Nuclear Fusion in the Sun

Sun is our star and the source of all energy on Earth. Solar energy is what sustains most of the life on earth through photosynthesis and sets the rhythm of our climate and seasons. Since ages, people have pondered about the source of Sun's extraordinarily high energy output which is, an emission of 1.74 X 10¹⁷ Joules of energy, per second. That is equivalent to the amount of energy that would be radiated by four thousand three hundred and fifty trillion 40-watt bulbs, in one second! What gives stars like our Sun, such limitless energy? Today, thanks to years of painstaking research, we know the answer! Nuclear fusion in the Sun, is the key which unlocks almost limitless power for the Sun!

Nuclear Fusion in the Sun's Core

Before we go into the details of nuclear fusion in the Sun, let us understand what we mean by 'Nuclear Fusion' and get acquainted with some fundamental ideas of basic physics and then stellar physics.

The Atomic Nucleus
This small introduction is for those, who are not familiar with atomic physics. Everything is made up of atoms. They are the smallest indivisible units of any object. The central core of an atom is called the nucleus, which is quite dense and packed with, most of the atom's mass, with electrons revolving around it.

The nucleus consists of two types of particles; protons and neutrons. A proton has a unit positive electric charge (1.6 x 10^-19 Coulomb) while the neutron is neutral! There are 92 different types of naturally occurring atoms. A type of an atom is decided by the number of protons in it. Hydrogen is the simplest type of atom that you could think of. Its nucleus is just a proton! Every atom is denoted by an abbreviation of its chemical name. When I want to denote Hydrogen, I use the symbol 'H'. Atomic weight is the total number of protons and neutrons in the nucleus.

What is Nuclear Fusion in the Sun?
Sun is a star and all stars are big balls of gas, primarily made up of gargantuan amounts of Hydrogen and Helium. To know what is nuclear fusion and how does it work in the Sun, one must know what are stars made of.

Nuclear fusion in the Sun is a process by which rapidly colliding nuclei, like those of Hydrogen, fuse together at very high temperatures, to form nuclei of higher atomic weight. In this process some mass is lost and converted into energy. That is the secret of Sun's energy production! The amount of energy obtained from conversion of 1 gm of matter into energy (given by Albert Einstein's celebrated equation, 'E = mc²) would be roughly 9 X 10¹³ Joules! So matter is just a form of energy! They are two manifestations of the same thing!

Where does Nuclear Fusion in the Sun Occur?
Nuclear fusion occurs in the Sun's core, which, not coincidentally, is also the hottest part in its whole constitution. The total radius of the Sun is 6.955×10⁵ km (about 109 times radius of Earth!). Its core extends from the center to about 1.8 lac kilometers, with a temperature of 14.5 million Kelvin!

Let me explain why does nuclear fusion in the sun occur only near the center. The reason for it, will be found, if you try to understand how was the Sun formed. Stars like the Sun are thermonuclear fusion reactors.

Nuclear fusion in the Sun, is a merger of smaller nuclei into heavier ones, releasing a lot of energy in the process. However, Hydrogen nuclei, which are protons, do not fuse easily. The reason for that is a fundamental fact of nature, which is, 'Like Charges Repel Each Other'. That is, a positively charged proton repels another one of its kind, because they have same charge. This is called 'Coulomb Repulsion'. So nuclear fusion can occur only at a high temperatures, at the central core of the Sun. Sun's core is hottest due to its phenomenally high density (150gm / cm³) caused by crunching under gravity!

Nuclear fusion in the Sun is only possible when this repulsion between protons is overcome. For that to happen, energy and temperature at the Sun's core has to be really high. However, nature has arranged it such that, the fusion in sun's core can occur at a much lower temperature, than that required to overcome Coulomb Repulsion! How does nature pull this trick off?

Nuclear Fusion is Possible Due to Barrier Penetration
The reason why protons with energy lesser than that required to overcome Coulomb Repulsion, fuse, is 'Barrier Penetration'! It is a quantum physics concept. Consider the following analogy. Imagine an adamant old man, trying to scale a wall. He doesn't have the energy or the tools to climb it. Still he thinks that, if he keeps banging and ramming into the wall, one day it will give in and he will be on the other side by 'tunneling' through. The chances of that happening in the 'Classical' (I mean the Non-Quantum Mechanical) world is zero. However, if our old man was the size of a proton (< 10^-15m) and the wall represented the coulombic energy required to overcome repulsion, then if he keeps hitting the wall, there is a chance he will tunnel through! In the weird, sub-microsopic world of quantum mechanics, there is always a probability that the protons will fuse together at an energy that will be, lower than required energy to climb the 'Coulomb Repulsion' hill.

Since the probability of 'tunneling' through Coulomb barriers is very low, fusion processes in low mass, relatively cooler stars like Sun, occurs very slowly. That is why star's like the Sun, burn or rather fuse their hydrogen fuel into Helium at a very low rate and have long lifetimes.

What Happens During Nuclear Fusion in the Sun?

The process of nuclear fusion in the Sun occurs by two major path ways! Here is a brief discussion of how these process power the nuclear furnace of our Sun.

Solar Nuclear Fusion Process #1: The Proton-Proton Chain Reaction
This is the dominant fusion process in the Sun. As I explained before, this phenomenon is possible due to 'Tunneling' or 'Barrier Penetration'. There are a number of stages through which nuclear fusion in the Sun occurs, in a 'Proton-Proton Chain Reaction'. Through this process, Hydrogen fuel is progressively converted into Helium, along with release of energy in the process.


A graphic illustration of proton-proton chain reaction (Image Credit: NASA)

The first stage in p-p chain reaction is the fusing of two hydrogen nuclei into a Deuterium (An isotope of Hydrogen) nucleus. I will not discuss the four alternative pathways by which the rest of the fusion process occurs to give us Helium-4, as it could easily fill up a book with all its details. The difference between the 'fusing masses' (the four protons) and 'fused mass' (Helium-4) is 0.7% of the total mass of 4 protons, which is converted into energy. The total energy produced by the fusion of 4 protons is 26.73 MeV. One electron volt (eV) is the energy gained by an electron as it passes through a potential difference of 1 Volt.

Solar Nuclear Fusion Process #2: Carbon-Nitrogen-Oxygen (CNO) Cycle
This nuclear fusion process occurs very marginally in the Sun, but is the dominant fusion pathway in stars 1.5 times more massive, than our Sun. This process also fuses four protons into a Helium nucleus, by using Carbon, Nitrogen and Oxygen nuclei as catalysts. The end products of both the processes are same. However, CNO cycle is dominant in stars with stellar cores much hotter than that of Sun (in the range of 13 Million Kelvin). However, despite occurring at higher temperature, overall energy released through the whole reaction is again 26.8 MeV, which is round about the same as p-p cycle. Through both these processes, about 3.6 X 10³⁸ protons are fused and converted into Helium Nuclei releasing 3.8 X 10²⁶ Watts of energy every second!

These processes of nuclear fusion in the Sun are the most dominant ones. There exist many more processes, but they occur marginally in the solar core. Nature always has many alternative ways to do the same thing. That is her way of giving us creative inspiration!

Hope this article has given you a good idea about nuclear fusion in the Sun and how it has been keeping our Sun, burning and bright for the past 4.5 billion years.