How have black holes influenced the production of precious metals on Earth?
For a long time, scientists have been fascinated by the forging of light and heavy materials. In reality, most elements are thought to have formed when hydrogen and helium were mixed to form stars. Those heavier elements were forged as the stars grew hotter and more massive, a process known as nucleosynthesis, and when the stars reached the end of their life, they vomited those elements into space as they went supernova.
Heavy elements, such as silver and gold, can be created in a variety of additional methods. It’s possible that these elements are created when neutron stars collide. If you’re unfamiliar with neutron stars, they’re the remains of enormous stars that weren’t massive enough to collapse into black holes, but were instead stopped by something called neutron degeneracy pressure, resulting in tremendously heavy objects that are small in size but highly dense. We’re talking about a teaspoon of neutron star material weighing four billion tons — all compacted into a ten-mile-wide mass!
Is there any other method that nature could have formed heavy elements? A recent publication, however, adds a fresh wrinkle to long-standing studies into how these atoms are produced. That will come later, but first…
What Causes Heavy Elements to Form Inside Stars?
We can’t really put a probe into a star to examine how it works, so modeling the innards of one is difficult. Cosmologists believe that, while the elements originated soon after the Big Bang, the conditions were not chilly enough for them to be stable. The early cosmos had expanded and cooled sufficiently for electrons to stay in orbit around atomic nuclei after around 3 or 4 minutes. Hydrogen, helium, and lithium, the lightest and most basic chemical elements, created during this time. These clumped together to form enormous gaseous clouds, which then collapsed under their own gravity to generate the first stars. When the cores of those stars grew large enough, nuclear fusion began.
The process by which light elements are changed into heavier ones is known as nuclear fusion. Hydrogen is also the lifeblood of the universe’s stars. A star’s stability is maintained via the process of fusing hydrogen into other elements. The mass of the star influences what kind of object it becomes once it hits a critical threshold and loses the hydrogen fuel it requires to survive. The Sun will become a white dwarf, while medium-sized stars will become neutron stars or pulsars, and the most massive stars will collapse into stellar-mass black holes.
To put it another way, “Heavier elements can only be produced by larger stars. This is due to the fact that larger stars may raise their temperatures faster than smaller stars like our Sun. After the hydrogen in these stars is depleted, they go through a sequence of nuclear reactions depending on the elements produced, such as neon, carbon, oxygen, or silicon burning. When carbon is burned, it undergoes nuclear fusion, yielding neon, sodium, oxygen, and magnesium. When neon is ignited, it fuses, releasing magnesium and oxygen. Oxygen, in turn, produces silicon and the other elements found in the periodic table between sulfur and magnesium “according to Sciencing.
“These elements, in turn, form the elements cobalt, manganese, and ruthenium, which are close to iron on the periodic table. The above-mentioned elements then generate iron and other lighter elements through continual fusion processes. Stable isotopes undergo radioactive decay as well. Nuclear fusion in the center of the star grinds to a halt once iron is produced.”
For the universe’s largest stars, this is the beginning of the end. Fusing other heavy elements, particularly iron, needs a tremendous amount of energy and heat. Once iron starts to fuse in the star’s core, it creates more energy than the fusion process needs to counteract the gravitational forces that keep the star stable and prevent it from collapsing in on itself. The core collapses and supernovae occur as a result. Within minutes of the supernova expulsion, atoms smash, particularly neutrons, when the gas is expelled into space.
When these atoms start to combine, radioactive decay becomes an issue since the neutrons must fuse quickly before being bombarded by more neutrons. Heavy elements are made up by a series of rapid neutron grabs by heavy seed nuclei, such as Fe-56, a common isotope of iron, or other heavier isotopes with more neutrons. The rapid neutron capture process, or r-process, is another name for this procedure. Around half of the “heavy elements” — atomic nuclei heavier than iron — are created through this process.
Where Does Gold Come From, Anyway?
As previously stated, gold and other heavy elements are thought to develop in a few different ways: in the cores of enormous stars, when two neutron stars crash, or amid the hot gas clouds that burst into space moments after a supernova occurs. However, a recent article suggests that black holes may have played a role in the creation of gold, one of the world’s most valuable metals.
This study, which was published in the Royal Astronomical Society’s Monthly Notices in November 2021, presents a fresh way to explaining the production of heavy elements. They believe heavy atoms were formed in the accretion disk, which is a churning mass of gas and dust encircling a nascent black hole. After the merger of two enormous neutron stars and the collapse and explosion of a spinning star, this system may develop.
“In these severe settings, the high neutrino emission rate should facilitate the conversion of protons to neutrons — resulting in an excess of the latter, required for the process that makes heavy elements,” according to ScienceAlert.
“In our study, we systematically investigated for the first time the conversion rates of neutrons and protons for a large number of disk configurations using elaborate computer simulations, and we found that the disks are very rich in neutrons as long as certain conditions are met,” says astrophysicist Oliver Just of the GSI Helmholtz Centre for Heavy Ion Research in Germany.
Furthermore, “A thick, hot ring of material is considered to encircle the embryonic black hole, spinning around it and feeding into it like water down a drain. Neutrinos are abundant in these conditions, and astronomers have long suspected that r-capture nucleosynthesis is taking place as a result.”
According to the researchers’ models and simulations, if certain parameters were met — such as the “baby black hole” meeting a certain mass and spin criteria, as well as the surrounding gas disk being sufficiently massive — neutrinos would have been abundant and fusion could have occurred within the accretion disk.
“The more massive the disk, the more neutrons are generated from protons by the capture of electrons under neutrino emission, and are available for the synthesis of heavy elements via the r-process,” says Dr. Oliver Just.
“If the disk’s mass is too great, however, the inverse reaction becomes more important, and more neutrinos are caught by neutrons before they leave the disk. The r-process is hampered by the conversion of these neutrons back to protons.”
“A disk mass of 1 to 10% of the mass of the Sun is the sweet spot where heavy metals are created most abundantly. This suggests that neutron star mergers in this mass range could be heavy element manufacturers. The judgement is still out on collapsar disks, according to the researchers, because it’s unclear how widespread they are.”
Researchers have discovered gold that is nearly the same age as the Earth, i.e., 4.5 billion years old, but they aren’t sure how it arose. The Sun isn’t quite big enough to melt metals into gold or silver. In fact, it is incapable of fusing anything heavier than oxygen, which has eight protons. Gold, on the other hand, has 79 protons in it. As a result, fusing takes a lot of energy. It’s possible that the swirling gas and dust surrounding a black hole contains enough energy.
So, the ring on your finger could have been created billions of years ago by a black hole and delivered to Earth through asteroids and comets (perhaps even existing in the massive cloud that collapsed and formed our Sun). I don’t know what else to say if that isn’t insanely fantastic.