Gold's value comes from its rarity, but it is actually puzzling why the Earth has as much as we do. Astronomers thought they had found the answer for where most of the universe's heavy metal comes from after the first observation of two neutron stars colliding. However, a new study throws doubt on that idea and puts the focus back on supernovas.
Until recently, the standard explanation for gold and nearby elements on the periodic table was their formation in core-collapse supernovas. However, theoretical models have cast doubt on this. When gravitational wave detectors picked up the ripple from a collision between two neutron stars for the first time, astronomers hurried to study the afterglow and saw the spectral lines of many heavy elements, gold included. Explanations were quickly updated, including in the form of handy graphics showing elements' origins.
However, Monash University's Dr Amanda Karakas doubts these conclusions. Karakas has been studying the composition of very old stars, which should offer an indication of the proportion of heavy metals at their formation.
Neutron star collisions take a long time to occur. In The Astrophysical Journal, Karakas concludes there simply wouldn't have been enough of such events in the early universe to seed the stars she has measured.
"Neutron star mergers did not produce enough heavy elements in the early life of the Universe, and they still don't now, 14 billion years later," Karakas said in a statement. "The Universe didn't make them fast enough to account for their presence in very ancient stars, and, overall, there are simply not enough collisions going on to account for the abundance of these elements around today."
Estimates of the frequency of neutron star collisions in the early universe and the amount of heavy metals they produced vary, but even the highest estimates for both don't match Karakas' observations.
Karakas told IFLScience that the source “has to be something really rapid" to have made the elements before the stars she studied formed, since they originated so early. Really massive stars' lifespans are so short they could have formed and exploded in time.
Although theoretical models of the processes Karakas calls “garden-variety supernovas” suggest they form only small quantities of elements heavier than iron, the same models suggest exotic supernovas have different matter. The combination of rapid spin and strong magnetic fields in some supernovas causes them to produce more heavy metals, gold included.
So far, Karakas told IFLScience, these models have been hard to confirm through observations. Supernovas produce so much oxygen and other light elements that the heavy element signals gets drowned out. However, if her calculations for neutron stars are right, fast-spinning, highly magnetized supernovae are the only remaining explanation.
Gold's deep cultural significance means it hogs the limelight, but Karakas and colleagues have compared the measured abundance of every element between carbon and uranium with what known sources should have produced. Of these, antimony is the only element for which she thinks neutron star mergers are the primary source, although smaller proportions of each heavy element do indeed originate in these events.
Some elements, gold included, are more abundant than can be easily explained. However, the Solar System appears to have less silver than supernova models would suggest it should. Karakas told IFLScience this is “probably because the nuclear physics or the theoretical models need improving.” Nevertheless, she laughed, “maybe there is some missing silver hiding somewhere in the Solar System,” making a space miner's dream.