Everything is made of something, and it seems like in the modern world there is an endless supply of "stuff" to make things out of. We have metals, plastics, ceramics, and wood to name a few and each of those categories themselves cover a wide range of subcategories (ie. "metals" includes copper, steel, zinc, bronze, aluminum etc.). It would seem at first that the universe has put a nearly limitless supply of materials at our disposal and one could be forgiven for assuming all of this good stuff came out of the Big Bang. As it turns out, the Big Bang didn't give us much.
Recall from the previous article that after the Big Bang, the universe was filled with about 75% hydrogen and 25% helium and that was it. Those were the only two types of atoms available in the early universe. Nowadays we have somewhere around 90 types of atoms that can be found in nature and that total swells to nearly 120 if you count the ones we have been able to artificially create in the lab. These are all arranged according to their size and properties on the periodic table. You'll notice that hydrogen (symbol H) and helium (symbol He) are right at the top. They are the smallest and simplest atoms so it's no surprise that they were the first to form in the universe.
The periodic table contains all 118 known types of atoms, 90 of which occur naturally
So if only two types of atoms could be found in the early universe and today we can find at least 90, where did the other 88 come from? This is a critically important question in piecing together our existence as human beings. We require numerous other types of atoms in order to exist: atoms like iron for your blood, potassium for your nervous system, and carbon for your DNA to name a few. So how do we turn a universe full of hydrogen and helium into a universe filled with the stuff we need to make planets and living things?
Recall in The Alchemist's Dream that although chemical reactions cannot transform one type of atom into another, nuclear reactions can. In fact nuclear reactions are the reason the universe had any helium to begin with. The 25% helium produced in the Big Bang was created by hydrogen undergoing nuclear reactions like the one below in the early moments of the universe.
Hydrogen can be converted into helium in a multi-stage reaction with other hydrogen atoms and neutrons
Don't get too bogged down with the details of the reaction. The point is that under intense heat and pressure, it is possible to convert one type of atom into another. Logically then, it must be possible to continue and convert helium into other larger atoms. Unfortunately, this requires more heat, pressure, and time than was available in the early universe. As a result, we're stuck with just hydrogen and helium to start with. We need another source of extremely high heat and pressure in order to make heavier atoms.
Although there may not have been much stuff to work with in the early universe, there was still gravity. Clouds of hydrogen and helium have mass and gravity will try to pull the material in these clouds towards each other. As the cloud becomes denser, the gravitational attraction between them strengthens and tries to pull them in tighter. Initially the cloud will be an irregularly shaped blob of gas but as gravity pulls it closer and closer together, this ever more massive blob will begin to take on a spherical shape. Gas that has not yet been packed into this growing sphere will be pulled towards it by its growing gravitational strength. Imbalances in the amount of gas coming from different directions will induce a spin and the system will begin to look a lot like a bathtub with the plug pulled as material drains into the spinning sphere at the centre.
As the system collapses, it begins to drain into the centre like a bathtub with the plug pulled
As more and more material is packed on, the pressure and temperature at the centre rise and will continue to do so as long as there is more gas available to add to the growing ball. Eventually it will reach a critical point where the pressure and temperature are high enough to cause nuclear reactions to happen between hydrogen atoms. Specifically, hydrogen-1 will begin to fuse to hydrogen-2 to form hydrogen-3 and then helium-4. This is basically the same reaction that occurred in the early universe to produce helium.
A ball of gas becomes a star once its core is hot enough to cause nuclear reactions
These nuclear fusion reactions release large amounts of energy which will push back against further collapse of the star. Whether this energy is enough to win out against gravity depends on the size of the star. Stars the size of our sun are small enough that hydrogen fusion can support the weight of the star against gravity. Larger stars however will continue to collapse, increasing the temperature and pressure in the core even further and allowing for helium to begin to fuse. It's at this point that we start seeing atoms like carbon that weren't created at the start of the universe.
Pressures and temperatures in large stars are high enough to cause helium to fuse
As new heavier elements like carbon form, they sink towards the centre of the star. Depending on the size of the star, there may be enough energy to cause them to undergo nuclear fusion reactions as well. In very large stars, the core will develop layers which fuse successively heavier atoms as the star ages.
Very large stars form layers as fusion reactions progress
It would seem at this point that we have a perfectly good explanation for how all the other types of atoms found on the periodic table formed. The Big Bang initially supplied hydrogen and helium, which were fused in very large stars to form successively heavier atoms. This explanation does a great job of explaining most of the table, right up until we reach iron. Iron has the symbol Fe and can be found in position 26 on the periodic table. Once a very large star has begun to produce iron, the structure of its core will look layered like the diagram below. Each layer as you move inwards contains heavier and heavier atoms undergoing fusion reactions.
Core structure of a fully developed massive star (not to scale)
But the star runs into a serious problem when it tries to fuse iron. Recall from the article on Nuclear Reactions that iron (specifically iron-56) and nickel (specifically nickel-62) are the most stable of all atomic nuclei because they are the most compact. All atoms ideally want to be like these two. When you fuse small atoms like hydrogen or helium, you bring them one step closer to being iron or nickel so energy is released. However trying to fuse iron into something heavier makes it less stable so it consumes energy instead of releasing it. Recall that stars rely on the energy produced by fusion reactions to support them against gravity so once a star begins fusing iron, it will begin to catastrophically collapse. Atoms heavier than iron prefer to be split into smaller parts since this gets them closer to their ideal size.
So if stars die shortly after trying to fuse nickel and iron then how do any of the heavier atoms in the periodic table exist? If this were the end of the story, we wouldn't have important atoms like gold (symbol Au, number 79) or silver (symbol Ag, number 47). Fortunately for us, the collapse of the star is not the end of the story.