How are the atoms that form us forged across the universe?

 For decades researchers have studied how heavy elements are produced in the cosmos – but there is still so much mystery to unravel, says astrophysicist Artemis Spyrou.

Where do we come from? This simple question stems from a basic desire to understand our origins and it has many meanings. It can refer to our ancestry and involve mapping our family tree. It can mean DNA testing to figure out which parts of the earth our forebears came from. But for me, it’s bigger than all that – it means figuring out where every atom in my body was forged. I know all atoms are made in stars, but how and where? I have spent my entire career trying to find out.

NGC 1858 is an open star cluster in the northwest region of the Large Magellanic Cloud, a satellite galaxy of the Milky Way that has an abundance of star-forming regions
NASA, ESA, G. Gilmore and Gladys Kober Copyright: NASA, ESA, G. Gilmore and Gladys Kober

For decades, scientists thought they already had the answers to these questions, but the more we learn about the universe, the more we realise that things are more complicated than they once seemed. The first description of how elements are formed came from Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle (B2FH for short) in 1957. The quartet consisted of an astronomer, a modeller, a nuclear experimentalist and a theorist, reflecting the scientific diversity required to address such a fundamental problem. Together, they described how the combination of intense pressure and heat inside stars could fuse atomic nuclei together, so that heavier elements could be produced from lighter ones. With this the field of nuclear astrophysics was born.

However, fusion reactions like this couldn’t explain how all elements are formed, only those with an atomic weight up to and including that of iron. Nuclei have a positive charge (because of their protons) and the heavier they get, the harder it is for them to overcome the repulsive electrostatic forces and fuse. Heavier elements do exist in the universe, of course, but the question of exactly how they get there has been open for decades.

B2FH suggested that heavier elements could be formed by heavy nuclei capturing neutrons, as the particles’ lack of charge makes them easier to grab than positively charged protons. B2FH proposed two ways that these neutron captures could take place inside stars.

The first involves stars with a relatively small number of neutrons (around 108 neutrons per cubic centimetre), where neutron-capture reactions take years to occur. This is the so-called s-process (s for slow). In this case, neutrons are captured only by stable isotopes – atoms of the same element with differing numbers of neutrons – and their closest neighbours. Stable isotopes have been studied by nuclear physicists for almost a century and so their properties and how they would work inside a star are well known.

The second involves stars with a huge number of neutrons (around 1022 neutrons per cubic centimetre), where neutron-capture reactions take only seconds to occur. This is the r-process (r for rapid), and at the time, it wasn’t known whether there were any suitable stars where this could actually occur. But around five years ago, a neutron-star merger was detected that could be the perfect place.

It was once thought that the s and r processes were each responsible for producing about half of the universe’s heavy elements, with some small contributions from a proton-rich process called the p-process. However, with more powerful telescopes came new astronomical observations, and some of them don’t fit this picture.

These observations are something like “stellar archaeology”. The goal is to look for old stars – those that created most of the elements they contain themselves or got them from a single close neighbour.

Initially, with a small number of observations of these old stars, we didn’t find any surprises. They could all be explained by s and r processes. However, in the last five to 10 years, a large number of observations have been made that do not fit the s and r process patterns, leading to an entire rethink of how heavy elements are made.

We already had slow and rapid processes, but it seems that the missing piece of the puzzle was something in between. The i-process (short for intermediate) involves a middling amount of neutrons (between 1013 and 1015 per cubic centimetre) and neutron-capture reactions happen over the course of minutes. Adding this process into the mix means that the models fit the new observation data, suggesting it could explain what is going on in those mystery stars uncovered by more powerful telescopes.

A lot of scientific developments needed to take place to bring us to this point. More advanced telescopes yielded observations that couldn’t be explained by current knowledge. At the same time, advanced computers allowed the development of 3D models of stars, which showed that stellar interiors are much messier than older models could describe. Finally, discoveries and progress in the science of rare isotopes meant that the properties of almost all isotopes involved in the i-process are known experimentally. Knowing the masses of these isotopes, how long they live, how they decay and what radiation they emit are all important pieces of information that go into the astrophysical models.

The only thing missing is the probability of each of these rare isotopes capturing neutrons. This is a property that is extremely challenging to measure in the lab because both parts (the neutron and the rare isotope) live for a short amount of time. This is exactly my own field of research: since we can’t directly measure these nuclear reactions, we design experiments that help us produce the product of the reaction in different ways. Once we produce this nucleus, we study its properties, like the type of radiation it emits and the amount of energy it releases. These experimental studies allow us to fine-tune the astrophysical models so we can learn how and where the i-process takes place.

We are still far from a full understanding of how heavy elements are produced in the universe. The more we learn, the more we realise that our simple nucleosynthesis picture is incomplete. The introduction of the i-process brings us a step closer to the truth, but we still don’t know where it could take place and how much it contributes to the mix of elements in the solar system. And there’s still much to learn about the r-process too, like whether there are places in the universe outside of neutron star mergers where it could occur. The main challenge comes from the fact that the nuclei involved can’t be produced by our current particle accelerators. However, next-generation facilities, like the Facility for Rare Isotope Beams (FRIB) at Michigan State University, will be able to discover hundreds of new rare isotopes, never-before-produced on Earth.

As a field, we are learning to keep our eyes open for surprises and to work together to attack these complex puzzles with the required creativity and scientific diversity. And with each new development, we get closer to figuring out where we really come from.

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