What Is the R-Process?
The r-process stands for "rapid neutron-capture process." This phenomenon, first theoretically described by nuclear physicists in 1957, creates elements in nature that are heavier than iron. In the
supernova explosions of massive stars and in neutron star collisions, tremendous numbers of freely moving neutrons bind with iron atoms. As more and more neutrons pile up in the atom's nucleus, the neutrons undergo a radioactive decay, turning into protons. Accordingly, new, heavier elements are formed, because elements are differentiated by the number of protons in their nucleus. As its name implies, this process must occur rapidly in order to build up to very heavy, neutron-rich nuclei that then decay into heavy elements, such as uranium, which has 92 protons compared to iron's 26. While a theoretical understanding of the r-process is sound, scientists have debated over the astrophysical conditions and sites where the process can actually occur.
TKF: Why has the provenance of these elements been such a tough nut to crack?
FREBEL: The question of the cosmic origin of all of the elements has been a longstanding problem. The precursor question was, “Why do stars shine?” Scientists tackled that in the early part of the last century and solved the mystery only around 1950. We found out that stars do nuclear fusion in their cores, generating heat and light, and as part of that process, heavier elements are created. That led to a phase where a lot of people worked on figuring out how all the elements are made.
Understanding how heavy, r-process elements, are formed is one of hardest problems in nuclear physics. The production of these really heavy elements takes so much energy that it's nearly impossible to make them experimentally, even with current particle accelerators and apparatuses. The process for making them just doesn't work on Earth. So we have had to use the stars and the objects in the cosmos as our lab.
JI: As Anna just mentioned, we have been mostly stuck with astronomy, trying to measure what could have made all of these elements out in the stars. But it's also very difficult to find stars that give you any information about the r-process.
RAMIREZ-RUIZ : Right, it is very difficult to see these elements shine when they're created in the universe because they are very rare. For example, gold is only one part in a billion in the Sun. So even though the necessary physical conditions needed to make these elements were clear to physicists more than 50 years ago, it was a mystery as to what sort of objects and astrophysics would provide these conditions, because we couldn't see r-process elements being produced in explosion remnants in our own galaxy.
Two competing theories did emerge, which are that these elements are produced by
supernovae and neutron star mergers. These phenomena are very different in terms of how often they should happen and in the amount of these elements they should theoretically produce. Just to give you an example, the explosion of a star with more than eight times the mass of the Sun is thought to produce about a Moon's mass-worth of gold. A neutron star merger, however, is thought to produce a Jupiter's mass-worth of gold. That's over 25,000 times more! So just one neutron star merger can provide the gold we would expect to find in about six million to 10 million stars.
Alex and Anna's observations are so unbelievably useful because they really show that the phenomenon which created these elements is something rare, but that produces a lot of these elements, as a neutron star merger should.
FREBEL: It took 60-something years of work to figure this out, and a variety of astronomers — observers as well as theorists — have all put in their share. That's exactly what we and Enrico are continuing to do.
TKF: Enrico, you study the ionized gas called plasma that composes stars. How is the material in neutron stars different than the plasma in run-of-the-mill stars like the Sun, and how does this provide the raw ingredients for making r-process elements?
RAMIREZ-RUIZ : Neutron stars are only about the size of San Francisco Bay, which I live close by, yet they pack in as much mass as the Sun — about 330,000 times the mass of the Earth. Neutron stars are the densest objects in the universe. A neutron star the size of a Starbucks cup would weigh as much as Mount Everest! We call them neutron stars because they are neutron-rich, and that's a key aspect for making r-process elements, as I'll let Alex and Anna explain.
JI: So the nuclear fusion in stars can only make the elements up to iron. That's because iron is the most stable nucleus. If you try to fuse two things to make elements heavier than iron, it actually takes more energy than the fusion reaction itself releases. A neutron that gets close enough to this dense iron nucleus can join it thanks to one of the fundamental forces of nature, the strong force, which binds protons and neutrons together.
You can keep increasing the size of this nucleus by adding more neutrons, but there’s a trade-off. That nucleus will undergo a radioactive decay called a beta decay. Specifically, one of those added neutrons will spontaneously release some energy and turn into a proton. The r-process is what happens when you capture neutrons faster than the beta decays happen, and in that way you can build up to heavier nuclei.
FREBEL: This process can only happen when you have lots and lots of free neutrons outside of an atomic nucleus, and that's actually a difficult thing to do, because neutrons only survive for about 15 minutes before they decay into a proton. In other words, almost as soon as you have free neutrons, they just disappear. So it's really hard to find places where there are even free neutrons to undergo this neutron capture. As far back as the 1930s, neutron stars had been postulated as something that could exist, and it wasn't until the late 1960s that we knew they were real.
RAMIREZ-RUIZ : As we learned more about neutron stars, we found out that about two percent of them have companion stars, and a very small fraction have another neutron star orbiting around them. If the neutron stars are close enough, they will merge within several billion years or less because they produce gravitational waves as they spin around each other. These waves simply carry off energy and angular momentum, so the stars get closer and closer, and eventually they touch each other.
"Neutron stars are the densest objects in the universe. A neutron star the size of a Starbucks cup would weigh as much as Mount Everest!" — Enrico Ramirez-Ruiz