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ANNA FREBEL: Have you ever wondered
how all the chemical elements are made?
Then join me as we are lifting all these data
secrets to understand the cosmic origin of the chemical
elements.
We talked a lot about the lighter elements
that are made in fusion processes
in the cores of stars.
But what about all the other elements from the bottom half
of the periodic table?
We haven't really talked about those yet.
Let's do that.
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What we need are so-called seed nuclei.
We have an iron nucleus here.
And if we are in a situation where there is a strong neutron
flux available-- and we'll talk about where that happens
in a sec--
then if we have little neutrons here,
and this seed nucleus is getting bombarded with these neutrons,
then it's going to swell and turn
into a much larger nucleus that is radioactive and neutron
rich.
And it's an isotope.
So it has lots of neutrons in it.
And what's happening, then, because this is radioactive,
it doesn't like to stay in this way.
It will, what we call, beta decay,
which is just a fancy word of saying
that all these neutrons here are being converted,
or a good fraction of them, into protons.
And so we end up with a stable element that's
much larger than this original one, the iron,
or it could also be a carbon atom.
So this is the basic idea of how all the other heavy elements
are made.
An example would be barium here or uranium.
Uranium 238 is technically not a stable element,
but its half-life is 4.7 billion years.
So for us humans, that's pretty stable.
But on cosmic timescales it is not.
If we want to consider it a stable,
it would be the heaviest element that we have
on Earth that's long lived.
And so they're all made by this so-called neutron capture
process, neutron capture process.
Now there are a few details that we should consider, mostly
that there are actually two different ways
that this neutron capture process can happen.
One is in a slow way, slow n-capture.
And the other way is rapid.
And that refers to how fast and over what
timescale this neutron bombardment is occurring.
And in the case of the slow neutron capture,
the timescale is about 10,000 years.
And what happens is that in evolved red giant stars,
evolved red giants, in the inner layers,
in some of the shell layers, where the nuclear fusion is
going on, there are a secondary nucleus into this fusion
processes operating, and as a result of that,
free neutrons are produced.
And so they provide a steady flux
of neutrons that then get essentially
shot onto these seed nuclei.
And so over the timescale of something like 10,000 years,
heavy elements are successively built up.
A neutron is added.
It turns into a radioactive isotope.
It decays and then we have a steady one.
You add another neutron.
It will decay again.
And so you build up one by one by one, all the way up to lead.
That's the heaviest stable, stable element,
if you take away thorium and uranium,
because they are radioactive, as I just mentioned.
Now in the case of the rapid neutron capture,
that really requires much more energetic and extreme
conditions.
And what recent research has shown
is that a rapid neutron flux only operates in two locations.
One is perhaps in supernovae.
When the iron core collapses at the end of a star's life,
it actually implodes and forms a neutron star.
So there's a really dense neutron star
in the middle that's a compact remnant left over
after the supernova.
And in the process of making this neutron star,
there are, of course, lots of neutrons floating around,
and they can provide this kind of flux,
operating on a one to two second timescale.
So huge neutron bombardment within a few seconds,
and that can lead up to a very, very fast
build up of a giant radioactive nuclei here, that then decays.
So you have enough seed nuclei.
All of them will do vroom and then slowly decay
back to the different elements that
make up the entire bottom of the periodic table.
Another option is an emerging neutron star.
So if you take two of these neutron stars
and you have them in a binary system where they orbit
each other, and if this system eventually, or the two
stars in the system eventually coalesce and merge,
then you also have some kind of firework of neutrons.
And that can also have this rapid neutron capture going on.
And so we have to add here.
So either in supernovae or the proton neutron star--
I'm going to abbreviate like that--
or in neutron star mergers.
So that are all the options.
And what we now want to figure out, really,
is how can we put all this theory to the test, right?
How can we observe this?
And that's why our old stars come back into play.
Imagine that in the very beginning of the universe when
the first stars emerged, and maybe the second generation
of stars, so not too much of all the heavy elements
was present at that time.
And so let's say you have a neutron star merger go off
at this very early time.
The rapid neutron capture process will occur.
All these new heavy elements get spilled into the surrounding.
And then you form a next generation star
from this enriched material.
And because the universe was not too much enriched
in all the other elements, we have this opportunity
to observe a clean nuclear synthesis
process of this R process.
We sometimes abbreviate it, rapid with R, so R process.
R process.
And actually this here is S process.
You could have guessed that.
And so at the earliest times, it is
possible to observe the signature of the R
process, a clean signature, as well as the S process.
That is not possible anymore today.
The universe has experienced 13 billion years
of chemical evolution.
So it's a pretty messy place out there.
And so if one more event goes off,
that signature just gets diluted into whatever
else is out there already.
But at the earliest times, we have this chance
to find these clean signatures.
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