One second to end what the universe has built in 10 million years

The human attitude toward gold is, to put it mildly, nothing short of strange. Of the 118 confirmed elements on the periodic table, gold is the one that is the focus of economic activity and probably the one that has involved the most blood, sweat, and tears. It certainly makes sense that it has been used as currency throughout history. If we discard from the periodic table, for practical reasons, the elements that are gases, liquids, or poisonous at room temperature; those that are too reactive with water; or those that explode, are corrosive, or radioactive, we are left with few useful elements and only one that is golden in color.

Alchemy, that esoteric belief linked to the transmutation of matter that was key to the origin of chemistry, also revolves around the golden noble metal. And judging by the number of manuscripts of his work found years ago, it’s no surprise that even Isaac Newton, to whom we owe so much for our understanding of the laws of gravity and the behavior of light, devoted much of his time and mental space to it.

We astrophysicists have long since discovered (although we don’t take much credit for it) the philosopher’s stone for transforming into gold: they’re called massive stars. It’s something we’ve never seen directly, because the stars inside them are opaque, but we know how the process works. That’s precisely what I’m going to describe here.

First, let’s keep in mind a couple of fundamental concepts we need to build the story. The life of a star begins when hydrogen fusion reactions ignite. Nuclear fusion stabilizes a star against the continued action of gravity and occurs primarily in the innermost part of the star, in its core. Only this place is hot enough to ignite hydrogen, transforming it into helium and releasing energy. Ten percent of every star is used as fuel, converting a tiny fraction—just 0.7 percent—of its rest mass into energy per gram, using what is probably the most famous formula in physics (E = m c2).

The more mass a star has at the beginning of its life, the less it lasts. One might think that the more mass, the more fuel it has to make it shine. And that’s true, but we also have to keep in mind that the greater the mass, the higher the temperature reached in the core, the more efficient the process, and the faster the fuel is consumed. More than 90% of a star’s lifespan is spent this way; it’s what the Sun is doing now. The amount of energy it loses is what it radiates, what we see, what makes it shine.

And as long as it has enough mass, the evolution of a star as seen from its engine room, its core, consists of successive episodes of nuclear consumption burning interrupted by states of gravitational contraction.

The core heats up as the star ages, a result of contraction in order to continue firing successive nuclear fuels. Each successive fusion reaction leads to the production of iron-group chemical elements and releases energy. But reactions that use iron or heavier elements as fuel absorb it.

And from here, the iron core, which has taken ten million years to build, will be destroyed in seconds. In literally a tenth of a second, the temperature reaches five billion degrees in an ultraheated core. The radiation produced at these temperatures is so high that it emits photons in the gamma ray range, which collide with the iron atoms and begin to photodisintegrate them, breaking them into the pieces that are helium particles (for every iron atom, 13 helium atoms are produced, and neutrinos are also released).

Photodisintegration, for the star, is like a heavy fly meeting a hand. Goodbye, au revoir, it’s over. Because then, literally in another tenth of a second, the nucleus becomes so dense that electrons and protons (which we’ve known since our earliest scientific education repel each other) have no choice but to get closer. And they do so so much that they form neutrons in the process, releasing an enormous amount of neutrinos that carry away energy.

When we reach this point, the contraction stops, and this is where the mess begins. Let’s mention beforehand that if there were enough mass, we would still continue contracting, building a stellar black hole. In 0.25 seconds from the beginning of its rapid contraction, the star that forms is a neutron star, reaching densities of atomic nuclei in its stellar core. This is not an alliteration: the largest has reached the densities of the smallest.

But let’s say that while the stellar core is busy contracting, its surroundings have suffered a drop in pressure, causing the external material surrounding this region, which is part of the star’s structure, to begin “falling” at high speeds—it can reach 15 percent of the speed of light. And again, when the core’s contraction stops, in a fraction of a second, a change occurs: the material that was falling into the core rebounds and is pushed outward, toward the star’s surface, aided by the energy and particles released within the core.

As it moves, it encounters increasingly less dense layers, which causes a shock wave when it exceeds the speed of sound in the medium. Within a few hours, it reaches the surface, and the energy escapes in a torrent of light. A supernova has been born, and we call it a core-collapse supernova because there are other types.

That shock wave is very important; it’s the only place in the universe capable of producing gold, mercury, uranium, zinc, silver, and tin. Let’s say that shock wave is the philosopher’s stone that alchemists were looking for.

Eva Villaver is deputy director of the Institute of Astrophysics of the Canary Islands, Spain.