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.
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.
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.
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
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.If you like our Elements
episodes you may also enjoy our Focal Point
series.
Check out this episode on a clock that could
redefine time.
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