Six protons, six neutrons and six electrons all forge the fundamental element of life: the carbon atom. Thanks to its electronic structure, carbon has the ability to form a large number of different chemical bonds, that makes it the perfect candidate to build complex molecules, which are the basis of organic chemistry. For this reason, there are millions of compounds containing carbon. In fact, along with hydrogen, oxygen and nitrogen, carbon makes up, among other things, 96% of the human body. But where do the elements that constitute us come from? Although the explosion of a star lost in the universe may seem, from our perspective, the furthest possible phenomenon; the reality is that we are made up of nothing more than the result of those explosions.
The elements of the periodic table are distinguished by the number of particles they consist of. The lightest element of all is hydrogen, whose nucleus consists of only one proton. Next, comes Helium with two protons. Then, lithium (three protons and four neutrons), beryllium (four protons and five neutrons) and on to carbon (six protons and six neutrons), oxygen (eight protons and eight neutrons), until we reach very heavy elements, such as uranium (ninety-two protons and one hundred and thirty-four neutrons).
It is therefore possible to think of a chemical element as the sum of other, lighter chemical elements. This adding-up process, called nuclear fusion, is exactly what happens inside stars and has granted the formation of the atoms we are made of.
If the temperature of the early universe had allowed the fusion of heavy elements, life as we know it would not have existed.
Before we start talking about what happens in stars, however, we need to take a step back of almost fourteen billion years, to just a few minutes after the Big Bang. The temperature of the universe at that time had dropped to about a billion degrees. Protons and neutrons began to join together, forming the first primordial atomic nuclei. Simple, light atoms such as deuterium (a special form of hydrogen consisting of a proton and a neutron), helium and very small amounts of lithium and beryllium were synthesised. The whole process took no more than twenty minutes, what stopped it was a further drop in temperature due to the expansion of the universe. This is a crucial detail for mankind. If for any reason the process had continued for longer, the union of the lighter nuclei would have caused the formation of even heavier nuclei. After a certain crucial threshold, the light elements would have been extinguished, which would have meant that life, based on them, would not exist.
The next step in our chronological history is guided by gravity, that occurred about a billion years after the Big Bang, at the end of what is known as the “dark age” of the universe. The now-formed matter was distributed throughout the universe, and the areas of greatest concentration began to attract the surrounding matter under the pull of gravitational force. As these concentrations increased in mass and density, they began to form larger conglomerates: gas clouds, then stars, and finally galaxies.
Therefore, the first stars are formed solely of the lightest atoms. The formation of heavier atoms, from carbon and oxygen, resides in them. The idea of the mechanism is the following: in order to fuse two atomic nuclei into a heavier nucleus, it is necessary to provide enough energy to overcome the electromagnetic repulsion of the nuclei. In stars, gravity is so strong that it induces a contraction of matter which results in an enormous rise in temperature, thus triggering the process of nuclear fusion .
The life of a star is a perfect balance between gravity, which pulls the star inwards, and the pressure of nuclear fusion, which pushes it outwards. If the star contracts, the temperature rises, thus increasing the nuclear pressure and restoring the balance.
Once the star has run out of lighter elements, which act as the star’s ‘fuel’, under certain conditions its contraction can be such that the temperature rises further and triggers the fusion of even heavier elements.
However, nuclear fusion in stars alone cannot synthesise all the elements in the universe. To synthesise elements heavier than iron, even more astonishing phenomena are required, such as ‘supernovas’: explosions of stars that can no longer sustain their internal equilibrium.
What is interesting to know is that the process of fusion of nuclei, from the lightest to the heaviest, is constantly operating within every star in the universe. Our Sun, for example, consumes about six hundred million tons of hydrogen every second in its nuclear reactions. If we consider that there are a hundred billion stars in our Milky Way alone and that there are estimated to be 125 billion galaxies in the universe, we can imagine the number of light elements that are converted into heavier elements every second. It is amazing to think of these numbers, and of the number of processes that have led to the current chemical distribution of the universe.
In the infinite complexity of cosmic events, we are the result of a fortuitous series of cases. As written above, if the temperature of the early universe had allowed the fusion of heavy elements, life as we know it would not have existed. Today, we live in a cosmic interlude, in which the essential elements for our existence are used in every instant to synthesise newer and heavier elements. That is why one day, light elements may be entirely consumed leaving a universe consisting solely of heavy elements. Regardless, we should not despair: the light elements are still in large majority in the universe. Alone, the two lightest atoms, hydrogen and helium, make up 98% of the total mass of the universe.
 Actually, stellar temperature is not only held responsible, there is also another phenomenon described by quantum mechanics that cannot be examined in depth here. For those interested, I recommend searching for “Gamow Peak”.