In 2017, scientists around the world were excited by the news that the Laser Interferometer Gravitational-Wave Observatory (LIGO) project had detected gravitational waves from the collision of two neutron stars. This discovery confirmed a pivotal prediction of general relativity, and eventually earned multiple Nobel Prizes. A new study now says this cataclysmic event may also have solved the mystery of why there are so many heavy elements in the universe. An analysis of the light from this event found strong evidence of strontium, which is too heavy for stars to produce through fusion.
If you remember your high school chemistry, you’ll recall there are more than 100 known elements, most of which occur naturally in the universe. Some have well-understood origins. Stars fuse hydrogen into helium, and the helium fuses into heavy elements later in a star’s life. However, stars can only produce elements up through iron (atomic number 26). Anything from the 27-proton cobalt and onward requires so-called “rapid neutron capture,” and stars don’t get dense or hot enough for that.
We know that supernovae can initiate some of these nuclear processes to create super-heavy elements, but that most likely can’t explain all the heavy elements out there. Several years ago, scientists proposed that the collision of ultra-dense objects like neutron stars could be responsible for some (or even most) of the heavy elements scattered across the stars. Following the gravitational wave detection, scientists turned their instruments to the site of the collision in search of, among other things, evidence of heavy elements like strontium.
Strontium is an alkaline earth metal, shown here purified.
During the initial observations, scientists reported some evidence of gold and uranium in the blast. However, the readings were unclear. After re-analyzing the light from the collision, the team from the University of Copenhagen identified a “strong feature” indicating the presence of strontium. That suggests that a kilonova produces the necessary conditions to fuel rapid neutron capture.
This doesn’t mean we’ve solved the mystery of where all the heavy elements came from, but it serves as a jumping-off point for future study. By understanding the forces at work in a kilonova, we may be able to determine which elements they produce and in what quantities. Carl Sagan famously said we’re made of “star stuff,” but we might also have some “kilonova stuff” in there.
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