The origins of the first chemical elements

If we take a moment in our busy schedules to look around, we will notice that the oxygen that fills our lungs, the iron that runs through our veins, the calcium in our teeth, the carbon in our genes, and the more than 50 chemical elements needed to make the mobile phone sitting on the desk were created millions of years ago inside a star.

The Big Bang and the first elements

When astronomers look at the sky, they see how galaxies are moving away from each other in an expanding universe. But, if we could reverse cosmic evolution by approximately 13.8 billion years, everything would come together in one dense, hot spot. As the clock went backwards in time, structures like galaxies would melt into a thick soup of primordial gas. If we went back even further, we would see this gas breaking down into a boiling sea of protons, neutrons and other subatomic particles. At this point, the universe would have a temperature of about 100 billion kelvins and a teaspoon of cosmic matter would weigh more than 100,000 tons. One millionth of a second after the Big Bang, the temperature of the universe will have cooled enough for the quarks to fuse into freely moving protons and neutrons. It will be necessary for the universe to begin to expand from 1032 to 109 kelvin for the first fusion reactions to begin. During the initial 3 minutes, the lightest and simplest chemical elements in the periodic table will be synthesized: hydrogen, helium and small amounts of lithium. Then the universe will expand and cool so much that the process of generating new elements will stop for millions of years, plunging it into darkness.

The birth of the first stars

It will take some 250 million years from the great explosion that gave birth to the cosmos for the first stars in the universe to be born. A star is an act of balance between two great forces of nature. On the one hand, there is the crushing force that the star's own gravity exerts, trying to squeeze the stellar matter and turn it into a dense and small sphere. On the other hand, there is an immense pressure derived from the fusion reactions that occur in the centre of the star and that try to push all that material outwards. Throughout its life it will be burning the fuel of its interior in different stages, in a constant fight against its own gravity. Its initial mass will mark its final destination, so that the most massive ones will manufacture elements faster, while the smaller ones will do it slowly, but for much longer. A young star is composed mainly of hydrogen, which is the simplest chemical element and the one that will give rise to all the others. At first, the two components of each hydrogen atom, proton and electron, are separated. However, the high pressure inside the star can bind two protons together, and sometimes a proton will capture an electron and form a neutron. When two protons join two of these neutrons they give rise to a helium nucleus, which becomes the second chemical element to appear. Similarly, when two helium nuclei fuse together they form the nucleus of a new element, called beryllium. This process continues, so that the fusion of beryllium with helium produces a carbon nucleus, the fusion of carbon and a helium nucleus leads to an oxygen nucleus, and so on. These fusion reactions are the origin of the nuclei of most chemical elements lighter than iron and are characterized by the release of energy, keeping the star alive. However, the fusion reactions that give rise to elements heavier than iron do not release energy, but rather consume it. If such reactions occurred, they would use up all the energy of the star and this would cause its immediate collapse. But not all stars produce iron. In less massive stars than the sun, the reactions are stopped by the creation of helium from hydrogen. In stars more massive than the sun but less than about eight solar masses, the additional reactions that convert helium into carbon and oxygen take place in successive stages before such stars explode. And only in very massive stars above eight solar masses does the chain reaction continue, producing the elements of the periodic table as far as iron.

El estallido de las supernovas

El núcleo de hierro es el núcleo más estable de la naturaleza, y resiste la fusión en cualquier núcleo más pesado. Cuando el núcleo central de una estrella muy masiva se convierte en núcleos de hierro puro, el núcleo ya no puede soportar la fuerza de aplastamiento de la gravedad resultante de toda la materia sobre el núcleo, y este termina por colapsar bajo su propio peso. A estas estrellas se las conoce como supernovas. Durante su rápido y violenta destrucción expulsan las capas superiores a velocidades de 15.000 a 40.000 kilómetros por segundo, enriqueciendo el medio interestelar de los elementos que lo forman. Además, en los pocos segundos posteriores a este proceso se dan condiciones de presión y temperatura tan elevadas que permiten la formación de elementos más pesados que el hierro, como el cobre, el zinc o el criptón. Las supernovas son asimismo capaces de acelerar algunas partículas hasta casi la velocidad de la luz, generando rayos cósmicos que propician la producción de elementos químicos, como el litio, el berilio o el boro, a través de la fisión nuclear.

The supernovae explosion

The iron core is the most stable core in nature, and it resists melting into any heavier core. When the central core of a very massive star is converted into pure iron cores, the core can no longer withstand the crushing force of gravity resulting from all the matter on the core, and the core ends up collapsing under its own weight. These stars are known as supernovae. During their rapid and violent destruction they eject the upper layers at speeds of 15,000 to 40,000 kilometres per second, enriching the interstellar medium of the elements that form it. Moreover, in the few seconds following this process, pressure and temperature conditions are so high that they allow the formation of elements heavier than iron, such as copper, zinc or krypton. Supernovae are also capable of accelerating some particles to almost the speed of light, generating cosmic rays that encourage the production of chemical elements, such as lithium, beryllium or boron, through nuclear fission.

The collision of neutron stars

The death of stars does not always end in a supernova. Sometimes the star collapses to a size of about 10 kilometres radius, with a mass twice that of our neighbouring Sun, and in which a teaspoon of matter can weigh up to 5 billion tonnes. All this, while spinning at up to 40,000 times a minute. In an object of these characteristics, matter is composed mainly of neutrons and a few protons and electrons. And, when two of these neutron stars collide, they not only cause ripples in space-time, known as gravitational waves, but also produce a powerful gamma-ray burst. As a result, some of the material that forms them is thrown out at high speed, giving rise to other heavy and rare elements such as gold, platinum or lead. It is possible to affirm that all the elements of the Mendeleev Periodic Table, and especially the fundamental atoms for life such as carbon, nitrogen and oxygen, come from the evolution of generations of stars that have been sowing the seeds for the formation of planets, moons and asteroids, as well as living beings. As the popularizer and astronomer Carl Sagan said, "we are made of stellar matter". We are simply stardust. After 13.8 billion years, the universe is now composed of 75% hydrogen, 23% helium and only 2% by mass of all other elements. However, this is a constantly evolving process, which will inevitably cause the presence in the universe of chemical elements heavier than helium to increase exponentially.

By Sebastián Jimenez.