The old trope that ‘we are starstuff’ has embedded itself in our minds to such an extent that it tends to lose some of its poetry. Yes, elements heavier than hydrogen and helium in our earthly environment were forged as part of the varied life-cycles of long-gone generations of stars. Many of these cosmic furnaces expunged their guts into the void, polluting our galaxy with traces of the atomic nuclei we call oxygen, carbon, iron and more. And over the eons gravity has caused the re-condensation of this interstellar matter. As a result the elements have been segregated, allowing the starstuff to become extraordinarily concentrated – making new stars, planets, and the clusters of heavy nuclei that constitute human beings and their ridiculous complexity.
It is truly fantastic, but repeat this tale enough times and it begins to sound a little ordinary. Part of the reason is that the narrative can become vague – from talking in broad terms about earlier generations of now unseen stars, to our loose descriptions of the nature of interstellar matter. It’s a bit like when an aged relative tells you about your extended family tree. There can be little to identify with, even though you really want to make the connection.
The story gets a lot more interesting when you look closer though. For one thing, not all elements are produced in the same way. Perhaps the most intriguing example is that of the so-called ‘r-process’ elements. These have nuclei heavier than iron and are built by a mechanism called rapid neutron-capture. As the name implies, you need something to capture the neutrons, in the form of ‘seed’ nuclei, and you need a fearsome flux of neutrons – enough coming in fast enough to build up nuclei beyond any highly unstable intermediate configurations.
But where do such environments exist?
In 2017 the gravitational wave observatories LIGO and Virgo made headlines by detecting the tell-tale signature of a binary neutron-star merger. Two stellar-mass balls of nuclear material spiralling together with an escalating shriek of spacetime oscillations.
Unlike binary black hole mergers this event churned out a prodigious amount of electromagnetic radiation in what’s termed a kilonova (literally a thousand times the output of an ordinary stellar nova). Telescopic study of the kilonova produced compelling support for a picture where merging neutron stars are an r-process heaven. This suggests that these cataclysmic events play a major role in supplying some of the heaviest elements to our galactic landscape. From gold, platinum, and iridium, to thorium, uranium, and short-lived elements like plutonium.
Now, a new piece of research by Bartos and Marka, published this week in Nature, provides an ingenious and somewhat startling insight to the origins of r-process elements in our own solar system. To achieve this they combine two key analyses. One is data from meteorites that preserve evidence of the elemental mix in our forming solar system some 4.6 billion years ago. The other is a clever statistical model of the Galaxy’s history of neutron star mergers.
What the research points to is a very nearby neutron star collision that took place at the dawn of our local cosmic history. Traces of this one event seem to be present in the details of radioisotopes coming from the r-process that got sprayed into our forming system after the neutron stars collided.
Reaching this conclusion requires some nimble thinking and tricky foot-work. Neutron star-on-neutron star mergers are cosmically rare in the Milky Way, with between one and a hundred occurring per million years across its entire expanse. Certain r-process elements, like the actinides (including Curium-247, Plutonium-244, and Iodine-129), not only have relatively short half-lives, measured in the tens of millions of years, but have left distinct signatures in ancient solar system meteoritic material that allow us to measure their original abundances. So, the amount of these elements that existed during the window of time that our solar system was forming offers a lever arm on not only how recently those elements had been forged, but also how nearby the forge must have been.
By constructing a simulation of neutron star mergers across our galaxy, and throughout its history leading up to our solar system’s formation (about 9 billion years into the Milky Way’s existence), Bartos and Marka are able to examine what scenarios could have produced the actinide mix inferred from meteoritic analyses.
The upshot is that it appears that there was a single kilonova from a neutron star merger that occurred within 80 plus-or-minus 40 million years of the formation of the solar system and was about 1,000 light years away. The researchers estimate that such a nearby kilonova event would outshine everything in the night sky for over a day. Four and a half billion years ago, as the merger's freshly made elements exploded outwards and diffused across interstellar space, a total of about 1020 kilograms wound up being deposited into our young system.
From there you can work out how much of the Earth’s repository of r-process elements came from that one event. For example, about one eyelash’s worth of the iodine in your body will have come from those neutron stars. A Tesla Model 3 contains a total of about 5 grams of the nuclei generated by this specific neutron star merger. A modern fission reactor, utilizing enriched uranium, will have about 200 kilograms of material that was produced in this singular cosmic explosion.
Critically, this study also seems to rule out events such as core-collapse supernova – where massive stars implode – as the primary producers of r-process elements across the Galaxy. Those events, which occur hundreds or even thousands of times more frequently than neutron star mergers, just don’t seem to fit the evidence.
Taken altogether it looks like we can update the tale of our origins in ‘starstuff’. Not only are we indebted to even more esoteric and extreme physics than we perhaps imagined, we now have two very specific members of our ancestral tribe to put on the family tree, a pair of neutron- star-crossed lovers whose embrace literally ended in fire.
[Full disclosure: I am acknowledged in the paper by Bartos and Marka, and they are both colleagues. But my contribution to their work was entirely in the form of making encouraging noises.]