Supermassive black holes

WEBVTT
Kind: captions
Language: en

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In the videos on massive
stars and on black holes,

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we learned that if the remnant
of a star, of a massive star,

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is massive enough, the
gravitational contraction,

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the gravitational
force, will be stronger

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than even the electron
degeneracy pressure,

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even stronger than the
neutron degeneracy pressure,

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even stronger than the
quark degeneracy pressure.

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And everything would
collapse into a point.

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And we called these
points black holes.

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And we learned there's
an event horizon

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around these black holes.

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And if anything
gets closer or goes

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within the boundary
of that event horizon,

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there's no way that it can never
escape from the black hole.

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All it can do is get closer
and closer to the black hole.

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And that includes light.

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And that's why it's
called a black hole.

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So even though
all of the mass is

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at the central point,
this entire area,

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or the entire surface
of the event horizon,

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this entire surface
of the event horizon--

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I'll do it in
purple because it's

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supposed to be black-- this
entire thing will appear black.

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It will emit no light.

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Now these type of black
holes that we described,

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we call those
stellar black holes.

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And that's because
they're formed

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from collapsing massive stars.

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And the largest stellar black
holes that we have observed

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are on the order of 33
solar masses, give or take.

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So very massive to begin
with, let's just be clear.

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And this is what the remnant
of the star has to be.

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So a lot more of the
original star's mass

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might have been pushed
off in supernovae.

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That's plural of supernova.

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Now there's another
class of black holes here

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and these are
somewhat mysterious.

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And they're called
supermassive black holes.

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And to some degree,
the word "super"

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isn't big enough,
supermassive black holes,

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because they're not just
a little bit more massive

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than stellar black holes.

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They're are a lot more massive.

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They're on the order of
hundreds of thousands

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to billions of solar masses,
hundred thousands to billions

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times the mass of our
Sun, solar masses.

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And what's interesting about
these, other than the fact

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that there are super
huge, is that there

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doesn't seem to be black
holes in between or at least

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we haven't observed
black holes in between.

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The largest stellar black
hole is 33 solar masses.

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And then there are these
supermassive black holes

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that we think exist.

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And we think they mainly exist
in the centers of galaxies.

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And we think most, if not
all, centers of galaxies

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actually have one of these
supermassive black holes.

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But it's kind of an
interesting question,

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if all black holes were
formed from collapsing stars,

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wouldn't we see
things in between?

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So one theory of how these
really massive black holes form

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is that you have a
regular stellar black hole

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in an area that
has a lot of matter

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that it can accrete around it.

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So I'll draw the-- this is
the event horizon around it.

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The actual black hole is going
to be in the center of it,

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or rather the mass
of the black hole

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will be in the center of it.

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And then over time, you have
just more and more mass just

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falling into this black hole.

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Just more and more
stuff just keeps

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falling into this black hole.

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And then it just keeps growing.

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And so this could be a plausible
reason, or at least the mass

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in the center keeps growing and
so the event horizon will also

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keep growing in radius.

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Now this is a
plausible explanation

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based on our current
understanding.

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But the reason why
this one doesn't

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gel that well is if
this was the explanation

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for supermassive
black holes, you

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expect to see more
black holes in between,

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maybe black holes
with 100 solar masses,

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or a 1,000 solar masses,
or 10,000 solar masses.

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But we're not seeing
those right now.

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We just see the
stellar black holes,

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and we see the
supermassive black holes.

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So another possible
explanation--

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my inclinations lean
towards this one

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because it kind of
explains the gap--

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is that these supermassive black
holes actually formed shortly

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after the Big Bang, that these
are primordial black holes.

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These started near the
beginning of our universe,

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primordial black holes.

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Now remember, what do you
need to have a black hole?

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You need to have an amazingly
dense amount of matter

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or a dense amount of mass.

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If you have a lot of mass
in a very small volume,

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then their
gravitational pull will

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pull them closer, and
closer, and closer together.

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And they'll be able
to overcome all

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of the electron
degeneracy pressures,

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and the neutron
degeneracy pressures,

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and the quark
degeneracy pressures,

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to really collapse into what
we think is a single point.

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I want to be clear here, too.

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We don't know it's
a single point.

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We've never gone into the
center of a black hole.

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Just the mathematics of the
black holes, or at least

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as we understand it right
now, have everything colliding

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into a single point where the
math starts to break down.

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So we're really not
sure what happens

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at that very small center point.

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But needless to say, it will
be an unbelievably, maybe

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infinite, maybe
almost infinitely,

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dense point in space, or
dense amount of matter.

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And the reason why
I kind of favor

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this primordial black hole
and why this would make sense

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is right after the formation
of the universe, all

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of the matter in the universe
was in a much denser space

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because the universe
was smaller.

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So let's say that this
is right after the Big

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Bang, some period of
time after the Big Bang.

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Now what we've talked
about before when

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we talked about cosmic
background is that

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at that point, the universe
was relatively uniform.

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It was super, super dense but
it was relatively uniform.

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So a universe like
this, there's no reason

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why anything would
collapse into black holes.

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Because if you look
at a point here,

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sure, there's a ton of
mass very close to it.

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But it's very close to
it in every direction.

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So the gravitational force would
be the same in every direction

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if it was completely uniform.

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But if you go shortly after
the Big Bang, maybe because

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of slight quantum
fluctuation effects,

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it becomes slightly nonuniform.

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So let's say it becomes
slightly nonuniform,

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but it still is
unbelievably dense.

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So let's say it looks something
like this, where you have areas

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that are denser, but
it's slightly nonuniform,

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but extremely dense.

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So here, all of a
sudden, you have the type

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of densities necessary
for a black hole.

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And where you have
higher densities,

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where it's less uniform,
here, all of a sudden,

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you will have inward force.

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The gravitational pull from
things outside of this area

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are going to be less than
the gravitational pull

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towards those areas.

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And the more things get pulled
towards it, the less uniform

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it's going to get.

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So you could imagine in
that primordial universe,

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that very shortly
after the Big Bang

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when things were very dense
and closely packed together,

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we may have had the
conditions where

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these supermassive black
holes could have formed.

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Where we had so much mass
in such a small volume,

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and it was just
not uniform enough,

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so that you could kind of
have this snowballing effect,

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so that more and more
mass would collect

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into these supermassive
black holes that

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are hundreds of thousands to
billions of times the mass

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of the Sun.

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And, this is maybe even
the more interesting part,

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those black holes would become
the centers of future galaxies.

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So you have these
black holes forming,

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these supermassive
black holes forming.

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And not everything would
go into a black hole.

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Only if it didn't have a
lot of angular velocity,

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then it might go
into the black hole.

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But if it's going
pass it fast enough,

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it'll just start going in
orbit around the black hole.

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And so you could
imagine that this

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is how the early galaxies
or even our galaxy formed.

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And so you might
be wondering, well,

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what about the black hole at
the center of the Milky Way?

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And we think there is one.

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We think there is
one because we've

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observed stars orbiting very
quickly around something

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at the center of the
universe-- sorry,

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at the center of our Milky Way.

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I want to be very clear, not
at the center of the universe.

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And the only
plausible explanation

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for it orbiting so
quickly around something

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is that it has to have a
density of either a black hole

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or something that
will eventually

00:08:34.690 --> 00:08:35.820
turn into a black hole.

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And when you do the
math for the middle

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of our galaxy, the
center of the Milky Way,

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our supermassive black hole is
on the order of 4 million times

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the mass of the Sun.

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So hopefully that gives you a
little bit of food for thought.

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There aren't just only
stellar collapsed black holes.

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Or maybe there are
and somehow they

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grow into supermassive
black holes

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and that everything in
between we just can't observe.

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Or that they really are a
different class of black holes.

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They're actually
formed different ways.

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Maybe they formed
near the beginning

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of the actual universe.

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When the density of things
was a little uniform,

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things condensed
into each other.

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And what we're going to
talk about in the next video

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is how these supermassive
black holes can help generate

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unbelievable sources
of radiation,

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even though the black
holes themselves

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aren't emitting them.

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And those are going
to be quasars.


Why gravity gets so strong near dense objects

WEBVTT
Kind: captions
Language: en

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In the video on black
hole several people

00:00:03.130 --> 00:00:05.620
asked what is actually a
pretty good question, which

00:00:05.620 --> 00:00:10.610
is if the mass of say a black
hole is only two or three

00:00:10.610 --> 00:00:13.630
solar masses, why is
the gravity so strong?

00:00:13.630 --> 00:00:16.090
Obviously the sun's
gravity isn't so strong

00:00:16.090 --> 00:00:18.700
that it keeps light from
escaping, so why would

00:00:18.700 --> 00:00:21.580
something, or even a star that's
two or three solar masses--

00:00:21.580 --> 00:00:24.480
its gravity isn't so strong that
it keeps light from escaping.

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Why would a black hole that has
the same mass, why would that

00:00:28.480 --> 00:00:30.160
keep light from escaping?

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And to understand
that, I'll just

00:00:33.410 --> 00:00:36.997
do Newtonian classical
physics right here.

00:00:36.997 --> 00:00:39.330
I won't get into the whole
general relativity of things.

00:00:39.330 --> 00:00:40.704
And this really
will just give us

00:00:40.704 --> 00:00:45.140
the intuition of why a smaller,
denser thing of the same mass

00:00:45.140 --> 00:00:48.520
can exert a stronger
gravitational pull.

00:00:48.520 --> 00:00:51.870
So let's take two examples.

00:00:51.870 --> 00:01:02.470
Let's say I have some star
here that has a mass m1.

00:01:02.470 --> 00:01:10.100
And let's say that its radius,
let's just call this r.

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And let's say that I have
some other mass right

00:01:15.470 --> 00:01:17.290
at the surface of
the star, somehow

00:01:17.290 --> 00:01:19.720
able to survive those
surface temperatures.

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And this mass over
here has a mass of m2.

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The universal law of
gravitation tells us

00:01:30.700 --> 00:01:35.370
that the force between
these two masses

00:01:35.370 --> 00:01:38.840
is going to be equal to the
gravitational constant times

00:01:38.840 --> 00:01:41.350
the product of the masses.

00:01:41.350 --> 00:01:53.190
So m1 times m2, all of that over
the square of the distance r

00:01:53.190 --> 00:01:53.750
squared.

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Now, let me be very clear.

00:01:54.833 --> 00:01:57.930
You might say, wait, this
magenta mass right here

00:01:57.930 --> 00:01:59.650
is touching this larger mass.

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Isn't the distance 0?

00:02:00.910 --> 00:02:02.410
And you have to be very careful.

00:02:02.410 --> 00:02:06.220
This is the distance between
their center of masses.

00:02:06.220 --> 00:02:08.830
So the center of mass of
this large mass over here

00:02:08.830 --> 00:02:13.870
is r away from this mass
that's on the surface.

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Now, with that said, let's
take another example.

00:02:17.070 --> 00:02:20.430
Let's say that this large
massive star, or whatever

00:02:20.430 --> 00:02:22.770
it might be,
eventually condenses

00:02:22.770 --> 00:02:26.380
into something
1,000 times smaller.

00:02:26.380 --> 00:02:28.160
So let me draw it like this.

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And obviously I'm not
drawing it to scale.

00:02:30.860 --> 00:02:34.540
So let's say we have
another case like this.

00:02:34.540 --> 00:02:36.570
And I'm not drawing it to scale.

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So this object, maybe
it's the same object

00:02:38.537 --> 00:02:40.120
or maybe it's a
different object, that

00:02:40.120 --> 00:02:43.170
has the exact same mass
as this larger object,

00:02:43.170 --> 00:02:44.840
but now it has a
much smaller radius.

00:02:48.500 --> 00:02:57.220
Now the radius is 1/1,000
of this radius over here.

00:02:57.220 --> 00:03:00.780
So maybe I'll just
call it r/1,000.

00:03:00.780 --> 00:03:04.620
So if this had a million
kilometer radius,

00:03:04.620 --> 00:03:07.690
so that would make it roughly
about twice the radius

00:03:07.690 --> 00:03:10.760
of the sun, if this was a
million kilometer radius right

00:03:10.760 --> 00:03:14.040
over here, this would be
1,000 kilometer radius.

00:03:14.040 --> 00:03:16.760
So maybe we're talking
about something

00:03:16.760 --> 00:03:18.180
that's approaching
a neutron star.

00:03:18.180 --> 00:03:20.040
But we don't have to think
about what it actually is.

00:03:20.040 --> 00:03:22.164
Let's just think about the
thought experiment here.

00:03:22.164 --> 00:03:23.940
So let's say I have
this thing over here.

00:03:23.940 --> 00:03:26.810
And let's say I have something
on the surface of this.

00:03:26.810 --> 00:03:29.380
So let's say I have
that same mass that's

00:03:29.380 --> 00:03:31.590
on the surface of this thing.

00:03:31.590 --> 00:03:34.970
So this is m2 right over here.

00:03:34.970 --> 00:03:39.240
So what is going to be the
force between these two masses?

00:03:39.240 --> 00:03:41.630
How strong are
they going to want

00:03:41.630 --> 00:03:44.600
to-- What's the force
pulling them together?

00:03:44.600 --> 00:03:48.870
So let's just do the universal
law of gravitation again.

00:03:48.870 --> 00:03:52.040
Let's just call this force one,
and let's call this force two.

00:03:52.040 --> 00:03:54.610
Once again, it's going to be
the gravitational constant times

00:03:54.610 --> 00:03:56.790
the product of their masses.

00:03:56.790 --> 00:04:02.690
So the big m1 times
the smaller mass, m2,

00:04:02.690 --> 00:04:06.540
all of that over this distance
squared, this radius squared.

00:04:06.540 --> 00:04:08.880
Remember, it's the distance
to the center of masses.

00:04:08.880 --> 00:04:11.020
This center of mass here,
we're considering m2

00:04:11.020 --> 00:04:13.430
to kind of be just a point
mass right over there.

00:04:13.430 --> 00:04:15.240
So what's the radius squared?

00:04:15.240 --> 00:04:22.070
It's going to be
r/1,000 squared.

00:04:22.070 --> 00:04:24.460
Or if we simplify this
what will this be?

00:04:24.460 --> 00:04:27.920
This is the same thing.

00:04:27.920 --> 00:04:29.550
And I'll just write
it in one color,

00:04:29.550 --> 00:04:31.420
just because it takes less time.

00:04:31.420 --> 00:04:36.570
Gravitational
constant m1 m2 over r

00:04:36.570 --> 00:04:41.030
squared over 1,000
squared, or over 1 million.

00:04:44.820 --> 00:04:46.350
That's just 1,000 squared.

00:04:46.350 --> 00:04:49.230
Or we can multiply the
numerator and the denominator

00:04:49.230 --> 00:04:55.270
by 1 million, and this is going
to be equal to 1 million--

00:04:55.270 --> 00:04:56.945
I'm going to write
it out, 1 million.

00:04:56.945 --> 00:04:59.220
Let me scroll to the
right a little bit--

00:04:59.220 --> 00:05:03.980
times the gravitational
constant, times m1 m2,

00:05:03.980 --> 00:05:05.990
all of that over r squared.

00:05:05.990 --> 00:05:09.420
Now, what is this
thing right over here?

00:05:09.420 --> 00:05:11.530
That's the same
thing as this F1.

00:05:11.530 --> 00:05:18.620
So this is going to
be 1 million times F1.

00:05:18.620 --> 00:05:21.460
So even though the masses
involved are the same,

00:05:21.460 --> 00:05:24.590
this yellow object right
here is the same mass

00:05:24.590 --> 00:05:26.930
as this larger object over here.

00:05:26.930 --> 00:05:31.280
It's able to exert a million
times the gravitational force

00:05:31.280 --> 00:05:32.170
on this point mass.

00:05:32.170 --> 00:05:33.170
And actually vice versa.

00:05:33.170 --> 00:05:34.550
They're both being attracted.

00:05:34.550 --> 00:05:37.940
They're both exerting
this on each other.

00:05:37.940 --> 00:05:40.670
And the reality is, because
this thing is smaller,

00:05:40.670 --> 00:05:44.420
because this m1 on the right
here, this one I'm coloring in,

00:05:44.420 --> 00:05:46.880
because this one is
smaller and denser,

00:05:46.880 --> 00:05:50.800
this particle is able to get
closer to its center of mass.

00:05:50.800 --> 00:05:53.940
Now, you might be saying,
OK, well, I can buy that.

00:05:53.940 --> 00:05:56.030
This just comes straight
from the universal law

00:05:56.030 --> 00:05:56.900
of gravitation.

00:05:56.900 --> 00:05:59.356
But wouldn't something
closer to this center of mass

00:05:59.356 --> 00:06:00.480
experience that same thing?

00:06:00.480 --> 00:06:06.420
If this was a star, wouldn't
photons that are over here,

00:06:06.420 --> 00:06:09.280
wouldn't this experience
the same force?

00:06:09.280 --> 00:06:16.180
If this distance right here is
r/1,000 wouldn't some photon

00:06:16.180 --> 00:06:20.710
here, or atom here, or molecule,
or whatever it's over here,

00:06:20.710 --> 00:06:22.610
wouldn't that experience
the same force,

00:06:22.610 --> 00:06:25.150
this million times the
force as this thing?

00:06:25.150 --> 00:06:27.200
And you've got to
remember, all of a sudden

00:06:27.200 --> 00:06:29.970
when this thing is inside
of this larger mass, what's

00:06:29.970 --> 00:06:31.960
happening?

00:06:31.960 --> 00:06:35.210
The entire mass is no
longer pulling on it

00:06:35.210 --> 00:06:36.720
in that direction.

00:06:36.720 --> 00:06:39.340
It's no longer pulling it
in that inward direction.

00:06:39.340 --> 00:06:43.760
You now have all of
this mass over here.

00:06:43.760 --> 00:06:45.780
Let me think of the best
way that's doing it.

00:06:45.780 --> 00:06:48.890
So you can think of it
all of this mass over here

00:06:48.890 --> 00:06:50.720
is pulling it in an
outward direction.

00:06:53.490 --> 00:06:55.080
It's not telling.

00:06:55.080 --> 00:06:56.640
What that mass out
there is doing,

00:06:56.640 --> 00:07:02.480
since that mass itself
is being pulled inward,

00:07:02.480 --> 00:07:04.650
it is pushing down on this.

00:07:04.650 --> 00:07:06.930
It is exerting
pressure on that point.

00:07:06.930 --> 00:07:09.600
But the actual
gravitational force

00:07:09.600 --> 00:07:12.630
that that point is experiencing
is actually going to be less.

00:07:12.630 --> 00:07:14.980
It's actually going to
be mitigated by the fact

00:07:14.980 --> 00:07:19.490
that there's so
much mass over here

00:07:19.490 --> 00:07:22.061
pulling in the other direction.

00:07:22.061 --> 00:07:23.560
And so you could
imagine if you were

00:07:23.560 --> 00:07:29.080
in the center of a
really massive object--

00:07:29.080 --> 00:07:30.610
so that's a really
massive object.

00:07:30.610 --> 00:07:32.400
If you were in the
center, there would

00:07:32.400 --> 00:07:35.610
be no net gravitational
force being pulled on you,

00:07:35.610 --> 00:07:37.860
because you're at
its center of mass.

00:07:37.860 --> 00:07:40.170
The rest of the mass is outward.

00:07:40.170 --> 00:07:44.510
So at every point it will
be pulling you outward.

00:07:44.510 --> 00:07:47.220
And so that's why if you were
to enter the core of a star,

00:07:47.220 --> 00:07:49.470
if you were to get a lot
closer to its center of mass,

00:07:49.470 --> 00:07:52.780
it's not going to be pulling
on you with this type of force.

00:07:52.780 --> 00:07:55.460
And the only way you can
get these types of forces

00:07:55.460 --> 00:08:00.010
is if the entire mass
is contained in a very

00:08:00.010 --> 00:08:02.890
dense region, in a
very small region.

00:08:02.890 --> 00:08:05.180
And that's why a black
hole is able to exert

00:08:05.180 --> 00:08:09.480
such strong gravity that
not even light can escape.

00:08:09.480 --> 00:08:12.730
Hopefully that clarifies
things a little bit.


Why cepheids pulsate

WEBVTT
Kind: captions
Language: en

00:00:00.624 --> 00:00:01.639
- [Voiceover] In the last video we learned

00:00:01.639 --> 00:00:06.516
that there are a class of
stars called Cepheid variables.

00:00:06.516 --> 00:00:08.101
And these are the super giant stars,

00:00:08.101 --> 00:00:10.641
as much as 30000 times
as bright as the sun.

00:00:10.641 --> 00:00:14.480
A mass, as much as 20
times the mass of the sun.

00:00:14.480 --> 00:00:16.086
And what's neat about them is, one,

00:00:16.086 --> 00:00:17.690
because they're so large and so bright,

00:00:17.690 --> 00:00:19.560
you can see them really really far away.

00:00:19.560 --> 00:00:21.286
And what's even neater about them

00:00:21.286 --> 00:00:23.643
is that they're variable,
that they pulsate.

00:00:23.643 --> 00:00:26.304
And because their pulsations are related

00:00:26.304 --> 00:00:28.052
to their actual luminosity,

00:00:28.052 --> 00:00:30.632
you know if you see a
cepheid variable star

00:00:30.632 --> 00:00:32.075
in some distant galaxy,

00:00:32.075 --> 00:00:35.143
you know what it's luminosity actually is

00:00:35.143 --> 00:00:37.012
if you were kind of at the star,

00:00:37.012 --> 00:00:38.821
because you know you can see

00:00:38.821 --> 00:00:41.299
how it's period of pulsation.

00:00:41.299 --> 00:00:43.127
And so if you know it's actual luminosity,

00:00:43.127 --> 00:00:45.586
and you know it's obviously
apparent luminosity,

00:00:45.586 --> 00:00:47.861
you know how much it's gotten dimmed.

00:00:47.861 --> 00:00:49.345
And the more dim it's gotten

00:00:49.345 --> 00:00:50.421
from its actual state,

00:00:50.421 --> 00:00:52.310
you know the farther away it is.

00:00:52.310 --> 00:00:53.408
So that's the real value of them.

00:00:53.408 --> 00:00:55.094
What I want to do in this video

00:00:55.094 --> 00:00:58.161
is to try to explain
why they're variables.

00:00:58.161 --> 00:00:59.625
Why they pulsate.

00:00:59.625 --> 00:01:02.165
And to do that, to do that,

00:01:02.165 --> 00:01:03.423
what we're gonna think about

00:01:03.423 --> 00:01:07.833
is doubly and singly ionized helium.

00:01:07.833 --> 00:01:09.478
And just to review, helium,

00:01:09.478 --> 00:01:12.830
so neutral helium, let
me draw a neutral helium,

00:01:12.830 --> 00:01:15.675
neutral helium's got two protons,

00:01:15.675 --> 00:01:18.803
it's got two protons, two neutrons,

00:01:18.803 --> 00:01:20.835
two neutrons, and then two electrons

00:01:20.835 --> 00:01:22.928
and obviously this is not drawn to scale.

00:01:22.928 --> 00:01:26.179
So this is neutral helium right over here.

00:01:26.179 --> 00:01:28.271
Now, if you singly ionize helium

00:01:28.271 --> 00:01:30.323
you knock off one of these electrons.

00:01:30.323 --> 00:01:31.706
And these type of things happen in stars

00:01:31.706 --> 00:01:32.741
when you have a lot of heat,

00:01:32.741 --> 00:01:34.570
easier to ionize things.

00:01:34.570 --> 00:01:36.114
So singly ionized helium

00:01:36.114 --> 00:01:37.658
will look like this.

00:01:37.658 --> 00:01:39.872
It'll have the same nucleus,

00:01:39.872 --> 00:01:41.722
two protons, two neutrons.

00:01:41.722 --> 00:01:43.551
One of the electrons gets knocked off

00:01:43.551 --> 00:01:45.318
so now you only have one electron.

00:01:45.318 --> 00:01:48.020
And now you have a net positive charge.

00:01:48.020 --> 00:01:51.372
So here, let me do this
in a different color,

00:01:51.372 --> 00:01:53.606
this helium now has a net charge,

00:01:53.606 --> 00:01:54.846
we could write one plus here,

00:01:54.846 --> 00:01:55.882
but if you just write a plus

00:01:55.882 --> 00:01:58.848
you implicitly mean a
positive charge of one.

00:01:58.848 --> 00:02:00.678
Now you can also double the ionized helium

00:02:00.678 --> 00:02:03.360
if the environment is hot enough.

00:02:03.360 --> 00:02:05.615
You can doubly ionize helium

00:02:05.615 --> 00:02:07.198
and doubly ionizing helium is essentially

00:02:07.198 --> 00:02:09.658
knocking off both of the electrons.

00:02:09.658 --> 00:02:12.684
So then it's really just a helium nucleus.

00:02:12.684 --> 00:02:15.915
It's really just a
helium nucleus like this.

00:02:15.915 --> 00:02:18.760
This right here is doubly, doubly

00:02:18.760 --> 00:02:20.100
ionized helium.

00:02:20.100 --> 00:02:22.559
Now I just said in order to do this

00:02:22.559 --> 00:02:24.875
you have to have a hotter environment.

00:02:24.875 --> 00:02:26.662
There has to be a hotter environment

00:02:26.662 --> 00:02:27.638
in order to be able to knock off

00:02:27.638 --> 00:02:30.097
both these, this electron
really doesn't want to leave,

00:02:30.097 --> 00:02:31.478
to take an electron off of something

00:02:31.478 --> 00:02:33.387
that's already positive is difficult.

00:02:33.387 --> 00:02:37.105
You have to have a lot of
really pressure and temperature.

00:02:37.105 --> 00:02:38.426
This is cooler.

00:02:38.426 --> 00:02:39.970
And this is all relative,

00:02:39.970 --> 00:02:42.612
we're talking about the insides of stars.

00:02:42.612 --> 00:02:43.586
So, you know, it's hot,

00:02:43.586 --> 00:02:45.192
this is a hotter part of the star

00:02:45.192 --> 00:02:47.203
versus a cooler part of the star

00:02:47.203 --> 00:02:48.910
I guess is a way you think about this,

00:02:48.910 --> 00:02:49.905
it's still a very hot environment

00:02:49.905 --> 00:02:52.952
by our traditional, every day standards.

00:02:52.952 --> 00:02:56.306
Now the other thing about
the doubly ionized helium

00:02:56.306 --> 00:02:58.784
is that it is more opaque.

00:02:58.784 --> 00:03:01.121
It is more opaque,

00:03:01.121 --> 00:03:03.416
which means it doesn't transmit,

00:03:03.416 --> 00:03:04.636
it doesn't allow light to go through it,

00:03:04.636 --> 00:03:06.281
it actually absorbs light.

00:03:06.281 --> 00:03:07.663
It is more opaque,

00:03:07.663 --> 00:03:09.857
it absorbs light.

00:03:09.857 --> 00:03:12.315
It absorbs light.

00:03:12.315 --> 00:03:12.823
Or in another way,

00:03:12.823 --> 00:03:15.038
it absorbs that light energy

00:03:15.038 --> 00:03:16.038
that energy will make it even hotter.

00:03:16.038 --> 00:03:17.948
So that's just something to think about.

00:03:17.948 --> 00:03:20.441
Now, the singly ionized helium

00:03:20.441 --> 00:03:22.575
is more transparent.

00:03:22.575 --> 00:03:25.439
This is more transparent.

00:03:25.439 --> 00:03:26.455
More transparent,

00:03:26.455 --> 00:03:28.731
it allows the light to pass through it.

00:03:28.731 --> 00:03:31.087
So it doesn't get heated as much

00:03:31.087 --> 00:03:33.790
by photons that are kind of going

00:03:33.790 --> 00:03:35.740
near it, or through it, or whatever.

00:03:35.740 --> 00:03:37.040
It allows them to go through it here

00:03:37.040 --> 00:03:38.848
the photons are going to actually heat up,

00:03:38.848 --> 00:03:40.169
heat up the ion.

00:03:40.169 --> 00:03:42.749
So let's think about how this might cause

00:03:42.749 --> 00:03:45.167
cepheid variable to pulsate.

00:03:45.167 --> 00:03:46.935
So assuming that cepheid variables

00:03:46.935 --> 00:03:50.470
have a large enough
quantity, I should say,

00:03:50.470 --> 00:03:52.075
of these ions,

00:03:52.075 --> 00:03:56.646
we can imagine that when
a cepheid variable is dim,

00:03:56.646 --> 00:03:59.105
so let me draw a dim cepheid variable,

00:03:59.105 --> 00:04:04.105
so I'll draw that like, I'll
draw this in a dim color

00:04:04.245 --> 00:04:07.658
so this is a dim cepheid
variable right here.

00:04:07.658 --> 00:04:09.893
It's dim state, and it's dim state

00:04:09.893 --> 00:04:10.766
just like this,

00:04:10.766 --> 00:04:15.259
you have a lot of the
doubly ionized helium,

00:04:15.259 --> 00:04:17.918
you have a lot of doubly ionized helium

00:04:17.918 --> 00:04:18.853
in the star,

00:04:18.853 --> 00:04:21.819
at least kind of the
outer surface of the star.

00:04:21.819 --> 00:04:23.851
Doubly ionized helium.

00:04:23.851 --> 00:04:25.313
And so this does not allow

00:04:25.313 --> 00:04:27.244
a lot of light to pass through.

00:04:27.244 --> 00:04:28.889
So this is the dim part of

00:04:28.889 --> 00:04:32.038
the pulsation of the cepheid variable.

00:04:32.038 --> 00:04:36.793
Now because this doubly
ionized helium is opaque

00:04:36.793 --> 00:04:38.540
it is absorbing the light,

00:04:38.540 --> 00:04:40.165
it is getting heated.

00:04:40.165 --> 00:04:43.538
It is getting heated.

00:04:43.538 --> 00:04:45.956
It is getting heated.

00:04:45.956 --> 00:04:47.337
And because it's getting heated,

00:04:47.337 --> 00:04:49.470
it'll cause the star to expand.

00:04:49.470 --> 00:04:51.014
So because it's getting heated

00:04:51.014 --> 00:04:52.782
it'll become more energetic

00:04:52.782 --> 00:04:55.687
and the star will actually expand.

00:04:55.687 --> 00:04:58.938
The star will actually expand.

00:04:58.938 --> 00:05:00.198
Now, as the star expands,

00:05:00.198 --> 00:05:02.047
because this doubly ionized helium

00:05:02.047 --> 00:05:03.855
is getting heated, what's going to happen,

00:05:03.855 --> 00:05:05.135
the further away you are from

00:05:05.135 --> 00:05:06.252
the core of the star,

00:05:06.252 --> 00:05:07.817
the cooler it gets.

00:05:07.817 --> 00:05:09.564
So this expanded because it was getting

00:05:09.564 --> 00:05:10.377
heated,

00:05:10.377 --> 00:05:11.616
but then because it expanded

00:05:11.616 --> 00:05:14.785
the outer layers of
the star become cooler.

00:05:14.785 --> 00:05:16.431
And since they're cooler,

00:05:16.431 --> 00:05:18.381
helium won't be doubly ionized anymore,

00:05:18.381 --> 00:05:20.088
it'll start to get a couple of,

00:05:20.088 --> 00:05:21.835
it'll get an electron from,

00:05:21.835 --> 00:05:23.745
each helium atom can now get an electron

00:05:23.745 --> 00:05:25.817
from the plasma, I guess you can say,

00:05:25.817 --> 00:05:27.565
to become singly ionized helium.

00:05:27.565 --> 00:05:31.709
So now we have singly ionized helium.

00:05:31.709 --> 00:05:34.310
We have singly ionized helium.

00:05:34.310 --> 00:05:36.707
And now the star is going
to be more transparent,

00:05:36.707 --> 00:05:39.369
it's going to allow more
light to pass through it.

00:05:39.369 --> 00:05:40.852
So now this is the bright part

00:05:40.852 --> 00:05:43.412
of the pulsation.

00:05:43.412 --> 00:05:45.688
It's going to allow more light through,

00:05:45.688 --> 00:05:47.902
so now it is bright.

00:05:47.902 --> 00:05:49.121
The star is bright.

00:05:49.121 --> 00:05:50.381
But what's happening now?

00:05:50.381 --> 00:05:52.250
Because the light is no longer

00:05:52.250 --> 00:05:53.896
or it's not being absorbed as well

00:05:53.896 --> 00:05:55.216
by the helium

00:05:55.216 --> 00:05:57.695
when it was a doubly ionized helium,

00:05:57.695 --> 00:05:58.975
now it's letting most of the light,

00:05:58.975 --> 00:06:00.539
or a lot more of the light,

00:06:00.539 --> 00:06:01.494
get through

00:06:01.494 --> 00:06:03.445
it's not going to get heated as much.

00:06:03.445 --> 00:06:05.050
And so it won't have the kinetic energy

00:06:05.050 --> 00:06:07.041
to kind of keep pushing out,

00:06:07.041 --> 00:06:08.321
to keep moving outward,

00:06:08.321 --> 00:06:10.881
and so it'll collapse back into the star.

00:06:10.881 --> 00:06:12.608
And so then this will cool down

00:06:12.608 --> 00:06:14.599
and collapse back in.

00:06:14.599 --> 00:06:16.041
And when it collapses back in

00:06:16.041 --> 00:06:17.849
what's going to happen?

00:06:17.849 --> 00:06:20.897
When it collapses back in,

00:06:20.897 --> 00:06:22.827
when these helium atoms get closer

00:06:22.827 --> 00:06:24.554
to the center of the star,

00:06:24.554 --> 00:06:25.854
to the core of the star,

00:06:25.854 --> 00:06:27.683
they're going to be heated again,

00:06:27.683 --> 00:06:29.328
because they're closer now to the core,

00:06:29.328 --> 00:06:30.365
and when they get heated

00:06:30.365 --> 00:06:32.227
they're going to become,

00:06:32.227 --> 00:06:34.651
they're going to become doubly ionized.

00:06:34.651 --> 00:06:38.288
So then we have doubly
ionized helium, again.

00:06:38.288 --> 00:06:39.304
Doubly ionized,

00:06:39.304 --> 00:06:40.503
and then the cycle will go again,

00:06:40.503 --> 00:06:41.864
it is now opaque,

00:06:41.864 --> 00:06:43.754
it will now absorb more energy,

00:06:43.754 --> 00:06:45.846
that'll cause it to
have more kinetic energy

00:06:45.846 --> 00:06:47.146
to expand,

00:06:47.146 --> 00:06:49.117
once it expands, it'll get cool again,

00:06:49.117 --> 00:06:51.149
and transparent and bright.

00:06:51.149 --> 00:06:53.729
And so this is the current best theory

00:06:53.729 --> 00:06:55.842
of why cepheid variable stars

00:06:55.842 --> 00:06:58.504
are variable to begin with.

00:06:58.504 --> 00:06:59.641
It's this whole notion of having

00:06:59.641 --> 00:07:01.754
the doubly ionized helium

00:07:01.754 --> 00:07:03.461
versus the singly ionized helium

00:07:03.461 --> 00:07:08.461
in kind of the outer
layers of the star itself.


Cepheid variables 1

WEBVTT
Kind: captions
Language: en

00:00:00.730 --> 00:00:09.210
This right here is a picture
of Henrietta Swan Leavitt.

00:00:09.210 --> 00:00:12.330
And she made, a little
over 100 years ago-- this

00:00:12.330 --> 00:00:16.420
is in the early 1900s, while
working for Edward Charles

00:00:16.420 --> 00:00:19.430
Pickering, who was a Harvard
astronomer, while working

00:00:19.430 --> 00:00:22.770
for his observatory, she
made what is arguably--

00:00:22.770 --> 00:00:26.130
well, definitely, one of the
most important discoveries

00:00:26.130 --> 00:00:28.380
in all of astronomy.

00:00:28.380 --> 00:00:31.840
And I would say it ranks their
top three, because it really

00:00:31.840 --> 00:00:33.820
enabled people like
Hubble to start

00:00:33.820 --> 00:00:35.730
realizing that the
universe is expanding

00:00:35.730 --> 00:00:38.920
or even being able to think
about how to measure distances

00:00:38.920 --> 00:00:42.840
to objects in space well
beyond the reach of our tools

00:00:42.840 --> 00:00:43.500
with parallax.

00:00:43.500 --> 00:00:46.370
We saw with parallax, you have
to have extremely sensitive

00:00:46.370 --> 00:00:48.880
instruments just to
even measure distances

00:00:48.880 --> 00:00:50.700
to stars relatively close to us.

00:00:50.700 --> 00:00:52.810
Very sensitive
instruments to get to

00:00:52.810 --> 00:00:55.390
stars maybe further
out into our galaxy.

00:00:55.390 --> 00:00:57.840
And we don't have the
instruments, even today,

00:00:57.840 --> 00:00:59.530
to measure things
beyond our galaxy.

00:00:59.530 --> 00:01:02.730
But because of
Henrietta Swan Leavitt,

00:01:02.730 --> 00:01:05.680
we were able to
approximate or get

00:01:05.680 --> 00:01:08.830
good senses of the distance
to objects beyond our galaxy.

00:01:08.830 --> 00:01:11.090
So let's just think
about what she did.

00:01:11.090 --> 00:01:15.360
So her job was literally
to classify stars

00:01:15.360 --> 00:01:20.640
in the Large Magellanic, I have
trouble saying that, Magellanic

00:01:20.640 --> 00:01:24.429
Cloud and the Small
Magellanic Clouds.

00:01:24.429 --> 00:01:26.970
And this is what they look like
from the Southern Hemisphere.

00:01:26.970 --> 00:01:29.630
This is the large,
right over here.

00:01:29.630 --> 00:01:32.950
And that this is the
small, right over here.

00:01:32.950 --> 00:01:36.840
And remember, this is before
Hubble realized that there--

00:01:36.840 --> 00:01:41.650
or showed the world-- that there
are stars beyond our galaxy,

00:01:41.650 --> 00:01:43.530
or that there are galaxies
beyond our galaxy.

00:01:43.530 --> 00:01:45.613
So at this point in time,
people didn't even fully

00:01:45.613 --> 00:01:47.560
appreciate that these
were separate galaxies.

00:01:47.560 --> 00:01:49.700
We just said, hey, these
are kind of these blobs

00:01:49.700 --> 00:01:51.810
or these clusters
of stars that we

00:01:51.810 --> 00:01:53.469
see in the Southern Hemisphere.

00:01:53.469 --> 00:01:55.010
And just to get a
sense of where they

00:01:55.010 --> 00:01:58.110
are relative to our galaxy,
the Milky Way Galaxy--

00:01:58.110 --> 00:02:00.020
this is obviously not
an actual picture.

00:02:00.020 --> 00:02:02.300
We can't take a picture
from this vantage point.

00:02:02.300 --> 00:02:04.050
This would have to be
very, very far away.

00:02:04.050 --> 00:02:06.220
But this is the
Milky Way right here.

00:02:06.220 --> 00:02:08.970
And this is the Small
Magellanic Cloud.

00:02:08.970 --> 00:02:11.640
And this is the Large
Magellanic Cloud.

00:02:11.640 --> 00:02:13.870
I'm getting better at saying it.

00:02:13.870 --> 00:02:20.070
So her job was literally just
to classify the different stars

00:02:20.070 --> 00:02:20.620
that she saw.

00:02:20.620 --> 00:02:22.140
But while she was
classifying, she

00:02:22.140 --> 00:02:23.879
looked at these things
called variables.

00:02:23.879 --> 00:02:25.420
And it turns out
what she was looking

00:02:25.420 --> 00:02:29.490
at were a class of stars
called Cepheid or Cepheid,

00:02:29.490 --> 00:02:34.470
Cepheid variable stars.

00:02:34.470 --> 00:02:37.360
And what's interesting
about them is two things.

00:02:37.360 --> 00:02:39.230
They're super-duper bright.

00:02:39.230 --> 00:02:42.650
They're up to 30,000 times
as luminous as the sun.

00:02:42.650 --> 00:02:45.330
And they're 5 to 20
times more massive

00:02:45.330 --> 00:02:50.720
than the sun, 5 to 20
times the sun's mass.

00:02:50.720 --> 00:02:53.420
But what makes them interesting
is one, they're really bright.

00:02:53.420 --> 00:02:55.190
So you can see them
from really far away.

00:02:55.190 --> 00:02:57.700
You can see these
Cepheid variable stars

00:02:57.700 --> 00:02:59.060
in other galaxies.

00:02:59.060 --> 00:03:01.980
In fact, we can see it
well beyond even the Small

00:03:01.980 --> 00:03:04.120
Magellanic Cloud or the
Large Magellanic Cloud.

00:03:04.120 --> 00:03:06.629
But you could see these
stars in other galaxies.

00:03:06.629 --> 00:03:08.420
And what's even more
interesting about them

00:03:08.420 --> 00:03:11.000
is that their
intensity is variable.

00:03:11.000 --> 00:03:13.010
That they become
brighter and dimmer

00:03:13.010 --> 00:03:15.290
with a well-defined period.

00:03:15.290 --> 00:03:17.790
So if you're looking at
a Cepheid variable star--

00:03:17.790 --> 00:03:20.340
and this is just kind of
a simulation, a very cheap

00:03:20.340 --> 00:03:23.520
simulation-- it
might look like this.

00:03:23.520 --> 00:03:26.130
And then over the course of
the next three, four days,

00:03:26.130 --> 00:03:28.990
it might reduce in intensity
to something like this.

00:03:28.990 --> 00:03:32.650
And then after three, four days
again, it will look like this.

00:03:32.650 --> 00:03:34.420
And then it'll look
like this again.

00:03:34.420 --> 00:03:37.630
So it's actual intensity
is going up and down

00:03:37.630 --> 00:03:39.040
with a well-defined period.

00:03:39.040 --> 00:03:41.510
So if this takes three
days and then this

00:03:41.510 --> 00:03:43.980
is another three
days, then the period,

00:03:43.980 --> 00:03:47.560
one entire cycle of its
going from low intensity

00:03:47.560 --> 00:03:50.820
back to high intensity,
is going to be six days.

00:03:50.820 --> 00:03:55.160
So this is a six-day period.

00:03:55.160 --> 00:03:59.190
And what Henrietta
Leavitt saw, and this

00:03:59.190 --> 00:04:02.640
wasn't an obvious
thing to do, she

00:04:02.640 --> 00:04:04.702
assumed that everything
in each of these clouds

00:04:04.702 --> 00:04:06.160
are roughly the
same distance away.

00:04:06.160 --> 00:04:08.440
Everything in the
Large Magellanic Cloud

00:04:08.440 --> 00:04:09.890
is roughly the
same distance away.

00:04:09.890 --> 00:04:11.920
And it's obviously not exact.

00:04:11.920 --> 00:04:13.310
This is an entire galaxy.

00:04:13.310 --> 00:04:15.680
So you have obviously things
further away in that galaxy

00:04:15.680 --> 00:04:16.555
and things closer up.

00:04:16.555 --> 00:04:18.170
You have stars here and here.

00:04:18.170 --> 00:04:20.670
And their distance isn't going
to be exactly the same to us,

00:04:20.670 --> 00:04:23.096
even though we're sitting
maybe over here someplace.

00:04:23.096 --> 00:04:24.220
But it's going to be close.

00:04:24.220 --> 00:04:26.290
It wasn't a bad approximation.

00:04:26.290 --> 00:04:29.730
And by making that assumption,
she saw something pretty neat.

00:04:29.730 --> 00:04:33.970
If she plotted-- so let me
plot this right over here.

00:04:33.970 --> 00:04:38.660
So she plotted on
the horizontal axis,

00:04:38.660 --> 00:04:46.790
if she plotted the
relative luminosity.

00:04:46.790 --> 00:04:49.030
So really, the only way
that she can measure this

00:04:49.030 --> 00:04:51.164
is just how bright
did they look to her?

00:04:51.164 --> 00:04:53.080
And she's assuming that
they're same distance.

00:04:53.080 --> 00:04:56.130
So obviously, if you have a
brighter star, but it's much,

00:04:56.130 --> 00:04:58.080
much further away, it's
going to look dimmer.

00:04:58.080 --> 00:05:01.132
So if you assume that they're
all roughly the same distance,

00:05:01.132 --> 00:05:02.590
then how bright it
is will tell you

00:05:02.590 --> 00:05:05.250
how bright it is
at the actual star.

00:05:05.250 --> 00:05:08.830
So she plotted relative
luminosity of the star

00:05:08.830 --> 00:05:09.950
on one axis.

00:05:09.950 --> 00:05:14.180
And on the other axis,
she plotted the period

00:05:14.180 --> 00:05:16.800
of these variable stars.

00:05:16.800 --> 00:05:18.879
She plotted the period.

00:05:18.879 --> 00:05:20.420
And what I'm going
to do is I'm going

00:05:20.420 --> 00:05:21.836
to do this on a
logarithmic scale.

00:05:21.836 --> 00:05:23.250
So let's say this is in days.

00:05:23.250 --> 00:05:25.200
So this is one day.

00:05:25.200 --> 00:05:27.220
This is 10 days.

00:05:27.220 --> 00:05:30.230
This is 100 days,
right over here.

00:05:30.230 --> 00:05:34.960
It's a logarithmic scale because
I'm going up in powers of 10.

00:05:34.960 --> 00:05:38.290
If we take the log of
these, this would be 0,

00:05:38.290 --> 00:05:40.384
this would be 1,
this would be 2.

00:05:40.384 --> 00:05:42.050
And so that's what
I'm using as a scale.

00:05:42.050 --> 00:05:44.010
So I'm using the
log of the period

00:05:44.010 --> 00:05:45.830
or I'm just marking
them as 1, 10, 100.

00:05:45.830 --> 00:05:49.170
But I'm giving each of these
factors of 10 an equal spacing.

00:05:49.170 --> 00:05:52.260
But when you plot it on this
scale, the relative luminosity

00:05:52.260 --> 00:05:53.800
versus the period,
she got a plot

00:05:53.800 --> 00:05:56.300
that looked something like this.

00:05:56.300 --> 00:05:59.670
And this is obviously not exact.

00:05:59.670 --> 00:06:02.030
She got a plot that looks
something like this.

00:06:02.030 --> 00:06:03.900
It was a fairly
linear relationship

00:06:03.900 --> 00:06:05.840
when you plot the
relative luminosity

00:06:05.840 --> 00:06:08.970
against the log of the period.

00:06:08.970 --> 00:06:11.630
So this is obviously a
logarithmic scale over here.

00:06:11.630 --> 00:06:14.770
And so you could fit a line.

00:06:14.770 --> 00:06:18.500
And why, I'd argue and I
think most people would argue,

00:06:18.500 --> 00:06:21.330
this is one of the most
important discoveries

00:06:21.330 --> 00:06:24.039
in astronomy is if you
know-- because think

00:06:24.039 --> 00:06:25.330
about what the problem here is.

00:06:25.330 --> 00:06:28.144
We can look at all of
these stars in space.

00:06:28.144 --> 00:06:29.935
Let's say you look at
a fraction of the sky

00:06:29.935 --> 00:06:32.380
and you look at something
that looks like that.

00:06:32.380 --> 00:06:33.580
So it's really bright.

00:06:33.580 --> 00:06:36.681
And then you see something
dim that looks like that.

00:06:36.681 --> 00:06:38.680
So if you have a very
superficial understanding,

00:06:38.680 --> 00:06:41.590
you say, oh, this
star is brighter.

00:06:41.590 --> 00:06:44.370
You would say that this is a
fundamentally brighter star.

00:06:44.370 --> 00:06:45.440
But how do you know that?

00:06:45.440 --> 00:06:47.890
Maybe, instead of
being brighter,

00:06:47.890 --> 00:06:50.880
maybe it's just a
dimmer, closer star.

00:06:50.880 --> 00:06:52.225
Maybe this is a closer star.

00:06:52.225 --> 00:06:54.060
Maybe this is an
entire galaxy, but it's

00:06:54.060 --> 00:06:56.410
so far away that
you can't even tell.

00:06:56.410 --> 00:07:03.652
But all of a sudden, by the
work that Henrietta Leavitt did,

00:07:03.652 --> 00:07:05.880
if you see one of
these Cepheid variable

00:07:05.880 --> 00:07:10.030
stars in another galaxy, you
know its relative brightness

00:07:10.030 --> 00:07:12.704
compared to other
Cepheid variable stars.

00:07:12.704 --> 00:07:15.120
And so if you can place just
one of these Cepheid variable

00:07:15.120 --> 00:07:17.411
stars, if you know exactly
the distance to one of them,

00:07:17.411 --> 00:07:19.580
and then you know its
absolute luminosity,

00:07:19.580 --> 00:07:21.830
you then know the
absolute luminosity

00:07:21.830 --> 00:07:24.370
of any other Cepheid
variable stars.

00:07:24.370 --> 00:07:29.580
So let's say using parallax,
which is our other tool,

00:07:29.580 --> 00:07:33.330
we find-- let's say there
are some star in our galaxy.

00:07:33.330 --> 00:07:37.690
And let's say using
parallax we're

00:07:37.690 --> 00:07:39.490
able to come up with
a pretty good measure

00:07:39.490 --> 00:07:42.740
that it is, I don't know, let's
say it's 100 light years away.

00:07:45.910 --> 00:07:50.640
And this star is a Cepheid,
this is a Cepheid variable star.

00:07:50.640 --> 00:07:54.580
And let's say its
period is one day.

00:07:54.580 --> 00:07:55.660
It's one day.

00:07:55.660 --> 00:07:59.500
So we now know
something interesting.

00:07:59.500 --> 00:08:02.570
We know variable stars
with a period of one day,

00:08:02.570 --> 00:08:05.780
at 100 light years away,
will look like this,

00:08:05.780 --> 00:08:08.690
will look like this
drawing right over here.

00:08:08.690 --> 00:08:14.530
So if we later on, if we later
on see a Cepheid variable

00:08:14.530 --> 00:08:22.500
star with a period of one day,
so it gets brighter and dim

00:08:22.500 --> 00:08:26.326
over the course of one day and
maybe it's red shifted as well,

00:08:26.326 --> 00:08:27.950
but maybe it looks
a little bit dimmer.

00:08:27.950 --> 00:08:29.290
It looks like this.

00:08:29.290 --> 00:08:34.750
We now know that if it
was 100 light years away,

00:08:34.750 --> 00:08:36.150
it would have this luminosity.

00:08:36.150 --> 00:08:38.419
So based on how
much dimmer it is,

00:08:38.419 --> 00:08:40.760
we can then figure out
how much further away

00:08:40.760 --> 00:08:42.674
this Cepheid variable star is.

00:08:42.674 --> 00:08:44.090
If that confuses
you a little bit,

00:08:44.090 --> 00:08:46.423
I'll do a little bit more
details in the next few videos

00:08:46.423 --> 00:08:49.172
so we can get a closer sense
of how the math would work.

00:08:49.172 --> 00:08:50.380
But this was a big discovery.

00:08:50.380 --> 00:08:53.957
Just discovering this class of
stars, this Cepheid variable

00:08:53.957 --> 00:08:56.040
class-- she wasn't on the
one who discovered them.

00:08:56.040 --> 00:08:57.600
People knew before
her that there

00:08:57.600 --> 00:08:59.870
were these stars that
got brighter and dimmer.

00:08:59.870 --> 00:09:01.650
But what her big
discovery was is

00:09:01.650 --> 00:09:05.150
seeing this linear relationship
between the relative luminosity

00:09:05.150 --> 00:09:07.020
of these stars and their period.

00:09:07.020 --> 00:09:09.450
Because then, if we see
Cepheid variable stars

00:09:09.450 --> 00:09:12.630
in completely different
galaxies or galactic clusters,

00:09:12.630 --> 00:09:14.290
by looking at their
period we know

00:09:14.290 --> 00:09:16.700
what their real
relative luminosity is.

00:09:16.700 --> 00:09:19.330
And then we can guess how
far those things really are.

00:09:19.330 --> 00:09:23.110
We could estimate how far
those things really are.


Black holes

WEBVTT
Kind: captions
Language: en

00:00:00.610 --> 00:00:02.640
In the last video, we
saw that if we started

00:00:02.640 --> 00:00:06.700
with a massive star
about nine to 20 times

00:00:06.700 --> 00:00:10.190
the mass of the Sun,
and when it finally

00:00:10.190 --> 00:00:13.740
matures the remnant of
the star is roughly,

00:00:13.740 --> 00:00:18.430
or that remnant core of the star
is roughly 1 and 1/2 to 3 times

00:00:18.430 --> 00:00:21.810
the solar mass or
the mass of the Sun,

00:00:21.810 --> 00:00:24.150
then this remnant right here--
and let me just be clear,

00:00:24.150 --> 00:00:28.420
this nine to 20 times is
the mass of that star when

00:00:28.420 --> 00:00:30.080
it's in its main sequence.

00:00:30.080 --> 00:00:32.340
This 1 and 1/2 to
three times is the mass

00:00:32.340 --> 00:00:35.175
once it's shed off
a lot of the, I

00:00:35.175 --> 00:00:37.340
guess, outer
material of the star.

00:00:37.340 --> 00:00:39.270
And this is really the
mass of the remnant

00:00:39.270 --> 00:00:41.730
of the star, kind of
the core of the star.

00:00:41.730 --> 00:00:44.060
But that remnant,
once it stops fusing,

00:00:44.060 --> 00:00:46.180
once it stops having
outward pressure,

00:00:46.180 --> 00:00:50.230
and once it has enough density,
this, we saw in the last video,

00:00:50.230 --> 00:00:52.180
will cause a supernova.

00:00:52.180 --> 00:00:53.951
It will cause a shock
wave to move out

00:00:53.951 --> 00:00:55.950
through the rest of the
material and essentially

00:00:55.950 --> 00:00:57.510
cause it to blow up.

00:00:57.510 --> 00:01:01.770
And this will condense
into a neutron star.

00:01:04.927 --> 00:01:06.760
Now, in this video what
I want to talk about

00:01:06.760 --> 00:01:10.476
is what if we're starting with
a star that has a mass more

00:01:10.476 --> 00:01:11.850
than-- and this
is give or take--

00:01:11.850 --> 00:01:13.990
but we don't know the
actual firm boundaries here.

00:01:13.990 --> 00:01:19.850
But what if we have a star
that is more than 20 times

00:01:19.850 --> 00:01:21.209
the mass of the Sun?

00:01:21.209 --> 00:01:22.750
And this is kind of
the original mass

00:01:22.750 --> 00:01:24.860
before the star
burns itself out.

00:01:24.860 --> 00:01:29.120
Or when that star is kind
of reached this old age,

00:01:29.120 --> 00:01:32.480
once it has an iron core,
it has more than-- so

00:01:32.480 --> 00:01:37.820
I could say the remnant--
the dense remnant has

00:01:37.820 --> 00:01:42.080
more than three to four
times the mass of the Sun.

00:01:42.080 --> 00:01:44.937
And remember, it's going
to have three to four times

00:01:44.937 --> 00:01:45.770
the mass of the Sun.

00:01:45.770 --> 00:01:46.915
But it's going to be far denser.

00:01:46.915 --> 00:01:48.123
It's just going to be a core.

00:01:48.123 --> 00:01:51.930
It's going to be in iron-nickel
core that's no longer fusing.

00:01:51.930 --> 00:01:53.280
So what happens to these stars?

00:01:53.280 --> 00:01:57.240
So it turns out that these
are so massive that even

00:01:57.240 --> 00:02:00.110
the neutron degeneracy
pressure will not

00:02:00.110 --> 00:02:02.860
be enough to keep the
mass from imploding.

00:02:02.860 --> 00:02:06.440
And these stars, all of
the mass in these stars

00:02:06.440 --> 00:02:09.060
will just keep imploding.

00:02:09.060 --> 00:02:12.180
So we imagine, in the first,
in kind of Sun-like stars,

00:02:12.180 --> 00:02:14.980
things would collapse
into white dwarfs.

00:02:14.980 --> 00:02:18.040
So maybe I should
draw that in white.

00:02:18.040 --> 00:02:20.805
So they would collapse
into white dwarfs.

00:02:20.805 --> 00:02:23.270
Now, that's not white either.

00:02:23.270 --> 00:02:24.120
There you go.

00:02:24.120 --> 00:02:26.490
They would collapse into
a white dwarfs eventually.

00:02:26.490 --> 00:02:27.670
So this is a white dwarf.

00:02:31.380 --> 00:02:33.280
And here, the pressure
that's keeping this

00:02:33.280 --> 00:02:36.550
from collapsing further is
electron degeneracy pressure.

00:02:36.550 --> 00:02:39.040
The atoms are squeezed so
much that the electrons are

00:02:39.040 --> 00:02:41.647
essentially keeping them
from squeezing anymore.

00:02:41.647 --> 00:02:43.230
But if the pressure
gets large enough,

00:02:43.230 --> 00:02:45.160
then you have the neutron star.

00:02:45.160 --> 00:02:48.120
So you have even more
mass and even a smaller--

00:02:48.120 --> 00:02:51.960
and I'm not drawing this to
scale-- neutron stars are tiny.

00:02:51.960 --> 00:02:54.010
White dwarf stars
are on the scale

00:02:54.010 --> 00:02:55.380
of an Earth-like like planet.

00:02:55.380 --> 00:02:57.300
Neutron stars, we learned
in the last video,

00:02:57.300 --> 00:02:58.780
are on the scale of a city.

00:02:58.780 --> 00:03:00.310
So these are
superdense, super tiny.

00:03:00.310 --> 00:03:02.566
And this has more mass
than this over here.

00:03:02.566 --> 00:03:04.440
In fact, maybe I should
just draw it as a dot

00:03:04.440 --> 00:03:06.320
just so you have a sense
of how dense it is.

00:03:06.320 --> 00:03:09.180
It's really just like
one big atomic nucleus

00:03:09.180 --> 00:03:11.000
or, well, it's still small.

00:03:11.000 --> 00:03:12.000
But it's size of a city.

00:03:12.000 --> 00:03:14.240
It's like a nucleus
the size of the city.

00:03:14.240 --> 00:03:18.706
But this right here
is a neutron star.

00:03:18.706 --> 00:03:20.580
And what's unintuitive
about what I'm drawing

00:03:20.580 --> 00:03:23.210
is each of these smaller
things have more mass.

00:03:23.210 --> 00:03:26.020
This overcame the electron
degeneracy pressure

00:03:26.020 --> 00:03:27.370
to collapse even further.

00:03:27.370 --> 00:03:29.520
But if the mass is
large enough, and this

00:03:29.520 --> 00:03:31.770
is what we're talking
about in this video,

00:03:31.770 --> 00:03:34.590
even the neutron
degeneracy pressure

00:03:34.590 --> 00:03:37.360
will not be able to keep
that mass from collapsing.

00:03:37.360 --> 00:03:40.550
And there's even
theoretical quark stars

00:03:40.550 --> 00:03:42.290
where the quark
degeneracy pressure--

00:03:42.290 --> 00:03:43.885
but if you get even
beyond that, then

00:03:43.885 --> 00:03:47.410
it all collapses
into a single point--

00:03:47.410 --> 00:03:49.560
and I'm simplifying
here-- but it

00:03:49.560 --> 00:03:54.200
collapses into a single
point of infinite density,

00:03:54.200 --> 00:03:59.920
infinite mass density.

00:03:59.920 --> 00:04:03.030
And this is really the
mass of a black hole.

00:04:03.030 --> 00:04:05.530
And I'm calling it the mass of
a black hole, because there's

00:04:05.530 --> 00:04:08.820
different ways how you could
view where a black hole starts

00:04:08.820 --> 00:04:11.570
and ends or what exactly
is the black hole.

00:04:11.570 --> 00:04:20.860
So this is all the
mass of the black hole

00:04:20.860 --> 00:04:23.310
or we could say of
the original star.

00:04:23.310 --> 00:04:26.260
So when we're talking about
that remnant being times three

00:04:26.260 --> 00:04:28.770
to four solar masses,
all of that mass

00:04:28.770 --> 00:04:29.987
is now being contained.

00:04:29.987 --> 00:04:30.820
Well, not all of it.

00:04:30.820 --> 00:04:33.409
Some of it is released as
energy during the supernova.

00:04:33.409 --> 00:04:35.200
And that was also true
of the neutron star.

00:04:35.200 --> 00:04:37.620
But most of that
mass is now being

00:04:37.620 --> 00:04:41.630
contained in this
infinitely small point.

00:04:41.630 --> 00:04:44.094
And you'll hear physicists
and mathematicians

00:04:44.094 --> 00:04:45.135
talk about singularities.

00:04:48.040 --> 00:04:50.850
And singularities
are really points,

00:04:50.850 --> 00:04:53.530
even in mathematics, where
everything breaks down,

00:04:53.530 --> 00:04:55.510
where nothing starts
to make sense anymore,

00:04:55.510 --> 00:04:59.130
where the mathematical
equations don't give you

00:04:59.130 --> 00:05:00.400
a defined answer.

00:05:00.400 --> 00:05:02.500
And this is a
singularity because you

00:05:02.500 --> 00:05:05.840
have a ton of mass in an
infinitely small space.

00:05:05.840 --> 00:05:09.050
You essentially have an
infinite density right here.

00:05:09.050 --> 00:05:10.440
And this is hard to visualize.

00:05:10.440 --> 00:05:13.190
But you have kind of an infinite
curvature in space/time right

00:05:13.190 --> 00:05:13.690
here.

00:05:13.690 --> 00:05:14.860
And I can't visualize that.

00:05:14.860 --> 00:05:16.920
So maybe we'll think
about that in more videos.

00:05:16.920 --> 00:05:20.310
But the reason why I said
that there's different ways

00:05:20.310 --> 00:05:22.080
to think about where
a black hole is,

00:05:22.080 --> 00:05:24.850
or where it starts and ends, is
that this is where the mass is.

00:05:24.850 --> 00:05:26.975
And if there was any other
mass that was over here,

00:05:26.975 --> 00:05:29.160
it would obviously be
attracted to this mass

00:05:29.160 --> 00:05:31.650
and then become part
of that singularity.

00:05:31.650 --> 00:05:35.430
It would add to that mass,
that already huge mass, that's

00:05:35.430 --> 00:05:38.449
in an infinitely
small point in space.

00:05:38.449 --> 00:05:40.490
But the reason why the
boundary is hard to define

00:05:40.490 --> 00:05:44.330
is because there's
some point in space

00:05:44.330 --> 00:05:47.820
around that singularity
at which no matter

00:05:47.820 --> 00:05:53.020
what that thing is, no matter
how much energy that thing has,

00:05:53.020 --> 00:05:56.400
it will not be able to escape
the gravitational influence

00:05:56.400 --> 00:06:00.510
of the black hole, of
that ultradense mass.

00:06:00.510 --> 00:06:03.700
So even if it was
electromagnetic radiation,

00:06:03.700 --> 00:06:06.050
even if it was light,
and even if it's

00:06:06.050 --> 00:06:09.000
a light that's shone
away from the mass,

00:06:09.000 --> 00:06:12.130
it will eventually
have to go back.

00:06:12.130 --> 00:06:15.960
It will not be able to escape
the gravitational influence.

00:06:15.960 --> 00:06:21.100
And so the boundary where if
you're within that boundary--

00:06:21.100 --> 00:06:27.900
that's really a sphere-- so that
boundary around the singularity

00:06:27.900 --> 00:06:29.420
where if you're
within the boundary,

00:06:29.420 --> 00:06:31.170
no matter what you do,
no matter if you're

00:06:31.170 --> 00:06:34.040
electromagnetic
radiation, you're

00:06:34.040 --> 00:06:36.360
never going to be able
to escape the black hole.

00:06:36.360 --> 00:06:38.310
If you are beyond
that boundary, you

00:06:38.310 --> 00:06:40.130
might be able to
escape the black hole.

00:06:40.130 --> 00:06:41.570
So this guy could escape.

00:06:41.570 --> 00:06:43.430
This guy over here, no
matter what he does,

00:06:43.430 --> 00:06:46.030
is going to have to go
back into the black hole.

00:06:46.030 --> 00:06:49.530
This boundary right here is
called the event horizon.

00:06:49.530 --> 00:06:52.010
This right here is
the event horizon,

00:06:52.010 --> 00:06:55.404
another word used in a lot
of science fiction movies.

00:06:55.404 --> 00:06:57.320
And for good reason,
because it's fascinating.

00:06:57.320 --> 00:07:00.130
And we'll actually learn in
future videos-- hopefully,

00:07:00.130 --> 00:07:02.740
about Hawking radiation-- we'll
see that that is not radiation

00:07:02.740 --> 00:07:04.360
from the black hole itself.

00:07:04.360 --> 00:07:06.850
It's the byproduct
of quantum effects

00:07:06.850 --> 00:07:09.520
that are occurring
at the event horizon.

00:07:09.520 --> 00:07:13.590
But the event horizon, it's
this kind of point in space,

00:07:13.590 --> 00:07:16.130
or this sphere in space,
or this boundary in space.

00:07:16.130 --> 00:07:19.460
Anything closer or
within the event horizon

00:07:19.460 --> 00:07:22.540
has to eventually end up in
the singularity, contributing

00:07:22.540 --> 00:07:23.110
to that mass.

00:07:23.110 --> 00:07:26.260
Anything on the outside
has a chance of escaping.

00:07:26.260 --> 00:07:27.820
So what would a
black hole look like?

00:07:27.820 --> 00:07:29.730
Well, not even light
can escape from it.

00:07:29.730 --> 00:07:31.670
So it will be black.

00:07:31.670 --> 00:07:34.540
It will be black in
the purest sense.

00:07:34.540 --> 00:07:36.700
It will not emit any
type of radiation

00:07:36.700 --> 00:07:40.170
from the black hole
itself, from that mass.

00:07:40.170 --> 00:07:45.554
And so here are some depictions
I got from NASA of black holes.

00:07:45.554 --> 00:07:47.470
And so just to be clear
what's happening here.

00:07:47.470 --> 00:07:49.550
What you're seeing
here is black.

00:07:49.550 --> 00:07:51.230
You can view that
as the black hole.

00:07:51.230 --> 00:07:52.380
When people talk
about the black hole,

00:07:52.380 --> 00:07:54.280
that's often what
they're talking about.

00:07:54.280 --> 00:07:56.700
But there's a point
of infinite density

00:07:56.700 --> 00:08:00.210
at the center of this
black sphere right here.

00:08:00.210 --> 00:08:01.970
And what you see as
that black sphere,

00:08:01.970 --> 00:08:05.060
that really is the boundary
of the event horizon.

00:08:05.060 --> 00:08:09.640
So this right here is the
boundary of the event horizon.

00:08:09.640 --> 00:08:11.890
And what we're seeing right
here is the accretion disk

00:08:11.890 --> 00:08:13.240
around the black hole.

00:08:13.240 --> 00:08:16.525
As all of this matter gets
closer and closer to it,

00:08:16.525 --> 00:08:18.401
it's being squeezed
more and more.

00:08:18.401 --> 00:08:20.900
It's moving faster and faster
and getting hotter and hotter.

00:08:20.900 --> 00:08:22.290
And that's why the way
this artist depicted it,

00:08:22.290 --> 00:08:23.980
it looks like the
stuff over here

00:08:23.980 --> 00:08:26.300
is redder and hotter than
the stuff further out.

00:08:26.300 --> 00:08:29.870
It's just accelerating as it
approaches that event horizon.

00:08:29.870 --> 00:08:33.399
Once it's in the event horizon,
we cannot even see the light it

00:08:33.399 --> 00:08:35.690
is emitting, even though it
would be starting to become

00:08:35.690 --> 00:08:37.780
unbelievably energetic.

00:08:37.780 --> 00:08:39.929
Here's some other pictures.

00:08:39.929 --> 00:08:41.490
This is a picture
of a star being

00:08:41.490 --> 00:08:42.434
ripped apart-- not a picture.

00:08:42.434 --> 00:08:43.830
This is actually an
artist's depictions.

00:08:43.830 --> 00:08:45.288
All of these are
artist depictions.

00:08:45.288 --> 00:08:47.800
We never were able to
get such a good pictures

00:08:47.800 --> 00:08:49.974
of actual action occurring
near black holes.

00:08:49.974 --> 00:08:51.140
These are artist depictions.

00:08:51.140 --> 00:08:54.320
But this is a star being
ripped apart by a black hole.

00:08:54.320 --> 00:08:58.410
So this star is getting pretty
close to this black hole.

00:08:58.410 --> 00:09:01.370
Already out here,
where the star is,

00:09:01.370 --> 00:09:03.930
it's very strong
gravitational attraction.

00:09:03.930 --> 00:09:07.680
So any mass that's being emitted
from the star in that direction

00:09:07.680 --> 00:09:09.730
is slowly being pulled
into the black hole.

00:09:09.730 --> 00:09:13.220
So the star is kind of being
ripped apart by the black hole.

00:09:13.220 --> 00:09:15.330
This is maybe a better
depiction of it.

00:09:15.330 --> 00:09:16.510
This is the star at first.

00:09:16.510 --> 00:09:20.980
And once it becomes under the
influence of the black hole's

00:09:20.980 --> 00:09:23.350
gravitation, it starts
to kind of elongate

00:09:23.350 --> 00:09:24.950
and gets ripped apart.

00:09:24.950 --> 00:09:28.019
And its matter starts
spiraling in closer and closer

00:09:28.019 --> 00:09:28.810
to that black hole.

00:09:28.810 --> 00:09:30.652
And then once it's
in the event horizon,

00:09:30.652 --> 00:09:31.860
we won't even see it anymore.

00:09:31.860 --> 00:09:34.500
Because even the light
from that matter,

00:09:34.500 --> 00:09:38.520
that intensely hot matter that's
entering into the black hole,

00:09:38.520 --> 00:09:41.250
cannot even escape
the black hole itself.

00:09:41.250 --> 00:09:43.420
Anyway, hopefully you
found that interesting.

00:09:43.420 --> 00:09:46.030
And I want to be
clear, we still don't

00:09:46.030 --> 00:09:48.230
understand a lot
about black holes.

00:09:48.230 --> 00:09:51.270
In fact, this whole
notion of a singularity,

00:09:51.270 --> 00:09:53.320
the fact that all the
math and all the theory

00:09:53.320 --> 00:09:56.680
breaks down at the singularity,
is a pretty good sign

00:09:56.680 --> 00:09:58.040
that our theory isn't complete.

00:09:58.040 --> 00:09:59.456
Because if our
theory is complete,

00:09:59.456 --> 00:10:02.030
we would maybe get something a
little bit more sensical than

00:10:02.030 --> 00:10:04.340
just all of our equations
not making sense

00:10:04.340 --> 00:10:06.340
at that infinitely dense point.

00:10:06.340 --> 00:10:08.850
Anyway, hopefully you
found that interesting.


Supernova (supernovae)

WEBVTT
Kind: captions
Language: en

00:00:00.590 --> 00:00:02.400
Where we left off
in the last video,

00:00:02.400 --> 00:00:05.530
we had a mature
massive star, a star

00:00:05.530 --> 00:00:08.860
that had started
forming a core of iron.

00:00:08.860 --> 00:00:12.770
It has enormous pressure,
enormous inward pressure

00:00:12.770 --> 00:00:13.350
on this core.

00:00:13.350 --> 00:00:17.140
Because as we form heavier and
heavier elements in the core,

00:00:17.140 --> 00:00:20.510
the core gets denser
and denser and denser.

00:00:20.510 --> 00:00:24.640
And so we keep fusing more
and more elements into iron.

00:00:24.640 --> 00:00:28.740
This iron core becomes
more and more massive, more

00:00:28.740 --> 00:00:29.460
and more dense.

00:00:29.460 --> 00:00:33.410
It's squeezing in on itself.

00:00:33.410 --> 00:00:34.880
And it's not fusing.

00:00:34.880 --> 00:00:36.750
That is not exothermic anymore.

00:00:36.750 --> 00:00:38.600
If iron were to fuse,
it would not even

00:00:38.600 --> 00:00:39.800
be an exothermic process.

00:00:39.800 --> 00:00:40.960
It would require energy.

00:00:40.960 --> 00:00:42.501
So it wouldn't be
even something that

00:00:42.501 --> 00:00:47.040
could be helped to fend off
this squeezing, to fend off

00:00:47.040 --> 00:00:49.470
this increasing
density of the core.

00:00:49.470 --> 00:00:51.170
So we have this iron
here, and it just

00:00:51.170 --> 00:00:54.240
gets more and more massive,
more and more dense.

00:00:54.240 --> 00:00:58.610
And so at some mass, already
a reasonably high mass,

00:00:58.610 --> 00:01:01.110
the only thing that's keeping
this from just completely

00:01:01.110 --> 00:01:05.069
collapsing is-- we could call
it electron degeneracy pressure.

00:01:05.069 --> 00:01:13.850
So let me write this here,
electron degeneracy pressure.

00:01:16.490 --> 00:01:23.590
And all this means is we have
all of these iron atoms getting

00:01:23.590 --> 00:01:25.920
really, really, really
close to each other.

00:01:25.920 --> 00:01:29.340
And the only thing that keeps it
from collapsing at this earlier

00:01:29.340 --> 00:01:32.130
stage, the only thing that keeps
it from collapsing altogether,

00:01:32.130 --> 00:01:33.672
is that they have
these electrons.

00:01:33.672 --> 00:01:35.130
You have these
electrons, and these

00:01:35.130 --> 00:01:36.504
are being squeezed
together, now.

00:01:36.504 --> 00:01:40.800
I mean, we're talking about
unbelievably dense states

00:01:40.800 --> 00:01:41.580
of matter.

00:01:41.580 --> 00:01:46.430
And electron degeneracy
pressure is, essentially--

00:01:46.430 --> 00:01:47.920
it's saying these
electrons don't

00:01:47.920 --> 00:01:49.990
want to be in the same
place at the same time.

00:01:49.990 --> 00:01:52.130
I won't go into the
quantum mechanics of it.

00:01:52.130 --> 00:01:55.680
But they cannot be squeezed
into each other any more.

00:01:55.680 --> 00:02:00.830
So that, at least
temporarily, holds this thing

00:02:00.830 --> 00:02:03.350
from collapsing even further.

00:02:03.350 --> 00:02:05.380
And in the case of
a less massive star,

00:02:05.380 --> 00:02:06.920
in the case of a
white dwarf, that's

00:02:06.920 --> 00:02:10.020
how a white dwarf actually
maintains its shape, because

00:02:10.020 --> 00:02:12.510
of the electron
degeneracy pressure.

00:02:12.510 --> 00:02:17.390
But as this iron core gets
even more massive, more dense,

00:02:17.390 --> 00:02:21.180
and we get more and more
gravitational pressure--

00:02:21.180 --> 00:02:23.650
so this is our core, now--
even more gravitational

00:02:23.650 --> 00:02:27.830
pressure, eventually even
this electron degeneracy--

00:02:27.830 --> 00:02:30.022
I guess we could call
it force, or pressure,

00:02:30.022 --> 00:02:31.980
this outward pressure,
this thing that keeps it

00:02:31.980 --> 00:02:36.500
from collapsing--
even that gives in.

00:02:36.500 --> 00:02:43.130
And then we have something
called electron capture,

00:02:43.130 --> 00:02:46.210
which is essentially
the electrons

00:02:46.210 --> 00:02:49.130
get captured by
protons in the nucleus.

00:02:49.130 --> 00:02:50.980
They start collapsing
into the nucleuses.

00:02:50.980 --> 00:02:56.990
It's kind of the opposite of
beta negative decay, where

00:02:56.990 --> 00:02:58.850
you have the electrons
get captured,

00:02:58.850 --> 00:03:01.170
protons get turned
into neutrons.

00:03:01.170 --> 00:03:03.210
You have neutrinos
being released.

00:03:03.210 --> 00:03:05.582
But you can imagine an
enormous amount of energy

00:03:05.582 --> 00:03:06.540
is also being released.

00:03:06.540 --> 00:03:08.248
So this is kind of a
temporary-- and then

00:03:08.248 --> 00:03:09.760
all of a sudden, this collapses.

00:03:09.760 --> 00:03:14.024
This collapses even more
until all you have--

00:03:14.024 --> 00:03:15.940
and all the protons are
turning into neutrons.

00:03:15.940 --> 00:03:18.110
Because they're
capturing electrons.

00:03:18.110 --> 00:03:21.440
So what you eventually
have is this entire core

00:03:21.440 --> 00:03:24.103
is collapsing into a
dense ball of neutrons.

00:03:28.860 --> 00:03:30.740
You can kind of view
them as just one

00:03:30.740 --> 00:03:33.130
really, really,
really, really, really

00:03:33.130 --> 00:03:36.850
massive atom because it's
just a dense ball of neutrons.

00:03:36.850 --> 00:03:40.940
At the same time, when
this collapse happens,

00:03:40.940 --> 00:03:43.560
you have an enormous
amount of energy

00:03:43.560 --> 00:03:47.520
being released in the
form of neutrinos.

00:03:49.829 --> 00:03:51.620
Did I say that neutrons
are being released?

00:03:51.620 --> 00:03:55.680
No, no, no, the electrons are
being captured by the protons,

00:03:55.680 --> 00:03:58.440
protons turning into
neutrons-- this dense ball

00:03:58.440 --> 00:04:00.690
of neutrons right here--
and in the process,

00:04:00.690 --> 00:04:03.054
neutrinos get released,
these fundamental particles.

00:04:03.054 --> 00:04:04.470
We won't go into
the details here.

00:04:04.470 --> 00:04:08.450
But it's an enormous
amount of energy.

00:04:08.450 --> 00:04:10.740
And this actually is
not really, really well

00:04:10.740 --> 00:04:12.500
understood, of all
of the dynamics here.

00:04:12.500 --> 00:04:16.220
Because at the same time that
this iron core is undergoing

00:04:16.220 --> 00:04:18.070
through this-- at
first it kind of pauses

00:04:18.070 --> 00:04:20.820
due to the electron
degeneracy pressure.

00:04:20.820 --> 00:04:23.600
And then it finally gives
in because it's so massive.

00:04:23.600 --> 00:04:28.325
And then it collapses into
this dense ball of neutrons.

00:04:28.325 --> 00:04:30.450
But when it does it, all
of this energy's released.

00:04:30.450 --> 00:04:32.780
And it's not clear how-- because
it has to be a lot of energy.

00:04:32.780 --> 00:04:34.488
Because remember, this
is a massive star.

00:04:34.488 --> 00:04:36.670
So you have a lot of mass
in this area over here.

00:04:36.670 --> 00:04:39.810
But it's so much energy that
it causes the rest of the star

00:04:39.810 --> 00:04:48.880
to explode outward in an
unbelievable, I guess,

00:04:48.880 --> 00:04:52.820
unbelievably bright or
energetic explosion.

00:04:52.820 --> 00:04:54.110
And that's called a supernova.

00:05:02.970 --> 00:05:04.470
And the reason why
it's called nova,

00:05:04.470 --> 00:05:06.260
it comes from, I
believe-- I'm not

00:05:06.260 --> 00:05:08.360
an expert here--
Latin for "new."

00:05:08.360 --> 00:05:10.839
And the first time
people observed a nova,

00:05:10.839 --> 00:05:12.130
they thought it was a new star.

00:05:12.130 --> 00:05:13.240
Because all of a
sudden, something

00:05:13.240 --> 00:05:14.730
they didn't see before,
all of a sudden,

00:05:14.730 --> 00:05:15.670
it looks like a star appeared.

00:05:15.670 --> 00:05:17.211
Because maybe it
wasn't bright enough

00:05:17.211 --> 00:05:18.650
for us to observe it before.

00:05:18.650 --> 00:05:22.310
But then when the nova occurred,
it did become bright enough.

00:05:22.310 --> 00:05:24.720
So it comes from
the idea of new.

00:05:24.720 --> 00:05:27.970
But a supernova is when you
have a pretty massive star's

00:05:27.970 --> 00:05:29.490
core collapsing.

00:05:29.490 --> 00:05:31.070
And that energy
is being released

00:05:31.070 --> 00:05:34.530
to explode the rest of the star
out at unbelievable velocities.

00:05:34.530 --> 00:05:38.150
And just to kind of fathom
the amount of energy that's

00:05:38.150 --> 00:05:40.410
being released in
a supernova, it

00:05:40.410 --> 00:05:43.686
can temporarily outshine
an entire galaxy.

00:05:43.686 --> 00:05:45.560
And in a galaxy, we're
talking about hundreds

00:05:45.560 --> 00:05:46.670
of billions of stars.

00:05:46.670 --> 00:05:48.045
Or another way to
think about it,

00:05:48.045 --> 00:05:49.580
in that very short
period of time,

00:05:49.580 --> 00:05:52.650
it can release as much
energy as the sun will

00:05:52.650 --> 00:05:54.580
in its entire lifetime.

00:05:54.580 --> 00:05:57.260
So these are unbelievably
energetic events.

00:05:57.260 --> 00:06:00.030
And so you actually
have the material

00:06:00.030 --> 00:06:02.830
that's not in the core
being shot out of the star

00:06:02.830 --> 00:06:06.310
at appreciable percentages
of the actual speed of light.

00:06:06.310 --> 00:06:10.900
So we're talking about things
being shot out at up to 10%

00:06:10.900 --> 00:06:14.410
of the speed of light.

00:06:14.410 --> 00:06:18.240
Now, that's 30,000
kilometers per second.

00:06:18.240 --> 00:06:21.640
That's almost circumnavigating
the earth every second.

00:06:21.640 --> 00:06:25.230
So that's, I mean, this is
unbelievably energetic events

00:06:25.230 --> 00:06:26.700
that we're talking about here.

00:06:26.700 --> 00:06:30.410
And so if the original star
was-- and these are rough

00:06:30.410 --> 00:06:30.910
estimates.

00:06:30.910 --> 00:06:32.743
People don't have kind
of a hard limit here.

00:06:32.743 --> 00:06:43.180
If the original star
approximately 9 to 20 times

00:06:43.180 --> 00:06:47.550
the mass of the sun,
then it will supernova.

00:06:47.550 --> 00:06:52.250
And the core will turn into
what's called a neutron star.

00:06:52.250 --> 00:06:56.580
This is a neutron
star, which you

00:06:56.580 --> 00:06:59.080
can imagine is just
this dense ball.

00:06:59.080 --> 00:07:01.500
It's this dense
ball of neutrons.

00:07:01.500 --> 00:07:03.380
And just to give
you a sense of it,

00:07:03.380 --> 00:07:07.820
it'll be something about maybe
two times the mass of the sun,

00:07:07.820 --> 00:07:10.090
give or take one and a half
to three times the mass

00:07:10.090 --> 00:07:11.530
of the sun.

00:07:11.530 --> 00:07:15.130
So this is one and a
half to three times

00:07:15.130 --> 00:07:21.210
the mass of the sun
in a volume that

00:07:21.210 --> 00:07:23.810
has a diameter of about
10-- on the order of tens

00:07:23.810 --> 00:07:25.040
of kilometers.

00:07:25.040 --> 00:07:29.990
So it's roughly the size of a
city, in a diameter of a city.

00:07:29.990 --> 00:07:34.336
So this is unbelievably
dense, diameter of a city.

00:07:34.336 --> 00:07:35.960
I mean, we know how
much larger the sun

00:07:35.960 --> 00:07:38.070
is relative to the Earth.

00:07:38.070 --> 00:07:41.200
And we know how much larger the
Earth is relative to a city.

00:07:41.200 --> 00:07:43.300
But this is something
large-- more mass

00:07:43.300 --> 00:07:45.600
than the sun being
squeezed into the density,

00:07:45.600 --> 00:07:48.520
or into the size of a city,
so unbelievably dense.

00:07:48.520 --> 00:07:51.670
Now if the original star
is even more massive,

00:07:51.670 --> 00:07:54.387
if it's more than 20
times the sun-- so

00:07:54.387 --> 00:07:55.470
let me write it over here.

00:07:55.470 --> 00:07:56.970
Let me scroll up.

00:07:56.970 --> 00:08:02.220
If it's greater than
20 times the sun,

00:08:02.220 --> 00:08:06.800
then even the neutron degeneracy
pressure, even the pressure,

00:08:06.800 --> 00:08:10.340
even the neutrons' inability to
squeeze further will give up.

00:08:10.340 --> 00:08:11.820
And it'll turn
into a black hole.

00:08:11.820 --> 00:08:14.574
And that's-- and I could
do many videos on that.

00:08:14.574 --> 00:08:16.740
And that's actually an open
area of research, still,

00:08:16.740 --> 00:08:19.330
on exactly what's going
on inside of a black hole.

00:08:19.330 --> 00:08:20.980
But then you turn
into a black hole,

00:08:20.980 --> 00:08:22.950
where essentially
all of the mass

00:08:22.950 --> 00:08:26.080
gets condensed into an
infinitely small and dense

00:08:26.080 --> 00:08:28.690
point, so something
unbelievably hard to imagine.

00:08:28.690 --> 00:08:30.230
And just to give
you a sense of it,

00:08:30.230 --> 00:08:34.381
so this will be more mass
then even three times the mass

00:08:34.381 --> 00:08:34.880
of the sun.

00:08:34.880 --> 00:08:39.240
So we're talking about an
incredibly high amount of mass.

00:08:39.240 --> 00:08:41.070
Just to kind of
visualize things,

00:08:41.070 --> 00:08:43.289
here is actually a
remnant of a supernova.

00:08:43.289 --> 00:08:45.524
This is the Crab Nebula.

00:08:45.524 --> 00:08:47.190
This is, right here,
is the Crab Nebula.

00:08:51.470 --> 00:08:54.760
And it's about 6,500
light years away.

00:08:57.490 --> 00:08:59.410
So it's still, from
a galactic scale--

00:08:59.410 --> 00:09:01.950
if you think of our galaxy
as being 100,000 light years

00:09:01.950 --> 00:09:05.460
in diameter-- it's still not
too far from us on those scales.

00:09:05.460 --> 00:09:06.710
But it's an enormous distance.

00:09:06.710 --> 00:09:09.220
The closest star to us
is four light years away.

00:09:09.220 --> 00:09:12.310
And it would take Voyager
travelling at 60,000 kilometers

00:09:12.310 --> 00:09:13.829
an hour, 80,000
years to get there.

00:09:13.829 --> 00:09:16.120
So this is a very, very--
that's only four light years.

00:09:16.120 --> 00:09:17.940
Now this is 6,500 light years.

00:09:17.940 --> 00:09:20.240
But this supernova,
it's believed

00:09:20.240 --> 00:09:23.530
happened 1,000 years
ago, right at the center.

00:09:23.530 --> 00:09:28.210
And so at the center here, we
should have a neutron star.

00:09:28.210 --> 00:09:32.080
And this cloud, the shock
wave that you see here,

00:09:32.080 --> 00:09:34.650
this is still the
material traveling

00:09:34.650 --> 00:09:39.020
outward from that
supernova over 1,000 years.

00:09:39.020 --> 00:09:43.120
This shock wave, or the diameter
of this sphere of material,

00:09:43.120 --> 00:09:45.410
is six light years.

00:09:45.410 --> 00:09:50.230
So we could say this distance
right here is six light years.

00:09:50.230 --> 00:09:53.320
So this is an enormously
big shock wave cloud.

00:09:53.320 --> 00:09:58.350
And actually, we believe
that our solar system started

00:09:58.350 --> 00:10:02.900
to form, started to condense
because of a shock wave created

00:10:02.900 --> 00:10:05.894
by a supernova
relatively near to us.

00:10:05.894 --> 00:10:07.560
And just to answer
another question that

00:10:07.560 --> 00:10:09.370
was kind of jumping
up, probably,

00:10:09.370 --> 00:10:12.910
in the last video-- and this is
still not really, really well

00:10:12.910 --> 00:10:13.810
understood.

00:10:13.810 --> 00:10:17.540
We talk about how elements
up to iron, or maybe nickel,

00:10:17.540 --> 00:10:20.727
can be formed inside of
the cores of massive stars.

00:10:20.727 --> 00:10:22.560
So you could imagine
when the star explodes,

00:10:22.560 --> 00:10:25.330
a lot of that material is
released into the universe.

00:10:25.330 --> 00:10:27.890
And so that's why we have
a lot of these materials

00:10:27.890 --> 00:10:28.710
in our own bodies.

00:10:28.710 --> 00:10:34.650
In fact, we could not exist if
these heavier elements were not

00:10:34.650 --> 00:10:38.650
formed inside of the cores of
primitive stars, stars that

00:10:38.650 --> 00:10:41.110
have supernova-ed
a long time ago.

00:10:41.110 --> 00:10:43.650
Now the question is, how do
these heavier elements form?

00:10:43.650 --> 00:10:46.650
How do we get all of this other
stuff on the periodic table?

00:10:46.650 --> 00:10:48.610
How do we get all these
other heavier elements?

00:10:48.610 --> 00:10:52.120
And they're formed during
the supernova itself.

00:10:52.120 --> 00:10:53.070
It's so energetic.

00:10:53.070 --> 00:10:56.650
You have all sorts of particles
streaming out and streaming in,

00:10:56.650 --> 00:10:58.940
streaming out because of
the force of the shock wave,

00:10:58.940 --> 00:11:00.940
streaming in because
of the gravity.

00:11:00.940 --> 00:11:03.961
But you have all sorts of
kind of a mishmash of elements

00:11:03.961 --> 00:11:04.460
forming.

00:11:04.460 --> 00:11:05.876
And that's actually
where you have

00:11:05.876 --> 00:11:07.680
your heavier elements forming.

00:11:07.680 --> 00:11:10.410
And because-- and I'll talk more
about this in future videos--

00:11:10.410 --> 00:11:12.130
most of the uranium,
or actually,

00:11:12.130 --> 00:11:14.720
all of uranium on
Earth right now,

00:11:14.720 --> 00:11:17.830
must have been formed in some
type of a supernova explosion,

00:11:17.830 --> 00:11:19.980
at least based on our
current understanding.

00:11:19.980 --> 00:11:23.900
And it looks to be about
4.6 billion years old.

00:11:23.900 --> 00:11:26.440
So given that it looks to be
about 4.6 billion years old,

00:11:26.440 --> 00:11:28.856
based on how fast it's decayed--
and I'll do a whole video

00:11:28.856 --> 00:11:33.870
on that-- that's why we think
that our solar system was first

00:11:33.870 --> 00:11:36.510
formed from some type
of supernova explosion.

00:11:36.510 --> 00:11:39.190
Because that uranium
would have been

00:11:39.190 --> 00:11:42.284
formed right at about the
birth of our solar system.

00:11:42.284 --> 00:11:44.200
Anyway, hopefully, you
found that interesting.

00:11:44.200 --> 00:11:45.450
This is a fascinating picture.

00:11:45.450 --> 00:11:49.536
And if you go to Wikipedia
and look up the Crab Nebula,

00:11:49.536 --> 00:11:50.660
keep clicking on the image.

00:11:50.660 --> 00:11:52.480
And eventually you'll
get a zoomed in picture.

00:11:52.480 --> 00:11:54.438
And that's just kind of
even more mind blowing.

00:11:54.438 --> 00:11:57.344
Because you could see all the
intricacy in this actual photo.


Supernova clarification

WEBVTT
Kind: captions
Language: en

00:00:00.499 --> 00:00:04.300
I want to make one
clarification on my video

00:00:04.300 --> 00:00:08.360
on supernovae, because that's
the plural for supernova,

00:00:08.360 --> 00:00:11.790
is that I said this supernova
occurred 1,000 years ago.

00:00:11.790 --> 00:00:13.810
That's actually not
precisely right.

00:00:13.810 --> 00:00:17.340
The supernova was first
observed 1,000 years ago,

00:00:17.340 --> 00:00:20.530
or the light from
that first explosion

00:00:20.530 --> 00:00:24.960
was observed by astronomers
we believe 1,000 years ago.

00:00:24.960 --> 00:00:26.510
But we have to be
very clear here.

00:00:26.510 --> 00:00:29.880
Because the Crab
nebula at its core

00:00:29.880 --> 00:00:33.560
is roughly 6,500
light years away,

00:00:33.560 --> 00:00:36.650
even this light, even this
image we see right here

00:00:36.650 --> 00:00:40.210
is that nebula as it
was 6,500 years ago.

00:00:40.210 --> 00:00:44.300
And so the supernova itself, if
we think about when it actually

00:00:44.300 --> 00:00:48.880
occurred, it actually must have
occurred about 7,500 years ago.

00:00:48.880 --> 00:00:55.050
So it must have occurred
about 7,500 years ago.

00:00:55.050 --> 00:00:57.730
And that first slide
from that first explosion

00:00:57.730 --> 00:01:01.250
from that first energetic
event reached us

00:01:01.250 --> 00:01:03.950
about 1,000 years ago.

00:01:03.950 --> 00:01:06.570
So it took 6,500
years to get to us,

00:01:06.570 --> 00:01:08.390
and reached us 1,000 years ago.

00:01:08.390 --> 00:01:15.769
So first light 1,000 years ago.

00:01:15.769 --> 00:01:17.060
Just wanted to make that clear.

00:01:17.060 --> 00:01:18.300
Might have been obvious
to some of you all,

00:01:18.300 --> 00:01:19.969
but always important
to think about it.

00:01:19.969 --> 00:01:21.510
When I said 1,000
years ago, I really

00:01:21.510 --> 00:01:23.200
should have said it
was first observed,

00:01:23.200 --> 00:01:25.680
the explosion was
observed 1,000 years ago.

00:01:25.680 --> 00:01:27.500
But since it's so
far, the actual event

00:01:27.500 --> 00:01:30.660
must have occurred
7,500 years ago.