author: none
Jan 21, 2025
In a simple depiction of the atomic theory of matter, all atoms are made up of three kinds of particle: neutrons and protons forming a core, and electrons which orbit that core. Neutrons and protons have nearly identical masses and electrons have a relatively tiny mass. Protons have a positive electric charge, neutrons no charge, and electrons are negatively charged.
Though modern theories of matter are more complicated, this simple description is enough for this story. The number of protons in an atom determine what element it is. Hydrogen has one, Helium has two .. counting up to element 94, Plutonium, the heaviest element that occurs naturally. Elements with atomic numbers above 94 are possible, but typically have a fleeting existence. An atom of Carbon is Carbon because it has 6 protons; an atom with one more proton would be Nitrogen, a gas that makes up most of the air we breath with a totally different chemistry.
However, atoms are lot less fussy about how many neutrons they contain; Carbon can have from 5 to 8 neutrons and it’s still Carbon, the black solid stuff in pencils. Almost all of the Carbon in the universe is Carbon-12 (6 protons + 6 neutrons). So, if neutrons don’t change the element they belong to, what use are they? The nucleus of an atom is governed by strange forces that, among other things, require neutrons to be mixed in with protons to prevent atomic nuclei from disintegrating.
The same forces govern the number of neutrons to be mixed in; too many or too few and the atom gets unstable. An example is the Carbon-14 isotope (same element, different number of neutrons), found in nature in trace quantities. C-14 has two neutrons more than C-12 and that makes the C-14 atom slightly unstable. To rectify matters, the C-14 atom transforms one neutron into an electron and a proton .. now it has 7 protons and 7 neutrons and is a stable isotope of Nitrogen (N-14).
Resolving the slight instability that forces this process is a rather relaxed matter .. for any amount of C-14, half of it will decay in the next 5,700 years. Some isotopes are too unstable to persist in nature .. C-11, for example, can be synthesised, but its half-life is only twenty minutes.
Heavier atoms tend to have more isotopes; Iron (chemical symbol: Fe), for example, has more than twenty isotopes (mostly synthetic); four of them are stable, of which one (Fe-56) accounts for just over 90% of all the Iron atoms in the universe. Fe-56 will appear again in this story.
If you were to travel from galaxy to galaxy making an inventory of the atoms, you would find about 98% of them were Hydrogen. Hydrogen is the simplest element .. one proton. When stars form, they are nearly pure hydrogen.
When a clump of this pervasive Hydrogen forms, it starts a run-away process .. its gravity pulls in more Hydrogen and the process feeds itself. As the atoms at center of this proto-star are squeezed closer together the pressure and temperature increases.
When the temperature reaches about 4-10 million degrees, everything changes! Up to this point our proto-star has been taking on more and more Hydrogen, getting heavier and heavier, collapsing in on itself and heating up. But eventually the pressure and temperaure become too great and the Hygrogen atoms start to get too close for comfort, and can no longer maintain their stability.
Two protons cannot merge in the absence of moderating neutrons, but it’s getting really hot down there and something has to give way. The impossible is dodged when one of the two protons in a pair of Hydrogen atoms turns into something else .. a neutron and two other small particles. The remaining non-suicidal proton now has a neutron buddy, and the pair are in a stable state, an isotope of Hydrogen: H-2.
This so called proton-proton cycle, ultimately results in four Hydrogen atoms (H-1) fusing into one Helium atom (He-4). In this fusion a little mass (~0.8%) is lost converted into energy (E=mc2) .. humans have managed to make this happen too; they call it a hydrogen bomb.
So the center of the new star is a huge continuous hydrogen bomb. This outpouring of energy pushes back against the crushing gravity and the star becomes a self-sustaining power source with a 15 million degree core and a relatively cool surface. Our sun, a mature star, is fusing 600 million metric tons of Hydrogen into Helium every second.
Stars come in many varieties but a critical distinguishing factor is their mass which determines its lifecycle. There’s a limit to how small a star can be .. protostars less than a tenth the mass of the sun may not become stars if conditions at their center aren’t extreme enough to initiate reliable nuclear fusion. Massive stars live short, violent lives and scatter the litter of their brief blaze in a terminal explosion. All stars are in a balance between inward gravitational force and an outward energy flow.
If the star is a little than bigger than our sun, it will be massive enough to press the Helium in its core into its own energy generating cycle. The process requires ten times higher temperatures to get started and creates the heavier elements Carbon, Nitrogen and Oxygen.
Yet larger stars take this fusion to higher elements in increasingly violent manners reaching temperatures of billions of degrees. As their lighter elements are exhausted the star collapses on its core produce very fast, violent, explosive reactions generating elements from Silicon to Nickel. Such stars explode so violently, sometimes multiple times, that they blow a significant part of their outer layers into space, leaving only their heavy cores intact.
These high mass stars have short lives and many of them were created, lived and exploded early in the life of the Universe. This process distributed critical heavy elements everywhere, some of which was eventually recycled into new stars and planets, and into the life that inhabits them.
The pressure in these remnant, heavy (mostly Iron) stellar cores can became so great that they are crushed down to pure nuclear material, all the atoms merged into one incrediblely dense object about ten miles across .. a neutron star.
The physical state and behavior of a neutron star is extreme. The forces pushing back against further gravitational collapse are exotic and describing neutron star properties in analogies to human experience is difficult. One measure we can, kind of, comprehend is that a neutron star’s mass is between about 1.5 and 2.5 times that of the Sun.
Above about 2.5 solar masses, a neutron star’s gravity becomes too much for even the nuclear forces to hold back further collapse. Current theory proposes another possible pushback on gravity when the nuclear core devolves to pure quarks, but before then, its gravity will overwhelm the ability of even light to escape and it will become a black hole.
But back to neutron stars .. these objects are incredibly dense, incredibly magnetic and they rotate very quickly, spinning a hundred times a second is not unusual. Despite these extreme conditions, and their violent past, they are relatively stable objects, but they are represent unimaginable stored energy. Left alone, over time, they will slow down and lose energy, but any short-term interruption of that slow decay will release enough energy to cause the nearby fabric of space-time to shudder.
What could disrupt a neutron star? Well, one possibility is another neutron star! Two neutron stars merging might be considered to be pretty unlikely, but stars are often formed in pairs. In the event that each member of such a binary system becomes a neutron star, and over time they gradually get closer to each other, they will eventually collide and merge. The first binary neutron star system was discovered by Russell Alan Hulse and Joseph Hooton Taylor, Jr., of the University of Massachusetts Amherst in 1974. This discovery and its subsequent analysis earned them the 1993 Nobel Prize in Physics.
Our story doesn’t follow the implications of this; the prediction of gravitational waves, and their subsequent detection, five years ago, from two colliding black holes nearly 1.3 billion light years away will go down in history as one of humanity’s greatest scientific achievements.
Earlier in the story we noted Iron was the ultimate element created inside stars, and Iron is the sixth most abundant element in the Universe. Iron is the 26th element and Uranium is the 92nd so there are 66 other elements found in nature that are not formed inside stars. How were they created?
Imagine an Iron atom in the vicinity of a neutron star collision. In the ensuing chaos the surrounding space is flooded with neutrons ejected from the stars’ disrupted cores. Under this deluge the Iron atom cannot avoid absorbing one of them to become a heavier, unstable Iron isotope. To relieve that instability, a neutron becomes a proton, and the Iron atom becomes a Colbalt atom. But, if left alone, that Cobalt atom will eventually decay back to Iron. However, if there is a sufficient barrage of neutrons, that Cobalt atom might absorb another one before it decays, finding solace in moving up the elemental ladder another step to become Nickel.
If the deluge of neutrons doesn’t let up the process continues: Cobalt to Nickel to Copper to Zinc .. and on and on, ever upward. The critical factor is whether there are enough neutron to drive the process, to keep adding neutrons fast enough to overcome the decay of most of those new atoms. If there is, when the environment eventually settles down, there will be atoms of every element floating around. If our Iron atom was driven all the way up to Uranium, there’s a chance it’s in your granite countertop!
How many is enough neutrons to drive this process? It has to be a lot because some atoms fall back down a rung of the ladder in fractions of a second and for the process to continue a good proportion of atoms need to get punched up to the next rung before falling back. This process requires about a hundred captures per atom every second. That, in turn, requires there to be about 1E24 (a million multiplied by itself four times) neutrons per cubic centimeter .. coincidentally, about the density of water .. an astonishing number, requiring an extreme environment.
Gold (element 79) is only thirteen rungs from the top of the ladder, every atom of Gold started down near Iron, 53 rungs below. In August 2017, the fifth gravitational wave event was detected .. two neutron stars spiraled into the each other. This was the first GW detection in which neither body was a black hole so it could be seen by telescopes; the light could escape to be observed on Earth 140 million light-years away. So-called, GW170817, was observed across the spectrum from gamma-rays to radio, and, in addition to a massive neutron flux, Gold was detected. Current astrophysical models suggest that this single neutron star merger event generated between 3 and 13 Earth masses of gold.
It is now believed that other violent astrophysical events which were theorized have produced Gold are probably not involved .. that neutron star mergers alone are suffient to generate all the Gold in the Universe .. including that in the ring on my left hand.