The Alchemy of Neutron Stars (Patreon)

Carl Sagan’s famous words: “We are star stuff” refers to a mind-blowing idea – that most atomic nuclei in our bodies were created in the nuclear furnace and the explosive deaths of stars that lived in the ancient universe.  In recent years it’s become clear that the truth is even more mind-blowing. Many heavy elements - includes most precious metals - were produced in an even more spectacular event: the collision of neutron stars. In fact, according to a recent study most of the Earth’s supply of these elements was created in a single neutron star merger that took place near our Sun’s birth nebula 80 million years ago before Earth formed. 

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When I was in astrophysicist school they taught us that all of the elements of the periodic table between carbon and iron were produced in onion shells by nuclear fusion in the cores of very massive stars during the last phases of their lives. And that the elements heavier than iron were synthesized in the following supernova explosion. That latter process is well understood – the star’s dead core collapses and protons are converted to neutrons. The surrounding shells ricochet outwards, along with a layer of the iron and nickel core. The latter is blasted by a wave of neutrons, which get rammed into the escaping nuclei. Some of those captured neutrons convert back to protons and so elements all the way up the periodic table can be made.  This is the rapid neutron capture or r-process. The rapid part is because neutrons are captured faster than nuclei can decay, making it possible to build very heavy nuclei.

It’s a cool story. It would be cooler if it were true. So the r-process is real, and must happen to some extent in supernovae. But there are some details – for one thing the r-process only produces the heaviest isotopes. Remember that element type – location on the periodic table - is defined by the number of protons in the nucleus. Neutron number is variable and defines the isotope of the element.  The r-process produces only the neutron-rich isotopes of most heavy elements. Neutron-poor isotopes are likely made by the slow-neutron-capture or s-process inside lower mass stars.

And actually, it turns out that even the heavy r-process isotopes are probably not made in supernovae. Given the rate at which supernovae go off in the Milky Way, the interstellar medium should have way more r-process elements than it appears to. In addition, models of supernova explosions have trouble producing the right conditions for substantial release of r-process elements. The only modern nearby supernova – 1987A – appeared to have no enhancement in r-process elements.

R-process elements exist, but their source doesn’t seem to be supernova explosions. One of the proposed alternatives shot to prominence last year when the LIGO and VIRGO gravitational wave observatories spotted the spacetime ripples from the merger of a pair of neutron stars. Many of the world’s great telescopes monitored the subsequent electromagnetic flash. As the ejected material from this collision expanded and faded, the spectral signatures of many r-process elements were seen in abundance. Neutron star merger is now the leading candidate for the production of most of these elements – including those here on earth.

The process is suitably awesome. Let’s take a look. Like I said, neutron stars are the dead cores of massive stars. They are composed almost entirely of neutrons at densities similar to the atomic nucleus. They also have a thin crust of iron. Densities so high, in fact, that they are on the verge of complete gravitational collapse into black holes. So take a pair of neutron stars in binary orbit – perhaps twin remnants of a once-binary pair of massive stars. They slowly spiral towards each other as gravitational radiation saps their orbital energy. In the last minute before merger that radiation is so strong that it will be detected by an as-yet-unborn civilization 100 million light years away. As will explosion of electromagnetic radiation that immediately follows as the neutron stars plow into each other.

At the instant of this collision the outer layers of the stars splash into a maelstrom of neutrons and iron in orbit around the merging neutron star interiors. The combined core has almost certainly pushed beyond the limits of gravity and collapses into a black hole within milliseconds. In the meantime, the surrounding vortex undergoes some crazy transformations. Prior to collision, the stars’ neutrons were stabilized by extreme pressure. But once released this nuclear goop expands and destabilizes; it breaks up into droplets of neutrons. Many neutrons rapidly undergo beta-decay, transform into a proton after ejecting an electron and a neutrino.

The droplets are now essentially nuclei, albeit hopelessly unstable ones. They break apart and beta-decay into semi-stable elements. Meanwhile, the inner part of the vortex is still bathed in a sea of neutrons. The r-process begins – newly formed nuclei and the older iron nuclei absorb neutrons, and so heavier and heavier elements are created as beta-decay transforms some of the absorbed neutrons into protons.

Some of these heavy elements are sprayed into surrounding space by the energy of the collision itself, but most remain trapped in the intense gravitational field of the newly formed black hole, presumably doomed to fall below the event horizon. But the same beta-decay process that converted neutrons back to protons also provides the mechanism for their escape. That beta-decay releases both electrons and neutrinos. In fact a wind of neutrinos so intense that in drives material outwards. We tend to think of neutrinos as ghostly particles that barely interact with matter; but here both the neutrino and matter densities are so high that our new nucleons can ride this neutrino wind to freedom.

Our best calculations suggest that neutron star collisions should be much better than supernovae at producing heavy elements and getting them out into the galaxy. Now that we’ve spotted these elements around the site of a neutron star collision, this story is looking better and better.

A recent study by Szabolcs Marka and Imre Bartos has clarified this rather awesome story. It turns out that most of the heavy elements on Earth may have been formed in a single neutron star collision around 80 million years before the formation of the solar system. It would be fair to ask how on Earth anyone could know that. Let me tell you.

Certain nuclei produced in the r-process are unstable. These isotopes undergo radioactive decay into lighter elements after being created in a neutron star collision. Now the average decay time or the half-life differs between different radioactive elements – some decay faster than others. That means the relative abundance between any two isotopes should change over time. So if you can measure the ratio in their abundances then you know how long ago they were formed.

There’s actually one more complication – if you want to find the event date to within a reasonable degree of accuracy then you need to look at isotopes with short half-lives. You want accuracy within millions of years then the half-life should be measured in millions of years. But Earth is billions of years old. If Earth’s r-process elements were produced by a neutron star merger that happened before Earth formed, then any short-lived isotopes from that merger should have completely decayed by now.

This is where the cleverness comes in. It turns out that the abundances of certain short-lived isotopes became locked into the very first minerals to form in our solar system. We’ve found ancient meteorites that coalesced when the Earth was still forming. When they formed, billions of years ago, they contained radioactive r-process isotopes in ratios that themselves were formed in a nearby neutron star merger millions of years prior. Those rocks eventually found their way to Earth and into the hands of scientists. The short-lived isotopes completely decayed long ago, but they decayed into other stable elements. The abundances of these daughter products exactly reflect the abundances of the parents at the moment the meteorite material was formed.

This study focused on Curium 247 with half-life of 15.6 million years compared in ratio to plutonium 244 with it’s longer half-life of 80.8 million years. Based on the relative abundance of their daughter products in meteorites we know the relative abundance of these isotopes in the nebula that our solar system formed from.

The researchers then did simulations to figure out how long ago and how far away the neutron star merger that formed these elements must have been. An important factor here is that neutron star mergers are rare. All short-lived r-process isotopes are likely to have formed from the same merger. Because of this, they were able to identify a single neutron star merger that must have happened between 40 and 120 million years before the formation of the solar system, and between 650 and 1300 light years away.

That one event produced most of the short-lived r-process elements that were present the early solar system. More stable elements were built up over multiple neutron star mergers, which the researchers conclude must happen every 20 million years or so galaxy-wide.

So what does all of this mean for the elements that make up the Earth – that make up YOU? Neutron star mergers are likely the dominant source of most elements with atomic masses 44 and up. That includes most of the lead, silver, gold, rare earth elements, and the radioactive stuff like uranium and plutonium. Also a good fraction of molybdenum and iodine which are essential for your biology. In fact, including the non-essential heavy elements, your body mass is something like 2 parts per million colliding neutron star material. That’s only a 10th of a gram or so, but it’s a pretty awesome 10th of a gram. It was, after all, synthesized on the rim of a black hole before surfing a wave of neutrinos into the nebula that would eventually collapse into our solar system. And those atoms would eventually find themselves part of a life-form that would figure out the very time and distance of their formation – a collision of ancient stellar corpses in an earlier and distant spacetime.