We are stardust. And Big Bang dust.
Neutron star collisions appear to be essential to our chemical origin story, writes Katie Mack.
When, in 2017, the LIGO experiment detected gravitational waves from two neutron stars colliding, it sent electromagnetic and gravitational ripples through the universe and the astronomical community.
This remarkable event, hotly anticipated but never before seen in this way, did more than give us a few new data points about the deaths of stars – it fundamentally changed our understanding of where we and our constituent atoms come from.
You may have heard before that “we are stardust”. This isn’t wrong. But it’s not the whole story, either.
A star is, fundamentally, an alchemy machine. It starts as a giant ball of mostly hydrogen gas, slowly crushing its central regions with the pressure of its own gravity. The core of a star eventually gets so hot and dense that it becomes a nuclear reactor, fusing hydrogen into helium.
In the core of our own sun, this process is converting hundreds of millions of tons of hydrogen into helium every second; what we receive as sunlight is essentially just the waste heat from the reaction.
This is how the vast majority of stars spend their lives: steadily burning themselves up, turning hydrogen into helium for billions of years. In their final death throes, as they become red giants ready to expel their outer layers, the fusion flares up in bursts, making lithium, carbon and nitrogen, and a smattering of heavier elements.
To fill in the rest of the periodic table, though, we need stars much more massive than our own. A star more than about eight times as massive as the sun contains at its centre a nuclear furnace that’s burning unimaginably hot.
After it tears through its supply of hydrogen in the core, it climbs up the list of elements, burning helium, carbon, neon, oxygen and silicon, until after only a few million years the centre of the star is iron and the fusion radiation that had been puffing the star up finally runs out.
At that point, nothing can stop the star from collapsing on itself, resulting in a spectacular supernova explosion. In the end, at the centre of the debris field will be either a super-dense neutron star or a black hole.
It’s this final explosion itself, rather than the interior burning, that creates the star’s ultimate chemical legacy. For a brief moment, a shock wave explodes through the layers of the star, creating heat and pressure so intense that a blast front of nuclear fusion carries a radioactive shell of new elements out into interstellar space.
The universe is seeded with stardust, ready to coalesce into new stars, new planets, new life. For years, it was thought that these stellar deaths were the main mechanisms by which the universe was enriched with metals and other heavy elements.
But evidence has been mounting that for heavy metals like gold, platinum and uranium, the supernova is just the beginning. It’s the tiny, dense, neutron star that carries within it the potential to explode across the rest of the periodic table.
Which brings us back to the LIGO detection. When the signal was first seen, astronomers around the world trained their telescopes on the same part of the sky. The resulting observations showed a clear sign in the brief flash that the stars had created enough gold to outweigh the Earth several times over.
Neutron star collisions appear to be essential to our chemical origin story. We are born of unimaginable violence in the stellar generations that came before our own. But there’s more to the story.
Most of the atoms in our bodies didn’t come from stars at all. They are, in fact, much more ancient. If you count up all the atoms in your body, more than 60% will be hydrogen, and the majority of the hydrogen in the universe has never been in a star at all.
Hydrogen, or, specifically, the protons that would later join with electrons to make neutral hydrogen atoms, was created in the primordial fire of the Big Bang itself.
In the first moments of the universe, every part of space was filled with a kind of prenuclear plasma hotter and denser than the centre of even the most massive star.
As this fire expanded and cooled, protons and neutrons, the building blocks of atomic nuclei, first came into being.
Hydrogen appeared in the form of solitary protons, along with small amounts of helium and lithium. These nuclei have persisted for the 13.8 billion years since those first moments, coming together in stars and, eventually, us.
So, yes, you are stardust. But you are also the ashes of the Big Bang: ancient atomic alchemy brought together by the inexorable flow of gravity and time.