We know that the element gold (AU, atomic number: 79) is a precious, dense, yellow metal. We make jewelries from gold because of its amazing malleable and ductile properties.
Electronic industries too widely use gold in circuit boards since it is not only a good conductor of electricity but also most resistant to corrosion.
But have you ever thought about gold’s origin? Though we get it from the gold mines on Earth, it is not its real origin. Let us explore how this precious heavy metal may have formed in the Universe.
At Big Bang
Mostly Hydrogen
At the Big Bang, 13.8 billion years ago, the Universe consisted mostly of hydrogen (H – atomic number 1, with just one proton in the nucleus) and a small portion of helium (He – atomic number 2 with two protons in the nucleus).
However, now we see more than 110 elements in the universe. Where did all the elements come from? How did they get created? Simple answer: stars created some of them, and cataclysmic events forged rest of them.
Interesting? Keep reading.
Sun Like Stars
Create Light Elements
Scientists have come to know that the thermonuclear process, occurring in and around the core of every star, creates some of the elements.
For instance, what powers the Sun now is the nuclear fusion process that is going on in the central core. Because of the high pressure and temperature in the core, two hydrogen nuclei fuse together to produce a helium nucleus releasing enormous energy in this process. An FYI : Our Sun burns 600 million tons of hydrogen every second!!
During the next stage, when the Sun runs out of hydrogen, it will start burning helium in the core producing carbon (C – atomic number 6, with six protons in the nucleus) and oxygen (O – atomic number 8, with eight protons in the nucleus). Once all the helium is used up in the core, the Sun is going to die after becoming a red giant leaving a white dwarf in the core. A white dwarf is a highly compressed dim star of carbon (and oxygen).
Our Sun will not be able to progressively continue the nucleosynthesis process beyond carbon and oxygen because its mass is not big enough to create the required pressure and temperature in the core to fuse carbon (or oxygen) nuclei to produce heavier elements.
However, the story of massive stars is different.
Massive Stars
Create Heavier Elements
Massive stars, with masses 8M☉ (M☉ is the mass of our Sun) or more, are able to take the nucleosynthesis process beyond carbon and oxygen. Such stars can continue synthesizing elements up to iron (Fe – atomic number 26 with twenty-six protons in the nucleus) in the core.
Even these huge stars cannot continue the nucleosynthesis process beyond iron because iron’s nucleus is, not only, very stable but also requires tremendous energy to fuse two iron nuclei. So, iron is the dead end for all these massive stars. If that is the case, how do all the elements heavier than iron get forged in the universe?
Cataclysmic Events
Create Very Heavy Elements
If iron core is the dead end, even in very massive stars, how do we explain the presence of much heavier atoms in the Universe? How did all the heavier elements get made?
Scientists think that the answer may lie in the cataclysmic events such as a supernova and the collision of two neutron stars wherein an incredible amount of energy is available to synthesize heavier elements through a process known as r-process (rapid neutronization process).
Let us now quickly understand what supernovae and neutron stars are.
Supernova
Supernova is a violent explosion (and death) of a huge star at the end stage of its life cycle.
The figure below shows how stars evolve from birth to death depending on their masses. Chandrasekhar limit is the mass threshold which determines their life cycle paths.
As mentioned above, stars like our Sun will end up as a white dwarf after using up hydrogen in the core first and helium next. However, a much bigger star continues the nucleosynthesis progressively in the core until its core fully becomes a huge ball of iron.
As fusion stops in the iron core, the weight of the outer layers takes over and crushes the iron core. Now the core turns into a neutron star accompanied by a violent and massive explosion. This cataclysmic explosion is named as a supernova.
The shockwave, during the supernova explosion, completely rips off the outer layers (of the star). It also increases the temperature and pressure of the outer layers so high that it causes explosive nucleosynthesis. Scientists believe that this explosive nucleosynthesis can create heavier elements through the r-process (rapid neutronization process).
The remnants of a supernova are a neutron star and a huge cloud of gas consisting of mostly hydrogen, and helium enriched with other heavy elements.
And from this huge cloud, next generation of stars and planetary systems are born.
Neutron stars merger
Creates Elements like Gold
As described above, some massive stars become neutron stars at the end of their lives after a supernova explosion. What happens to the binary system of two such massive stars? That could become a binary system of neutron stars at the end of their life cycles. These neutron stars spin at extremely high rate and are known as pulsars. They lose energy over a period of millions of years, and they slowly come closer to each other, and finally merge. This merger is another kind of cataclysmic event.
Scientists have recently witnessed the merger of a pair of such neutron stars. In this collision, they have seen evidences for the presence of much heavier elements. This collision too may result in r-process which creates much heavier elements such as gold.
Solar System
From Interstellar Clouds
Our solar system formed about 4.6 billion years ago, that is, about 9.2 billion years after the Big Bang. During those 9.2 billion years, many generations of stars, both massive and not massive ones, must have formed and died. Some of the massive stars and binary system of such massive stars left behind huge clouds of gases after supernova explosions and subsequent neutron stars mergers.
Those clouds, enriched with heavy and much heavier elements, were the breeding grounds for the next generation of stars.
Our solar system was one of the next generation of stars that formed from the relics of the earlier generation of huge stars.
So, when our solar system formed from the interstellar gas clouds, containing heavy elements such as gold, our solar system inherited gold and other heavy elements from those clouds. Earth too inherited some portion of the heavy elements.
The bottom line is supernova explosions create some heavy elements, and neutron star mergers forge much heavier elements such as gold.
Asteroid Collisions
Still one puzzle remains. Now we understand that our Earth inherited heavy metals from the interstellar gas clouds. But, when the Earth formed, gold being a heavy element should have sunk to Earth’s core as the element iron did. Right? If so, how are we able to get gold or iron from Earth’s crust which is only 30 kilometers deep?
Scientists believe that the constant collisions by asteroids, during the formation period of the Earth, should have brought some portion of the heavier elements to the crust.
This is the story on the origin of gold, and other heavy elements.
References
- Black Holes, Cambridge University Press, by J-P Luminet
- Helmenstine, Anne Marie, Ph.D. “How Is Gold Formed? Origins and Process.” ThoughtCo, Feb. 17, 2021, thoughtco.com/how-is-gold-formed-4683984.
- https://www.pnas.org/doi/10.1073/pnas.2026110118
- https://en.wikipedia.org/wiki/Gold