Scientists Hunt for the Source of the Universe’s ‘Missing Gold’

Heavy elements like gold are created by the deaths, explosions, and collisions of stars. A new study breaks down these diverse origins.
September 18, 2020, 1:00pm
​Illustration of a neutron star collision. Image: NASA Goddard Space Flight Center/CI Lab
Illustration of a neutron star collision. Image: NASA Goddard Space Flight Center/CI Lab

We are made of “star stuff,” in the words of astronomer Carl Sagan, which is a poetic reminder that life on Earth is interlinked with cosmic processes that date back billions of years.

But exactly which stars make what stuff is still a matter of some debate. While we know that heavy elements such as gold, silver, and iron are forged by the deaths of stars, or in cataclysmic stellar collisions, it’s not clear what proportion of elements originate from each source.

That’s why a team of astrophysicists developed an updated Periodic Table that includes the stellar origins of elements ranging from carbon through uranium. The results reveal a mysterious gap in our knowledge about the genesis of gold in the cosmos, among many other insights published on Tuesday in The Astrophysical Journal.

“We were interested in testing theoretical predictions for various ‘sites’ of element production,” said co-author Amanda Karakas, an astrophysicist at Monash University in Australia, in an email. “By ‘sites’ I mean types of stars, as different types of stars can make elements depending on their mass.”

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The Periodic Table, showing naturally occurring elements up to uranium. Shading indicates stellar origin. Image: Content: Chiaki Kobayashi et al Artwork: Sahm Keily

Karakas, along with authors Chiaki Kobayashi of the University of Hertfordshire in the U.K. and Maria Lugaro of Konkoly Observatory in Hungary, also emphasized that these elemental lineages are key to understanding the intricate evolution of the galaxies as a whole.

“Stars in the present-day galaxy are fossils that retain the information on the properties of stars from the past,” the team said in the study. “This approach is called Galactic archaeology and can be applied not only to our Milky Way Galaxy but also to other galaxies.”

Scientists are able to estimate the abundance and origin of elements through a variety of techniques. Details about the chemical composition of a star can be directly observed in its light spectrum, and stellar evolution models predict which elements should be created by stars depending on their mass, age, and location.

The Big Bang, the event that kicked off the universe, is the main source of the lightest elements: hydrogen, helium, and lithium. Stars, meanwhile, forge complex heavy elements, including those that are essential for life, such as carbon, oxygen, and nitrogen, or flashy metals like gold, silver, and iron.

The origins of nearly all elements can be traced to several sources, but the new study constrains the contribution of each type of stellar death, explosion, or crash. Modeling such a complex problem requires estimates of the abundance of various star types across time and space, as well as the frequency of the powerful cosmic processes that produce them.

“We wanted to know the origin of elements exactly, ie. how much fraction of each element is produced from what kind of stars,” said Kobayashi in an email.

Previous studies have suggested that collisions between neutron stars, the dead husks of some massive stars, could be a major source of heavy elements, including gold. But the new study challenges the assumption that neutron star mergers play a dominant role in gilding the universe.

“We were interested in heavy elements because neutron star mergers were recently hypothesised to produce half of the elements heavier than iron by the ‘rapid’ neutron capture process,” Karakas said.

“We wanted to know if that was the case, using state-of-the art neutron star merger theoretical calculations,” she continued. “We found that neutron star mergers do not occur quickly enough to account for the observed heavy element composition of very ancient stars nor can it produce enough to account for the amounts of heavy elements observed in our Solar System.”

The researchers suggest that a special type of rapidly spinning supernova, with strong magnetic fields, might explain this “mystery of the missing gold,” as it was described in a release.

“We found that a rare class of rapidly rotating, highly magnetic supernovae makes most of the heavy elements via the rapid process,” Karakas said. “However, we still don't know how many massive stars end their lives this way, nor many of the details of the explosions.”

“We are already seeing considerable progress with 3D models and I suspect in the next five years we will have a better handle on these questions,” she added.

To shed more light on elemental origins, scientists will need to crack the weird nuclear physics that govern the rapid neutron capture process that creates gold and other heavy elements. Next-generation nuclear facilities may be able to constrain the dynamics of this particular process.

In addition to nuclear experiments in laboratories on Earth, Kobayashi noted that “the observation of ancient ‘metal-poor’ stars that were born just after the Big Bang, much earlier than the Solar System, is very useful.”

“In stellar spectra, it is very difficult to measure very heavy elements in general, and is extremely hard to measure gold,” she said. “Gold has been observed only for a handful metal-poor stars and in the Sun so far, but more measurements will be obtained in the near future (with the Hubble Space Telescope).”

It’s also possible that the team underestimated the number of neutron star collisions, given that astronomers have only just begun to directly observe these wild encounters within the past few years using new gravitational wave detectors called LIGO and Virgo.

“Our predictions regarding the amount of heavy elements associated with neutron star mergers depends on how quickly two neutron stars can merger, and also the rates of these events in the Galaxy,” Karakas said. “Hopefully, neutron star merger rates will be much better constrained in the next 10 years by results from the LIGO/Virgo collaborations, which look for the gravitational wave signatures of such events.”

The breakdown of elements that living stars will create, and eventually sow into the universe, are helpfully visualized in the above Periodic Table. Stars that are over eight times as massive as the Sun are destined to explode into pyrotechnic supernovae. This process is the main source of certain elements, such as neon, silver, and iridium.

Smaller stars, like the Sun, don’t undergo a supernova event. As they are dying, these stars shed their outer gassy layers into their surroundings, a dynamic that enriches the next generation of stars with new elements. This process is a major source of nitrogen, lead, and strontium.

The burned-out corpses of stars like the Sun, known as white dwarfs, sometimes explode in a special type of supernova that supplies much of the universe’s manganese, iron, and nickel.

The phrase “star stuff” encapsulates all of these intricacies in a lovely and succinct way. But behind those simple words lies a mountain of fascinating and ever-evolving research into the mind-boggling variety of elemental origin stories out there in the stellar wilds.

Update: This article has been updated with comments from Dr. Karakas and Dr. Kobayashi.