Original Link : https://medium.com/predict/from-neutron-stars-to-heavy-metal-s-c0135b8f23a7

Scientists have identified the heavy element strontium in colliding neutron stars. We are one step closer to discovering our origins.

A team of researchers has been able to confirm the theory, that some elements heavier than iron are produced in neutron stars:

With this discovery, we are getting closer to filling the gap that has been in our understanding of how some elements heavier than iron, could have been created.

Creation of the lightest elements

We have known for a long time that hydrogen and helium, the two lightest elements, have been created at the beginning of the Universe during the big bang. Approximately 75% of the mass of the known ordinary matter is from hydrogen, while 23–25% is from helium. All heavier elements are in total accountable for only a small percentage of the known mass of matter in the universe.

We also know the fusion process in stars that has created all the heavier elements until iron in the periodic table. Fusion is the process where two elements given enough energy from the high temperature in the core of a star will fuse together into elements with a higher atomic number. High temperature is needed to give the elements enough energy to overcome the repulsion between protons. The high pressure in the core will push the elements close enough together for the fusion to happen.

All lighter elements until iron have increasing nucleus binding energy. But when iron is created in a very massive star, there are no heavier elements with a more stable nucleus. Hence no heavier elements can be manufactured in the fusion factory of a star.

The nucleus of iron (Fe-56) has the highest binding energy and hence is one of the most stable elements. Lighter elements fusion together to create heavier elements to release energy (this is what makes stars shine). This process can lead to the manufacturing of iron in the most massive stars, but no element heavier than iron can be formed in the fusion process. Image credits: Public Domain.

Elements between oxygen and iron

Even with elements lighter than iron, the creation is not that simple. Between oxygen and iron in the periodic table, mainly elements with an atomic mass number that is a multiple of 4 are created. This is because the elements are created by capturing an alpha-particle (the nucleus of helium: two protons and two neutrons). After oxygen with the atomic mass 8u, we have carbon, oxygen, neon, magnesium, silicon, sulfur, argon, calcium and iron. Carbon and oxygen are the most abundant elements heavier than hydrogen and helium.

The periodic table, for reference. Strontium is seen as Sr, atomic number 38. Iron is Fe, atomic number 26. Image by ExplorersInternational from Pixabay.

Elements with a higher atomic mass than iron

So where and how are the rest of the elements created? Above is a periodic table just for a reference. We have strontium with atomic number 38, silver with the number 47 and gold number 79. Strontium has many uses, among some, it gives the red color in fireworks, makes paint glow in the dark and blocks X-ray radiation from TV picture tubes.

One theory has been that heavier metals are created in supernovas, which are dying exploding massive stars (more than 25 solar masses), but although a good theory, none of the heavier elements have been detected in spectra from supernovae. So this still left us with a gap for heavier elements.

A spectrum in astronomy is a plot of light intensity as a function of wavelength. Different elements will have different absorption or emission lines in different wavelengths, which makes it possible to detect elements in observed objects.

Some of those heavy elements have been detected in dying low mass stars (like our own). Technetium was detected in 1952 (Tc, atomic number 43) by Astronomer Paul Merrill. The production of heavy elements in dying low mass stars can be explained with an s-process (slow-capture process), where the nucleus will capture a neutron once in a while and if the isotope of the atom with a larger amount of neutrons becomes unstable, one neutron in the nucleus will decay into a proton, creating an atom with one atomic number one up the scale in the periodic table. Elements up to polonium (atomic number 84) can be created in the s-process, which spans a time range of thousands of years.

Another neutron capturing process is the r-process (rapid-capture process), which requires different conditions than the s-process. While there can pass years between a neutron capture in an s-process, the r-process requires at least 100 neutrons per second. This can happen in high temperatures and high densities of nucleus such as is in a neutron star (dead massive stars, that weren’t massive enough to become supernovae) containing only neutrons pushed very closely together. In the r-process there are a lot of neutrons captured at once and some of them will decay into protons creating isotopes of heavier elements.

Until recently it has been an unproven theory, that heavier elements can be created by neutron stars. In 2017, scientists have been able to capture light from two merging neutron stars. This was a great achievement. Now, scientists from Copenhagen have finally been able to identify one of the heavier elements in the astronomical spectra from the kilonova — the strontium signatures.

kilonova (also called a macronova or r-process supernova) is a transient astronomical event that occurs in a compact binary system when two neutron stars or a neutron star and a black hole merge into each other — wikipedia.

While only this heavy metal has been identified so far, the researchers are optimistic, that other heavy metals are created under the same conditions.

We are all made of stardust — with a twist

This is exciting news for all of us, who are curious about where we are from. After all — we are made of those exact elements that were once made in some violent astronomical event billions of years ago.

Since all of those elements can be found here on Earth, it must mean that there must have been some violent event nearby before our solar system and Earth where born. We already hypothesized that a supernova explosion must have happened nearby for the shock waves to start a gravitational instability in a cold gas cloud to start the birth of our star. Now maybe some of our known elements could also come from kilonovas.

Some of the heavier elements in the early forming Earth might have partially heated the crust of the Earth, maybe making it possible for life to emerge.

We are all made of stardust — that’s the pretty version of us. But like in all families, we also have our dirty secrets: we are a result of a violent relationship. Heavy metal style!

Reference Article :

First Identification of a Heavy Element Born from Neutron Star Collision

Newly created strontium, an element used in fireworks, detected in space for the first time following observations with ESO telescope

For the first time, a freshly made heavy element, strontium, has been detected in space, in the aftermath of a merger of two neutron stars. This finding was observed by ESO’s X-shooter spectrograph on the Very Large Telescope (VLT) and is published today in Nature. The detection confirms that the heavier elements in the Universe can form in neutron star mergers, providing a missing piece of the puzzle of chemical element formation.

In 2017, following the detection of gravitational waves passing the Earth, ESO pointed its telescopes in Chile, including the VLT, to the source: a neutron star merger named GW170817. Astronomers suspected that, if heavier elements did form in neutron star collisions, signatures of those elements could be detected in kilonovae, the explosive aftermaths of these mergers. This is what a team of European researchers has now done, using data from the X-shooter instrument on ESO’s VLT.

Following the GW170817 merger, ESO’s fleet of telescopes began monitoring the emerging kilonova explosion over a wide range of wavelengths. X-shooter in particular took a series of spectra from the ultraviolet to the near infrared. Initial analysis of these spectra suggested the presence of heavy elements in the kilonova, but astronomers could not pinpoint individual elements until now. 

“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,” says the study’s lead author Darach Watson from the University of Copenhagen in Denmark. On Earth, strontium is found naturally in the soil and is concentrated in certain minerals. Its salts are used to give fireworks a brilliant red colour. 

Astronomers have known the physical processes that create the elements since the 1950s. Over the following decades they have uncovered the cosmic sites of each of these major nuclear forges, except one. “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.”

Rapid neutron capture is a process in which an atomic nucleus captures neutrons quickly enough to allow very heavy elements to be created. Although many elements are produced in the cores of stars, creating elements heavier than iron, such as strontium, requires even hotter environments with lots of free neutrons. Rapid neutron capture only occurs naturally in extreme environments where atoms are bombarded by vast numbers of neutrons.

“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.

Scientists are only now starting to better understand neutron star mergers and kilonovae. Because of the limited understanding of these new phenomena and other complexities in the spectra that the VLT’s X-shooter took of the explosion, astronomers had not been able to identify individual elements until now.

“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. 

The GW170817 merger was the fifth detection of gravitational waves, made possible thanks to the NSF‘s Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and the Virgo Interferometer in Italy. Located in the galaxy NGC 4993, the merger was the first, and so far the only, gravitational wave source to have its visible counterpart detected by telescopes on Earth. 

With the combined efforts of LIGO, Virgo and the VLT, we have the clearest understanding yet of the inner workings of neutron stars and their explosive mergers.