This article first appeared in Issue 12 of our free digital magazine CURIOUS.
Carl Sagan said: “The cosmos is within us. We are made of star stuff. We are a way for the universe to know itself.” It’s a quote so unbelievably good it makes you stay up at night to take it all in. But of those words, let’s focus on two in particular: star-stuff. Sagan did not actually say stardust. Star-stuff might be a more vague description, but, in a way, it is more precise. We are about to find out why.
On one hand, you can easily argue that we are made of stardust. Roughly 99 percent of the human body is made of just six elements. In order of fraction of mass, they are: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Around 65 percent of your mass is from oxygen, 18.5 percent from carbon. Hydrogen makes up 90 percent of all atoms and is the only element in the human body that does not come from stellar processes or events.
To make a human from scratch
To find the origin of hydrogen, we need to start at the very beginning of the universe. Within a few minutes following the Big Bang, the cosmos was experiencing nucleosynthesis. The whole visible universe a few minutes after the Big Bang was relatively small. If we centered it from the Sun, it would not stretch much further than a few dozen of the closest star systems. And it was hot, a state called quark-gluon plasma. Quarks are the bits that make protons and neutrons, the particles at the center of atomic nuclei, and gluons make them stick together.
The universe was undergoing massive expansion, and as it became larger, it also became cooler – at least cold enough for protons and neutrons to pop into existence. Hydrogen in its simplest form is just a single proton, so as the protons emerged from the quark-gluon plasma, voila, you got hydrogen.
Some of the calcium in your bones and iron in your blood would come from the death of these stars.
The less common form of hydrogen is called deuterium, which has one proton and one neutron, and this ended up fusing into helium (two protons and two neutrons) and a tiny amount of other elements but nothing as heavy as carbon (in its most common form, six protons and six neutrons). At that point, the universe was 75 percent hydrogen and 25 percent helium (plus traces of other elements).
To make carbon and anything heavier, you’d need another source of nucleosynthesis: stars. Stars create heavier elements than hydrogen and helium by nuclear fusion. First hydrogen is turned into helium, then helium into carbon and oxygen, and then heavier elements if the star is massive enough.
When they get to iron, the system breaks down. It takes more energy to fuse iron than you get out. Without the energy production, the star collapses on itself, making more elements in the process and spreading those elements around the universe as it explodes in a supernova.
These violent ends have violent delights
To make us, you need all of that. Not just the Big Bang, not just stars fusing elements; you also need their evolution and eventual death, especially in the most dramatic and explosive fashion.
Some elements are produced in the explosion of white dwarfs. White dwarfs are stars that were not massive enough to go supernova by themselves, so their collapsed core ends up exposed. But if they have a companion, they might steal enough material to cross a critical threshold and go boom – in the process creating new elements. Some of the calcium in your bones and iron in your blood would come from the death of these stars.
The final process that forms the heavier natural elements in the periodic table is neutron star collisions. It’s the only way you can have the rapid neutron capture, also known as the r-process. This is a set of nuclear reactions that create half of the elements that are heavier than iron. It is responsible for stuff such as gold and platinum, but also for elements fundamental to human physiology. The crucial contribution of neutron star collisions to our composition is iodine. There is not much, but it’s a key element for our metabolism.
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So, to make us, you need all of that. Not just the Big Bang, not just stars fusing elements; you also need their evolution and eventual death, especially in the most dramatic and explosive fashion. You need big stars and small stars. Maybe even the biggest stars that ever existed, objects up to 10,000 times the mass of our Sun that lived for just 2 million years at the very dawn of star formation in the universe.
With all of this in mind, it is quite obvious we are made of star-stuff. We are here because stars have died. But the question we’re addressing is slightly different: are we made of star dust? The answer is partly – and now this is where we can properly nerd out.
Dust to dust, ashes to ashes
Stardust, cosmic dust, extraterrestrial dust – whatever you want to call it, the universe is peppered with it. But it’s not like Earth dust. First of all, it’s usually much finer, mostly between the consistency of smoke and structures made of just a handful of molecules. The biggest structures are micrometeoroids about 100 microns across (around 0.1 millimeters). And here we get to the issue: stardust has a very specific meaning in astrophysics.
Everything around you comes from stars. We are the product of those cosmic interactions.
It is dust created by stars, but not all dust is created there. So, it is a nitpicking argument but it is an important one. Firstly, it’s interesting to know that some stars created molecules, even some complex carbon-based ones. Stars make these molecules and they are then ejected into space. Some carbon molecules might eventually find their way to planets, where they might be used as the building blocks of living things.
So you see, we are stardust, but only in part. A common estimate of how much stars directly contribute to all the dust in the universe is around 6 percent. This might seem like a disappointing number, but we’re going to argue that it’s not. Being nitpicky about the question “are we made of stardust?” allows us to better appreciate all the sources that eventually made our planet and, in time, us.
Cosmic dust is created near stars, as they lose the outer layers of their atmospheres. It is created in the ejected material of novae, the ephemeral state where astronomers observe the sudden brightening of a white dwarf stealing material from a companion star. And of course, as supernovae explode, dust also forms in the ejecta of these cataclysmic events.
From the stars to our bodies
Dust is then processed in the interstellar medium, the matter that exists in the space between stars. Here, dust can be destroyed and reconstituted, it can be turned into something new. The interstellar medium is filled with complex molecules such as polycyclic aromatic hydrocarbons, precursor molecules to important organics like amino acids, the building blocks of proteins. The evolution of these dust grains occurs not only when stars are dying or have died, but also at their birth.
“Star stuff” is more matter-of-fact and less defined, but it encompasses all the shades of the stellar processes that directly or indirectly ended up making us.
Intense energy from the protostellar phase can create a reaction in the cloud of gas and dust that surrounds a fledgling star. Those processes will eventually lead the grains of dust to interact with others, to accumulate into pebbles, and from pebbles to planets – well, it’s just a short step in cosmic terms. Everything around you comes from stars. We are the product of those cosmic interactions.
But there is a molecule that we have not considered yet, not exactly stardust but stardust adjacent (sometimes literally attached to it): water. Water ice is a byproduct of star formation and it is the most common solid material in the universe.
The water in our rivers, oceans, drinks, and bodies formed long before the Solar System in the cooler parts of interstellar space where it can condense. But water vapor can also form in cool stars, those that have a temperature less than the value that causes water molecules to split. So, among that dust released by stars, there’s also water. Water that eventually found its way to Earth and to us.
Stuff is better than dust
We are a hopelessly romantic species. “Stardust” to us has connotations of something sparkly and magical, like fairy dust; a sprinkling of cosmic magic that makes it a bit more special. “Star stuff” is more matter-of-fact and less defined, but it encompasses all the shades of the stellar processes that directly or indirectly ended up making us.
Maybe “stuff” has less magical connotations but the grit of it has more power. Stars burned to make us, became giants, and went supernova. The densest stars in the universe merged to make bits of us, and the molecules that make us formed in burning atmospheres and freezing interstellar clouds. We are star-stuff, a patchwork of elements and molecules formed in a huge variety of cosmic environments. But we are more than just dust or stuff, we are the sum of universal processes that came together in what we call a human being.