Astronomy Jargon 101: R-Process

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll quickly see what we’re talking about this week: r-process!

How do you make heavy elements? Some can be forged deep inside a star, where the intense pressures and temperatures are enough to fuse elements. Right now, at this very moment, the sun is turning hydrogen into helium. Towards the end of its life, it will convert helium into carbon and oxygen. Even heavier stars can forge silicon, magnesium, and iron.

It’s through this process that the stars of our universe have turned the primordial hydrogen and helium of the big bang into more elements. But stars themselves are incapable of fusing elements beyond iron, because that fusion process sucks up energy rather than releasing it.

To make elements heavier than iron, you need a few key ingredients. One, you need a lot of seeds, a lot of nuclei roaming around, ready to get heavy. Two, you need neutrons. A lot of them. Three, you need way more energy than is strictly reasonable.

These are the ingredients behind the r-process, which is short for rapid neutron-capture process. It happens in extreme environments. On Earth, it can happen briefly during a nuclear bomb detonation. In space, it happens when stars go supernova or neutron stars collide.

What happens during the r-process is that the seed nuclei get absolutely blasted with neutrons. The neutrons crash into the nuclei so quickly that they can build up to heavier elements before they naturally radioactively decay into lighter ones. Through this process, the majority of the periodic table can be filled out in an extremely brief amount of time. While the entire process may take a week or so to sort out, the initial neutron blast is over and done with in a matter of seconds. After that, any radioactive elements decay into more stable isotopes.

This process happens during Type-II (core collapse) supernovae, but the majority of heavy elements in the universe come from neutron star collisions. While not nearly as powerful as a supernova, there are a whole heck of a lot of neutrons involved, and so all the conditions are just right.