We have a pretty good understanding of how a lot of elements in the periodic table formed. Hydrogen, Helium and Lithium, the three lightest elements, formed shortly after the big bang. Heavier elements, up to Iron, were forged billions of years later in the hearts of stars. But the origin of the naturally occurring elements heavier than iron is less certain. Latest Space News
That was until October of 2019, when researchers analyzing data from a neutron star collision announced they were certain how about half of those heavier elements form. They witnessed something called the rapid neutron capture process, or r-process, which was first proposed about 60 years ago. It was thought to occur only in extreme environments where atoms were bombarded with huge numbers of neutrons. Latest Space News
This allows the atom’s nucleus to capture neutrons quicker than it can decay, forming a heavier element. But we couldn’t confirm where in the universe would bombard elements with enough neutrons to make the r-process possible. Now I realize I may have spoiled the big reveal here when I said that scientists figured it out after watching neutron stars collide. After all, where are you going to find more NEUTRONS than when two NEUTRON stars crash into each other?
Aside from that episode where Jimmy Neutron cloned himself a whole bunch. But the truth is scientists weren’t certain neutron stars had, um, neutrons in them. We were pretty sure that’s what’s left over after massive stars go supernova and their surviving cores are dense enough to crush protons and electrons together yet not dense enough to further implode and become a black hole. But we weren’t 100% sure. Then, in 2017, scientists caught a break. Latest Space News
Gravitational-wave detectors LIGO and Virgo sensed waves coming from somewhere in the southern sky. About two seconds later, two instruments detected a gamma-ray burst from the same area. These phenomena together tipped off scientists that a neutron star merger and it’s predicted explosive aftermath called a kilonova was likely occurring. Telescopes scrambled to find the source, until they spotted a new point of light about 130 million light-years away.
One instrument in particular, the European Southern Observatory’s X-shooter, studied the kilonova for days, recording its spectrum from ultraviolet to the near-infrared. By analyzing the spectrum, scientists could look for the distinct fingerprint elements that leave as they absorb parts of the spectrum. But at first, the heavier elements weren’t easy to suss out, because their spectrums can create complex blends of tens of millions of spectral lines, making them hard to tell apart. It wasn’t until scientists reexamined the data that they spotted a distinct line at the boundary of visible light and infrared.
This 810 nm spectroscopic feature told the scientists they witnessed the creation of the heavy element strontium through rapid neutron capture. What’s surprising about spotting strontium is it’s actually one of the lighter of the heavy elements formed by the r-process. In order for it to occur, neutrinos have to bombard neutrons to break them down into protons and electrons. Strontium’s discovery told astronomers that a wide range of heavy elements form during kilonovae, from the lighter to the very heaviest. The discovery of strontium hiding among the kilonova’s spectrum filled in several gaps in our knowledge. It confirms the r-process takes place when neutron stars merge, and shows conclusively that neutron stars in fact contain neutrons. Not a bad day’s work, but the science is never finished.
Next, the researchers will look to expand their knowledge of the spectral lines of heavier elements, hoping to conclusively identify more products of the neutron star collision. In case you were wondering, there is also a slow neutron capture process, or s-process, which is thought to occur in the outer layers of old stars and is responsible for the other half of heavy elements.
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