Nuclear Fusion In Stars: What,Equations,Steps,Causes


The application of binding energy illustrates the nuclear fusion in stars, as the production of stellar energy is caused due to nuclear fusion process.

The difference in the binding energy between the lighter atomic nuclei leads to the nuclear fission process by combining them. During this, either absorption or release of energy occurs, and stars possess energy mainly due to fusion. Based on this theory, a brief note on nuclear fusion in stars and its consequences is given in this post.

What is nuclear fusion in stars?

Combining more than two lighter nuclei together to form a single heavy nucleus is called fusion. A study predicts that the core of the stars consists of lighter nuclei, which are ready to combine.

The stars consist of a huge number of hydrogen in their core, and the nuclei of hydrogen combine to form a deuterium nucleus. The deuterium nuclei thus generated merged to form a helium nucleus. The combined mass of the helium nucleus is less than the original mass of the hydrogen nuclei creating a difference in the mass, which leads to the release of a certain amount of energy. Overall reaction together is called nuclear fusion in stars.

File:Pict--proton-proton-chain-reaction-diagram-proton-proton-chain-reaction-diagram.png  - Wikimedia Commons
Nuclear fusion in lighter star
Image credits: Wikimedia commons
  • This nuclear fusion empowers the sun and other stars to glow end emit energy.
  • Nuclear fusion initiates the generation of supernovae.
  • The nuclear fusion in stars also gives an account to estimate the lifetime of the stars.

What triggers nuclear fusion in stars?

We know that the star’s core has hydrogen and helium in a large amount. The abundance of these lighter nuclei triggers the nuclear fusion in stars.

The hydrogen and helium nuclei are packed very tightly inside the stars like a huge gas cloud. A force of gravity will shrink the cloud at a very high temperature. The hydrogen nuclei fuse to manifest a helium nucleus as the temperature jumps up to a sufficient amount. The helium nuclei also form a dense cloud, so some pressure is exerted to initiate the nuclear fusion called as thermonuclear fusion reaction in stars.

To initiate hydrogen nuclei fusing, a temperature of about 15 million degrees must require.

File:Massive star cutaway pre-collapse (pinned).png - Wikimedia Commons, nuclear fusion in stars
Structure of star containing fusionable hydrogen and helium in inner core
Image credits: Wikimedia commons

Nuclear fusion in stars equation

Since the star consists of only hydrogen in its core at the very beginning stage, thus only the proton-proton reaction cycle will occur at the initial stage.

H+H→D+β++ν at Q=1.44MeV

Q is the value of positron annihilation by an electron in the star.

The deuterium-deuterium reaction does not occur because there is still a large amount of hydrogen nucleus left for the fusion process; thus, deuterium undergoes fusion with the hydrogen nucleus available for the fusion reaction.

H+D→3He+γ; Q=5.49MeV

The helium isotope thus produced combines with another helium isotope to give normal helium and hydrogen.

3He+3He→4He+2H; Q=12.8MeV.

The above equations predict the entire process of nuclear fusion in stars.

Steps of nuclear fusion in stars

Three major steps carry out the nuclear fusion in stars; the proton of the hydrogen is the basic particle available for the process.

  • Initially, two protons from the hydrogen fuse together. In most cases, the fused protons break apart, but sometimes, the proton itself transforms into a neutron through weak nuclear interaction intended for the formation of positron and neutrino. This forms proton-neutron pain inside the star, intended to produce deuterium.
  • There exists a third proton available for fusion. This proton collides with the deuterium to form a helium-3 isotope. Along with that, gamma rays are also generated inside the stars. The gamma rays thus released by the process emerge out of the core and are released in the form of light.
  • All helium-3 isotopes generated by the fusion reaction collide with one another to form ordinary helium, and along with that, two protons are also released in the form of hydrogen.
File:FusionintheSun sv.svg - Wikimedia Commons
Stages of nuclear fusion in stars
Image credits: Wikimedia commons

Before forming helium from the fusion of helium-3 nuclei, beryllium-6 nuclei are generated, which is quite unstable. Thus Be-6 undergoes a disintegration process to give a stable helium-4 nucleus. The helium-4 nucleus has a mass lesser than the original mass of the 4-proton, which insisted on the fusion reaction. This difference in mass is released as heat and light.

The energy released from the fusion process in stars has to cross many layers to emerge out into space.

What is released through nuclear fusion in stars?

The occurrence of nuclear fusion in stars induces energy to release out of the core in the form of heat or light, or sometimes some other nucleus. So it is said that nuclear fission in stars releases all elements virtually.

When the hydrogen nuclei induce the proton-proton cycle, it releases deuterium along with β+ particles with a certain amount of energy. As deuterium fuse with a proton, an isotope of helium and gamma rays are released. After the reaction of helium isotopes with one another, release helium-4 and hydrogen nucleus. These are the main product released during the nuclear fusion in stars.

File:Proton-Proton II chain reaction.svg - Wikimedia Commons
Release of elements and subatomic particle during nuclear fusion in stars
Image credits: Wikimedia commons

Some subatomic nuclei such as positron and neutrino are released. Some of the elements like Lithium, Beryllium, Nitrogen, and carbon are also released as a byproduct of the reaction. At the end of the luminous stage, the star iron is released and available for further fusion reaction. The production of elements from nuclear fusion is virtually called “Nucleosynthesis.”

Some interesting facts about nuclear fusion in stars

  • Nuclear fusion is the lead source of stellar energy; this theory insists on establishing a contraction hypothesis regarding the cause of the star’s rotation. It says that rotation speed is characterized by the conservation of angular momentum in stars. But some observation on stellar energy denies the prediction of the hypothesis.
  • Conservation of matter to energy in stars provides a road to explain the theory of how a small amount of matter can generate a large amount of energy given by Einstein.
  • In a recent study, the calculated combined mass of helium in stars is 0.8% less than the mass of 4-proton that initiated the fusion to form helium. This is evident that even a small difference in the mass can release enormous energy.
  • If a star consists of only 5% of hydrogen available for fusion, it is adequate to release a sufficient amount of energy in the form of heat and light.

Frequently Asked Question

What is the important significance of nuclear fusion in stars?

Nuclear fusion is stars explain the origin of a star and the luminosity and lifetime of the star. It is the main significance of nuclear fusion reaction in stars.

Nuclear fusion in stars also demonstrates the origin and occurrence of other elements of nature. Nuclear fusion helps to maintain the high temperature of the star’s inner core, which further tends to keep the fusion from occurring.

Do other elements released during nuclear fusion reactions in stars affect the further fusion?

If other elements generated during the fusion process are also involved, it would be affected further. But this situation never arises.

Since nuclear fusion in stars releases gamma rays which destroy the other elements generated; thus, it balances other elements not involved in the process. In some cases, the element thus produced during the reaction disintegrates to give the required proton and neutron for the fusion process.

Does nuclear fusion in stars stop?

As long as helium-4 generates at the stars’ inner core, the fusion reaction does not stop.

Suppose the production of helium-4 in the inner core stops, then there will be no available proton to cause further fusion reaction. At that instance, the nuclear fusion in stars stops, leading to the collapse of stars called a white dwarf.

Does gravity involve nuclear fusion in stars?

Yes, the involvement of gravity plays a vital role in the fusion process in the stars.

At the initial stage, when two protons are ready to fuse, the gravity compresses the inner core so that the core’s temperature becomes high and creates an environment for the fusion process to occur. And also inward pull of the gravity balances itself to push the energy released by the fusion outward till it reaches the outer core of the star.

Keerthi Murthi

I am Keerthi K Murthy, I have completed post graduation in Physics, with the specialization in the field of solid state physics. I have always consider physics as a fundamental subject which is connected to our daily life. Being a science student I enjoy exploring new things in physics. As a writer my goal is to reach the readers with the simplified manner through my articles. Reach me – keerthikmurthy24@gmail.com

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