If you've spent any time perusing the periodic table of elements, you've probably noticed that there aren't too many gaps. As the chart progresses, the number of protons, which define an element, march neatly from 1 to 2 to 3 and so on. This orderly transformation can't be made by fusion in stars alone. If you've ever enjoyed lithium, gold, or our entire physical universe, you have a phenomenon called neutron capture to thank. Here's why.
Eskimo Nebula image via NASA
Most science students will get the "we are all made of stars," speech at some point in their lives. Stars shine with the power of fusion in their cores. Gravity crushes a ball of hydrogen together until the hydrogen atoms in the center overcome the forces pushing the protons apart and fuse into helium. This process will continue until the star is pretty much made up of helium atoms. If the star is massive enough, they will then fuse together to form larger elements and larger ones, until the star goes supernova and disperses chunks of those elements across the universe. Every atom, besides hydrogen, is made by those stars. We're made of stardust.
This is true, but not complete. Two hydrogen atoms, each with one proton, make helium, which has two protons. When the star progresses to the next state, these two helium atoms can fuse and make beryllium, with four protons. But wait, what about, lithium, number three on the periodic table of elements? For that matter, what about numbers five through seven, since two beryllium atoms would fuse to make oxygen? It's not that these elements would be completely skipped over. There are many different kinds of elements inside a star at any one time, and which ones fuse depends on a number of factors. Depending on the mass, temperature, and composition of the star, there are many pathways that lighter elements take to make heavier ones. But straight fusion wouldn't make the universe we live in. Scientists don't believe we'd have the proportion of elements that we do if there weren't other processes at work.
One of these critical processes is neutron capture. Protons are only standoffish towards each other due to their similar charge. What draws electrons to a proton keeps protons apart. It's only when the protons are muscled close to each other that the nuclear strong force kicks in that the particles keep together. To get them that close generally takes the crushing mass and extreme heat of the inside of a star. A neutron, however, has no charge. It can wander into a nucleus full of protons relatively easily. But a neutron can't change one element into another. It takes a proton for that. Fortunately, neutrons are unstable. They degrade, spontaneously, into protons, while ejecting an electron. The electron heads out of the nucleus, and the proton bumps the atom up one on the periodic table of elements. The insides of stars have a surprising amount of neutrons swimming around in them. This helps build elements up one proton at a time, and gives us our physical universe.
One flashy example of neutron capture is the amount of gold that's sitting around. Gold is a heavy element, with an atomic number of seventy-nine. It is not easy to get that by fusion, both because of the odd number of protons and the overall heaviness of the element. But stars don't only push atoms together. In a supernova, they rip atoms apart. Specifically, they rip apart iron nuclei, which tend to have even more neutrons than protons. These neutrons shoot out through the surrounding elements, building up a deposits of neutrons on them. When some of these neutrons decay into protons, they create elements like gold, lead, and uranium. If you have any gold jewelry, at least part of it was formed via neutron capture during a supernova.
Gold Image: Alchemist HP