Where do atoms come from?

[This is a transcript of the video embedded below.]

Matter is made of atoms. You all know that. But where do atoms come from? When were they made and how? And what’s the “island of stability”? That’s what we will talk about today.

At first sight, making an atom doesn’t seem all that difficult. All you need are some neutrons and protons for the nucleus, then you put electrons around them until the whole thing is electrically neutral, done. Sounds easy. But it isn’t.

The electrons are the simple part. Once you have a positively charged nucleus, it attracts electrons and they automatically form shells around the nucleus. For more about atomic electron shells, check my earlier video.

But making an atomic nucleus is not easy. The problem is that the protons are all positively charged and they repel each other. Now, if you get them really, really close to each other, then the strong nuclear force will kick in and keep them together – if there’s a suitable amount of neutrons in the mix. But to get the protons close enough together, you need very high temperatures, we’re talking about hundreds of millions of degrees.

Such high temperatures, indeed much higher temperatures, existed in the early universe, briefly after the big bang. However, at that time the density of matter was very high everywhere in the universe. It was a mostly structureless soup of subatomic particles called a plasma. There were no nuclei in this soup, just a mix of the constituents of nuclei.

It was only when this plasma expanded and cooled, that some of those particles managed to stick together. This created the first atomic nuclei which could then catch electrons to make atoms. From this you get Hydrogen and Helium and a few other chemical elements with their isotopes up to atomic number 4. The process of making atomic nuclei, by the way is called “nucleosynthesis”. And this part of nucleosynthesis that happened a few minutes after the big bang is called “big bang nucleosynthesis”.

But the expansion of plasma after the big bang happened so rapidly that only the lightest atomic nuclei could form in that process. Making the heavier ones takes more patience, indeed it takes a few hundred million years. During that time the universe continued to expand, but the light nuclei collected under the pull of gravity and formed the first stars. In these stars, the gravitational pressure increased the temperature again. Eventually, the temperature became large enough to push the small atomic nuclei into each other and fuse them to larger ones. This nuclear fusion creates energy and is the reason why stars are hot and shine.

Nuclear fusion in stars can go on up to atomic number twenty-six, which is iron, but then it stops. That’s because iron is the most stable of the chemical elements. Its binding energy is the largest. So, if you join small nuclei, you get energy out in the process until you hit iron, after which pushing more into the nucleus begins to take up energy.

So, with the nuclear fusion inside of stars, we now have elements up to iron. But where do the elements heavier than iron come from? They come from a process called “neutron capture”. Some fusion processes create free neutrons, and the neutrons, since they have no electric charge, have a much easier time entering an atomic nucleus than protons. And once they are in the nucleus, they can decay into a proton, an electron, and an electron-antineutrino. If they do that, they have created a heavier element. A lot of the so-created nuclei will be unstable isotopes, but they will spit out bits and pieces until they hit on a stable configuration.

Neutron capture can happen in stars just by chance every now and then. Over the course of time, therefore, old stars breed a few of the elements heavier than iron. But the stars eventually run out of nuclear fuel and die. Many of them collapse and subsequently explode. These supernovae distribute the nuclei inside galaxies or even blow them out of galaxies. Some of the lighter elements which are around today are actually created from splitting up these heavier elements by cosmic rays.

However, neutron capture in old stars is slow and stars only live for so long. This process just does not produce sufficient amounts of the heavy elements that we have here on Earth. Doing that requires a process that’s called “rapid neutron capture”. For this one needs an extreme environment of very high pressure with lots of neutrons that bombard the small atomic nuclei. Again, some of the neutrons enter the nucleus and then decay, leaving behind a proton, which creates heavier elements.

For a long time astrophysicists thought that rapid neutron capture happens in supernovae. But that turned out to not work very well. Their calculations indicated that supernovae would not produce a sufficient amount of neutrons quickly enough. The idea also did not fit well with observations. For example, if the heavy elements that astrophysicists observe in some small galaxies –called “dwarf galaxies” – had been produced by supernovae, that would have required so many supernovae that these small galaxies had been blown apart and we wouldn’t observe them in the first place.

Astrophysicists therefore now think that the heavy elements are most likely produced not in supernovae, but in neutron star mergers. Neutron stars are one of the remnants of supernovae. As the name says, they contain lots of neutrons. They do not actually contain nuclei, they’re just one big blob of super-dense nuclear plasma. But if they collide, the collision will spit out lots of nuclei, and create conditions that are right for rapid neutron-capture. This can create all of the heavy elements that we find on Earth. A recent analysis of light emitted during a neutron star merger supports this hypothesis because the light contains evidence for the presence of some of these heavy elements.

You may have noticed that we haven’t checked off the heaviest elements in the periodic table and that there are a few missing in between. That’s because they are unstable. They decay into smaller nuclei in times between a few thousand years and some micro-seconds. Those that were produced in stars are long gone. We only know their properties because they’ve been created in laboratories, by shooting smaller nuclei at each other with high energy.

Are there any other stable nuclei that we haven’t yet discovered? Maybe. It’s a long-standing hypothesis in nuclear physics that there are heavy nuclei with specific numbers of neutrons and protons that should have life-times up to some hundred thousand years, it’s just that we have not been able to create them so far. Nuclear physicists call it the “island of stability”, because it looks like an island if you put each nucleus on a graph where one axis is the number of protons, and the other axis is the number of neutrons.

Just exactly where the island of stability is, though, isn’t clear and predictions have moved around somewhat over the course of time. Currently, nuclear physicists believe reaching the island of stability would require pushing more neutrons inside the heaviest nuclei they previously produced.

But the maybe most astonishing thing about atoms is how so much complexity, look around you, is built up from merely three ingredients neutrons, protons, and electrons.

I hope you enjoyed this video. You can now support this channel on Patreon. So, if you want to be part of the story, go check this out. I especially want to thank my super-supporters on Patreon. Your help is greatly appreciated.