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[MUSIC PLAYING]

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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 this data

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secrets to understand the
cosmic origin of the chemical

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elements.

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Let's look at the early
chemical evolution.

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Some 14 billion
years ago, everything

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started with a big bang.

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A big bang.

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And it left behind
a universe that

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was made from just
hydrogen and helium gas.

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I mentioned before, there's
a tiny, tiny little trace

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of lithium, but we're not
going to worry about that.

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So we had a universe, but
there were no stars yet.

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So it was actually dark.

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And some people call that
the cosmic dark ages.

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But structure began to form,
and gas began to clump.

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And so eventually,
in a gas cloud,

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the very first stars formed.

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Here are two of the first stars.

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And they were made just
from hydrogen and helium,

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because the gas was just
hydrogen and helium.

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But for energy
generation purposes,

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these stars needed to
have nuclear fusion go

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on in their cores
to produce energy.

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And so the first
elements heavier

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than hydrogen and helium were
created inside of these stars.

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And later, when they
exploded in supernovae,

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they were expelled
into this gas cloud.

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So we had the first
elements here.

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And of course, they also
gave the first light.

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So they lit up the
universe for the first time

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and changed it in
a number of ways,

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not just by bringing in
light, but also by producing

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the first heavier elements.

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And that set in motion
the chemical evolution

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of the elements that's an
ongoing process until today.

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Now, these stars
here, because they're

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formed from just
hydrogen and helium gas,

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they're very massive.

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Rather large.

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Maybe something like
100 solar masses.

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So we use the unity
of solar mass--

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this is the symbol
for the sun here--

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as a unit.

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So 100 times more-- a ton of
times as heavy as the sun.

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And the reason for that is
that hydrogen and helium

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gas has trouble getting really
cold, making small stars.

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The important thing
with massive stars

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is that they have
really short lifetime.

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So they have a short lifetime of
maybe a few million years only.

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A few million years is not
much on a cosmic timescale.

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Which means they exploded pretty
quickly as gigantic supernovae.

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And as already indicated,
what happened at a later time,

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these stars were
gone, but they left

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behind all the heavy
elements that that created.

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And I should be specific here.

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These heavier elements
were all the elements made

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in fusion processes up to iron.

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So we have fusion
elements in the gas.

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And with the onset of
these little other elements

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being in the gas, now the gas
could form a clump much better

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and actually make small stars.

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So the next
generation of stars--

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so this was the
first generation.

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First generation.

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And here now, we
have the formation

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of second-generation stars.

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Here's a second-generation star.

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Here's a second-generation star.

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So small stars
similar to the sun,

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and perhaps a little bit less
massive than the sun, actually.

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But of course, you're not
going to make just small stars.

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You're also going
to make big stars.

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So we have a few big
stars here again,

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and a few intermediate ones.

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And the big ones,
the massive ones,

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will again explode on a
pretty fast time scale

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and make more elements.

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But these little guys--

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and that's the interesting
part for us here now.

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So the low-mass
second-generation stars,

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they have very long lifetimes.

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Long lifetimes of something
like 15 to 20 billion years,

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because they have--

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and I can just write that here.

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So low mass, they had maybe
masses-- or at least some

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of them had masses between
0.6 to 0.8 solar masses.

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So less massive than our sun.

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And as a good rule of thumb--

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I can write this here--

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the sun has, by
definition, one solar mass.

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And that has a
lifetime, or the sun

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has a lifetime of
10 billion years.

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And so everything
that has less mass

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will have a longer lifetime.

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And everything that is more
massive than one solar mass

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will have a much
shorter lifetime.

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Now, what does this imply,
having a long lifetime?

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It means that these
stars are still around.

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They're around today.

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We can observe them today.

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We can see them today.

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And that means that
we can use them

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to study the composition
of this early gas

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here, because that's
when they're formed.

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They have incorporated
in all their layers

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throughout the
composition of this gas

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cloud, that was enriched
by these very first stars.

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So we have means to study the
first stars, and what came out

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of the supernovae
explosions, and what

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happened at this
very early phase here

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of star formation, galaxy
formation, and the formation

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of all the chemical elements.

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If you then wind
the clock forward,

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many things happened after.

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Well, of course,
more stars formed.

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Many, many more stars.

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More supernovae.

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And with every
generation of supernovae,

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more of all the
elements was provided.

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More of all the elements.

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Because, as we will
see later, in some

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of these very early processes--
nuclear physics processes,

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all the elements from the
periodic table could have been

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or were created already
at the earliest times.

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Then with time, more and more
of all of them were created.

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It's not like that the universe,
because it was just hydrogen

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and helium in the
beginning, then

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it took time to build
up all the elements

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and march through the
periodic table with time.

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That is not the case.

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Everything was produced
from early on, and then

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just more and more often.

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And so, well, we have more stars
and more chemical enrichment.

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But also, the formation of
the first bigger structures--

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formation of larger structures,
including the Milky Way.

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Just going to
abbreviate it like this.

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Because this all happened in
small gravitationally-bound

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systems.

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And they were then
gobbled up later

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by slightly bigger neighbors.

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And then those were maybe
gobbled up by the proto Milky

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Way, and eventually,
the Milky Way

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formed, and formed
from the remnants

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of all these little systems.

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And so all of this here happened
as time went on until today.

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Today, we have a
universe that's full

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of structure, and full
of all the elements.

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Well, a whole 2%, but I think
we can consider this full,

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at least if you're
an astronomer.

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And chemical evolution
is an ongoing process.

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So in the following,
we're going to look

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in more detail at chemical
enrichment and chemical

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evolution.

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And also, stellar archeology,
which is the way how we use old

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stars to trace these
different early stages

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of chemical evolution.

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