Astronomy Jargon 101: R-Course of – Universe Immediately

In this series, we explore the weird and wonderful world of astronomy jargon! You will quickly see what we are 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 sufficient to merge elements. At that moment, the sun turns hydrogen into helium. Towards the end of its lifespan, it converts helium into carbon and oxygen. Even heavier stars can forge silicon, magnesium, and iron.

Through this process, the stars of our universe have transformed the original hydrogen and helium of the Big Bang into more elements. But stars themselves are unable to fuse elements other than iron because this fusion process sucks up energy instead of releasing it.

In order to make items heavier than iron, you need a few key ingredients. First, you need lots of seeds, lots of kernels running around, ready to go heavy. Second, you need neutrons. Many of them. Third, you need a lot more energy than is strictly reasonable.

These are the ingredients behind the r-process, which stands for Rapid Neutron Capture Process. It happens in extreme environments. On earth it can happen briefly during an atomic bomb detonation. In space it happens when stars become supernovae or neutron stars collide.

What happens during the r-process is that the seed kernels are absolutely bombarded with neutrons. The neutrons hit the nuclei so quickly that they can build up into heavier elements before they naturally decay radioactively into lighter ones. Through this process, most of the periodic table can be filled in in no time. While the entire process can take about a week, the first neutron explosion is complete in seconds. After that, all radioactive elements decay into more stable isotopes.

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

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