“We’re made of star stuff,” astronomer Carl Sagan famously said.
And it is true — the atoms that make you a living, breathing being; the screen you are reading these words on; the stuff you can see — it was all created out there in the cosmos.
This is what physicists call ordinary or “baryonic” matter. It makes up a fraction of the universe — less than 5 per cent — with the rest comprising dark matter and dark energy.
You have seen ordinary matter on the periodic table of elements.
First up on the grid is hydrogen, with its single proton and electron. At the tail end is oganesson, a monster of an atom created when calcium and californium are smashed together in particle accelerators.
The thing is, physicists do not know exactly how and where all the naturally occurring elements — such as hydrogen, carbon and oxygen — are made.
But they are filling in the blanks. Recent observations, experiments and modelling hint at a whole range of matter-makers, from cataclysmic explosions of stars to cosmic rays bouncing off individual atoms.
So let’s take a look at some of the cosmic events that produced these elements, starting with — appropriately — the Big Bang.
Big Bang fusion
As the universe expanded and started cooling just minutes after the Big Bang, the first building blocks of everything we see — such as protons and neutrons — were born. These, in turn, aggregated to create the nucleus of atoms.
Eventually, as the universe continued to cool, electrons orbited these nuclei. Bingo! The first atoms came to be.
Only a few elements were made this way, said Amanda Karakas, a theoretical astrophysicist at Monash University.
“Hydrogen and helium and lithium, and that’s it.”
Hydrogen is still the most abundant element in the universe, making up around three-quarters of normal matter.
In this dense fog of primordial gas, over millions of years, the first stars formed, converting hydrogen into helium in a process known as nuclear fusion.
Exploding massive stars
If a star is at least eight times the mass of the Sun, it fuses hydrogen to create helium, then carbon, nitrogen, oxygen and elements up to (and including) iron.
The heaviest elements sink deep into the core, which grows so massive that the star begins to collapse on itself.
Eventually, the implosion bounces off the core and the shockwave heats the outer layers of the star, producing temperatures so high that elements heavier than iron are formed.
In the explosion, called a Type II supernova, these elements are sprayed into space.
Dying low mass stars
Low mass stars, like the Sun, also produce heavy elements as they run out of puff.
Rather than explode in a supernova, these stars bloat into a red giant, which can be up to a thousand times bigger than the Sun.
Helium burns to create carbon and oxygen before shrugging off layers of material as a planetary nebula, said Brad Tucker, an astrophysicist at the Australian National University.
As well as lithium, carbon and nitrogen, they also release much heavier elements, such as tin and lead, into interstellar space.
Exploding white dwarfs
After a low mass star dies, what is usually left is the dead remnant of its core: a small, hot, very dense white dwarf star.
If a white dwarf is close enough to another star, it can slurp hydrogen gas off its neighbour.
Over time, this hydrogen shroud builds up, ignites and explodes, bringing the white dwarf back to life in what is called a classical nova.
It blasts out elements such as iron, nickel and zinc.
These elements are also produced by another spectacular explosion, known as a Type Ia supernova.
Unlike a nova, which fires matter from the white dwarf’s surface, a Type Ia supernova’s white dwarf detonates from the inside in a violent eruption so bright it can outshine its host galaxy.
Merging neutron stars
When a star around eight to 15 times the mass of the Sun explodes, it too leaves a super dense remnant of its inner core behind: a neutron star.
And when the gravitational waves produced by the collision between two neutron stars was picked up last year, telescope observations of the event also helped fill in some of the blanks on the periodic table of the origin of the elements, Dr Tucker said.
While physicists pretty much knew that elements in the fourth line of the period table — from potassium to krypton — were created by exploding stars, “the latter part of the table has always been, ‘Well, we think we know what happens there’.
The light signature thrown off by the neutron star merger provided clues that the heaviest naturally occurring elements, including gold, platinum, radium, thorium and uranium may be produced in the cataclysmic collision known as a kilonova.
Cosmic ray fission
Some elements are formed when cosmic rays — super energetic particles that barrel through space at near the speed of light — crash into and split bigger atoms, Dr Tucker said.
A cosmic ray in the form of a proton that crashes into a carbon or oxygen atom, for instance, could carry enough energy to bust it apart.
“This leaves smaller, lighter elements such as boron and beryllium in the debris,” Dr Tucker said.
While small proportions of other elements are made by cosmic ray fissions, pretty much all boron and beryllium is created this way.
Mystery of the missing lithium
On Earth, physicists use particle accelerators to recreate star-like conditions and discover what is produced, Dr Karakas said.
But there remains a mystery that physics is yet to explain: the lithium problem.
According to some estimates, the amount of lithium we see in the universe is around a third of what should have been produced by the Big Bang.
“Observations of amounts of hydrogen and helium and their isotopes [which contain extra neutrons] match what we predict what should have been made in the Big Bang,” Dr Karakas said.