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ANNA FREBEL: Have
you ever wondered

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how all the chemical
elements are made?

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Then join me as we are
lifting all these data

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secrets to understand the
cosmic origin of the chemical

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elements.

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We talked a lot about
the lighter elements

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that are made in
fusion processes

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in the cores of stars.

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But what about all the other
elements from the bottom half

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of the periodic table?

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We haven't really
talked about those yet.

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Let's do that.

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What we need are
so-called seed nuclei.

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We have an iron nucleus here.

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And if we are in a situation
where there is a strong neutron

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flux available-- and we'll
talk about where that happens

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in a sec--

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then if we have
little neutrons here,

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and this seed nucleus is getting
bombarded with these neutrons,

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then it's going
to swell and turn

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into a much larger nucleus
that is radioactive and neutron

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rich.

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And it's an isotope.

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So it has lots of
neutrons in it.

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And what's happening, then,
because this is radioactive,

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it doesn't like to
stay in this way.

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It will, what we
call, beta decay,

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which is just a
fancy word of saying

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that all these neutrons
here are being converted,

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or a good fraction of
them, into protons.

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And so we end up with
a stable element that's

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much larger than this
original one, the iron,

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or it could also
be a carbon atom.

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So this is the basic idea of
how all the other heavy elements

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are made.

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An example would be
barium here or uranium.

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Uranium 238 is technically
not a stable element,

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but its half-life is
4.7 billion years.

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So for us humans,
that's pretty stable.

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But on cosmic
timescales it is not.

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If we want to
consider it a stable,

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it would be the heaviest
element that we have

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on Earth that's long lived.

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And so they're all made by
this so-called neutron capture

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process, neutron
capture process.

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Now there are a few details
that we should consider, mostly

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that there are actually
two different ways

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that this neutron capture
process can happen.

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One is in a slow
way, slow n-capture.

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And the other way is rapid.

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And that refers to
how fast and over what

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timescale this neutron
bombardment is occurring.

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And in the case of the
slow neutron capture,

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the timescale is
about 10,000 years.

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And what happens is that
in evolved red giant stars,

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evolved red giants,
in the inner layers,

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in some of the shell layers,
where the nuclear fusion is

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going on, there are a secondary
nucleus into this fusion

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processes operating,
and as a result of that,

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free neutrons are produced.

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And so they provide
a steady flux

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of neutrons that
then get essentially

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shot onto these seed nuclei.

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And so over the timescale of
something like 10,000 years,

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heavy elements are
successively built up.

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A neutron is added.

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It turns into a
radioactive isotope.

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It decays and then
we have a steady one.

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You add another neutron.

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It will decay again.

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And so you build up one by one
by one, all the way up to lead.

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That's the heaviest
stable, stable element,

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if you take away
thorium and uranium,

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because they are radioactive,
as I just mentioned.

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Now in the case of the
rapid neutron capture,

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that really requires much
more energetic and extreme

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conditions.

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And what recent
research has shown

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is that a rapid neutron flux
only operates in two locations.

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One is perhaps in supernovae.

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When the iron core collapses
at the end of a star's life,

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it actually implodes and
forms a neutron star.

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So there's a really
dense neutron star

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in the middle that's a
compact remnant left over

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after the supernova.

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And in the process of
making this neutron star,

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there are, of course, lots
of neutrons floating around,

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and they can provide
this kind of flux,

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operating on a one to
two second timescale.

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So huge neutron bombardment
within a few seconds,

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and that can lead up
to a very, very fast

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build up of a giant radioactive
nuclei here, that then decays.

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So you have enough seed nuclei.

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All of them will do vroom
and then slowly decay

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back to the different
elements that

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make up the entire bottom
of the periodic table.

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Another option is an
emerging neutron star.

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So if you take two of
these neutron stars

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and you have them in a binary
system where they orbit

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each other, and if this
system eventually, or the two

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stars in the system
eventually coalesce and merge,

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then you also have some kind
of firework of neutrons.

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And that can also have this
rapid neutron capture going on.

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And so we have to add here.

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So either in supernovae or
the proton neutron star--

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I'm going to
abbreviate like that--

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or in neutron star mergers.

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So that are all the options.

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And what we now want
to figure out, really,

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is how can we put all this
theory to the test, right?

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How can we observe this?

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And that's why our old
stars come back into play.

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Imagine that in the very
beginning of the universe when

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the first stars emerged, and
maybe the second generation

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of stars, so not too much
of all the heavy elements

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was present at that time.

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And so let's say you have a
neutron star merger go off

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at this very early time.

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The rapid neutron capture
process will occur.

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All these new heavy elements get
spilled into the surrounding.

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And then you form a
next generation star

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from this enriched material.

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And because the universe
was not too much enriched

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in all the other elements,
we have this opportunity

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to observe a clean
nuclear synthesis

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process of this R process.

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We sometimes abbreviate it,
rapid with R, so R process.

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R process.

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And actually this
here is S process.

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You could have guessed that.

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And so at the
earliest times, it is

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possible to observe
the signature of the R

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process, a clean signature,
as well as the S process.

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That is not possible
anymore today.

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The universe has
experienced 13 billion years

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of chemical evolution.

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So it's a pretty
messy place out there.

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And so if one more
event goes off,

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that signature just gets
diluted into whatever

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else is out there already.

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But at the earliest
times, we have this chance

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to find these clean signatures.

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