Scientists have proven that the heavy chemical elements classed as metals appear when two neutron stars collide. Find out how they did it and whet it means for all of us.
In an earlier story (Where Do Heavy Metals Come From? Not Ozzy Osbourne!), we talked about a new hypothesis from researchers at the University of Guelph. They suggested that, contrary to conventional wisdom, the heavier chemical elements come from a special kind of supernova called a collapsar.
Collapsars are rare because most stars aren’t heavy enough to turn into them when they die. This challenged the conventional wisdom that heavy elements form when neutron stars collide.
Last week, a team of researchers from the Chile-based European Southern Observatory (ESO) published a new discovery in the journal Nature. Their findings were consistent with the older, standard theory.
They found strontium where two neutron stars had collided
Astronomers using the Very Large Telescope (VLT) and its X-shooter spectrograph instrument, spotted newly formed strontium floating in space. They found the strontium near where two neutron stars had collided. Strontium quickly bonds into molecules with other elements, so it’s extraordinary to see it in nature in its pure form.
Scientists found this neutron star collision in 2017 when they detected gravity waves for only the fifth time in history. As a follow-up, the ESO aimed its telescopes, including the VLT, at the collision point.
The aftermath of these explosive star mergers is called a kilonova. Scientists suspected that, if these mergers caused heavy elements to form, they could pinpoint their chemical signatures in the kilonova.
They could tell they were seeing heavy elements
All of the ESO’s telescopes monitored the kilonova explosion at all available wavelengths. In particular, the X-shooter focused on wavelengths from the infrared through to ultraviolet. They could tell that they were seeing heavy elements. The researchers thought they might be seeing strontium right from the start, but they couldn’t isolate it.
The team decided to take another look at the data from 2017. Then, as lead author Darach Watson from the University of Copenhagen explains, “By re-analyzing the 2017 data from the merger, we have now identified the signature of one heavy metal element in this fireball, strontium, proving that the collision of neutron stars creates this element in the Universe.”
Readers have probably heard of Strontium-90 from news reports about Fukushima or Chernobyl. Nuclear weapons also release Strontium-90. It’s one of the deadliest things we find in nuclear fallout because our bodies readily absorb it, and it’s highly radioactive.
We all have a bit of stable strontium in our bones
Most strontium isn’t like that. Stable strontium, like Strontium-88, is a lot like calcium, and it’s not radioactive. We used to mine it to make picture tubes for old school TVs. It also goes into signal flares and fireworks to make them red. Potters use it in ceramics. We all have a bit of stable strontium in our bones, just like calcium.
Doctors use radioactive Strontium-89 to treat bone cancer. Some places, like Russia, use Strontium-90 to generate electricity. It’s cheaper to use in generators than plutonium, and it’s a by-product of other nuclear reactions.
The ESO research team agrees that some of the heavy metal elements come from supernova explosions, just as the University of Guelph scientists found. Average stars and the outer layers of old stars also produce heavy elements. These new findings show that the most massive heavy metal elements arise from a previously undiscovered process called rapid neutron capture.
A previously undiscovered process called rapid neutron capture
If we think back to high school chemistry, we will remember that atoms have protons and sometimes neutrons in the nucleus. Elements with higher atomic mass have more neutrons than lighter elements. For example, the most common standard isotope of Strontium has 50 neutrons in its core.
Rapid neutron capture happens when the nucleus of an atom pulls in free-floating neutrons so fast that it traps them. The new atom is now a more massive element. For this to happen, atoms have to be bombarded with an unimaginably vast number of neutrons.
Where do all these free-floating neutrons come from? That’s what cosmologists are working out. Rapid neutron capture can only happen in extreme conditions with very high temperatures.
The merger of two neutron stars provides the heat
This can happen in supernovae. Still, the temperatures aren’t hot enough to produce the neutrons needed for the most massive heavy metal elements like strontium with its 50 neutrons. However, the kilonova from the merger of two neutron stars can provide the heat required to allow for rapid neutron capture.
These new direct observations give scientists the empirical evidence they need to confirm two things. As Camilla Juul Hansen from the Max Planck Institute for Astronomy in Heidelberg explains, “This is the first time that we can directly associate newly created material formed via neutron capture with a neutron star merger, confirming that neutron stars are made of neutrons and tying the long-debated rapid neutron capture process to such mergers.”
Professor Watson added, “This is the final stage of a decades-long chase to pin down the origin of the elements.” This was a challenging project for scientists because they are only beginning to understand the processes behind neutron star mergers and their kilonovae.
“We are made of star-stuff”
Scientists have recognized the spectra of stars for decades. Stars are made of the light elements hydrogen and helium. So far, they have a sketchy grasp of what the spectral patterns look like when they go beyond iron. They rarely see these elements in space.
Carl Sagan famously said, ““The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star-stuff.” He wasn’t trying to be poetic. Well, maybe he was, but anyway it’s objectively true.
We will never fully understand our place in the new story of the universe until we know the origins of all the chemical elements. This discovery by the ESO lifts us very close to the finish line.
We always have more to learn if we dare to know.
European Southern Observatory (ESO)
Identification of strontium in the merger of two neutron stars