By Adam McMaster
Look at a nearby object. Any object – it doesn’t matter what it is. It could be the device you’re reading this on, a mug, a desk, or the ground. Just look at it as closely as you can. You might be able to make out bumps in plastic or lines in wood grain, small cracks or imperfections. Maybe you can start to see how the material is structured, but no matter how closely you look you won’t be able to see the smallest constituent parts. The molecules. The atoms. The protons and neutrons. The quarks and electrons. Everything is made from these same particles and you are surrounded by an uncountable number of them. Have you ever wondered just where they all came from?
The Universe is great at recycling. Everything around you came from something which was here before. That’s true at every scale. Your clothes, for example, are made from various fibres which were ultimately all made from plants and animals (even the synthetic ones were made from oil, the result of long-deceased organisms). All those materials are made from hydrocarbons, meaning molecules of hydrogen and carbon. At some point, a living being created them by combining carbon from the air with hydrogen from water. The hydrogen and carbon were, most likely, here when the planet formed. The story of how they got here is long, but interesting. It spans the entire history of the Universe. Let’s start at the beginning.
You will have heard of the Big Bang. That’s the name given to the earliest moment that we can study, immediately after the Universe was created.
At this time, the Universe was mostly filled with photons and neutrinos, with a smaller number of electrons that the photons could interact with, and elementary particles that would soon give rise to protons and neutrons. For a few minutes, the Universe was hot and dense enough for protons and neutrons to overcome their natural repulsion and fuse together into nuclei, forming hydrogen, helium, and lithium (plus a few isotopes of other elements which aren’t stable and didn’t last long). We can measure the amounts of these early elements in the most distant (and so the oldest) galaxies, quasars, and gas clouds, providing an important piece of evidence that the Big Bang theory is correct.
At this point the Universe was a few light years across, but it wasn’t yet transparent – light could not move more than a short distance without being absorbed by some particle or another. It was still too hot for electrons to bind with atomic nuclei, so the Universe was filled with a mix of positively charged nuclei and negatively charged electrons. All these charged particles, so densely packed together, impeded the free movement of photons. It wasn’t until the Universe expanded enough for things to cool down, after 377,000 years, that the background radiation cooled enough so that electrons, protons, and nuclei could stably bind together. At this moment, the clutter of charged particles was cleared, and light was free to travel the length of the Universe. We see the evidence for this in the cosmic microwave background (CMB). Whichever way you look, space is filled with a low frequency background of microwave radiation. This is the leftover light from the moment the Universe became transparent. We can detect the CMB with radio and microwave telescopes, and by doing so, we capture photons which have been travelling unhindered for 13.8 billion years.
At the moment the Universe became transparent it was suddenly filled with matter as we know it – atoms consisting of a nucleus (protons and neutrons) surrounded by electrons, with no overall electrical charge. With no repulsive charges to keep them apart, small differences in the density of matter at different positions caused it to start clumping together under its own gravity. The first large-scale structures started forming. Matter came together first into slightly dense patches, and then into molecular clouds. The clouds continued to fall into themselves, collapsing further. And then, no later than 250 million years after everything began, the clouds started reaching a critical density, the hydrogen in them started to fuse together, and the first stars switched on. These stars lit up the Universe and pushed away the excess material around them, becoming bright islands in the fog of primordial hydrogen. After the first stars formed, they started to group together into larger and larger groups, eventually forming small dwarf galaxies. These dwarf galaxies then merged, forming larger and larger galaxies, eventually resulting in the huge galaxies we see in the Universe today.
All the structure that we know and observe today is powered by two things: gravity, which pulls matter together, and stars, which provide the starlight illuminating the cosmos. The longest period of a star’s life is called the main sequence. This is the most uneventful part of the star’s life, during which not much really changes and the star is stable. In the star’s core, nuclear fusion turns hydrogen into helium through a series of reactions known as the proton-proton chain reaction. This lasts as long as there is enough hydrogen in the core – how long exactly depends on how big the star is. Stars are mostly made of hydrogen, so there is plenty of fuel to sustain fusion for a long time. For a star like our Sun, which is a medium-sized (G type) star, this will last eight or nine billion years – our Sun is half way through this. Stars which are more massive than the Sun live shorter lives, because big stars are brighter and use up their fuel more quickly. Stars which are smaller than the Sun will live a lot longer: the smallest will live for trillions of years.
Eventually, though, the star’s supply of hydrogen starts to run out and fusion starts to slow down. That’s when things start to get interesting. While fusion is still in full swing, the outward (explosive) force of the fusion counteracts the inward (crushing) force of gravity. The star’s main sequence life is marked by these two vast forces being in balance with each other. When the star’s fuel starts to run low, that balance is broken. Fusion slows down, and gravity starts to win. The star starts to collapse and the core comes under more pressure than it’s ever felt before. Initially fusion can stage something of a comeback, as the increased pressure starts fusing heavier elements (heavier elements require more energy to fuse together, and contracting more pushes their atoms together with more force, providing that energy) – first turning helium into carbon, and then working its way up the periodic table – but that can only keep the balance for so long.
The smallest stars, red dwarfs, end their lives in the least dramatic way. Because they’re so small, convection thoroughly mixes their materials during their lifetime. This means their cores are continuously provided with a fresh supply of hydrogen, as long as there is hydrogen left in the star’s outer layers. The process of mixing new fuel into the core means red dwarf stars are the only ones which use up their entire supply of hydrogen, and that’s why they live for so long. When their fuel eventually runs out the star stays stable and becomes a white dwarf: a dense, dead star with no fusion in its core.
In Sun-like stars, this bigger, hotter fusion takes things too far in the other direction and starts heating up the layers of material just outside the core. These layers eventually get hot enough that fusion starts happening there as well as in the core. This provides a kick of extra force that causes the star to expand, cooling and getting redder as it does (as with fire, red is colder than blue). The outer layers expand so much that they will engulf any planets close to the star – this fate awaits the Sun, the Earth, and the planets interior to us. Eventually, the star will blow off its outer layers once it has fused the heaviest elements it can, leaving behind only the dense core, now a white dwarf, surrounded by a cloud of gas known as a planetary nebula. (They are called this for historical reasons, because their similarity to planets, as seen through early telescopes). The planetary nebula, whose materials will eventually return to the interstellar medium to help form new generations of stars, contains some of the elements which were created inside the star.
For bigger stars the end is somewhat more dramatic. These stars don’t push away their outer layers. Instead, they keep contracting because the crushing force from gravity is so much stronger. As the material in the core contracts a little further, it starts fusing heavier and heavier elements, slowing down the contraction for a while each time until the new fuel starts to run out and everything contracts further still. This cycle repeats until it suddenly hits a dead end. Energy is released when you fuse atoms together, because it turns out that when you fuse two atoms together, their total combined mass is slightly less than when they were separated. That missing mass is converted to energy and released as photons of light. But there’s a big caveat – that’s not true for all elements. It’s only true for elements which are lighter than iron. That’s why we can also generate energy through fission: by splitting apart elements like uranium. Those elements are a lot heavier than iron, and their combined mass is actually slightly higher than the mass of the elements they split into, so splitting them apart releases energy and fusing them absorbs energy. Iron is where the balance flips and fusion starts absorbing energy.
So what does that mean for a large, elderly star, gradually working its way up the periodic table, fusing heavier and heavier elements? It means that eventually that star is going to create iron. And when it does, it’s all over. It will be able to create iron atoms just fine, but doing so will use up energy rather than releasing it. That means once the star starts producing iron, there is no longer anything stopping the relentless crushing force of gravity. The core collapses under its own weight and nothing can stop it. This unhindered collapse results in a supernova, wherein the core collapses so rapidly that it triggers a sudden burst of fusion, and the star explodes leaving a neutron star or a black hole in place of the core. During the collapse and the explosion, and just afterwards as everything starts to cool back down, the star produces the gamut of heavy elements in the Universe.
Supernovae aren’t the only place that heavy elements can be created. Mergers between neutron stars are actually the main source of these elements. Neutron stars are so dense (they’re a little more massive than the Sun, but compressed into a sphere a few miles across) that when two of them collide, they produce an event known as a kilonova (famously detected in 2017 through both gravitational waves and electromagnetic radiation). During the kilonova, nuclei fuse together into the heaviest, unstable elements, and then decay into more stable ones.
Everything heavier than iron – the gold in your jewelry, the copper in your electronics, the americium in your smoke detectors – it’s all created in those fractions of a second when giant stars collapse and neutron stars merge, and in the surrounding material afterwards. Supernovae and kilonovae create all that material and send it out into the Universe.
In the somewhat arcane parlance of astronomy, everything other than hydrogen and helium is called a “metal”. The amount of metals in a star (i.e. how much of it is made of stuff other than hydrogen and helium) is called the star’s metallicity.
We’ve seen that each time a star dies it releases metals into space, and we’ve seen that new stars are formed from the remains of old ones. That means that each new generation of stars contains a higher fraction of metals than the previous generation. This is something we can measure with spectroscopy (looking at which wavelengths of light are emitted and absorbed) and it is one way of estimating how old stars are. Stars with low metallicity formed earlier in the Universe’s life (they’re older) and stars with higher metallicity are younger.
Astronomers divide stars into three populations by their metallicity. Population I are the youngest, high metallicity stars. Population II are old, low metallicity stars. Population III are virtually metal-free – they’re the first stars that formed (though none of these have ever actually been discovered yet).
The size and colour of stars is different in later generations compared to earlier ones. Earlier generations of stars tended to be bigger than today’s stars, so their lives tended to be shorter. Their bigger size also tends to make them bluer than we’d otherwise expect. All stars start off bluer and get redder as they age and cool down, but older stars stay bluer for longer because they start hotter.
There’s nothing special about the Sun; it’s a Population I star like any other and it formed like all the others: from a cloud of gas and dust left over from the stars which came before. Almost all that material ended up in the Sun itself – the Sun comprises 99.8% of the mass of the solar system. What was left formed the planets. As the Sun began to form – as the gas cloud was collapsing – the conservation of angular momentum caused the cloud to start rotating and to flatten into a disk. This disk of material is known as a protoplanetary disk, and it was from this that all the planets and their moons formed, along with the comets and asteroids, and everything else.
A star’s metallicity tells us something about any possible planets orbiting the star today, since the metallicity of the protoplanetary disk determines what material was around when the planets formed. A star with lower metallicity is less likely to have as many large planets around it. Planets – including gas giants – need a solid core to form around. Metals in the dust cloud act as a seed for new planets to form. The protoplanetary disk evaporates during the violent, early part of the star’s life, known as the T Tauri phase, when there are extremely strong stellar winds, with lighter elements being blown away from the star. The disk will evaporate more quickly during the T Tauri phase if it has a lower metallicity, allowing less time for planets to form (so they can’t get as big). A correlation between metallicity and the number of gas giants has been confirmed by comparing metallicity (spectra) with exoplanet surveys, such as Kepler. Kepler has found small planets around stars with varying metallicities, but there don’t seem to be as many big planets where the metallicity is lower. That means that a solar system like ours could only form after several generations of stars had been and gone.
Once the solar system’s protoplanetary disk formed, small differences in density caused some of the material in it to start to clump together. Heavier materials started to form rocky cores around which gas and lighter solids started to gather. The clumps collided and merged, gathered more loose material, and kept getting bigger. First to the size of pebbles, then boulders, then on to asteroids, and then planets. In some areas, closer to the Sun, there was a higher proportion of rocky material (because of the T Tauri winds), and this is where planets like Earth and Mars formed. Further out things were dominated by hydrogen and helium gas, and this is where the gas giants formed.
After the planets formed they migrated around and there were lots of collisions – times were turbulent. “Planet” comes from the Greek for wanderer and the planets in the early solar system really lived up to this name. The gas giants interacted with each other, changing one another’s orbits and scattering the smaller bodies, sending large numbers of asteroids falling into the inner solar system and triggering a period known as the Late Heavy Bombardment. Some would-be planets were thrown into the Sun. Others were ejected from the solar system, banished to interstellar space. We don’t know where they are now – they’re lost in the abyss, but undoubtedly still out in the galaxy somewhere. Planets collided with other planets. Some were destroyed, smashed into countless small pieces. Others held themselves together and merged into bigger planets. The planets we see today are the ones which survived this chaos. The lucky ones.
Even the Earth was nearly destroyed at one point. There is evidence that the Earth of today is the result of a giant collision between two smaller planets – Gaia and Theia. Theia was a similar size to Mars, and Gaia was much larger, but when the two collided head-on they were both nearly shattered to pieces. The collision threw enough material into space that it went into orbit around the Earth and coalesced to form the Moon. Chemical analysis of rock samples from the Apollo missions confirmed that the Moon is formed from two distinct sources.
At first Earth was a hot, dry planet, covered in molten rock. The high frequency of early impacts during the Late Heavy Bombardment, and the size of the impacting bodies, triggered volcanic activity that kept the surface of the Earth molten for hundreds of millions of years. Eventually, as things calmed down in the wider solar system, the really big impacts became less frequent (though collisions of all sizes continue to this day – think shooting stars and dying dinosaurs). The Earth began to cool once the collisions slowed down. That’s when things on Earth started to get really interesting. As things cooled, oceans began to form. Exactly where all the water came from is still up for debate, but the water we have today is a mix of water from comets (brought here during the Late Heavy Bombardment) and water that was originally chemically bound inside rocks, and released as vapour during volcanic episodes. Wherever it came from, the condensation of water out of the atmosphere and into oceans opened the way for life to form.
Everything on Earth – the rocks, the air, the water, your own body – are made from material which used to be part of a star. That star died and then our Sun formed from the leftover material 4.6 billion years ago. The Earth formed 100 million years later. In a literal sense, we really are all made of stardust. The very atoms inside us were part of stars that died long before our planet existed. It’s hard for us to comprehend the scale of the Universe. It’s easy to think of the Universe as something out there, something separate from us. That separation is an illusion. There is no here and there. Everything is the same. The same laws of physics. The same atoms. The same Universe. It’s as much as part of us as we are a part of it.