That Gold Ring You're Wearing? It Could Have Come From a Black Hole

How have black holes played a role in the production of elements like gold, on Earth?
Jaime Trosper

The forging of light and heavy elements has been a source of study among scientists for a very long time. in fact, It has long been believed that most elements came into existence as hydrogen and helium combined to form stars. As those stars got hotter and more massive, those heavier elements were forged — a process called nucleosynthesis — and when the stars reached the end of their lives, they spewed those elements out into space as they went supernova. 

There are other ways in which heavy elements, such as silver and gold, can come into existence. It may be that these elements are produced as a consequence of neutron stars colliding. If you are unfamiliar with neutron stars, they are essentially the remnants of massive stars that weren't massive enough to condense into black holes, but instead, the collapse was halted by something called neutron degeneracy pressure, leading to the creation of extremely heavy objects that are small in size, but extremely dense. We're talking, one teaspoon full of neutron star material would weigh roughly four BILLION tons — all condensed into an object only ten miles across!

Are there other ways nature created heavy elements? Well, a recent paper puts a possible new twist on longheld research into how these elements are formed. We will get to that, but first... 

How Exactly Are Heavy Elements Formed Inside Stars?

The inside of a star is very complex to model, as we can't exactly send a probe inside one to see how it functions. However, cosmologists believe that although the elements first formed soon after the Big Bang, conditions were not cool enough for the elements to remain stable. After about 3 or 4 minutes, the early universe had expanded and cooled enough for conditions to favor electrons staying in orbit around atomic nuclei. This is when the lightest and simplest chemical elements formed — hydrogen, helium, and lithium. These collected to form large, gaseous clouds, and those clouds eventually collapsed under their own gravity and became the first stars. Within the cores of those stars, nuclear fusion began when they became massive enough.

Nuclear fusion is the process through which light elements are transformed into heavier elements. Hydrogen also happens to be the lifeblood of stars themselves. The process of fusing hydrogen into other elements is what keeps a star stable. Once it reaches a critical threshold and the star loses the hydrogen fuel it needs to survive, the mass of the star determines what kind of object it becomes. A smaller object like the Sun will become a white dwarf; medium-sized stars become neutron stars or pulsars, and the most massive stars collapse into stellar-mass black holes.

Artistic rendering of a pulsar
Artistic rendering of a pulsar Source: NASA’s Goddard Space Flight Center

To put it simply, "Only the bigger stars can produce heavier elements. This is because these stars can pull up their temperatures higher than the smaller stars like our Sun can. After hydrogen is used up in these stars, they go through a series of nuclear burning depending on the types of elements produced, for example, neon burning, carbon burning, oxygen burning, or silicon burning. In carbon burning, the element goes through nuclear fusion to yield neon, sodium, oxygen, and magnesium. When neon burns, it fuses and produces magnesium and oxygen. Oxygen, in turn, yields silicon and the other elements found in between sulfur and magnesium in the periodic table," per Sciencing.

"These elements, in turn, produce the ones that are near iron on the periodic table — cobalt, manganese, and ruthenium. Iron and other lighter elements are then produced through continuous fusion reactions by the above-mentioned elements. Radioactive decay of unstable isotopes also occurs. Once iron is formed, nuclear fusion in the star’s core comes to a stop."

This is the beginning of the end for the biggest stars in the universe. It requires an extraordinary amount of energy and heat to fuse other heavy elements, especially iron. Once iron begins to fuse in the star's core, it produces more energy than the process of fusion uses to counteract the forces of gravity, which keep the star stable and prevent it from collapsing in on itself. Core collapse and supernova events then occur. While the gas is ejected into space, atoms collide, neutrons in particular, within moments of the supernova expulsion.

Once these atoms begin to combine, radioactive decay poses a problem: The neutrons must fuse very quickly before the nucleus is bombarded by more neutrons. Heavy elements are formed by a succession of rapid neutron captures by heavy seed nuclei, such as Fe-56, a common isotope of Iron, or other more neutron-rich heavy isotopes. This process is otherwise known as the rapid neutron capture process, or r-process. This process is responsible for the creation of around half of the "heavy elements" — atomic nuclei heavier than iron.

So, Where Does Gold Come From?

As we mentioned, it has long been speculated that gold and other heavy elements can be formed in a few different ways: to recap, either in the cores of massive stars, when two neutron stars collide, or within the hot clouds of gas that spew into space moments after a supernova event. However, a recent paper postulates that black holes may have had a role in creating one of the most precious elements on Earth: gold.

This research, which was published in the November 2021 issue of the Monthly Notices of the Royal Astronomical Society, proposes a novel approach to understanding the formation of heavy elements. They suggest heavy elements were created in the swirling masses of gas and dust encircling a newborn black hole, the accretion disk. This system may be formed after the merger of two massive neutron stars and during the collapse and explosion of a rotating star.  

Per ScienceAlert, "In these extreme environments, the high emission rate of neutrinos should facilitate the conversion of protons to neutrons — resulting in an excess of the latter, required for the process that produces heavy elements."

Astrophysicist Oliver Just, from the GSI Helmholtz Centre for Heavy Ion Research in Germany, further notes: "In our study, we systematically investigated for the first time the conversion rates of neutrons and protons for a large number of disk configurations by means of elaborate computer simulations, and we found that the disks are very rich in neutrons as long as certain conditions are met."

Moreover, "The baby black hole is thought to be surrounded by a dense, hot ring of material, swirling around the black hole and feeding into it, like water down a drain. In these environments, neutrinos are emitted in abundance, and astronomers have long hypothesized that r-capture nucleosynthesis could be taking place as a result."

Models and simulations indicated to the researchers that if certain parameters are met — like the "baby black hole" meeting a certain mass and spin criteria, along with the surrounding gas disk being sufficiently massive — neutrinos would have existed in abundance, and fusion could have taken place within the accretion disk. 

"The more massive the disk, the more often neutrons are formed from protons through the capture of electrons under emission of neutrinos, and are available for the synthesis of heavy elements by means of the r-process," explains Dr. Oliver Just.

"However, if the mass of the disk is too high, the inverse reaction plays an increased role so that more neutrinos are recaptured by neutrons before they leave the disk. These neutrons are then converted back to protons, which hinders the r-process."

"This sweet spot in which heavy elements are produced most prolifically is a disk mass between 1 and 10 percent of the mass of the Sun. This means that neutron star mergers with disk masses in this range could be heavy element factories. Since it's unknown how common collapsar disks are, the jury is still out for collapsars, the researchers said."

Researchers have found gold roughly the same age as Earth, so about 4.5 billion years old, but aren't exactly sure how it was formed. The Sun isn't nearly massive enough to fuse anything into gold or silver. In fact, it cannot fuse anything heavier than oxygen, which has 8 protons. Gold, on the other hand, is comprised of 79 protons. So it requires a LOT of energy to fuse. It is possible that enough energy might exist in the swirling gas and dust surrounding a black hole.

So, the ring you are sporting on your finger may have been generated by a black hole billions of years ago and delivered to Earth via asteroids and comets (perhaps even existing in the massive cloud that collapsed and formed our Sun). If that isn't amazingly cool, I don't know what is. 

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