Where the elements come from

Everything you can touch is made of atoms, and there are ninety or so kinds of atom in nature. Each kind is called an element. Oxygen is one, iron is another, gold is another. Nearly all of them were made in space, long before the Earth existed.

The universe began with almost none of them. Its first few minutes were hot enough to make only the two lightest: hydrogen and helium. By weight, about three quarters of what came out was hydrogen and one quarter was helium. For the next 200 million years or so there was nothing else to make anything from.

Stars made the rest. The middle of a star is hot enough to press small atoms together into bigger ones. Hydrogen becomes helium. Later, three heliums make a carbon, and a carbon and a helium make an oxygen. The heaviest stars carry on, step by step, all the way to iron.

A star like the Sun stops early and dies gently. At the end it puffs its outer layers off into space, and a good deal of carbon and nitrogen goes with them. The carbon in every living thing came out of old stars in this way.

Heavy stars end in a supernova, which throws out the oxygen, silicon and other atoms they have built. White dwarfs that blow up make much of the iron.

Gold needs something rarer still. In August 2017 astronomers watched two dead stars, called neutron stars, crash into each other in a galaxy 130 million light years away. The glow that followed had the marks of freshly made heavy atoms in it. Most of the gold and platinum on the Earth is thought to have been made in crashes like that one.

So you are a mixture. The hydrogen in the water you drink is as old as the universe. The oxygen you breathe came from heavy stars, the carbon in your body from small ones, and much of the iron in your blood from exploding white dwarfs.