WEBVTT

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PROFESSOR: 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 summarize what we've
learned so far about the older

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stuff and how they can
be used in our concept

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of stellar archeology to
understand what happened soon

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after the big bang in terms
of the chemical enrichment

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and chemical evolution.

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Still we have old stars,
and we call them metal-poor.

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And they are our tool.

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They are our tool to
study the early universe.

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These stars are long-lived, so
they have a low mass, something

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like 0.6 to 0.8 solar masses.

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And that means
they have lifetimes

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of 15 to 20 billion years.

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And that means that they
are still observable.

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And that is very lucky for us.

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And they are not
just observable.

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They are actually fairly
easily observable,

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because they're now
located in the Milky Way.

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Let's look at this again.

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So this is very quick
drawing of our Milky Way.

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This is the bulge,
the inner part

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of our galaxy with a
supermassive black hole

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in the center.

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And this here--
actually two disks.

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So this is the disk.

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And we're about here,
2/3 on the way out.

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And the bulge contains
a lot of young stars.

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There's a lot of
gas, which means

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you have formed a
lot of stars, which

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means you make a lot of elements
and you form more stars.

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So the bulge is very metal-rich.

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And the disk here is not
quite as metal-rich but still

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pretty enriched.

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Now, this is not the
only part of our galaxy.

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This is just the most
visible part, namely

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the Milky Way band
on the night sky.

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That's when you
look into the spiral

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arms that make up the disk.

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But we look in a different
place for the older stars,

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because they are
kind of located up

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here and below the disk
in something that's

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called the halo.

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It's actually much larger than
what I'm drawing right now.

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And so that's called
the halo of the disk.

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It's a spherical envelope
of the disk here.

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And all the old stuff
is parked there.

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It's a bit of a junkyard.

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Because when a galaxy forms,
you have small systems

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that actually come together
and form a bigger system.

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And then here you have
a bigger system too.

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And then they come together
and make an even bigger one.

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So that's called the
hierarchical structure

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formation paradigm.

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And so this is the Milky Way.

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And which means that these
little guys here kind of end

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up in the outskirts, or
at least a good amount

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of these little guys
end up in the outskirts.

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But they are completely
shredded apart.

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And what is left
are all the stars

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that are being spilled
into the Milky Way.

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And so this is how
old stars actually

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get into the outer
halo of the Milky Way.

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I should mention here that
little dwarf galaxies are also

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actually in the
halo of the galaxy.

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So they are also pretty old.

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So these are entire
little systems here

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that are not completely
shredded yet.

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So they are just in
the gravitational field

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here of the Milky Way.

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And they're orbiting
around the disk.

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And we also have
globular clusters.

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These are clusters of stars
with up to a million stars.

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They are also located
here and down here.

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And they are also really old.

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We don't really know
where they come from.

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So the halo contains
mostly three things--

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globular clusters, dwarf
galaxy, and lots of old stars.

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And so with our telescope we can
peek from here kind of here up

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into the halo and down here
and observe all the old stars

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that are in this range.

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So all in all, our
metal-poor stars

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are the local equivalent to
what we call the high redshift

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universe.

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In a very complementary
way, both metal-poor stars

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and the furthest
most distant galaxies

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are used to study
the early universe.

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These faraway galaxies, when
their light comes to us,

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we receive it from
this early time.

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And this way, we can figure out
what this galaxy can tell us

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about the early
universe, because it

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formed at that early time.

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Our metal-poor
stars, their light

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hasn't traveled for a long time.

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It has traveled maybe
just from here to us.

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That's a negligible
amount of time.

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Because these stars are today
located in our Milky Way.

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But they are really old.

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We see them as
when they are old,

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not as when they were
young, as in the case

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of these distant galaxies.

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But the fact that we see them
all doesn't matter to us.

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Because these stars don't
get wrinkly or anything.

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They just sit there.

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And they are just waiting
for us to observe them.

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And as we will see
in the following,

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these stars are
really undisturbed.

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And today, they
look like just what

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they did 13 billion years ago.

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So that's a huge
advantage for us.

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And of course, we're
going to make use of it.