WEBVTT

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We've already talked about
the life cycle of stars

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roughly the same mass as our
sun, give or take a little bit.

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What I want to do
in this video is

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talk about more massive stars.

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And when I'm talking
about massive stars,

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I'm talking about stars that
have masses greater than 9

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times the sun.

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So the general idea
is exactly the same.

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You're going to start off
with this huge cloud of mainly

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hydrogen.

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And now, this cloud
is going to have

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to be bigger than the clouds
that condensed to form stars

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like our sun.

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But you're going
to start with that,

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and eventually gravity's
going to pull it together.

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And the core of it is going
to get hot and dense enough

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for hydrogen to ignite, for
hydrogen to start fusing.

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So this is hydrogen,
and it is now fusing.

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Let me write it.

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It is now fusing.

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Hydrogen fusion.

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Let me write it like this.

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You now have hydrogen
fusion in the middle.

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So it's ignited,
and around it, you

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have just the other
material of the cloud.

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So the rest of the hydrogen.

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And now, since it's so
heated, it's really a plasma.

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It's really kind of a soup
of electrons and nucleuses

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as opposed to well-formed atoms,
especially close to the core.

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So now you have hydrogen fusion.

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We saw this happens at
around 10 million Kelvin.

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And I want to make
it very clear.

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Since we're talking
about more massive stars,

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even at this stage,
there's going

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to be more gravitational
pressure, even at this stage,

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during the main
sequence of the star,

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because it is more massive.

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And so this is going to
burn faster and hotter.

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So this is going to be faster
and hotter than something

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the mass of our sun.

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And so even this stage
is going to happen over

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a much shorter period of
time than for a star the mass

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of our sun.

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Our sun's life is going to be
10 or 11 billion total years.

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Here, we're going to
be talking about things

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in maybe the tens of
millions of years.

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So a factor of 1,000
shorter life span.

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But anyway, let's think
about what happens.

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And so far, just the
pattern of what happens,

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it's going to happen
faster because we

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have more pressure, more
gravity, more temperature.

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But it's going to happen
in pretty much the same way

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as what we saw with a
star the mass of the sun.

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Eventually that helium--
sorry, that hydrogen

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is going to fuse into
a helium core that's

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going to have a hydrogen
shell around it.

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It's going to have a
hydrogen shell around it,

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hydrogen fusion shell around it.

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And then you have the rest
of the star around that.

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So let me label it.

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This right here is
our helium core.

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And more and more
helium is going

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to be built up as this
hydrogen in this shell fuses.

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And in a star the size of our
sun or the mass of our sun,

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this is when it starts
to become a red giant.

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Because this core is getting
denser and denser and denser

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as more and more
helium is produced.

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And as it gets denser
and denser and denser,

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there's more and more
gravitational pressure

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being put on the hydrogen,
on this hydrogen shell

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out here, where we have
fusion still happening.

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And so that's going to release
more outward energy to push out

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the radius of the actual star.

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So the general
process, and we're

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going to see this as the star
gets more and more massive,

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is we're going to have heavier
and heavier elements forming

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in the core.

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Those heavier and
heavier elements,

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as the star gets
denser and denser,

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will eventually ignite,
kind of supporting the core.

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But because the core itself
is getting denser and denser

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and denser, material is getting
pushed further and further out

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with more and more energy.

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Although if the star
is massive enough,

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it's not going to be
able to be pushed out

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as far as you will have
in kind of a red giant,

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with kind of a sun-like star.

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But let's just think
about how this pattern is

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going to continue.

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So eventually, that helium,
once it gets dense enough,

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it's going to ignite and it's
going to fuse into carbon.

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And you're going to have
a carbon core forming.

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So that is carbon core.

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That's a carbon core.

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Around that, you
have a helium core.

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And near the center
of the helium core,

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you have a shell
of helium fusion--

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that's helium, not hydrogen--
turning into carbon, making

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that carbon core
denser and hotter.

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And then around that,
you have hydrogen fusion.

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Have to be very careful.

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You have hydrogen fusion.

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And then around that, you
have the rest of the star.

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And so this process is just
going to keep continuing.

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Eventually that carbon
is going to start fusing.

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And you're going to
have heavier and heavier

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elements form the core.

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And so this is a
depiction off of Wikipedia

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of a fairly mature massive star.

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And you keep
forming these shells

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of heavier and heavier
elements, and cores

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of heavier and heavier
elements until eventually, you

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get to iron.

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And in particular, we're
talking about iron 56.

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Iron with an atomic mass of 56.

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Here on this periodic table
that 26 is its atomic number.

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It's how many protons it has.

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56, you kind of view it
as a count of the protons

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and neutrons, although
it's not exact.

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But at this point, the reason
why you stop here is that you

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cannot get energy
by fusing iron.

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Fusing iron into heavier
elements beyond iron

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actually requires energy.

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So it would actually be
an endothermic process.

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So to fuse iron actually
won't help support the core.

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So what I want to do in this--
So just to be very clear,

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this is how the heavy
elements actually formed.

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We started with
hydrogen, hydrogen fusing

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into helium, helium
fusing into carbon,

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and then all of these things
in various combinations--

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and I won't go into
all the details--

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are fusing heavier
and heavier elements.

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Neon, oxygen, and you
see it right over here.

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Silicon.

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And these aren't the only
elements that are forming,

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but these are kind of the main
core elements that are forming.

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But along the way, you have
all this other stuff, lithium,

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beryllium, boron.

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All of this other
stuff is also forming.

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So this is how you form
elements up to iron 56.

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And also, this is actually how
you can form up to nickel 56,

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just to be exact.

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There will also
be some nickel 56,

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which has the same
mass as iron 56,

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just has two fewer neutrons
and two more protons.

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So nickel 56 will
also form, can also

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be, it'll be like
a nickel-iron core.

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But that's about
how far a star can

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get, regardless of how massive
it is, at least by going

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through traditional
fusion, through

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the traditional
ignition mechanism.

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What I want to do
is leave you there

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just so you can think about
what might happen next,

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now that we can't fuse
this star anymore.

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And what we're actually going to
see is that it will supernova.