After long believing that exploding stars forged the coveted metal, researchers are now divided over which extraordinary cosmic event is truly responsible.
Across history and folklore, the question of where Earth’s gold came from—and maybe how to get more of it—has invited fantastical explanation. The Inca believed gold fell from the sky as either the tears or the sweat of the sun god Inti. Aristotle held that gold was hardened water, transformed when the sun’s rays penetrated deep underground. Isaac Newton transcribed a recipe for making it with a philosopher’s stone. Rumpelstiltskin, of course, could spin it from straw.
Modern astrophysicists have their own story. The coda, at least, is relatively clear: About four billion years ago, during a period called the “late veneer,” meteorites flecked with small amounts of precious metals—gold included—hammered the nascent Earth. But the more fundamental question of where gold was forged in the cosmos is still contentious.
For decades, the prevailing account has been that supernova explosions make gold, along with dozens of other heavy elements on the bottom few rows of the periodic table. But as computer models of supernovas have improved, they suggest that most of these explosions do just about as well at making gold as history’s alchemists. Perhaps a new kind of event—one that has traditionally been difficult, if not impossible, to study—is responsible.
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In 1957, the physicists Margaret and Geoffrey Burbidge, William Fowler, and Fred Hoyle laid out a set of recipes for how the lives and deaths of stars could fill in almost every slot in the periodic table. That implied that humans, or at least the elements making up our bodies, were once stardust. So was gold—somehow.
“The problem itself is rather old, and now for a long time has been the last stardust secret,” said Anna Frebel, an astronomer at the Massachusetts Institute of Technology.
The Big Bang left behind hydrogen, helium, and lithium. Stars then fused these elements into progressively heavier elements. But the process stops at iron, which is among the most stable elements. Nuclei bigger than iron are so positively charged, and so difficult to bring together, that fusion no longer returns more energy than you have to put in.
To many astronomers, those requirements implicate one specific kind of object: a supernova.
A supernova erupts when a massive star, having fused its core into progressively heavier elements, reaches iron. Then fusion stops paying off, and the star’s atmosphere crashes down. A sun’s worth of mass collapses into a sphere only about a dozen kilometers in radius. Then, when the core reaches the density of nuclear matter, it holds firm. Energy rebounds outward, ripping apart the star in a supernova explosion visible from billions of light years away.
But as supernova models got more and more sophisticated, the situation got worse, not better. Temperatures in the neutrino-driven wind didn’t seem to be high enough. The wind might also be too slow, allowing seed nuclei to form so abundantly that they wouldn’t find enough neutrons to build up heavy elements all the way up to uranium. And the neutrinos could also convert neutrons back into protons—meaning there might not even be a lot of neutrons to work with.
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In 1974, radio astronomers found the first binary neutron-star system. With each orbit, the pair were losing energy, implying that one day they would collide. The same year, the astrophysicists James Lattimer and David Schramm modeled what would happen in such a situation—not specifically the clash of two neutron stars, since that was too complicated to calculate at the time, but the similar merger of a neutron star and a black hole.
While supernova explosions can briefly outshine the entire galaxies that host them, neutron stars are extremely difficult to see. The supernova that produced the Crab nebula was observed by many different cultures in the year 1054; the neutron star it left behind wasn’t detected until 1968. A merger of two neutron stars would be still more difficult to find and understand. But although nobody had ever seen one, this kind of exotic event could be responsible for the r-process elements, Lattimer and Schramm said.
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Neutron star mergers and supernovas are both capable of making making r-process elements. But there’s a big difference in just how much each of those options can make. Supernovas produce perhaps our moon’s worth of gold. Neutron star mergers, by contrast, make about a Jupiter-size mass of gold—thousands of times more than in a supernova—but they happen far less frequently. This allows astronomers to search for the distribution of r-process elements as a way to track their origins.
“Think of r-process [elements] as chocolate,” Ramirez-Ruiz said. A universe enriched in the r-process elements predominantly by supernovas would be like a cookie with a thin, evenly spread glaze of chocolate. By contrast, “neutron star mergers are like chocolate chip cookies,” he said. “All of the chocolate, or the process, is concentrated.”
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To advocates of the neutron-star merger model, all of this fits nicely. Neutron star mergers are naturally rare. Unlike a single massive star collapsing and going supernova, they require two neutron stars to form, to be in a binary orbit, and to merge perhaps a hundred million years later. But critics also point out that they might be too rare.
In our galaxy, neutron star mergers could happen as rarely as once every hundred million years, or as often as once every 10 thousand years—rates that differ by a factor of 10,000. “The thing that shook me is: The people who were saying neutron star mergers are going to explain the r process were also taking this highest rate,” said Christopher Fryer, an astrophysicist at Los Alamos National Laboratory.
That’s where supernovas may see their stock rise again. If perhaps 1 percent of core collapse supernovas behave differently than the standard simulations predict, they might also be able to make considerable amounts of r-process elements in a chocolate chip pattern. One way to salvage a supernova explosion is if a star detonates with massive, magnetically powered jets instead of neutrinos, argues Nobuya Nishimura, an astrophysicist at Keele University in England, and his colleagues in a recent paper. That would create a rapid explosion of neutron-rich matter, allowing seed nuclei to grow into at least some of the r-process elements. “It’s not like you can have a tea party there,” Fryer said. “You just need to stay [in that region] for 100 milliseconds.”
The answer, many astronomers believe, will end up being some kind of compromise. That shift may already be happening. “R process is really not r process anymore now,” Frebel said. Maybe it can be broken in half, with the “weak” r-process elements lighter than barium coming from supernovas, and the heavier ones like gold coming from neutron star collisions.
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And there’s one more dark horse still lurking out there: the merger of a neutron star and a black hole, which Lattimer and Schramm had originally considered. The neutron star in the pair would still eject material, just as before. But the rate of those events is even fuzzier. “Maybe even they are the dominant ones producing r-process elements,” Janka said. “We don’t know. We need better data.”
That data may already be on its way. The last few orbits of a neutron star merger or a merger between a neutron star and a black hole warp and drag space-time so much that gravitational waves roar out of the system. LIGO (the Laser Interferometer Gravitational-Wave Observatory), which has already succeeded at “hearing” such a crescendo between merging black holes, is approaching a sensitivity that should let it start picking up neutron star mergers in distant galaxies. The longer it doesn’t, the less often it seems these events occur. Once LIGO reaches its full design sensitivity, a nondetection could spell doom for neutron star merger models. “If they still have not found something, there will be a moment in which Enrico [Ramirez-Ruiz] and Brian [Metzger], etcetera, should wonder, and get back to the board,” said Selma de Mink, an astrophysicist at the University of Amsterdam.
The dream, though, is to go beyond making inferences about r-process events and see one actually in action. Two teams may have already done so. In 2013, the Swift satellite picked up a short gamma-ray burst: a type of event also attributed to colliding neutron stars. Other telescopes zoomed in on the aftermath.
In simulations, an observational signature called a kilonova follows neutron-star mergers. The radioactive nuclei made through the r process spread and glow, causing the system to ramp up in brightness for about a week before starting to fade. And these elements are so opaque that only red light can penetrate out. The 2013 event matched both predictions, but it was so far away that it was hard to fully interpret. “It’s not compelling, but it’s suggestive,” Metzger said.
Many of the astronomers who made that discovery are now part of teams hoping to find a closer, more definitive kilonova. That entails pouncing on a LIGO signal from merging neutron stars and quickly finding its source in the sky with more traditional telescopes—perhaps even measuring its light spectrum using something like the upcoming James Webb Space Telescope. In doing so, it may be possible to see a cloud of newborn r-process elements—or to infer something from their absence. “The world of gamma-ray bursts has trained us very well,” said Wen-fai Fong of the University of Arizona. “It is definitely like a race. How quickly can you react?
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