From King Midas to Goldfinger, something about the 79th element has always captivated the human mind. Indeed, Isaac Newton devoted a great deal of his time and his writings to the study of alchemy – turning base metals into gold. But recent astronomical discoveries reveal just how difficult gold formation really is, and give clues as to why it is so precious to us.
Gold is a precious metal used by humans for thousands of years for jewellery, currency and art. Its popularity likely stems from its unusual and appealing colour, standing out from the usual metallic greys and silvers. Gold is also a particularly soft metal, making it easy to reshape, and can be beaten into the incredibly thin gold leaf found in art pieces and on the pages of bibles. It is estimated that all the gold ever discovered on Earth would fit comfortably into three Olympic swimming pools. But why is it so rare? How is gold formed in the universe?
A brief history of the entire universe would of course start with the Big Bang. About 10 seconds after the start of time itself, the expanding universe began a long career in atomic chemistry. Hydrogen, Helium and a small amount of Lithium dominated during early times, formed by a process of “nucleosynthesis” in the first few minutes of existence. These simple elements were an improvement on the nothingness that came before them, but for carbon-based life forms like you and me, not enough to build a world.
Enter the stars. No, not the red-carpet Hollywood types, but the original giant-ball-of-plasma variety. Upon sufficient cooling of the early universe, primordial clouds of predominantly-hydrogen gas began to succumb to their own self-gravity, condensing down into compact spheres known as protostars. Once a threshold mass was exceeded, the pressure and temperature in the protostars’ cores becameso great that the atomic nuclei present within themwere fused together – this process, known as nuclear fusion, releases a huge amount of energy and powers all stars in the universe. It is being heavily researched as a possible renewable energy source of the future.
A consequence of nuclear fusion is the creation of new elements. Atoms are made up of smaller units known as protons, neutrons and electrons. The number of protons in an atom is the defining characteristic of an element – for example, any atom containing 26 protons is called iron, and atoms containing 8 protons are called oxygen. Gold atoms are, by definition, composed of 79 protons. The product of fusing two Hydrogen atoms (each containing one proton) will be a new element containing two protons, known as Helium. By fusing atoms together in their cores, stars can build heavier elements from lighter ones, making them the most prevalent atom factories in the universe.
The picture now is one of a universe filled with left-over atomic gas from the Big Bang, which in turn collapses into primordial stars. These stars burn by nuclear fusion, creating heavy elements from their starting supply of Hydrogen. Broadly speaking, this is what the early universe would have looked like. But for every star birth, there must be a star death.
Once the core of a star has converted all its Hydrogen into Helium, the fusion process stops. There is not enough heat or pressure in the core to fuse Helium into heavier elements, since with greater mass comes a greater energy requirement. Since the primary source of energy has ceased, the star cools; but with cooling comes contraction, which dramatically increases the pressure in the core. The threshold for Helium fusion is then surpassed, and a new phase begins. The same occurs when the last of the Helium has been converted to Carbon (and a wide range of other elements). With each successive step on the fusion ladder, the required initial mass of the protostar increases, such that only stars roughly eight times the mass of the Sun can support Carbon burning.
This fusion chain cannot go on forever, though; there comes a point where the thermal pressure generated by the fusion process can no longer support the immense force of gravity. So begins a “core-collapse”, where, depending on the exact process at play, the outer layers of the star fall inwards at an ever-increasing speed until one of three things happens. First, if the star is big enough and the core is dense enough, literally no force in the universe can prevent the collapse and the star becomes a black hole – the star collapses into a point so dense that the concepts of time and space become meaningless, and even light is prevented from escaping. Second, for a fractionally less massive star, the collapse drives the core temperature so high that electrons and protons combine to form neutrons (and an explosion of fundamental “neutrino” particles); a quantum mechanical effect called the Pauli Exclusion Principle, which states that no two particles can exist in the same quantum state, prevents the neutrons from touching each other, resulting in a ball of incompressible neutrons known as a Neutron Star. The outer layers of the star collide with the neutron core and are blown apart by the force of the collision, sending their constituent elements flying off into the universe. These spectacular events, known as supernovae, can emit more energy than the Sun will in its entire lifetime. Finally, for low mass stars, the core only partially collapses, forming what is known as a White Dwarf star.
While a little complicated, the processes listed above are vital to understanding how heavy elements are distributed across the universe. For the most part, only elements up to iron can be created by stellar fusion, and these are dispersed through the universe by the explosive deaths of their parent stars.
This is good news for life on Earth. The human body does not rely on elements heavier than iron for its chemistry, so the stellar atom factories are sufficient to explain our existence. But iron is only the 26thelement in the periodic table, so how can gold and its heavy-element siblings exist?
It was previously thought that gold and other heavy elements were formed in the very instant of a star’s death, during the supernova itself. A complex nuclear process, whereby atoms can rapidly capture free neutrons to increase their mass (known as the r-process), allows for the production of heavy elements in these extreme environments. Indeed, this is still regarded by astronomers as a possible, though less common, path to the creation of gold. But recent observations by the LIGO and Virgo gravity wave detectors in the US and Italy reveal that the predominant gold-forming mechanism is even more special.
Since LIGO first came online in 2015, the astronomical community (and, dare I say, the world) has kept its ear firmly to the ground, awaiting new results. A marvel of physics and engineering, the LIGO detector uses two laser beams, each travelling over 1000 km before interference, to measure the ripples in the very fabric of space caused by large gravitational events. That’s right, these gravitational waves, caused by the collisions of black holes and other dense objects many millions of light years away, can cause the actual distance between two points on Earth to wobble by a few hundred sextillionths of a metre. This change in distance, equivalent to changing the distance to the nearest star by a hair’s width, is astonishingly detected by the finely tuned sensors at LIGO, and from the oscillation pattern a fingerprint is identified.
The first LIGO result in 2015 was identified as the collision between two black holes, weighing in at 36 and 29 solar masses respectively. Since then, the detector has been refined significantly, and can now pick up collisions with a total mass of around 2.75 solar masses. It was the first such detection, GW170817, which really cracked the golden code. At 12:41:04 on 17thAugust 2017, the collision of two neutron stars was recorded by LIGO/Virgo. 1.7 seconds later, a burst of high-energy gamma rays was observed by the Fermi space telescope, and within 11 hours of the signal, optical telescopes had found the object. The remnant of the collision was a fast-moving cloud of neutron-rich gas, cooling rapidly as it blew away from its point of origin.
Electromagnetic observations of this remnant cloud revealed signatures of heavy elements, including gold. A 2018 paper in the Astrophysical Journal estimated that the quantity of gold created in this one event was equivalent to between 3 and 13 Earth-masses, which, at today’s prices, would sell for about one nonillion US dollars. They proposed that the gold-forming process in the neutron star merger was the same r-process as in supernovae, but in much greater quantities. Even more interestingly, after taking into account the estimated rate of these neutron star merger events in observable galaxies, they concluded that such events could account for almost all the gold formation in the universe.
It is no wonder gold is so rare then, when its formation relies on the incredibly rare mergers of neutron stars. The quantities of gold discovered in the 2017 observation show that even the powerful supernova deaths of stars contribute very little to the total gold population, relative to these mergers. Gold formation requires stars of precisely the right masses to collapse into neutron stars, and for these neutron stars to merge in collisions powerful enough to cause ripples in the very fabric of space. These merging events are estimated to have occurred only a handful of times in the history of our galaxy.
The moral of this story is that, behind every gold ring, there are billions of years of cosmic blacksmithing. Our most precious metals were forged from the elemental building blocks of the universe, transformed from primordial hydrogen to carbon, and oxygen, and other elements; compressed into neutrons during their parents’ deaths, they spent millions or billions of years alone until, one day, the conditions were just so that they found a partner and merged. Perhaps that is why gold, bound in the heart of dead stars, binds us until death do us part.
