Our story begins in a vast cloud of protons, about 14 billion years ago. The cloud consisted almost entirely of hydrogen atoms, with one proton each, and a trace amount of helium, with two protons in each atom. With the mass of 100 suns, the cloud spread over a sphere in space about a light year in diameter. It had separated from an even larger cloud with the mass of 1000s of suns, and now began to shrink, drawn to its center by its own gravitational attraction. In this original cloud of protons, let us follow the progress of 26 of them.
The protons in the cloud moved extremely slowly at first, with a temperature only a few degrees above absolute zero. As the cloud gradually shrank, the protons falling toward the center of the cloud and traveling steadily faster, the temperature rose, and collisions between protons became more frequent. After millions of years had passed, the vast cloud had shrunk to a diameter a few times larger than our sun, the protons had fallen an immense distance and were now traveling at speeds of thousands of miles per second, and the temperature in the core of the cloud approached 12 million degrees.
At such high collision speeds, the single protons of hydrogen atoms fuse (after a rather complicated process) into helium atoms of two protons each, releasing enormous amounts of energy. With thermonuclear fusion now occurring in the core of the cloud, the shrinking of the cloud had stopped, and a stable star had formed.
For the next 100 million years this colossal star continued to convert hydrogen to helium, and along the way our 26 protons fused into 13 helium atoms. This star did not have rocky planets in its solar system – at this time in the history of the universe, only hydrogen and helium existed, and no rocks could form.
Gradually, the supply of hydrogen in the core was converted to helium, and the star began to run low on fresh hydrogen fuel. The rate of reaction fell, the star began to lose temperature and pressure, and the ever-present gravity forced the center of the star to shrink dramatically. Again gas fell toward the center of the star, causing the speed of the atoms to soar, raising the temperature to higher extremes, approaching 100 million degrees.
The combination of drawing in fresh hydrogen from the lower atmosphere of the star, and the beginning of helium fusion – three helium atoms combining to form one carbon atom with 6 protons – created a new source of energy to stop the further collapse of the star. This surge of energy caused the enormous atmosphere of the star to expand to fill a vast volume of space – about the diameter of the orbit of Mars in our solar system. The star became a red supergiant. Of our 26 protons in 13 helium atoms, three helium atoms (6 protons) were fused into a carbon atom, resulting in 1 carbon atom and 10 helium atoms.
After another few hundred thousand years of burning helium into carbon at the core of the aging star, helium became depleted at the core, and the reaction rate again began to fall. This caused another round of core collapse, speed-up of atoms, and soaring temperatures, enabling fusion of carbon to helium, producing oxygen (8 protons).
The same cycle repeated several more times, but with the protons rapidly forming larger and larger groups, there were correspondingly fewer and fewer possible reactions, and the time of each cycle was reduced dramatically. Oxygen atoms combined with helium to create neon, neon with helium to create magnesium, and similarly through the production of silicon, sulfur, argon, calcium, titanium, chromium, and finally – iron. Each iron atom contains 26 protons. The entire sequence starting at silicon lasted less than a day. As each larger atom fused, less and less energy was released in the reaction relative to the amount of energy needed to cause the reaction.
When, at last, iron started fusing to helium, more energy was consumed than was produced – and the star was dramatically COOLED. This caused the final, disastrous collapse of the star’s core, with the entire core rushing to the star’s center in a matter of seconds. The temperature exceeded several hundred billion degrees, and a chaotic fusion process in which any atom can fuse to any other atom ensued. In less than a second the energy equivalent of billions of stars was released, and the star became a supernova.
This star died dramatically. Our 26 protons in the iron atom were blown clear of the star, while the center collapsed into a black hole.
For the next 9 billion years, the iron atom traveled through interstellar space and intermingled with other clouds of hydrogen, helium, and the remains of other supernovas. At last, it found itself in another vast cloud of gas — this time laced with atoms heavier than hydrogen and helium, and dust. And this cloud slowly began to shrink upon itself, attracted by its own gravity toward its center. And the iron atom fell inward as well…
Hydrogen fusion started in the core of this shrinking cloud, and a smaller star was created. The dust and heavier elements formed rocks orbiting this star, and those rocks crashed into one another, slowly building planets. Our iron atom became a part of one of those rocks. After about a billion years, a system of eight planets had stabilized, but our iron atom remained part of a large rock for another billion years.
About 2 billion years ago, the rock containing our 26 protons in an iron atom crashed through the atmosphere of the third innermost planet around the star, and exploded in mid-air, sending our iron atom into the sea. After two billion years of geological transformations, the iron atom entered the surface soil in a farming area, and about a year ago made it into the feed on a beef farm. The atom, bonded into a complex bio-organic compound in the muscles of a steer, was inside the hamburger I ate last night, and now flows in the blood of my veins.
Although parts of this tale are approximated due to uncertainty in the exact history of iron atoms reaching Earth, the basic fact remains — all of the atoms of which you and I are made, and which surround us on this planet (apart from hydrogen) were formed in ancient supernovae of stars formed very shortly after the emergence of the universe. Fusion of atoms occurs only within stars (ignoring the few fused by Man), and of atoms larger than carbon, only within stars that become supernova.