Orange or K-type star.

Thus, we end today's post. We hope you learned a lot of things. See you soon in upcoming posts

Ba-bye.