All That Glisters

29 October 2022

Artist’s conception of a neutron star merger. Tohoku University
Artist’s conception of a neutron star merger.

In the beginning, there was hydrogen and helium. Other than some traces of things such as lithium, that’s all the matter the big bang produced. Everything other than those two elements was largely produced by astrophysical rather than cosmological processes. The elements we see around us, those that comprise us, were mostly formed within the hearts of stars. They were created in the furnace of stellar cores, then cast into space when the star died. But there are a few elements that are created differently. The most common one is gold.

While gold can be produced in a stellar core, the gold we have on Earth wasn’t produced that way. Gold is a very heavy element, so when a star explodes most of the gold stays in the core. So where does our gold come from? Neutron star collisions. When two neutron stars collide, they are ripped apart creating a kilonova. All that nuclear matter within the neutron stars is freed from the crushing weight of gravity and quickly forms into elements such as gold. We know this because the amount of gold we see in the galaxy agrees with the rate of neutron star collisions.

Spectrum showing various elements. Domato, et al
Spectrum showing various elements.

For a while now astronomers have assumed neutron star collisions are also the primary source of other heavy elements, particularly the lanthanide series, also known as rare earth elements. But that’s just been a theory. We don’t have a good measure of the cosmic abundance of rare earth elements, so it’s a hard idea to prove. But that has changed, thanks to a recent study.1

Back in 2017, gravitational wave observatories captured an event known as GW170817. Unlike gravitational events that were the merger of two black holes, this one was a merger of two neutron stars. The resulting kilonova was observed by 70 observatories across the world, making it the first great multi-messenger observation, combining data gathered from electromagnetic and gravitational waves. Some of the electromagnetic observations included spectral line data, so in principle, we should be able to identify which elements were formed by the collision.

This is fairly easy for lighter elements but more challenging for heavier ones. In this study the team ran supercomputer simulations of kilonova explosions, calculating where absorption lines should appear based on different elements. When they compared their calculations to the observed spectra of GW170817, they were able to identify several rare earth elements, including strontium, lanthanum, and cerium. It’s the first time these elements have been confirmed as by-products of a neutron star merger.

This is just the first multi-messenger observation of colliding neutron stars. In time we will have several more, and that will give this team and others a chance to discover even more rare earth elements in the debris.

  1. Domoto, Nanae, et al. “Lanthanide Features in Near-infrared Spectra of Kilonovae.” The Astrophysical Journal 939.1 (2022): 8. ↩︎