Need Help?

Get in touch with us


Nuclear Fusion in Sun’s Core

Aug 20, 2022

Key Concepts

• Core

• Radiation zone

• Convection zone

• Chromosphere

• Photosphere


• Corona

• Sunspots

• Nuclear fusion


Earth’s primary energy source is the Sun; however, the Earth only gets a small portion of its energy, and the Sun is only an ordinary star. Many stars produce more energy than the Sun. Nuclear fusion is the source of energy for all-stars. 

1: Sun

Structure of Sun 

The three main parts of the Sun’s interior are the core, the radiative zone, and the convective zone. 

2: Structure of the sun 


The core is situated at the center. It is the hottest part of the Sun, where nuclear fusion reactions occur. The core is the power generator in the Sun present in its center. In the Sun’s core, nuclear fusion takes place. It produces the energy that reaches Earth. The fusion reactions convert hydrogen nuclei into helium nuclei. The temperature in the core region is around 15 million ﹾK. The temperature is so high that atoms have been exposed to their electrons. Due to this, the core is a gas made of charged particles. The state of matter is made of charged particles in plasma. The core is about 150 times as dense as water, and it has a burning temperature of around 15 million ﹾC or 28 million ﹾF. 

Radiative zone:

This layer of the Sun is above the super-dense core. The density slowly decreases when it moves away from the core. Light emitted by nuclear fusion in the core region travels out in the next zone of the Sun called the radiative zone. This layer is not as dense as the core, but it is still so dense that light from the core region bounces around, which takes around 100,000 years to move through the radiative zone. 

Convection zone:

This layer of the Sun is present above the radiative zone. When the radiative zone’s density becomes low, energy from the core region, which is present in the form of light, is converted into heat. Energy (in the form of heat) is like the bubbles in a pot of boiling water. The heat from the side of the radiative zone rises till it cools sufficiently, then it sinks. This pattern of heated material rising, and cooling takes place in giant bubbles called convection cells. 

Sun’s Atmosphere 


When the material reaches the top of the convection zone, it cools by giving light. The photosphere is the first part of the Sun visible to us. This is the region from where the light we see from the Sun originates. We should never stare at the Sun directly without proper glasses. However, if we could look at the Sun directly, we would see the photosphere. The solar atmosphere starts from the photosphere, and its temperature is about 5,800 ﹾC or 10,000 ﹾF. 


The chromosphere layer is present above the photosphere and is around 2,000 km thick. As we move higher, the temperature rises to around 20,000 ﹾC (36,032 ﹾF) at the top of the chromosphere. The chromosphere is primarily red in visible light. It can be seen as red flashes in the period of a total solar eclipse. 

3: Structure of sun 

Transition zone:

The area between the chromosphere and the uppermost layer of the Sun’s atmosphere is called the transition zone. In the transition zone, the temperature rapidly rises. 

4: Structure of sun – Transition zone 

Corona :

The outermost part of the Sun’s atmosphere is Corona. The area of Corona starts around 10,000 km above the Sun’s photosphere. The Corona is usually hidden by the bright light of the Sun’s surface. That makes it very difficult to see without using special instruments. However, the Corona can be viewed in a total solar eclipse.  

The temperature in Corona is very high. It reaches up to 2 million ﹾC. The reason for this high temperature is still not known. 

The Coronal Mass Ejection, Solar Flares, and Solar Winds are associated with Corona. 

Coronal Mass Ejection (CME): The plasma and magnetic field release in a large amount are called CME (Coronal Mass Ejection). 

Solar Flares: These are the sudden flash of increased brightness due to the sudden release of magnetic energy. 

Solar Wind: The flow of energized, charged particles at a very high speed. The solar wind is made up of plasma and mainly contains electrons, protons, and alpha particles. 

Nuclear Fusion on Sun  

The Core:

The core is the power generator in the Sun. The core radius is close to one-fourth of that of the star. Inside the core, pressures and temperatures are high enough to make fusion. In a nuclear fusion reaction, the nuclei of two atoms come together to make a new atom. In the core of a sun, two hydrogen atoms combine to form a helium atom. The proton-proton cycle is an essential reaction that occurs within the core of the Sun. 

In the proton-proton chain reaction, hydrogen nuclei are converted to helium nuclei through many intermediates. This reaction generates high-energy photons (gamma rays) that move through the “radiative layer” surrounding the core. This layer takes up 60% of the Sun’s radius. 

 5: Proton-proton chain 

The helium nuclei make up 62% of the mass in the core region, and the remaining is still hydrogen. The radiative and convective layers have around 72% hydrogen, 26% helium, and 2% heavier elements by mass. The energy generated by the fusion reaction is then transferred to the solar surface and produced as light or emitted as high-energy particles. 

When the energy reaches the surface of the Sun, the temperature has come down to 6000 ﹾK. This [6000 ﹾK] is a temperature related to the sunlight we see. On average, the energy released from the hot surface is close to 230 million watts per square meter.  

Nuclear fusion is the source of all the energy emitted by the Sun, does two things: 1. It converts hydrogen atoms into helium (or, in other words, creates helium nuclei from protons), and it transforms mass into energy. 

Einstein’s equation explains the mass-to-energy conversion:  

E = mc2 


Energy = mass × the square of the velocity of light. 

As the velocity of light is a vast number, this equation tells that lots of energy can be obtained from using up a small mass. 

The energy produced by the process of nuclear fusion within the core of the Sun (or any other star) exerts outward pressure. If not contained, such pressure can cause an explosion. The internal pressure that keeps a star from exploding is the gravitational attraction of the gas mantle that surrounds the core (It is the majority of the volume of the Sun and is very hot but does not burn away by itself). 

The outward pressure generated from the nuclear fusion reactions prevents the stars from collapsing. The inward pressure from gravitation prevents the stars from bursting. If the nuclear fusion reactions in the core turn out to be very weak, a star can collapse. Such collapse can provide new situations in a core that develop in new types of nuclear fusion reactions; as a result, expansion follows. If the nuclear fusion reactions in the core turn out to be more strong, then a star can explode. Such incidents can be seen. 

When a star explodes, it shines with intense brightness; it turns from an unseen to a “new” star, called a “nova”. Stars, such as our Sun, where inward and outward pressure is appropriately balanced, fluctuate but little in brightness and give off a continuous energy flow. This balance is accomplished by self-regulation: a slight decrease in fusion energy can result in contraction, which can heat the Sun’s core and result in an increased fusion rate, and vice versa. However, in other stars, where the balance is not so perfectly tuned, vibrate noticeably. 

Therefore, the Sun neither expands (due to the constant explosion within) nor collapses (due to its weight) because the two forces keep the balance. In the long future, when this balance is disturbed because most of the hydrogen is used up, the Sun will expand and then, as we know, this will be the end of the solar system. 

So, on the whole, our star shines with continuous light. However, it has altered its output over geologic time. Also, it differs a little bit on a number of cycles. The most noticeable indication of this cycle is called the “sunspot cycle”, which explains periodic changes in the abundance of sunspots on the face of the Sun. The large quantity of spots is related to the brightness of the Sun. If there are more spots on the Sun, there will be more brightness. 

 6: Structure of the Sun 


• Earth’s primary source of energy is the Sun.

• Nuclear fusion is the source of energy for all-stars.

• The Sun can be categorized into different layers—the core, the radiation zone, and the convective zone.

• The inner layer of the atmosphere is called the chromosphere, and the outer layer is called the Corona.

• Proton proton cycle is an essential reaction that occurs within the core of the Sun.

• The darker regions of the Sun’s photosphere are called sunspots.

• The core is the power generator in the Sun.

• The Sun neither expands (due to the constant explosion within) nor collapses (due to its weight) because the two forces keep the balance

• Strong magnetic fields linked with sunspots can cause massive bending columns of gas called prominences.

• A sudden increase in the Sun’s brightness is called a solar flare.


Related topics

Define Position Time Graph and its Types

Key Concepts • Slope of a graph • Position time graph • Slope of s-t graph = Velocity • Types of position time graphs Introduction An object in a uniform motion covers equal distances in equal intervals of time. This also indicates that it moves at a constant velocity. When its position at different instants […]


Magnetic Field Lines: Definition, Explanation and Q&A

Key Concepts Magnetic Field Magnetic Field Lines properties of magnetic field lines Uniform and non uniform magnetic lines Introduction Two magnets when placed close to each other attract and stick to each other. However, if we go on increasing the distance between them, the attraction between them reduces gradually to such an extent that they […]


The Life Cycles of Stars: Meaning and Example

Key Concepts Stars Analysis of starlight Composition of stars Stars’ temperature Size and mass of stars Stages of life cycle of a star Introduction Stars are huge, shining balls of extremely hot gas (known as plasma) in space. The Sun is our nearest star. During the nighttime, many other stars are visible to the naked […]


Mirror Formula

Key Concepts New cartesian sign convention Mirror formula Solving problems using the mirror formula Introduction When dealing with the reflection of light by spherical mirrors mathematically, a set of sign conventions is followed, called the New Cartesian Sign Convention. According to this convention, the pole of a spherical mirror is taken as the origin and […]


Other topics