A very, very long time ago, there was a star. It was much more massive than the star that lights our sky, around twenty times as massive. It was composed almost entirely of hydrogen, as was most of the universe when it was formed. Gravity collected its hydrogen together, and gravity compressed the hydrogen fuel at its core, until it was so hot that the hydrogen fused into helium. The star began to shine.
The heat of the fusion resisted its massive gravity, and it shone in this way for ten million years, a very short time in the history of stars. But after this time, the helium in its core accumulated, the hydrogen in its core depleted, and the star dimmed. Without the heat to resist its gravity, the core compressed, and under the pressure, the helium began to fuse into carbon and oxygen, the building blocks of our life.
Hydrogen surrounding the core was under enough pressure to begin fusing into helium, and the star shone again with two lamps instead of one. This carried on for only a few million years, and then the helium was exhausted and the light dimmed again.
The star was still very massive, and its gravity compressed the core again, and now the carbon began to fuse into neon. Surrounding this core was a layer of helium fusing into carbon and oxygen, and surrounding this, a fresh layer of hydrogen was now fusing into helium.
This state of affairs lasted but a few hundred years, and then the light dimmed, gravity asserted itself with another contraction, and now the neon began to fuse into more oxygen, surrounded by layers of fusing carbon, helium, and hydrogen.
When the neon in the core was exhausted, there was another contraction, and now the oxygen-rich core was under enough pressure to begin fusing into silicon. Surrounding this were successive layers of fusing neon, carbon, helium, and hydrogen. This lasted but a single of our earth seasons, there was a penultimate contraction, and the star reached the final phase of its life.
The silicon at its core, under the tremendous pressure of gravity and heat of fusion, fused into nickel, which decayed into a dense, iron-rich core. Surrounding this core were layers of fusing oxygen, neon, carbon, helium, and hydrogen. The star’s life had come to an end.
The silicon within the star’s core burned out in just five days, and without the heat of fusion to resist the pressure of gravity, the core shrunk. Unlike previous contractions, there was no next phase of fusing iron atoms, so it did not restart. The core shrunk and kept shrinking, until the only thing preventing it from further collapse was electron-degeneracy pressure, the quantum resistance of electrons from occupying the same place and state.
As the outer layers of fusion surrounding the core continued to burn, they deposited even more mass upon the core, and then suddenly the massive gravitational force overcame the electron-degeneracy pressure, and the core collapsed even further. An enormous amount of energy was released, heating it to the point where the iron atoms disintegrated into alpha particles.
The temperature continued to rise, and these alpha particles captured the electrons, forming neutrons. The neutrons packed together more tightly, forming a much smaller, hyper-dense neutron sphere. The rest energy of alpha particles and electrons is higher than the rest energy of neutrons, so the energy freed by the formation of the neutron sphere was released as a blast of neutrinos. This blew what was left of the active shell surrounding the core out into space, creating a supernova.
We did not see the supernova.
Neither did the dinosaurs, nor any other life on earth. Although the explosion lasted for 100 days, and although it happened far away from where we are now, it happened so long ago that its visible light passed this place long before our star was even born.
What remained behind was a neutron star. Its mass was maybe twice that of our sun, packed into a sphere perhaps 20 kilometers across. Very massive, but not enough for gravity to collapse it even further and create a black hole. But very massive!
The neutron star’s tremendous gravity affected everything around it, not just the material within it. It would have been impossible to stand on its surface, but if somehow I travelled back in time to this neutron star, I would have found that while I have a mass of about 86 kilograms on Earth, I would have felt like I “weighed” more than eight trillion kilograms on this neutron star.
If I was wearing a headlamp for illumination, I would have been able to see all the way around the nutron star to the other side, because its gravity would have bent light into a semi-circle. And if I decided that this much gravity was not to my liking and I wanted to fly away, I would have needed to accelerate to as much as half the speed of light, just to escape its gravity well.
There are many other incredibly interesting things about neutron stars. They spin. They emit beacons of x-ray energy that sometimes fall upon our earth in regular pulses, creating “pulsars.” The remnants of the supernova that they created may envelop them creating a “nebula” that they illuminate with their energy.
One such nebula is the Crab Nebula, pictured above.
Neutron stars do not hang, fixed in space. Nothing really does. Gravity and momentum moves everything around in the dance of the heavens. And from time to time, a neutron star encounters another. Their massive gravities pull them towards each other. They may bend their paths and then continue on their way. Or their immense gravities may create a deadly embrace, causing them to spiral towards each other, closer and closer, until they collide.
When they collide, a massive gravitational wave is created. Gravitational waves were first proposed by Henri Poincaré in 1905, but most people associate them with Albert Einstein, who predicted them ten years later within the framework of his General Theory of Relativity, one of the great human intellectual achievements. Humans have been looking for evidence of gravitational waves ever since, but it was almost exactly 100 years later, in 2015, that we detected them, and the results were announced in early 2016.
In August of 2017, the Laser Interferometer Gravitational-Wave Observatory detected a gravitational wave, GW170817, produced by two neutron stars spiralling together just before they collided. When neutron stars merge, they are thought to either create a newer, more massive neutron star, or a black hole if their combined masses are large enough.
But there is more to the story than gravitational waves. A massive magnetic field, trillions of times stronger than that of Earth, is created in milliseconds. And kilonovae occur, gamma ray bursts and electromagnetic radiation from material that decays as it is ejected from the merging neutron stars.
Kilonovae are very interesting. The neutrons in neutron stars are held together by gravity. But material ejected in the cataclysmic merger is no longer held together by gravity. It “decays” back into ordinary matter.
Physicists have long been interested in heavy metals like gold. The “normal” fusion process within a star does not create heavy substances like gold or platinum. The abundance of such metals in the universe was puzzling. Astrophysicists realized that one way heavy metals might be created would be if an extremely old star, like a red giant, contained lots and lots of iron.
As with the first star, iron is created when a sufficiently large star reaches the final stage of fusion. Not all such stars become neutron stars, some supernova in such a way that their iron is blown out into the nebula, where it could become part of a different kind of star. So for a red giant to contain lots of iron, it would have had to have been created out of stellar materials that contained iron left over from a previous supernova.
As the red star burned, the iron within it would slowly “capture” neutrons thrown off by the fusion reaction. Some of those neutrons would become protons, and the iron would transmute into heavier elements. Some of those would be unstable and decay, and the process would continue until it reached a particularly stable material like lead or bismuth.
This process, called the s-process, or “slow” process, explained one way that stars might create materials that fusion could not create. But it didn’t explain a lot of the matter we find in the universe. It explained strontium, but not gold or uranium. It explained silver, but the predicted amount of silver created was only a fraction of the silver found in the universe. There must have been another process that created heavy elements.
Astronomers came up with another possibility, the r-process. What if matter ejected from events like the merger between two neutron stars decayed into elements like uranium, gold, or silver? Kilonovae could explain the amounts of materials like uranium, gold or silver observed in the universe.
Remember gravitational wave GW170817? This was produced by the merger of two neutron stars, and unlike the four gravitational waves detected before it, it was independently observed by seventy observatories around earth: Its event was accompanied by a massive amount of electromagnetic radiation.
When two black holes collide, there is no observable radiation because their gravity does not allow light to escape. Gravity waves are our only hint that anything has happened. But neutron stars are different. They are massive enough to create gravity waves, but not massive enough to prevent light from reaching us alongside the gravity waves.
So when a gravity wave is accompanied by a blast of electromagnetic radiation, we know that the bodies colliding were smaller than black holes, like neutron stars. And that’s what happened.
Light tells us a lot about the process that created it. Spectroscopy is the study of the interaction between matter and electromagnetic radiation. One form of spectroscopy, atomic emission spectroscopy, identifies the atomic composition of a material that gives off radiation by measuring the amplitude and frequencies of the radiation it emits.
One of the great early accomplishments in science was when astronomers, observing our Sun, noted an unknown yellow spectral line signature. In 1868, Norman Lockyer predicted that it must be created by a hitherto unknown element, which he named “Helium” after the Greek Titan of the Sun, Helios.
In 1895, two Swedish chemists detected helium in ore samples here on Earth, and in the great tradition of the scientific method, we had a theory, a prediction, and a confirmation of the theory by test.
And this very year, we repeated the process. Astrophysicists had predicted that the merger of two neutron stars would create kilonovae, and that the r-process decay would produce heavy elements. And when those seventy observatories focused on the source of the gravitational wave GW170817, what did spectroscopy reveal?
Gold. Other heavy elements, like platinum. And plenty of them! The theory had passed its test, we knew that gold was produced from the matter ejected in the collision of neutron stars. This was a glorious observation, and you and I were alive to witness the popping of champagne corks around the world.
Remember our neutron star? It wandered, it collided with another star, it ejected some of its matter, perhaps the very chunk that I had–in a thought experiment only–stood upon. It formed a new neutron star, and carried on. But what of its ejected matter?
As we noted, that decayed into various heavy elements, some of which were gold. The matter formed interstellar dust. And while some of it stayed within the gravitational field of the newly merged neutron star, some of it, propelled by the vast energies created in the merger, flew away from the neutron stars to wander, alone, in the universe.
In time, it was attracted by other matter, in a great cloud we call a nebula. At the center of this great cloud, gravity pulled the matter together into lumps. Our matter was gold, but most of the nebula was lighter elements, predominately hydrogen, the lightest element.
Gravity acted to make the lumps lumpier. They sucked nearby material into them. The gravity compressed them together, swirling and spinning, flattening into discs with a massive bump in the center. As they compressed, they heated. At a certain point, when the temperatures at the cores of these super-compressed lumps reached about 10,000 Kelvin, the hydrogen began to fuse into helium.
Stars were born. The nebula had become a nursery.
One of these stars was about one solar mass. Spinning around it was a huge accretion disc. Gravity within that disc made it lumpy, and the lumps coalesced into spinning spheres orbiting the star. Other lumps passing by were sucked into the star’s gravity well and took up irregular orbits. Some begin to orbit the spheres orbiting the stars, forming satellites.
The star was our sun. The third lump from the star was our earth. Our earth was compressing itself under its own gravitational field. The heaviest elements fell to its centre, just as they do in a star. A core of mostly iron formed, just like that star that would birth the neutron star. Our gold fell into it, along with lots of passing uranium.
The uranium was radioactive and decaying. This caused a chain reaction. But not the kind that explodes, there were too many other elements all together, acting like the moderating rods of a nuclear reactor. So the uranium just got hot, and it is still hot to this day. The heat of compression and of this slow burning nuclear decay turned the core to molten metal, with a solid crust on the outside.
At some point, a satellite the size of Mars came crashing into the earth, so hard and so fast that the mantle surrounding the earth was blasted into space, along with the mantle surrounding the satellite. It didn’t go far–much of the debris formed a ring around the earth, and gravity eventually compacted it into a single satellite, our moon. This explanation for how the moon came to be is called the giant-impact hypothesis, and it accounts for the moon’s unusual size, density, composition, and other matters.
The earth developed another mantle out of its magma, including heavy materials like uranium and gold. The earth’s iron core formed a huge magnet. An atmosphere formed. Life formed. The magnet bent electromagnetic rays such that it helped to shield life from the universe. Another shield was formed by ozone in the atmosphere.
Our moon is a satellite much larger than our planet’s gravity would normally be able to capture. The large moon formed a third shield, attracting meteors and other flying debris so that extinction-event meteorites would strike the earth much less frequently.
Our world had become a nursery for life. The seas filled with creatures. Many tiny such creatures, when they died, sunk to the bottom of the seas. Their corpses piled up, eventually to be compressed into oil. Other creatures crawled onto the land, and then the land and skies filled with life. Plants developed the wood fibre lignin. For a very long time–the Carboniferous period–nothing evolved to eat or break down lignin, so when trees died, they did not decay and they too, piled up and would be compressed into coal.
Life continued to evolve. The most dominant (as we define dominance in our own image) came to be the dinosaurs. Remember the extinction-event meteors that the moon was shielding us from? One got through sixty-six million years ago, the Cretaceous–Paleogene extinction event, forming the Chicxulub crater.
The dinosaurs were nearly wiped out. The survivors became avians, and ecosystems the world over changed drastically. A small homeothermic creature that specialized in night-hunting filled the now available opportunities, multiplied, and became a minor success story.
One of its descendants walked on its hind legs and became us. We explored, grew, fought each other, became superstitious, discovered science and reason, rejected it, and fought some more. We took the bounty of coal and leveraged it into an explosion of industry and transportation. Later, we mastered oil, electricity, and the terrible nuclear fires.
Having mastered the other creatures, our dominant behaviour became manipulating each other through social engineering. During the Cold War, the two dominant superpowers got into a “Space Race” that was part propaganda contest, part claim-staking. We turned oil into a rocket fuel called RP-1, and sent humans to the moon.
There, we found evidence supporting the giant-impact hypothesis. Once again, science confirmed what reason suggested.
Today, the battle between superstition and science is still hotly contested. But science hasn’t gone away. Science made it possible for us to learn this story, and us to share it in this form. We make our machines with science, and in turn, our machines have made us who we are today.
One day a mere twenty years ago, one of us humans–Raganwald–had a pain in his jaw. He went to a dentist, who used radiation to see what the naked eye could not, to look into solid matter, and to see that Raganwald had an infection in the root of a tooth.
The dentist sent him to a specialist, who drilled a hole in the top of the tooth, and then extracted the root without removing the tooth. The specialist closed the hole with a temporary filling, and when all had healed satisfactorily, Raganwald returned to the dentist, who fitted the tooth with a crown.
All was well for about twenty years, but in 2017, the infection returned. This time, there was nothing to do except remove the tooth. The dentist removed Raganwald’s crown, then the tooth. The dentist gave him the crown as a keepsake.
Here it is, Raganwald’s crown:
The surface of the crown is made of gold, matter from an ancient giant star. A star that burned brightly but briefly, before becoming a neutron star in an explosive supernova. The neutron star wandered, then collided with another neutron star, and in their fiery coupling, created this gold.
The gold was expelled into a nebula, that became a nursury, and then it drifted into an accretion disc around a new young star, our sun. The gold became part of our planet’s second mantle, where it sat until life evolved, dug it out of the earth, and formed it into Raganwald’s crown.
This crown’s journey from star to planet to keepsake in this simple plastic box–made from the oil laid down by life, millions of years ago–is the journey of our universe. And it is fitting that Raganwald lost his crown the very same year that we confirmed that its gold had been forged in the collision of two neutron stars, long before our own sun began to shine.
Raganwald treasured his crown. It reminded him that although the creation of a thing may be hundreds of millions or even billions of years in the past, the relentless march of science leads us to an explanation for how things came to be.
Ragnwald’s crown stands as a testament to how small and fragile we are, and yet how great is our thirst for learning.