Nuclear Fusion in Stars

Nuclear fusion processes that occur in stellar cores are one of the most important reasons which make life on Earth 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 star dust! Fusion in stellar cores is a subject that merges stellar physics with nuclear physics.

The Cooking Elements of Stellar Nucleosynthesis

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 form 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.

What is Fusion?

Nuclear fusion is the fusion or unification of light element nuclei into heavier element nuclei, at a temperature going into 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. Nuclear fusion and fission are different in another sense that fusion takes a lot of energy to be made possible. The efficiency of the reaction is very low. However, fission releases far more energy compared to what it is required to initiate it.

Imagine heating a gas slowly from absolute zero of temperature, 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 is into millions of Kelvins, the nuclei do not collide and glance away from each other, but they fuse. That is, they get combined to form a heavier element! If you keep an account of mass of combining nuclei in nuclear 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 form lost mass equals mass times the velocity of light squared!). This is the trick of the trade upon which the big business of nuclear fusion in stellar cores is based on, to create energy that keeps the stars shining.

Stars: Natural Fusion Reactors Powered by Gravity

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 matter, depends on what is the mass of the matter content. Stars form from 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.

What are the Starting Particles For Fusion?

The initial starting particles for fusion in stars are Hydrogen and Helium nuclei. You may ask, where were these initial starting particles of nuclear fusion 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 nuclear 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 Creation

Helium is created through nuclear 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 majorly 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 more massive than the Sun.

Carbon Creation

One day, after millions of years, Hydrogen fuel in 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 nuclear fusion against crunch of gravity, gives in again to gravitational collapse. The core again starts heating up, 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 thus by nuclear fusion in a stellar core, depends entirely on their mass. A star which is more than eight times mass 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 mass), 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.

Hope this article has whetted your appetite to study the beautiful objects that stars are and helped you understand how they create building blocks of our world, the elements or atoms, through nuclear fusion.