The Universe was probably born from what cosmologists call the Big Bang. Thirteen, fourteen billion of years ago its now scientifically observable region was contained in a volume smaller than a pin head. The density of matter and energy was incredibly high, and consequently the temperature was too. Matter was different from the one we know today: heavy chemical elements, from carbon up to iron and the heaviest and radioactive materials, simply didn’t exist. The temperature was too high for atoms to be formed and the Universe was a sort of soup of matter and energy.
During the first second, due to the extremely rapid expansion of the Universe, the temperature plunged to a hundred billion degrees and neutrons, protons and electrons started stirring at too high a speed for atomic nuclei to be formed. After a hundred seconds, the temperature fell to about a billion degrees: neutrons and protons slowed down forming nuclei of deuterium, a kind of hydrogen with a neutron more. Occasionally, these nuclei collided forming nuclei of helium, and even more rarely they formed nuclei of lithium. In a few minutes, all the matter in the Universe – which was cooling down due to the expansion process – came to be exclusively made of deuterium, helium, and lithium. 95% of the atoms were atoms of deuterium, 5% were atoms of helium and only scattered traces of lithium did exist.
As the Universe kept expanding, atoms formed gigantic gas clouds that were the only existing matter for millions of years. After about 200 million years, gravity started to attract the atoms of the clouds giving life to stars, which essentially formed due to density variation in the clouds. At the beginning, stars were just gigantic gas spheres, but then gravitational attraction slowly started to compress these protostars causing an extremely high pressure in their centres, which in turn caused a tremendous temperature rise. Atoms intensified their excitement until their collisions set in motion a fusion process that started to transform a part of the matter into energy. This is a known process of atomic physics called hot fusion: when an equilibrium is reached between the energy flow from the centre of the star and the pressure exerted by gravity, the star can start its life cycle where the central fusion produces a massive energy flow towards the outside and the void surrounding the star. When the light atoms population completes its fusion process, it engages the products in a new fusion process that, following a predictable pattern, generates heavier atoms and the chemical elements that we find in the matter all around us. Through different fusion processes, stars of different sizes came to a critical state that forced them to explode scattering into the void huge clouds of materials that not only included deuterium, helium, and lithium, but also carbon, iron, and radioactive materials. These clouds started forming new stars, but a percentage of the heavy materials remained in the form of new clouds.
All the matter that we are made of comes from the fusion processes in the stars: our body exists and is made of this stardust. If stars hadn’t appeared during a phase of the Universe’s evolution we wouldn’t physically exist, or we would essentially be made of hydrogen and thus we would evaporate in a few seconds.
The birth of a star due to gravity, its long history during which it shines in a corner of the Universe and, after going through its specific fusion cycle in the inner nucleus, explodes in a gigantic nebula of galactic dust where new stars are born, is a cycle that keeps happening in every corner of the Universe populated by stars. Matter as we see it on our planet, which includes our own body, is nothing more than the material created in the nuclei of the stars and scattered all over the Universe at the end of their extremely long life. We are all stardust.