The first unequivocal evidence of where the heaviest elements were forged has now been found by a research group led by the University of Copenhagen. For the first time, an element heavier than iron has been clearly detected in the collision of two neutron stars, resolving one of the fundamental questions about the history of the universe.

Since the 1950s, we have known that hydrogen and helium were formed during the Big Bang, and that heavier elements up to iron are created by nuclear fusion in stars and when stars explode as supernovae. But iron is only no. 26 out of about 90 naturally occurring elements in the periodic table. Where the other elements heavier than iron came from has long been a mystery. For some time now we have known that some of them form in the envelopes of low-mass stars, so-called AGB stars. But only half of the elements heavier than iron are created this way. So where do the rest come from?

Now a research team led by astrophysicist Darach Watson of the Niels Bohr Institute has, for the first time, found spectroscopic evidence that heavy elements are created in the explosion that happens when two neutron stars collide. The researchers have identified the metal strontium in a spectrum from a neutron star collision observed in 2017. The result is published in the scientific journal Nature.

"Before this we were unable to identify any specific element created in a neutron star merger. There were strong indications and good circumstantial evidence that heavy elements were created in these events, but the unequivocal evidence was missing until now," says astrophysicist Darach Watson of the Niels Bohr Institute at the University of Copenhagen, adding:

"One of the most fundamental questions about the universe has been: where do the elements of the periodic table come from? You could say that this is the last piece of the puzzle of the formation of the elements."

Unique stellar crash in 2017 helped the researchers

The only way to create substances heavier than iron is by a process called neutron capture, where neutrons penetrate an atomic nucleus - for example, an iron atom - which absorbs the neutrons, creating a new, heavier atomic nucleus and thus a new element. Neutron capture can be either fast or slow, in the so-called r-process (rapid) or s-process (slow). About half of the substances created by neutron capture are primarily formed by the r-process. Elements formed almost exclusively by the r-process are typically very heavy and near the end of the periodic table: gold, platinum, uranium.

It is this rapid process whose location has never been established. In recent years, the scientific consensus has evolved toward the idea that much of the r-process happens when two neutron stars collide - but the definitive evidence has thus far been missing. The neutron star collision triggers a phenomenon called a kilonova, where a fraction of the neutron stars' combined mass is released and spread into the universe in a giant explosion.

The only time the phenomenon was well-observed was in August 2017, when two neutron stars collided in a galaxy approx. 140 million light years from Earth; a collision first discovered through its gravitational wave signature and then followed-up by observatories such as the European Southern Observatory (ESO) in the Atacama desert in Chile. The spectra gathered back then at ESO are what Darach Watson and his colleagues have been analyzing ever since. However, no one at the time was able to identify any specific elements. Using a so-called black body spectrum, Darach Watson and colleagues succeeded in reproducing the early spectra of that kilonova, in which the element strontium is prominent. Curiously, strontium is one of the lighter of the heavy elements, and this in itself is important:

"It was thought that perhaps only the heaviest elements, such as uranium and gold, formed in neutron star mergers. Now we know that the lighter of the heavy elements are also created in these mergers. And so it tells us that neutron star collisions produce a broad range of the heavy elements, from the lightest to the very heaviest," says astrophysicist and co-author Jonatan Selsing, who until recently was a postdoc at the Niels Bohr Institute.

The researchers' next step is to try to identify more elements in the spectra of the kilonova. If successful, they expect to find elements heavier than strontium - possibly barium and lanthanum.

FACTS:

• A neutron star is an extremely compact star consisting mainly of neutrons. It is typically only about 20 km. in diameter, but can weigh one and a half to two times more than the Sun.

• The identification of strontium also proves for the first time that neutron stars actually consist of neutrons, since strontium can only be formed so quickly by having an enormous number of free neutrons available. While it has been widely believed since the early 1970s that these incredibly dense stars were likely neutron-rich, no direct spectroscopic evidence of large amounts of neutron-rich matter had been found before now.

• When two neutron stars collide, a phenomenon known as a kilonova occurs, believed to be the outward explosion of radioactive material thrown off in the collision. In this explosion, large amounts of heavy elements are thrown out as a hot, luminous plasma. The phenomenon was first unequivocally observed in 2017.

• When two neutron stars merge, a huge mass of heavy elements is produced. In this work, a minimum of 10 Earth masses of strontium alone is required to explain the observed features and the amount of strontium actually produced is likely far larger.

• An atomic spectrum is like a fingerprint. Spectroscopy is the technique that enables the composition of elements in a star to be determined.

• Strontium is a metal, the 38th element in the periodic table. It appears in solid form at room temperature. Its main practical application is in the production of cathode ray tubes for television screens. It has chemical properties similar to calcium and is therefore often incorporated into human bones, teeth, and hair, and is often used for this reason in archaeology to trace the origin and lives of prehistoric people.

• Darach Watson realized that the kilonova spectrum could be roughly approximated using a so-called "blackbody spectrum", the simplest kind of thermal spectrum. Removing this blackbody spectrum left a series of features that, to the researchers' own surprise, could then be explained by the spectrum of a gas of heavy elements at the temperature of the blackbody.

• The research article is published in the scientific journal Nature