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Stellar Interiors and Fusion Processes

Stars are incredibly large matter creating factories. Inside them hydrogen is fused into larger, and larger elements. In fact, all matter that makes up our body, that is heavier than iron, was once produced inside a massive, dying star. Stars are hot on the surface, especially massive blue stars, but inside the star it gets hotter and hotter as you approach the core. In the core of our sun, temperatures are as high as 15 000 000 degrees Kelvin, which is enough for fusion between hydrogen atoms to occur. However, in the cores of massive supergiants temperatures can rise up to several billion degrees when heavy atoms are produced. This happens when the star approaches its' death.

The Interior of Stars

On the surface stars can appear to be violent creations of nature, but inside raging powers reside. The closer one gets to the core of the star, the hotter it gets. In the core the main energy (in the form of light: powerful gamma rays) is produced. This energy is transported to the surface through different methods. In massive stars, energy is mainly transported (outside the core) by convection. This zone is therefore called the Convective Zone. Hot plasma rises from the core, because of its' lower buoyancy, cools off and then descends into the depths again. This energy transport is usually in circular convection currents. Once the energy produced in the core reaches a certain distance (and thereby a certain temperature) the plasma turns into solid atoms. Therefore energy is now transported through photons colliding with atoms. The powerful gamma ray photons that are produced inside the core take very long to reach the surface because they bounce off of atoms that are in the way. They may take millions of years to reach out into space.
Stars smaller than the sun, such as red dwarfs may not even have a radiative zone. Energy is solely transported through convection. This means that the helium that is produced in the center will be mixed up with hydrogen even close to the surface. Because red dwarf stars lack enough mass to produce heavier atoms than helium, they will die only when they have used up all the hydrogen in the star, contrary to other stars. This enables red dwarfs to live for trillions of years!

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Proton-Proton Chain

The Proton-Proton Chain is the primary way to fuse protons to form hydrogen in stars the size of the sun, or smaller. To make them fuse is no easy task, as it requires very high temperatures. The whole sequence takes about 109 years on average to complete. The fact that the sun is still shining brightly is because there are extremely many reactions that are occurring at any given time. The whole chain looks like this:

1: 11H + 11H 21D + e+ + Ve

2: e- + e+ 2 γ

3: 21D + 11H 32He + γ

From here the reaction could go in three possible ways. Only one will be mentioned here:

4: 32He + 32He 42He + 2(11H)

What happens is this: in step 1, which one average takes 109 years, two protons overcome their mutual repulsion and fuse together to form an isotope of hydrogen: deuterium, which means there is one proton in the hydrogen core, and one neutron. As this happens, a neutrino, an almost massless particle, and a positron (β), an antielectron is emitted. In step 2, the positron, β, meets an electron and is annihilated. The result is 2 gamma rays. The energy released here is what keeps the sun warm and stable, else it would collapse under its' own weight. Step 3 consists of the deuterium merging with a proton to create a helium isotope and yet another gamma ray. After this has happened the reaction can go in several ways, one way is when two helium isotopes fuse to create one stable helium atom, and two free protons. When a star converts hydrogen into helium it is located on the main sequence on the Hertzsprung-Russell Diagram.

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The CNO Cycle

Helium is produced mainly through the CNO cycle (Carbon, Nitrogen and Oxygen) in stars heavier than 1.5 MSun. As soon as the temperatures inside the core reach about 17 million degrees Kelvin, the CNO-cycle becomes the dominant helium producing reaction, but the cycle starts at a few million degrees less. This is what the reaction looks like:

1: 126C + 11H 137N + γ

2: 137N 136C + e+ + ve

3: 136C + 11H 147N + γ

4: 147N + 11H 158O + γ

5: 158O 157N + e+ + ve

6: 157N + 11H 126C + 42He

Step 1 creates a nitrogen isotope, and a gamma ray from a carbon atom and a hydrogen atom. In step 2, the nitrogen atom decomposes into a carbon isotope, a positron and a neutrino. In the next stage, the carbon isotope acquires a second hydrogen atom to create a stable nitrogen atom, and a gamma ray. Later, the nitrogen atom fuses together with a proton. Now we have an oxygen isotope, and a gamma ray. Furthermore, the oxygen isotope decomposes into a nitrogen isotope and thereby releases another set of a positron and a neutrino. In the final stage the nitrogen isotope acquires a hydrogen atom to create one carbon atom, and one helium atom, aka an alpha particle.

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Triple-Alpha Process

Once the hydrogen in the core is used up, the star starts collapsing because no energy is not enough energy produced in the core to counter the gravitaty. As the star starts to collapse, the heat inside the core starts to increase. Once the temperature reaches 100 000 000 degrees Kelvin, helium fusion creates carbon. This procedure is simple:

1: 42He + 42He 84Be

2: 84Be + 42He 126C

A pair of helium atom create one beryllium atom, which fuses with another helium atom to create a carbon atom. In massive stars, a very small fraction of carbon atoms may continue adding helium atoms to create larger, and larger atoms. If the carbon atom continues, then the process is called the Alpha-Process.

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Other Fusion Reactions

How heavy elements that will be created inside the core of a star is entirely dependent on how massive it is. Each time the core starts fusing a new element it accelerates the time till its' death. Along with that, temperatures rise in the core.
Stars that have less than 3 solar masses stop fusing heavier elements when the helium is digested. If the star is more massive it may start fusing carbon in a process called the Carbon Burning Process, which requires a core temperature of atleast 600 x 106 degrees K. The Neon Burning Process requires a core temperature of 1.2 x 109 K, whereas the Oxygen Burning Process requires 1.5 x 109 K.
The Silicon Burning Process starts somewhere between 2.7-3.5 x 109 K. At that point the core fuses silicon (2814Si) with helium atoms in the alpha process, until the core consists of nickel, 5628Ni. The core now resembles an onion due to its' many layers of different elements. At this point the fusion in the core stops, because in order to fuse heavier elements, the core must provide each reaction with energy, rather than releasing it. The star collapses under its' own weight and goes supernova. During this process everything is shed into space, except for the core, which collapses into either a neutron star, or a black hole. The matter which is into space may create a planetary nebula, and create new, smaller stars.

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Previous: Nebulae - Stellar Nurseries.
Next: H-R Diagram.


Quick links

The Interior of Stars - Illustration (P)
The Proton-Proton Chain - Illustration (P)
The CNO Cycle - Illustration (P)
The Triple Alpha Process - Illustration (P)
Other Fusion Reactions

Space Art

Above: A massive star exploding in an event which will outshine the rest of the entire galaxy, combined.

This illustration is available upon request, as a print (5000x3000 pixels, 300 dpi), and as a .PSD-document.

Space Art

Above: The Proton-Proton Chain, which creates helium through fusion of hydrogen atoms.

This illustration is available upon request, as a print (5000x3000 pixels, 300 dpi), and as a .PSD file, customizable.

Space Art

Above: The CNO-Cycle. It creates helium while fusing hydrogen to carbon, nitrogen and oxygen.

This illustration is available upon request, as a print (5000x3000 pixels, 300 dpi), and as a .PSD file, customizable.

Space Art

Above: The Triple-Alpha Process. It takes place in the interiors of stars as soon as temperatures rise to 100 million K. At much higher temperatures the process of adding alpha particles can create even heavier atoms.

This illustration is available upon request, as a print (5000x3000 pixels, 300 dpi), and as a .PSD file, customizable.


All content Copyright , 2005- by Fahad Sulehria, unless stated otherwise.
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