NEWS FEATURE Tracing gold’scosmicoriginstory NEWS FEATURE Astronomers thought they had finally figured out where the gold, platinum, and other heavy elements in the universe came from. In light of recent results, they’re not so sure. Ken Croswell, Science Writer Desperate phone calls made in the dead of night amounts of europium, an element that’s even rarer in rarely convey good news, much less first word of a the cosmos than gold. Frebel told Ji to observe an- major scientific discovery. Alex Ji made such a call in other star in the same small galaxy. That star also 2015 from atop a mountain in Chile, where he was turned out to be rich in europium, as did a third. In using one of the world’s largest telescopes. “This was the end, seven of the nine stars Ji observed had ex- actually the first time that I had taken data on a tele- treme levels of the rare element. It would take some scope ever,” says Ji, then a graduate student at the time for the astronomers to interpret their surprising Massachusetts Institute of Technology in Cambridge. result, but when they did, it bore on one of the most “I was absolutely convinced that I had done pressing questions in astronomy: How does the uni- something wrong.” verse make its heaviest elements? So, just after midnight, Ji called his advisor Anna It’s a question astronomers have wrestled with for Frebel to describe what he had just seen and could more than half a century. Elements such as gold, silver, barely believe: A faint star in a dim nearby galaxy and platinum don’t normally form in the nuclear reactions named Reticulum II seemed to have extraordinary that power stars. So most researchers had long thought How does the universe make heavy elements such as gold? It’s a question astronomers have wrestled with for more than half a century—and the answers are only starting to emerge. Image credit: Science Source/Tom McHugh. Published under the PNAS license. Published January 20, 2021. PNAS 2021 Vol. 118 No. 4 e2026110118 https://doi.org/10.1073/pnas.2026110118 | 1of4 Downloaded by guest on September 30, 2021 only up to iron. Iron is a nuclear dead end because it is the most stable element: Fusing it to create heavier ones actually requires energy, which means stars don’t normally make them. Elements slightly heavier than iron, such as copper and zinc, can be forged in the turmoil of a supernova explosion. But what about even heavier elements? In the 1950s, astronomers and physicists identified two processes as the source of these elements (1, 2). One, called the s-process (“s” for slow), involves nuclear reactions near the end of a star’s life that release neutrons. Unlike protons, neutrons can sail into the positively charged nucleus of an atom without getting repelled. As a result, the iron nuclei the star inherited at birth occasionally capture these neutrons; because the neutron flux is low, the newly formed atomic nuclei have plenty of time to decay if they are radioactive. During this process, neutrons in the newly formed nuclei can turn into protons, thereby creating ele- ments with greater atomic numbers. Astronomers have long observed s-process elements such as technetium on the surfaces of the aging stars that are making them. But the s-process didn’t explain everything. In particular, most gold, silver, and platinum, as well as all thorium and uranium, are synthesized when a rapid Although supernova explosions, such as the one that created the Crab Nebula flux of neutrons bombards iron nuclei. New neutrons (shown here based on 24 exposures from the Hubble Telescope), were once accumulate in nuclei before those captured earlier can thought to forge gold and other heavy elements, astronomers now know that decay, leading to even heavier elements. This is the ’ this isn t so for most of these cataclysmic events. Image credit: NASA, ESA, and r-process, as in “rapid.” It occurs in nuclear bombs, Allison Loll/Jeff Hester (Arizona State University). which is why for decades astronomers thought that supernova explosions drove the r-process and that these elements were created when massive stars accounted for the universe’s gold and platinum. explode at the ends of their lives as fiery supernovae. Whereas an aging star can spend millions of years Ji’s 2015 discovery helped overturn that conven- churning out s-process elements, a supernova rams tional wisdom. Two years later, a dramatic observation neutrons into iron nuclei and forges r-process material seemed to confirm the new thinking. Astronomers in mere seconds. saw heavy elements actually forming when two The rapidity of the process, however, means that dense neutron stars spiraled into each other, which it’s much more difficult to study the r-process than the implied that merging neutron star binaries and not s-process. Until recently, no one had ever seen the supernovae were the main source of the heaviest r-process actually operate in space, and no one has elements. ever seen a supernova create r-process elements. Now, however, doubts have arisen, suggesting To study the r-process, astronomers have long fo- that the original solution—supernova explosions— cused on europium. Unlike gold, whose spectral lines may be an important part of the answer after all. lie in the ultraviolet region of the electromagnetic spectrum and are blocked by Earth’s atmosphere, Rare Finds europium’s spectral lines appear in the visible part of These heavy elements barely exist. If you add up every the spectrum. The high europium abundance that Ji atom in the universe from gallium (atomic number 31, found in the galaxy Reticulum II yields a key clue to the which is the number of protons in the nucleus) r-process (3): “Whatever produced this r-process ele- to uranium (atomic number 92), you’dhaveonly ment is very rare,” Ji says. In particular, it must be 1/2,300th of the total number of iron atoms (atomic much rarer than an ordinary supernova. number 26). Europium (atomic number 63) and gold This conclusion follows from the nature of Reticu- (atomic number 79) belong in this same category. lum II. It is an ultra-faint dwarf galaxy, with just a few The heaviest elements are rare because stars tens of thousands of stars, and it is so dim that as- hardly make them. To survive, every star must gener- tronomers spotted it only a few years ago—even ate energy so as not to collapse under its own weight. though it’s in our backyard, just 100,000 light-years This energy comes from nuclear fusion reactions ini- from Earth. The nine stars Ji observed told the tiated by intense heat and pressure. The reactions history of the numerous supernovae that had occurred start as hydrogen fuses to form helium, which later in the galaxy. Two stars are extremely iron-poor, be- gets transformed into heavier elements, such as car- cause they formed earliest, after only a few stars had bon and oxygen. But these reactions produce energy exploded to supply the iron. These two iron-poor stars 2of4 | PNAS Croswell https://doi.org/10.1073/pnas.2026110118 News Feature: Tracing gold’s cosmic origin story Downloaded by guest on September 30, 2021 also lack europium. Then additional supernova ex- Flash of Inspiration plosions occurred, raising the level of iron in the gal- All that changed on August 17, 2017. Astronomers axy and in stars that formed later. Somewhere along detected signals from the merger of two neutron stars the way, a rare r-process event showered the galaxy in a galaxy 130 million light-years away. The resulting with europium. The seven europium-rich stars Ji ob- gravitational radiation reached Earth (6), along with a served formed from this material, which also had burst of gamma rays (7) and—crucially—a flash of higher levels of iron. Thus, whatever drove the visible light (8): “The holy trinity all at once,” says Stan r-process was a lot rarer than a typical supernova—so Woosley at the University of California, Santa Cruz. rare it had happened only once in Reticulum II and not Woosley, a theorist, had long struggled to get su- at all in the previously studied ultra-faint dwarf galaxies. pernova models to make r-process material, so he had The rare r-process event might have been an exotic already concluded that neutron star mergers were supernova. But Ji and his colleagues favored a dif- probably the solution. By observing the visible light of ferent idea, one that another graduate student had the 2017 merger, which came from the radioactive explored four decades earlier. decay of the many r-process elements produced during the event, astronomers deduced the amount of Star Turns material made: roughly 5 percent of a solar mass, in In 1973, James Lattimer was pursuing his doctorate in line with the predictions from Schramm and his stu- astronomy at the University of Texas, Austin. Although dents. It was the first time, and still the only time, that supernovae were then thought to be the source of anyone had seen the r-process in action beyond Earth. r-process material, Lattimer’s advisor, David Schramm, Unfortunately, Schramm never lived to see the historic suspected that these explosions might not actually discovery; he died piloting a small plane in Colorado produce the heaviest elements. So Schramm sug- in 1997. gested that Lattimer look at something else: a neutron The 2017 neutron star merger that confirmed ’ star, the tiny but super-dense core of a dead massive Schramm s idea convinced many astronomers that star.