Nuclear Fusion in Stars

Nuclear fusion processes, that occur in stellar cores, are one of the most important reasons why life on Earth is possible. How is that so? 65% of your body is made up of oxygen and 18% is carbon. Guess which is the only process by which these two elements can be created?

That is right, they are created through fusion, in the stellar core. All that we are made of, was cooked in the belly of a star, millions of years ago. So, we all are indeed made up of stardust. Fusion in stellar cores is a subject that merges stellar with nuclear physics.

Nuclear fusion is the art of creating newer and heavier atoms, by fusing smaller nuclei together. The universe is like a great sculptor's workshop, who keeps on creating complex shapes and sculptures from clay. The kilns of this sculptor, where he creates new elements, are stars.

Matter can be converted into energy and energy back into matter and they are clay in the hands of that great sculptor (whatever you wish to call him). Thus, energy and its other form, matter, keep molding themselves into more and more complex forms, to create beings like us, who question the raison d'etre (Purpose of Being) of all this. It is the creation, questioning the creator.

Withdrawing ourselves from this anthropomorphic philosophy, let us get a bird's-eye view of creation of elements through fusion in stellar cores, and understand, how these great furnaces called stars, create new elements like helium, carbon, oxygen, silicon, and so on.

It is the unification of light element nuclei into heavier element nuclei, at a temperature reaching up to millions of Kelvins, liberating energy in the process. Nuclear fusion and fission are opposite reactions.

As opposed to fusion, fission is breaking up of heavier nuclei into smaller ones. Fusion absorbs a lot of energy, in its operation. The efficiency of the reaction is very low. However, fission releases far more energy, compared to what is required to initiate it.

Imagine heating a gas slowly, from absolute zero, to millions of Kelvins. As the gas is heated, the nuclei of the gas crash into each other, more and more harder. More the temperature, greater is the energy of collision. At one point, when the temperature has reached millions of Kelvins, the nuclei do not collide and bounce away from each other, but they fuse.

That is, they get combined to form a heavier element. If you keep an account of the mass of combining nuclei in fusion, and mass of combined nuclei after fusion, you'll discover that the mass of the fused nucleus is lesser than the mass of fusing nuclei. So what happened to the lost mass? It got converted into energy.

How much energy was it converted into? The answer to that question can be found from Einstein's relation 'E = mc2' (Energy created from lost mass equals mass times the velocity of light squared.). This is the trick of the trade, upon which the big business of fusion in stellar cores is based, to create energy that keeps the stars shining.

The sites of fusion reactions are the bellies or cores of stars, which are heated up to millions of kelvins. Why are the core of the stars so hot? How do stars generate that kind of temperature in their cores, which are the furnaces, where elements are cooked? The answer is 'Gravity'.

Gravity is an attractive force, which clumps matter together into tighter and tighter bunches. How hard it crunches it, depends on what the mass of matter content is. Stars form out of massive clouds of matter, that get crunched by gravity and heat up as they are compressed.

The cloud of gas is crunched into denser and denser clumps by gravity. As it gets denser, it heats up, until finally, the temperature at the core of the cloud reaches millions of kelvins, at which point, fusion starts. This turns stars into natural fusion reactors, powered by gravity. Sun is also a star formed through a similar process.

The starting particles for fusion in stars are hydrogen and helium nuclei. You may ask, where were these initial starting particles created? Hydrogen was not created in the belly of a star. Hydrogen and helium were formed in Big Bang nucleosynthesis. Hydrogen is the only element which is not created through fusion.

Even helium was created from fusion of primordial hydrogen nuclei, created through big bang nucleosynthesis. So, the starting particles of fusion are hydrogen and helium and stars are almost totally made up of these two elements.

Helium is created through fusion of hydrogen nuclei in stars. The process begins with the fusion of hydrogen into helium. All stars spend a major part of their lifetime in fusing of hydrogen into helium. Our Sun is also currently fusing hydrogen into helium and derives its energy from this process.

Helium is created through two separate processes, from the fusion of hydrogen nuclei. They are

• P-P Chain Reaction: This is the fusion of four hydrogen nuclei into a helium nucleus. This reaction is dominant in stars like our Sun.

• Carbon-Nitrogen-Oxygen Cycle: This process also leads to creation of helium from hydrogen nuclei, but it uses carbon, nitrogen, and oxygen as catalysts. This is the dominant process of helium creation in stars, which are more massive than the Sun.

One day, after millions of years, the hydrogen fuel in a stellar core will get exhausted, by its conversion into helium. Then, the star, which has till then maintained a hydrostatic equilibrium, by balancing thermal pressure generated through fusion, against the crunch of gravity, gives in again to gravitational collapse.

The core starts heating up again until it reaches a temperature, where three helium nuclei start fusing into carbon. In this process, carbon, the element on which all of our organic life is based, is created. Through a separate pathway, some oxygen is also created through helium fusion. Thus, two of the life-supporting elements on Earth are created through fusion.

How many elements are created by fusion in a stellar core, depends entirely on the mass of the star. A star which has a mass of more than 8 times, that of the Sun, will fuse elements from carbon, neon, oxygen, silicon up to iron, where fusion ceases. The iron core, being very stable, cannot fuse into any heavier elements.

Such a core (greater than 1.4 times solar masses), which cannot fuse iron, further implodes in a massive event called the Supernova. The core becomes a neutron star and the rest of the heavy elements after carbon, are created in neutron capture reactions, triggered by the Supernova. Thus, in life and in death, stars create the elements that make our world possible.