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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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Why do stars shine?

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Why do we have
sunlight every day?

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Well, it's because
of nuclear fusion.

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Nuclear fusion is going
on in the core of the sun.

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Hydrogen to helium
gets converted there.

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And that gives the
sun enough energy

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to sustain its luminosity
for billions of years.

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How does this work?

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Let's have a look.

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So we want to reproduce
what is going on in the sun.

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And what's basically
happening is

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that four protons, which also
are just four hydrogen atoms,

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come together in
a series of steps

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that we're going to
leave out for now.

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And they form a helium atom.

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And that's made from two
protons and two neutrons.

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So we have some conversion of
protons here into neutrons.

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--one helium.

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This actually works
only because there

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is quantum mechanical
tunneling going on.

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That's a really cool thing.

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Ordinarily, these positively
charged protons would actually

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repel themselves.

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But in the sun, it's really
quite hot, not quite hot

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enough for them to
all fuse straight up.

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But because of this
tunneling effect,

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it's hot enough,
just hot enough,

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so that these protons can
combine to eventually form

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a helium nucleus.

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This kind of tunneling
effect is important

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for all subsequent
fusion processes,

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namely if we have
another helium here

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and another one-- so
we'll put all of those

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together-- we're going to
get the carbon nucleus.

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This is the carbon nucleus.

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And if we're going to add
another helium to that,

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we're going to get oxygen. And
if we add more so-called alpha

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particles-- helium
nuclei are often

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called alpha particles--
then eventually we're

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going to get to iron.

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Now, how does this
help us understanding

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why the sun shines?

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As it turns out, these
lightest nuclei here,

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they're less tightly bound
than the big ones like iron.

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And that means we're going to
get a little bit more energy

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out of this than that.

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Now, let's look at some details
and then come back to this.

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So if we're going to
look at the constituents

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here for protons, which
make up one helium nucleus,

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and then we know a
helium nucleus consists

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of two neutrons and
two protons, and if we

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make a little experiment and
we weigh one helium nucleus,

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and then we weigh two neutrons
and two protons separately,

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we're going to find out that
the helium nucleus actually

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weighs a little bit less than
my initial constituents here.

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And actually, it's 0.73%.

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That our final helium
nucleus here weighs less

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than these constituents.

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And that's really fantastic.

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So this is called a mass defect.

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And you've all
seen the equation E

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equals mc squared, usually with
a picture of Einstein attached.

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And this here, this is a little
mass, a little mass difference.

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And if you stick
that in here, you

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multiply it with the speed
of light, c squared--

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which is just a constant,
so just a number--

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you're going to get out energy.

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And that is the
energy that the sun

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is using to shine every day.

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So this is the nuclear
energy that stars produce.

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Now, this amount of
energy that gets out

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become successively
less if you go

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to heavier and heavier nuclei.

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And if you were to try to
fuse to iron atoms together,

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you're not going to
get out anything.

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So iron atoms will not give
you any fusion energy with this

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here, because this is zero.

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Actually, you would
need to put energy

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in if you wanted to
fuse two iron atoms.

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So obviously, the star is
going to have a big problem.

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Because it doesn't
want to put energy in.

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It wants to get to energy out.

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And that's why, in the end, the
star ends up with an iron core.

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So this is an iron core here.

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These fusion processes have
been going on in the center

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and growing larger and larger
as more and more elements are

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being made.

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And eventually, there is a big,
fat iron core sitting there.

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And since it can't get
any energy out anymore,

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the star has a big problem.

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Because it needs to
have an energy source.

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And so without that, it
explodes as a supernova.