Part A · What stuff is · Interlude

2 · Made in stars

Chemistry rearranges atoms but never makes new ones. So where did the hundred-odd kinds come from? The hydrogen in your water is about 13.8 billion years old. The carbon in your cells, the oxygen you breathe and the iron in your blood were made inside stars. The gold in a ring probably came from colliding neutron stars.

Making elements means changing the nucleus, which is a different game from chemistry. Chemical bonds involve a few electronvolts per atom (chapters 1 and 5). Nuclear reactions involve millions of electronvolts. That's why they need the inside of a star, and why stars shine for billions of years on what they burn.

The first three minutes

In the first few minutes after the Big Bang the whole universe was hot and dense enough to fuse protons and neutrons, but it expanded and cooled too fast to get far. It came out about 75% hydrogen and 25% helium by mass, with a trace of lithium. Since helium is four times heavier, that's roughly 12 hydrogen atoms for every helium atom. Those proportions are still visible today in the oldest stars, and matching them is one of the Big Bang model's strongest tests.

Stars as element factories

A star is a ball of gas squeezed by its own gravity until its core reaches about 15 million degrees, hot enough for hydrogen nuclei to overcome their electrical repulsion and fuse into helium. That's chapter 10's barrier again, except the barrier is the Coulomb wall between two positive nuclei, crossed partly by quantum tunnelling. Each helium nucleus weighs 0.7% less than the four protons that made it, and that missing mass is the energy that makes sunlight (E = mc2). The Sun converts about four million tonnes of mass into energy every second.

When the hydrogen in the core runs out, the core contracts and heats until helium fuses into carbon (three helium nuclei at once) and oxygen. Heavier stars continue with neon, magnesium, silicon and sulfur, each stage hotter and shorter, until the core is iron. Then it stops. Why iron? Because of this curve:

Binding energy per nucleon (MeV), against the number of protons and neutrons in the nucleus. Dots: measured values for selected nuclei (hover for details). Dashed line: the semi-empirical mass formula, a liquid-drop model of the nucleus with five fitted constants. The higher the point, the more tightly bound each particle is.

The liquid-drop formula is a smooth approximation. It misses the extra stability of "magic" nuclei like helium-4 (the spike on the left) and gets light nuclei wrong, which is why the measured dots matter. The peak is at iron-56 and nickel-62, within a hair of each other.

Building nuclei up toward the peak releases energy, and that's fusion. Breaking very heavy nuclei down toward the peak also releases energy, and that's fission, which powers nuclear reactors. A star's core can't get energy by fusing iron, so it has nothing left to burn and collapses. In a massive star the collapse rebounds into a supernova, scattering the oxygen, silicon, calcium and iron it made across the galaxy.

Beyond iron: neutrons

Heavier elements are made by adding neutrons, which have no charge and so no Coulomb wall to climb. A nucleus captures neutrons, some of them turn into protons by beta decay, and the nucleus climbs the table.

How much of the r-process happens in neutron-star mergers versus rare kinds of supernova is still debated. The broad picture above is well established; the exact shares element by element are an active research topic. Jennifer Johnson's "periodic table of origins" (linked below) is the best summary of current thinking.

A few light elements have odd stories. Lithium, beryllium and boron are rare because stars destroy them rather than make them. Most of what exists was chipped off heavier nuclei by cosmic rays crossing space. And elements past uranium don't last: they're radioactive and have mostly decayed away, or are made only in laboratories.

Chapter 3 told how we know: every element leaves its barcode in starlight, so we can read the composition of stars, nebulae and the gas between galaxies, and watch it get richer in heavy elements as generations of stars live and die.

Physics bridge

The scale separation is what makes chemistry possible as a separate science. Nuclear binding energies are about 8 MeV per nucleon; chemical bonds are a few eV. At chemical temperatures (kT ≈ 0.026 eV) nuclei are frozen: they're inert, labelled point charges, and their identity (atomic number) is conserved by every chemical reaction. That's the clean factorisation that lets chemistry ignore nuclear physics, just as the Born–Oppenheimer approximation lets it treat nuclei as slow and electrons as fast.

Not quite perfectly: isotopes (same element, different neutron count) have slightly different masses, which shifts vibrational energies and reaction rates a little. That's useful in practice, for dating rocks and tracing climate. Nuclear decay also happens on its own clock regardless of chemistry: carbon-14 dating works because no reaction can speed it up or slow it down.

Tell it to a 6-year-old
Look
On a clear night, go outside and look at the stars. (Or look at a picture of a colourful nebula, a cloud in space.)
Wonder
"Where do you think the tiny pieces that make you came from?"
Try
Find the brightest star you can. Then hold up your hand: tell her that some of the tiny pieces in her hand were made inside a star like that, a very long time ago.
True sentence
"Almost all the tiny pieces in your body were made inside stars, long before the Earth existed. When those stars got old, they blew their pieces out into space, and some of them ended up in the Earth, and in you."

This one is story only; there's no safe kitchen experiment for nuclear fusion. It's true at every level: the hydrogen in her body dates from the Big Bang, the carbon, oxygen and calcium from stars. Avoid saying atoms were "born" or are "alive"; "made" is right. "We are made of star stuff" (Carl Sagan) is a lovely line to share.

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