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So here we're talking about--

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you use the [? Meikle ?] model
to model a hot, transparent--

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and before yesterday, I said
I wasn't sure if it was opaque

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or not, but in this case, you
do need it to be not opaque.

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And few people had asked
about that yesterday

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in their written reflections.

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So it's a hot,
transparent, diffuse--

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which means a low-density--

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gas or plasma with
emission lines.

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So the [? Meikle ?]
model is used

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to predict what kind of
a spectrum we would get.

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So this is a spectrum from a
hot, transparent, diffuse gas

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or plasma with emission lines.

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Emission lines are
these extra peaks

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that we get above
the background which

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astronomers call the continuum.

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The continuum--
and what I didn't

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tell you yesterday is, where
does the continuum come from?

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Because you're right,
if we just had silicon

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and we heated it up,
we would just have--

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it would be 0, 0, 0, and then
there would be a peak here--

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0, 0, 0.

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And I think there's another
silicon line somewhere else,

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and it peak up, and it
would be 0, 0, 0, 0,

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if we just had pure silicon.

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But where does all the rest
of this background shape

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come from?

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Well, it actually comes from
if you have these atoms that

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have bound electrons
that are going

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a little orbits around
them, those electrons

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can be excited by
bumping into each other.

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They can be excited by
adding energy to the system.

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And they jump up to
higher energy levels,

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and then the electrons naturally
relax back down to a lower

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energy level and emit a photon.

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But if you have two atoms
that bump together too hard--

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remember, we talked about
ionization a couple of days

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ago.

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What is ionization?

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Anybody remember?

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Bianca?

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AUDIENCE: When atoms are
separated from each other.

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So the electron breaks off
or the protons and neutrons

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are broken from each other.

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PROFESSOR: OK.

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In this case, it's when
atoms are broken up.

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If you hit two atoms together
hard enough, instead of

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just exciting an electron
to a higher energy level,

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somebody actually asked
in the reflection, well,

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what's the highest energy level
that you can be excited to?

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If you have a really
big bump, you won't just

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excite the electron
to a higher orbit,

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you'll actually kick
the electron out.

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AUDIENCE: Whoops.

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PROFESSOR: Yeah.

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Imagine that, you get kicked out
of your nice, electron orbital

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home.

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So if you have collisions
that are too hard--

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and remember, we said in
a thermal gas, or in a gas

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where there's thermal motion,
some of the bounces are hard

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and some of the
bounces are soft.

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So sometimes, you just
bump those electrons up

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to high energy levels and
then they fall back down.

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Sometimes, if you
bump hard enough,

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you'll knock the electron out.

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And then what you
have is an ion,

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which may still have some
extra electrons around it.

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Because each atom doesn't
just have one electron,

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it has many electrons.

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But then you'll have ions
and you have free electrons.

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And Peter-- oh, before we go
on, Juan, you have a question.

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AUDIENCE: Yeah.

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What happens to the electron
that got kicked out?

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PROFESSOR: What happens to it?

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AUDIENCE: Yeah.

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PROFESSOR: That's
exactly what we're

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going to take a look at now.

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So the electron gets kicked out.

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So now, let's replay
the simulation.

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So can you stop and then
start from beginning.

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OK.

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So in this case, this
is a little simulation.

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The purple spheres
represent these ions.

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You should just be able to
hit play at the bottom again.

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There you go.

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So the purple spheres
represent ions.

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And then this
little yellow sphere

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represents the free electrons.

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And you'll see that they still
accelerate around each other.

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They still bounce around.

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And every time one of
them bounces around,

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just like in the
black body model,

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we get a photon that's produced.

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So now, instead of just
jumping up and down

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between specific levels,
they're all just kind of

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bouncing around
again, And remember,

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if we said we had
lots of bouncing,

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we get lots of different
energies of photons.

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If we turn the temperature up,
we get the bouncing harder,

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just like we had with
the black body model.

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Except now the difference
is, in the black model

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we had an opaque object,
so all those photons had

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to bounce around
inside of an object,

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and then they got
to the surface.

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Whereas in this case,
since it's a diffuse gas,

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we're actually just
getting those photons,

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and now those are photons of a
bunch of different energies--

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a continuous range of energies.

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All of those photons
come out and they

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make that background
shape that you see.

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In fact, this process is
called thermal bremsstrahlung.

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Bremsstrahlung is a German word
that means braking radiation.

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Braking, like when
you break in your car.

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So in each case here,
you've got these electrons

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that are braked, or
slowed down, or sped up

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as they go around another ion.

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And when they do that,
they emit a photon.

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And the photons are of
all different energies,

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because some of those
collisions are fast and hard,

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some those collisions are slow.

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Steve?

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AUDIENCE: Are they going through
the orbit and then jumping out?

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PROFESSOR: That's
a good question.

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Are they going
through the orbit--

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like through a
particular energy orbit,

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and then kind of jumping out?

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And the answer is, no.

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With electrons and
nuclei, you would

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have to have the
electrons actually

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get bound to the
object, and it would

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have to go into a closed orbit.

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In this case, it's just
getting close enough

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that the electromagnetic
field from the ion

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is causing the electron to
move in a different direction.

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So what we're having
is a mixture of stuff.

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We're having both this, which
is just all of these things

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are kind of moving around
randomly so they have

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00:07:13,630 --> 00:07:16,900
a bunch of different
energies, but then

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from inside of these atoms--

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this guy-- well,
I guess they don't

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collide in this simulation.

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But from the atoms themselves,
you get extra photons.

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And Peter, can we
go back to the--

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this.

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So the thermal bremsstrahlung
part of the [? Meikle ?] model

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produces just a nice
smooth curve like this.

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That's because you've got a
bunch of random collisions

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between free electrons and
ions, but at certain energies

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there's extra.

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There's extra photons
here, because there are

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those extra atoms of silicon--

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or I'm sorry, this
is calcium, there's

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extra atoms of calcium--

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that are being bounced
around, and those electrons

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are jumping up and down.

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And they're giving you
just a little bit more

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than what you would expect
from that nice smooth curve.

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So what I want you to do--

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and let's take like
two or three minutes--

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I want you to write
down, what did

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you learn from us doing this
little review this morning.

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So you guys made
this prediction,

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you looked at what the
intensities were here,

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and we learned a little bit more
about the [? Meikle ?] model.

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What did you learn
about the Meikle model?

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What do we remember about
what we just talked about?