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NARRATOR: Have you ever wondered
how all the chemical elements

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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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ANNA FREBEL: We just talked
about the fusion processes

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and how elements are
made in stars, mostly

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for energy generation purposes.

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Now let's look at
how that actually

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manifests in the star as a whole
because, as astronomers, we

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can observe stars but we can't
really look inside of stars.

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We can only see the surface.

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There are a number of
ways we can get clues

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from the surface of the star as
to what's going on in the core.

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And so overall this is really
a nice example of how nuclear

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physics and astrophysics--
nuclear and astrophysics--

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come together because the
nuclear physics governs what's

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happening inside the core and
then the astrophysics provides

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what we can actually observe.

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And both kind of need to
come together and work out.

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So over the last several
decades a lot of progress

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has been made to put
these two together

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and to understand what is
now called stellar evolution.

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That is actually governed
by the nuclear physics

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processes, specifically
fusion, in the core.

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I wanted to share this with you
because it's very insightful.

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So I'm going to
draw what we call

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a Hertzsprung-Russell diagram.

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It basically shows how a star
evolves during its lifetime.

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We can use the
sun as an example.

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And what we're going
to have is, we're

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going to have hot stars
here and cool stars here.

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And then we have
more luminous stars

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up here and less
luminous stars down here.

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And there is a
certain track that

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looks like this, half
of a Christmas tree.

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And the sun actually sits
right now about here.

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One can put any star
in this diagram.

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And you will see in a
moment how we can then

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learn about the evolutionary
phase of the star and hence

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what's going on inside its core.

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So the sun is sitting here.

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And we know on this
branch here, which

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we call the main sequence, that
stars burn hydrogen to helium.

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How does this look?

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If we draw a star here,
in the core hydrogen

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is burned into
helium just like what

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we had in the previous section.

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The star, when it moves on--

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oh, and I should
say, every star will

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start somewhere along
the main sequence here

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and it will stay there for
about 90% of its lifetime.

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Which means coming back to
the old stars for a second,

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90% of 15 billion years is about
the age of the universe which

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means the stars that started
here when they were born

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in the early universe,
they are just

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at the end of this hydrogen
to helium process which really

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means they haven't
done anything else

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but burning hydrogen
into helium which

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really is the key to why these
stars don't show their age.

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They are just like what
they've always done.

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And we can observe
them today and infer

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things about the early
universe from them

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today because they
haven't changed.

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That's the key.

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But if we look at a star
that has a shorter lifetime

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and wants to evolve,
it will move up here.

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And it will move up
here when the core--

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let's see, this
was hydrogen here

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and it has been
converted to helium--

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when we indeed have just
helium in the core and there

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there's no hydrogen
left in the core.

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Then the star will get
a little bit rumbly.

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And so it's going to
start moving along here.

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And there are all sorts of
things going on in the core

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because the thermostat is out.

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There is no energy being
produced right now.

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And so what happens
is, the star actually

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inflates to counteract that
and it will move up here

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and become very luminous.

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And up here we have
the red giants.

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They're called red
giants because they

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are much bigger
and more luminous

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but they're also
cooler because--

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they're bigger and
so they turn red.

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And so they have just a helium
core and what's happening

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is, in an outer
shell here they still

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burn hydrogen to helium
burning going on in the shell.

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And that provides a little
bit of substitute energy,

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a little interim energy to
the star as it moves up here.

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And then up here,
we have something

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called the helium
flash which means

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the helium here in the core is
now being converted to carbon.

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How can I draw this?

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I'll make this go away.

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So we eventually get--

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helium gets converted to carbon.

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So eventually we're going
to get to a carbon core.

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And then we have helium
burning further out

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and hydrogen burning
yet further out.

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When helium burning starts here,
by the time it reaches here

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it has this carbon core.

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So here it reaches a
helium core, this region.

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And then helium starts to burn.

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And then by the time it gets
here we have a carbon core,

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and then it moves up here.

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And this last part
here, it really

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depends on the mass of the star.

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The sun is actually
not going to do much.

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It's going to just stick it
out with a carbon oxygen core

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here and then turn into white
dwarf and just cool down.

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So the sun is actually
pretty boring star

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that has a pretty boring fate.

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If we make the sun
much more massive,

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let's say 10 times more
massive, it would move up here

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in this carbon burning phase.

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And if we have variety
of later burning stages

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that lead to iron.

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And then it would have
an iron core up here.

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And you already know
what's going to happen.

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If a star has an iron core
it has lost its energy source

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and it will explode
as a supernova.

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And so before it explodes,
what's it going to look like?

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We have a whole bunch of
these so-called shells.

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Sometimes they're referred
to as onion shells.

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In the center you
have iron and then

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there's silicon and all the
other elements folding out

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here--

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oxygen, carbon, helium.

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Let's draw another one.

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Hydrogen.

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There are a few
other elements being

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produced in minority processes.

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So some of these shells are
not pure in these elements.

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But that's the basic
idea that you go--

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this is oxygen and silicon.

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This is sulfur and others.

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That you have a star
that looks like that.

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So what you see here is
that whatever is happening

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in the core has a direct
impact of where the object sits

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on this diagram here.

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And so by measuring the
luminosity of a star

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as well as its temperature we
can place it on this diagram

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and then learn which
evolutionary state

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the star is in, which tells us
what is going on in its core.

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