Astronomers See Strontium in the Kilonova Wreckage, Proof that Neutron Star Collisions Manufacture Heavy Elements in the Universe

Astronomers have spotted Strontium in the aftermath of a collision between two neutron stars. This is the first time a heavy element has ever been identified in a kilonova, the explosive aftermath of these types of collisions. The discovery plugs a hole in our understanding of how heavy elements form.

In 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European VIRGO observatory detected gravitational waves coming from the merger of two neutron stars. The merger event was named GW170817, and it was about 130 million light years away in the galaxy NGC 4993.

The resulting kilonova is called AT2017gfo, and the European Southern Observatory (ESO) pointed several of their telescopes at it to observe it in different wavelengths. In particular, they pointed the Very Large Telescope (VLT) and its X-shooter instrument at the kilonova.

This chart shows the sprawling constellation of Hydra (The Female Sea Serpent), the largest and longest constellation in the sky. Most stars visible to the naked eye on a clear dark night are shown. The red circle marks the position of the galaxy NGC 4993, which became famous in August 2017 as the site of the first gravitational wave source that was also identified in light visible light as the kilonova GW170817. NGC 4993 can be seen as a very faint patch with a larger amateur telescope. Image Credit: ESO, IAU and Sky & Telescope

The X-shooter is a multi-wavelength spectrograph that observes in Ultraviolet B (UVB,) visible light, and Near Infrared (NIR.) Initially, X-shooter data suggested that there were heavier elements present in the kilonova. But until now, they couldn’t identify individual elements.

“This is the final stage of a decades-long chase to pin down the origin of the elements.”

Darach Watson, Lead Author, University of Copenhagen.

These new results are presented in a new study titled “Identification of strontium in the merger of two neutron stars.” The lead author is Darach Watson from the University of Copenhagen in Denmark. The paper was published in the journal Nature on 24 October 2019.

“By reanalysing the 2017 data from the merger, we have now identified the signature of one heavy element in this fireball, strontium, proving that the collision of neutron stars creates this element in the Universe,” said Watson in a press release.

This artist’s impression shows two tiny but very dense neutron stars merging and exploding as a kilonova. Such objects are the main source of very heavy chemical elements, such as gold and platinum, in the Universe. The detection of one element, strontium (Sr), has now been confirmed using data from the X-shooter instrument on ESO’s Very Large Telescope.

The forging of the chemical elements is called nucleosynthesis. Scientists have known about it for decades. We know that elements form in supernovae, in the outer layers of aging stars, and in regular stars. But there’s been a gap in our understanding when it comes to neutron capture, and how heavier elements are formed. According to Watson, this discovery fills that gap.

“This is the final stage of a decades-long chase to pin down the origin of the elements,” says Watson. “We know now that the processes that created the elements happened mostly in ordinary stars, in supernova explosions, or in the outer layers of old stars. But, until now, we did not know the location of the final, undiscovered process, known as rapid neutron capture, that created the heavier elements in the periodic table.”

There are two types of neutron capture: rapid and slow. Each type of neutron capture is responsible for the creation of about half of the elements heavier than iron. Rapid neutron capture allows an atomic nucleus to capture neutrons quicker than it can decay, creating heavy elements. The process was worked out decades ago, and circumstantial evidence pointed to kilonovae as the likely place for the rapid neutron capture process to take place. But it was never observed at an astrophysical site, until now.

This animation is based on a series of spectra of the kilonova in NGC 4993 observed by the X-shooter instrument on ESO’s Very Large Telescope in Chile. They cover a period of 12 days after the initial explosion on 17 August 2017. The kilonova is very blue initially but then brightens in the red and fades.
Credit:ESO/E. Pian et al./S. Smartt & ePESSTO/L. Calçada

Stars are hot enough to produce many of the elements. But only the most extreme hot environments can create heavier elements like Strontium. Only those environments, like this kilonova, have enough free neutrons around. In a kilonova, atoms are constantly bombarded by massive numbers of neutrons, allowing the rapid neutron capture process to create the heavier elements.

“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,” says Camilla Juul Hansen from the Max Planck Institute for Astronomy in Heidelberg, who played a major role in the study.

Even though the X-shooter data has been around for a couple years, astronomers weren’t certain that they were seeing strontium in the kilonova. They thought they were seeing it, but couldn’t be sure right away. Our understanding of kilonovae and neutron star mergers is far from complete. There are complexities in the X-shooter spectra of the kilonova that had to be worked through, specifically when it comes to identifying the spectra of heavier elements.

On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Interferometer both detected gravitational waves from the collision between two neutron stars. Within 12 hours observatories had identified the source of the event within the lenticular galaxy NGC 4993, shown in this image gathered with the NASA/ESA Hubble Space Telescope. The associated stellar flare, a kilonova, is clearly visible in the Hubble observations. This is the first time the optical counterpart of a gravitational wave event was observed. Hubble observed the kilonova gradually fading over the course of six days, as shown in these observations taken in between 22 and 28 August (insets). By ESA/Hubble, CC BY 4.0,

“We actually came up with the idea that we might be seeing strontium quite quickly after the event. However, showing that this was demonstrably the case turned out to be very difficult. This difficulty was due to our highly incomplete knowledge of the spectral appearance of the heavier elements in the periodic table,” says University of Copenhagen researcher Jonatan Selsing, who was a key author on the paper. 

Up until now, rapid neutron capture was much debated, but never observed. This work fills in one of the holes in our understanding of nucleosynthesis. But it goes further than that. It confirms the nature of neutron stars.

After the neutron was discovered by James Chadwick in 1932, scientists proposed the existence of the neutron star. In a 1934 paper, astronomers Fritz Zwicky and Walter Baade advanced the view that “a super-nova represents the transition of an ordinary star into a neutron star, consisting mainly of neutrons. Such a star may possess a very small radius and an extremely high density.”

Three decades later, neutron stars were linked and identified with pulsars. But there was no way to prove that neutron stars were made of neutrons, because astronomers couldn’t obtain spectroscopic confirmation.

But this discovery, by identifying strontium, which could only have been synthesized under extreme neutron flux, proves that neutron stars are indeed made of neutrons. As the authors say in their paper, “The identification here of an element that could only have been synthesized so quickly under an extreme neutron flux, provides the first direct spectroscopic evidence that neutron stars comprise neutron-rich matter.”

This is important work. The discovery has plugged two holes in our understanding of the origin of elements. It confirms observationally what scientists knew theoretically. And that’s always good.


Evan Gough

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