s only number 26 out of about 90 naturally occurring elements in the periodic table. Where the elements heavier than iron came from has long been a mystery.
“Before this we were unable to identify any specific element created in a neutron star merger. There were theoretical motivation and good circumstantial evidence that heavy elements were created in these events, but the unequivocal proof was missing until now,” explains astrophysicist Darach Watson at the Niels Bohr Institute at the University of Copenhagen, and leader of the international team behind the new discovery.
A long standing question has now been answered
One of the most fundamental questions about the universe has always been: where do the elements of the periodic table come from? For some time researchers have known that some of them form in the envelopes of low-mass stars called AGB stars. But only half of the elements heavier than iron are created this way.
With the new results based on the unique neutron star collision in 2017, the scientists have disclosed the origin of at least one of these. The discovery is based on analysis of the light spectra from the event, that is the decomposition of the detected radiation in its single elements, splitting each wavelength separately.
When a neutron star collision occurs it triggers a phenomenon called a kilonova. Here a fraction of the neutron stars’ combined mass is released and spread into the surrounding space in a giant explosion. Darach Watson and colleagues succeeded in explaining the early light spectra of that kilonova in which the element strontium is prominent.
The researchers next step is to try to identify more elements in the spectra of the kilonova.
“The data have yet more to reveal. A more sophisticated analysis may indicate the presence of other elements, including some heavier than strontium, such as barium and lanthanum,” said Giorgos Leloudas, another astrophysicist at DTU Space and co-author of the new study.
Detecting presence of strontium by analyzing spectres of a kilonova resulting from a neutron star collision
• A neutron star is an extremely compact star consisting mainly of neutrons. It is typically only about 20 km in diameter, but can weigh up to two times more than the Sun.
• When two neutron stars collide, a phenomenon known as a kilonova occurs. Darach Watson and his colleagues realized that the kilonova spectrum could be overall approximated using a so-called ‘blackbody spectrum’, which is the simplest radiation emitted by any hot material. However there were deviations from a pure blackbody which the researchers could then identify as being due to the presence of the heavy element strontium.
• When neutron stars merge, a huge mass of heavy elements is produced. A minimum of 10 Earth masses of strontium is required to explain the observed features, and the amount of strontium produced was likely larger.
• The element strontium is a metal, number 38 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.
The figure shows four spectra of a kilonova indicationg the presence of strontium. Here the light is decomposed into its spectral components (split wavelength by wavelength).
From left to right is shown (in angstroms) how much light there is in the blue, in the visible, and in the near-infrared spectre. At all epochs, the spectra show a departure from the simple, thermal kilonova spectrum, due to the presence of the element strontium, which both absorbs and re-emits some of the light.
These deviations are indicated by the red shaded areas. These areas highlight the difference between the observed data (the grey line) and the kilonova model without strontium (the dotted blue line). The kilonova model including the effect of strontium is indicated by the red line.
(Figure: Niels Bohr Institute/DTU Space)
In addition to scientists from the University of Copenhagen (Niels Bohr Institute) and DTU Space, researchers from institutions in Iceland, Italy, England, The Netherland