Welcome to the Steel Story! In this blog, I hope to guide you through the history of steelmaking: from prehistoric man’s use of meteoric iron, up to the steel skyscrapers being built today. Since this is the beginning of the blog, I thought it only fitting to take you back to the beginning of iron – to the beginning of the universe.
Math, science, history, unravelling the mysteries that all started with the big bang!
The Barenaked Ladies – Big Bang Theory Theme
It all started with a big bang – an explosion of energy and matter which, within a few minutes, created all the hydrogen (H) and dark matter that the universe would ever contain. Einstein predicted that this could happen because of his famous equation, E = mc2. This equation states that not only are mass (m) and energy (E) related, they are interchangeable – with enough energy, matter (particles and atoms) can be generated, and this is how the very first atoms were formed.
Atoms are made up of a nucleus of protons and neutrons, surrounded by electrons (see the atom of beryllium (Be) above). A common notation for talking about elements is: where X is the element’s symbol on the periodic table, Y is the proton number (the number of protons in the nucleus) and Z is the nucleon number (the number of protons AND neutrons in the nucleus). The proton number of an element defines an element and its chemistry – Hydrogen (H) has one proton, Helium (He) has two protons and so on. Protons are positively charged particles, whereas neutrons have no charge. Two positively charged particles will repel each other (as would two negative particles), and so the protons within the nucleus are constantly repelling each other and should blow the atom apart – the mysterious force holding together is called the strong nuclear force.
This strong nuclear force means that forming an atom releases energy (the process is exothermic – it releases energy as heat), and that breaking apart an atom takes energy (the process is endothermic – it requires heat taken in to occur). The amount of energy it takes to form an atom is defined as the nuclear binding energy. It is often more useful to talk about the binding energy per nucleon – a nucleon is a proton/neutron, and this energy term represents the stability of an atom (higher binding energy = more stable).
The very first hydrogen and helium ‘atoms’ to be produced didn’t really look how we’d expect – they consisted of protons and neutrons, but their electrons were missing! This is because the early universe was exceedingly hot, and these elements were ions (atoms which have lost their electrons) in an ultra-hot plasma (a gas made up of ions). This plasma expanded, and in doing so cooled (because volume and temperature are related by the gas law PV=nRT). Fast forward about 400,000 years after the Big Bang, and the first atoms form when H and He capture electrons.
But where did the rest of the elements come from? This useful periodic table gives us a clue:
H and He atoms attract each other gravitationally in huge numbers to form stars; once large enough, the centre of the star becomes very hot and the process of fusion begins. Fusion is the high-speed collision of atoms, forming a single heavier atom and releasing some energy – this energy fights against gravity which wants the star to collapse in on itself and shrink to a tiny ball. The key to the process is binding energy – lighter atoms with lower binding energies per nucleon form atoms with higher energy per nucleon – this difference in energy is released as heat.
The collision of 2 H atoms produces He, and He and H atoms colliding produces lithium (Li). These are the two main reactions in a young star – but as they age, they run out of H and start burning heavier atoms – for example, it could fuse Li atoms together to create carbon (C), oxygen (O), neon (Ne), and silicon (Si). A star fusing these atoms is classified as a red dwarf star.
These dying stars have layers, with the lightest elements (starting with H) on the outside and the heaviest in the centre. Most red dwarfs will have a core of Si, and will not be heavy enough (and therefore hot enough) to fuse it – it will eventually fizzle out and die. Our Sun will eventually undergo this cycle. However, much heavier stars such as Betelgeuse (pronounced “beetle juice”) will be able to burn Si and heavier elements.
Very heavy red dwarf stars (those who were initially about 8 times larger than our Sun) will go on to burn even these elements, forming iron, chromium, manganese, cobalt and nickel. A star in this phase of its life is a white dwarf. The graph of binding energy vs. nucleon number above shows that iron has the highest binding energy per nucleon. This means that it cannot fuse as this would form a less stable atom – this is not energetically favourable. Therefore, once a star runs out of these elements, it cannot fuse Fe, Cr, Mn etc. and so it collapses in on itself, forming a neutron star. This incredibly dense, very, VERY hot star leads to shock waves – massive explosions which produce Ni and Fe atoms.
So all of the iron in the universe is produced in stars in two main ways: in the heart of a silicon-burning white dwarf, or in the shock waves of a collapsing neutron star.
Astrophysicists have modelled how the elements formed in dying stars then form planets, including our Solar System; their findings are summarised in the nebular hypothesis. It suggests that when our Sun formed (and similar stars), a large dust cloud gathered around it, consisting of elements from other dead stars – this dust is known as the circumstellar disc. Particles of dust collide with each other, causing them to combine together and grow. These larger, growing dust granules are called planetesimals, and their gravity can attract more dust, causing even more growth.
Eventually large planetesimals collide and their high energy impact melts them together, to form planetary embryos. This process takes 100 million years all told, until planetary size objects form. Since planets closer to the sun are hotter, they are necessarily made up of higher melting point materials – iron and silicates. This is why Mercury, Venus, Earth and Mars are ‘rocky planets’ rather than the gas giants further from the Sun.
Our Earth consists of a silica rich crust with a molten iron core, but due to the variety of planetesimals and dust grains that formed the planet, there is an abundance of iron and other metals in the silica crust too. This is what enables humans to create tools made of iron.
In the next blog post we will whizz forward several billion years to early Neolithic man, and how they were able to manufacture basic iron tools. See you in the next blog!
References & Further Reading:
Sixty Symbols – Creating the Elements
Nucleosynthesis and Chemical Abundance of Galaxies – Bernard E.J. Pagel
An Introduction to the Solar System – Neil McBride, Iain Gilmour
Abundances of the Elements: Meteoric and Solar – Edward Anders, Nicolas Grevesse
Sloan Digital Sky Surveys Blog