EVERYONE knows that ancient alchemists were obsessed with making gold. They never cracked it. Everyone knows that too. But here’s something not a lot of people know: thousands of years later, we still don’t fully understand where gold comes from. We can find it on Earth, of course. But where in the universe it was first made has long been a cosmic-scale mystery.
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For most of the elements we are familiar with – carbon, oxygen, nitrogen – their origins are clear. These atoms were cooked up in the roiling hearts of stars through nuclear fusion. But bog-standard fusion is only powerful enough to produce relatively light elements. To create heavier stuff, a more powerful process is needed. The thing is, we don’t know exactly how or where it happens. This applies not just to gold, but to dozens of the heavier, more exotic elements. You could say that the origins of half the elements on the periodic table are unknown.
There is no shortage of possible explanations, each of which involves the fiery deaths of stars or other celestial implosions. Now, though, we finally have some hard evidence. For the first time, we have spotted heavy elements actually being made. In exploring further, we have learned that their origins are probably far more subtle than we ever suspected.
The chemical elements are the basic building blocks of everything, including you. Along with rather a lot else, an adult body contains somewhere in the region of 16 kilograms of carbon, 780 grams of phosphorus and 0.2 milligrams of gold. Deep down, every element is made of the same three particles. There is a nucleus of positively charged protons and neutral neutrons, orbited by much smaller, negatively charged electrons. The number of protons in a nucleus determines the element: six protons means you have carbon; gold is much heavier with 79. The number of neutrons is usually the same as the number of protons, although this can vary a bit, producing slightly heavier or lighter versions of the same element, called isotopes.
Stars begin their lives as a great mass that consists almost entirely of the lightest chemical element, hydrogen, made of one proton and one electron (see “The first elements“). Soon, the process of nuclear fusion sparks into life and hydrogen atoms merge to form helium, which has two protons. This process keeps going to produce steadily heavier elements.
But only up to a point. As atomic nuclei get bigger, the electromagnetic force begins to push them apart more strongly, and eventually they can’t get close enough to fuse into heavier elements. Beyond iron, which has 26 protons, further fusion almost never happens in a star.
So where do we get the heavier elements from? In the 1950s, physicists including Fred Hoyle . It involves neutrons, which, having no charge, are unhindered by the electromagnetic force. If an atom captures a neutron as it whizzes past, it will grow into a heavier isotope of the same element. Once inside the nucleus, a neutron can decay into a proton and an electron in a radioactive process called beta decay. This transforms the atom into an element that is one proton heavier.
Hoyle showed that there were two ways this could happen. The first is called the slow neutron-capture process (or s-process). It requires only a gentle drip of neutrons. Hoyle reckoned it could happen in dying stars called red giants, and we now have little doubt that this is true. We can look at light from red giants and spot the characteristic ways that different elements in the star absorb it – and this spectroscopic data shows that these stars do indeed contain some heavy elements. But this can’t produce each and every one of the heavier elements. And anyway, the pace is glacial: a nucleus undergoing the s-process only captures a neutron about once a month. This isn’t enough to explain all the heavy elements we see in the universe.
Hoyle’s second mechanism is called the rapid neutron-capture process (or r-process). Here, atoms get deluged with neutrons and quickly grow to huge sizes. They then radioactively decay into a series of lighter, if still heavy, elements. We think that this must be the process responsible for making the majority of the universe’s heavy elements, including gold.
But where could a big enough tidal wave of neutrons come from? Astrophysicists long believed the answer had to be a supernova, the cataclysmic explosion of a star. But in the past 20 years, there have been signs that this is wrong. For one thing, we have started using computers to simulate the r-process during a supernova, and the abundances of different elements produced in these simulations don’t match real spectroscopic observations of the explosions.
Then there is a more surprising line of evidence. Earth occasionally passes through patches of dust left behind by supernovae; the dust drifts into our atmosphere and settles at the bottom of the ocean and in Antarctic ice. It is easy enough to confirm the origins of this dust because it contains a telltale isotope of iron created in supernovae. – a fairly convincing sign that supernovae don’t produce this precious metal.
Even before such evidence emerged, back in 1974, David Schramm and James Lattimer at the University of Texas at Austin . If what we need to make heavy elements is a lot of neutrons, then perhaps their origins involve neutron stars? These are incredibly dense balls of matter – mostly neutrons, as the name suggests – left behind after certain kinds of supernovae. Neutron stars often form in pairs, then spin around each other until eventually smashing together in a cataclysmic explosion called a kilonova. The tide of neutrons that results, reckoned Schramm and Lattimer, could be enough to kick-start the r-process.
In the early 2000s, Brian Metzger, now at the Flatiron Institute in New York, was taken by the idea that gold might come from kilonovae. But he was frustrated that there had been precious little thought about how we might test the idea. He began wondering how to get some hard data. “What could we actually go out and measure with a telescope?” he says.
The r-process within a kilonova would involve an absolutely massive cascade of radioactive decay, and Metzger realised that this would throw out a characteristic flare of light across the cosmos. In 2010, he predicted . Some colleagues at the University of California, Berkeley, developed this further to show that the flare would start off blue, as lighter elements were produced, and then shift to red, as heavier elements like gold were produced. This so-called light curve would be the smoking gun.
The search for it would prove to be tricky, to say the least. We did see likely looking flashes in the sky from time to time, but these were usually so far away that we couldn’t measure the light curve with any precision. This happened in 2013, for instance, when the Hubble Space Telescope saw a burst of gamma rays thought to be caused by a kilonova. There was a promising bump of light at the end of this burst, but it was too faint to properly compare it with Metzger’s predictions.
A hum and a chirp
Meanwhile, other physicists were setting up machines that could observe the universe in an entirely different way. The Laser Interferometer Gravitational-Wave Observatory, or LIGO, has detectors that can measure gravitational waves, vibrations in space-time caused by powerful cosmic collisions. In 2017, LIGO observed the clear signal of two neutron stars colliding: there was a “hum” of increasing frequency as the stars spiralled closer, then a “chirp” as they crashed together.
This was the moment Metzger had been waiting for. The gravitational waves arrived at Earth before the light from the explosion, and physicists scrambled to pinpoint the source and train optical telescopes on it, ready to check for Metzger’s blue-to-red signal. When the data came in, it looked just right. The kilonova had spat out a wave of exotic elements. “For the first time in humanity’s history, we’ve directly seen the synthesis of these heavy elements,” says Metzger.
But was there gold in that thar explosion? The most undeniable evidence would be to see the element’s characteristic spectroscopic fingerprint in the light emitted from the kilonova. Unfortunately, the fingerprints of various elements overlap in the observations and it isn’t possible to distinguish one from another. However, Metzger sees that as nitpicking. “When you create one of these heavy elements, it takes a lot of fine-tuning not to create a whole spectrum,” he says.
Most physicists now agree that kilonovae do create gold. But that isn’t the end of the story. It seems these events aren’t enough to explain all of the heavy elements in the universe.
That much has become apparent from studies of certain stars in the diffuse halo that shrouds our Milky Way galaxy. These stars are thought to be exceedingly old, from an era when hardly any had died, whether in supernovae, kilonovae or any other manner. They shouldn’t contain any r-process elements; if those elements are created by kilonovae, and none of those had happened before the star was born, then that star simply can’t have any inside it. Strangely, however, a small group of astronomers called the R-Process Alliance that has been surveying these stars since 2018 has shown that .
“For the first time ever, we have directly seen the synthesis of heavy elements”
This could mean something seriously weird is going on, says Terese Hansen at Texas A&M University, who is part of the alliance. If the stars are as old as we think and there is no mistake about the heavy elements, then something other than a kilonova must have created those elements.
That said, the universe was unrecognisably different at the time of the first stars. Stars grew many times bigger than they do today and they span much faster. When they reached the end of their lives, they didn’t necessarily explode in a typical supernova. Some might have collapsed under their colossal gravity and shot out incredibly powerful bursts of radiation. These events, known as collapsars, were first proposed in 1993 by Stan Woosley at the University of California, Santa Cruz. “Collapsars produce these epic shock waves,” says astrophysicist Emma Chapman at Imperial College London. “Within this wave, heavier elements past iron are formed because the material is compressed to levels far beyond what the star achieved in the core during its lifetime.”
Maybe gold comes partly from the fiery demise of the universe’s first stars, then. The idea is supported by research from Daniel Siegel at the University of Guelph in Canada. He has argued, based on the data from the 2017 kilonova and theoretical work on collapsars, that . But Imre Bartos at the University of Florida and Szabolcs Márka at Columbia University, New York, have calculated .
We now know a lot more about the origins of gold. Some of this most coveted metal was created in kilonovae and more was probably made during the deaths of the first stars. But even these two sources can’t account for all the gold in the universe. It is something to ponder next time you see someone wearing gold jewellery. We still don’t really know where it came from.
The first elements
Some say we are made of stardust – and that is mostly true, as the major elements in our bodies, like carbon and oxygen, were forged inside stars. But what were the very first stars made of? It turns out that all the universe’s hydrogen and most of its helium were produced in the first 20 minutes after the big bang, when things were hot and dense enough to sustain fusion. The first stars were made of just these two elements, plus a little bit of lithium.
The missing lithium
Our best understanding of the big bang suggests that a certain quantity of lithium should have been created then along with hydrogen and helium. From this, we can calculate how much of these elements there should be in the cosmos. Our predictions match observations for hydrogen and helium well, but about two-thirds of the lithium that should be there is missing. No one can confidently explain this long-standing niggle, known as the cosmological lithium problem. Even more strangely, certain types of stars have an inexplicably large amount of lithium. One idea is that this might come from novae, a special class of cosmic explosion not to be confused with supernovae. These can happen when a white dwarf is orbited by a much larger star. Gas from that star falls onto the white dwarf, causing an explosion that may produce large amounts of lithium.
It’s raining boron
Elements number four and five, beryllium and boron, weren’t produced in stars. These light elements are used up in stellar fusion reactions more quickly than they can be produced. But there is plenty of both on Earth – so what gives? It turns out that ultra-high-speed protons from space collide with nitrogen and oxygen in Earth’s atmosphere, causing them to split apart into beryllium and boron. From there, the elements dissolve in water and rain down onto the surface.
