Gravitational waves, nuclear fire, rocks and love

The old saying “we are stardust” is so penetrated into our mind that we risk losing part of his poetry. Yes, elements heavier than hydrogen and helium present in the terrestrial environment have been forged by various ancient life cycles of generations of stars. Many of these cosmic furnaces have expelled their contents into the void, polluting our galaxy with traces of the atomic nuclei we call oxygen, carbon, iron and more. And in the course of the aeons gravity has caused the recondensation of this interstellar matter. As a result, the elements have been separated, allowing the star matter to become extraordinarily concentrated, creating new stars, planets, and clusters of heavy nuclei that make up human beings and their absurd complexity.

Gravitational waves, nuclear fire, rocks and love
Artistic representation of the collision between two neutron stars. (Courtesy Dana Berry, SkyWorks Digital, Inc.)

This is fantastic, but repeat the story a great many times and it will start to sound a little trivial. Part of the reason is that the narrative can become vague – from speaking in general terms of previous generations of stars now invisible to our extensive descriptions of the nature of interstellar matter. It’s a bit like when an elderly relative tells you about the family tree of your family up to the fifth generation. There may be little to identify with, even if we would like to do so. 

The story becomes much more interesting when you look closer. First, not all elements are produced in the same way. Perhaps the most interesting example is that of the elements of the so-called ” process r “.

These elements have heavier nuclei than iron and are built by a mechanism called rapid neutron capture. Such as

 the name suggests, something is needed to capture neutrons in the form of “seed” nuclei, and a tremendous neutron flux is needed, which is fast enough to go and form nuclei beyond any highly unstable intermediate configuration. 

But where do you find such environments? 

In 2017, observers of gravitational waves LIGO and Virgo caused a sensation by detecting the signature of a fusion of two neutron stars. Two stellar mass spheres of nuclear material spiraled towards one another with a cry of space-time oscillations of increasing intensity.

Unlike the fusion of a binary black hole, that event produced a prodigious amount of electromagnetic radiation in the so-called kilonova (literally, a thousand times the emission of a normal nova star). The kilonova’s telescopic study has provided convincing support for the idea that the neutron star fusone represents a paradise for the r process. This suggests that these cataclysmic events play an important role in supplying our galactic landscape with some of the heavier elements. From gold, platinum and iridium to thorium and uranium, up to short-lived elements such as plutonium. 

Now, new research by Bartos and Marka, published in recent days on “Nature”, offers a creative and rather surprising view of the origins of the elements of the r-process in our solar system. The researchers combined two key analyzes. One of the data on meteorites that preserve evidence of the mix of elements in our solar system in formation, about 4.6 billion years ago. The other is an ingenious statistical model of the history of neutron star fusions in the galaxy. 

Research indicates that at the dawn of our local cosmic history a very close neutron star collision occurred. Traces of this unique event seem to be present in the details of the radioisotopes due to the r process that sprayed our system in formation after the collision of neutron stars.

Gravitational waves, nuclear fire, rocks and love
The nucleosynthesis of some elements can occur within the stars, but even more energetic processes are needed for the elements heavier than iron. (© Science Photo Library / AGF)

Reaching this conclusion requires some mental flexibility and hard work. Neutron star fusions are cosmically rare in the Milky Way, ranging from one to one hundred events per million years in all its extension. Some elements of the r process, such as the actinides (including curio-247, plutonium-244 and iodine-129), have relatively short half-lives, in the order of tens of millions of years, but have left specific traces in the meteorite material of the ancient solar system, which allow us to measure their original abundances.

Therefore, the quantity of these elements that existed during the time window in which our solar system was forming offers a tool to evaluate not only the era in which those elements were forged, but also the distance at which the element must have been forge. 

By building a simulation of neutron star fusions in our galaxy, in the course of its history up to the formation of our solar system (in the approximately 9 billion years of existence of the Milky Way), Bartos and Marka have been able to examine which scenarios could have produced the mixture of actinides obtained from meteorite analysis.

From the result of the analysis it seems that there was only one kilonova produced by a fusion of neutron stars that would be verified within 80 million years (more or less 40) by the formation of the solar system and about a thousand light years away. The researchers estimate that a kilonova event so close would have hidden all the night sky for over a day. Four and a half billion years ago, when the newly generated elements of the fusion were projected outside and spread in interstellar space, about 10 ^ 20 kilograms of them ended up depositing in our young system.

From there you can understand how much of the terrestrial deposit of elements of the process came from that one event. For example, the equivalent of an eyelash of iodine in your body will have come from those neutron stars. A Tesla Model 3 car contains a total of about 5 grams of the nuclei generated by this specific neutron star fusion. A modern fission reactor, which uses enriched uranium, will contain about 200 kilograms of material that was produced in that one cosmic explosion.

Significantly, the study also seems to exclude that among the primary producers of process elements r throughout the galaxy there have been events such as nuclear collapse supernovae, linked to the implosion of massive stars. Those events, which occur hundreds or even thousands of times more frequently than neutron star mergers, do not seem to match the data. 

Overall, it seems that we can update the story of our origins from “stardust”. Not only are we indebted to an even more exotic and extreme physics than perhaps we imagined, but now we have to place two very specific members of our ancestral tribe on the family tree: a pair of lovers neutron stars, whose embrace was literally on fire.