February 28, 2020 0

Supermassive black holes | Stars, black holes and galaxies | Cosmology & Astronomy | Khan Academy


In the videos on massive
stars and on black holes, we learned that if the remnant
of a star, of a massive star, is massive enough, the
gravitational contraction, the gravitational
force, will be stronger than even the electron
degeneracy pressure, even stronger than the
neutron degeneracy pressure, even stronger than the
quark degeneracy pressure. And everything would
collapse into a point. And we called these
points black holes. And we learned there’s
an event horizon around these black holes. And if anything
gets closer or goes within the boundary
of that event horizon, there’s no way that it can never
escape from the black hole. All it can do is get closer
and closer to the black hole. And that includes light. And that’s why it’s
called a black hole. So even though
all of the mass is at the central point,
this entire area, or the entire surface
of the event horizon, this entire surface
of the event horizon– I’ll do it in
purple because it’s supposed to be black– this
entire thing will appear black. It will emit no light. Now these type of black
holes that we described, we call those
stellar black holes. And that’s because
they’re formed from collapsing massive stars. And the largest stellar black
holes that we have observed are on the order of 33
solar masses, give or take. So very massive to begin
with, let’s just be clear. And this is what the remnant
of the star has to be. So a lot more of the
original star’s mass might have been pushed
off in supernovae. That’s plural of supernova. Now there’s another
class of black holes here and these are
somewhat mysterious. And they’re called
supermassive black holes. And to some degree,
the word “super” isn’t big enough,
supermassive black holes, because they’re not just
a little bit more massive than stellar black holes. They’re are a lot more massive. They’re on the order of
hundreds of thousands to billions of solar masses,
hundred thousands to billions times the mass of our
Sun, solar masses. And what’s interesting about
these, other than the fact that there are super
huge, is that there doesn’t seem to be black
holes in between or at least we haven’t observed
black holes in between. The largest stellar black
hole is 33 solar masses. And then there are these
supermassive black holes that we think exist. And we think they mainly exist
in the centers of galaxies. And we think most, if not
all, centers of galaxies actually have one of these
supermassive black holes. But it’s kind of an
interesting question, if all black holes were
formed from collapsing stars, wouldn’t we see
things in between? So one theory of how these
really massive black holes form is that you have a
regular stellar black hole in an area that
has a lot of matter that it can accrete around it. So I’ll draw the– this is
the event horizon around it. The actual black hole is going
to be in the center of it, or rather the mass
of the black hole will be in the center of it. And then over time, you have
just more and more mass just falling into this black hole. Just more and more
stuff just keeps falling into this black hole. And then it just keeps growing. And so this could be a plausible
reason, or at least the mass in the center keeps growing and
so the event horizon will also keep growing in radius. Now this is a
plausible explanation based on our current
understanding. But the reason why
this one doesn’t gel that well is if
this was the explanation for supermassive
black holes, you expect to see more
black holes in between, maybe black holes
with 100 solar masses, or a 1,000 solar masses,
or 10,000 solar masses. But we’re not seeing
those right now. We just see the
stellar black holes, and we see the
supermassive black holes. So another possible
explanation– my inclinations lean
towards this one because it kind of
explains the gap– is that these supermassive black
holes actually formed shortly after the Big Bang, that these
are primordial black holes. These started near the
beginning of our universe, primordial black holes. Now remember, what do you
need to have a black hole? You need to have an amazingly
dense amount of matter or a dense amount of mass. If you have a lot of mass
in a very small volume, then their
gravitational pull will pull them closer, and
closer, and closer together. And they’ll be able
to overcome all of the electron
degeneracy pressures, and the neutron
degeneracy pressures, and the quark
degeneracy pressures, to really collapse into what
we think is a single point. I want to be clear here, too. We don’t know it’s
a single point. We’ve never gone into the
center of a black hole. Just the mathematics of the
black holes, or at least as we understand it right
now, have everything colliding into a single point where the
math starts to break down. So we’re really not
sure what happens at that very small center point. But needless to say, it will
be an unbelievably, maybe infinite, maybe
almost infinitely, dense point in space, or
dense amount of matter. And the reason why
I kind of favor this primordial black hole
and why this would make sense is right after the formation
of the universe, all of the matter in the universe
was in a much denser space because the universe
was smaller. So let’s say that this
is right after the Big Bang, some period of
time after the Big Bang. Now what we’ve talked
about before when we talked about cosmic
background is that at that point, the universe
was relatively uniform. It was super, super dense but
it was relatively uniform. So a universe like
this, there’s no reason why anything would
collapse into black holes. Because if you look
at a point here, sure, there’s a ton of
mass very close to it. But it’s very close to
it in every direction. So the gravitational force would
be the same in every direction if it was completely uniform. But if you go shortly after
the Big Bang, maybe because of slight quantum
fluctuation effects, it becomes slightly nonuniform. So let’s say it becomes
slightly nonuniform, but it still is
unbelievably dense. So let’s say it looks something
like this, where you have areas that are denser, but
it’s slightly nonuniform, but extremely dense. So here, all of a
sudden, you have the type of densities necessary
for a black hole. And where you have
higher densities, where it’s less uniform,
here, all of a sudden, you will have inward force. The gravitational pull from
things outside of this area are going to be less than
the gravitational pull towards those areas. And the more things get pulled
towards it, the less uniform it’s going to get. So you could imagine in
that primordial universe, that very shortly
after the Big Bang when things were very dense
and closely packed together, we may have had the
conditions where these supermassive black
holes could have formed. Where we had so much mass
in such a small volume, and it was just
not uniform enough, so that you could kind of
have this snowballing effect, so that more and more
mass would collect into these supermassive
black holes that are hundreds of thousands to
billions of times the mass of the Sun. And, this is maybe even
the more interesting part, those black holes would become
the centers of future galaxies. So you have these
black holes forming, these supermassive
black holes forming. And not everything would
go into a black hole. Only if it didn’t have a
lot of angular velocity, then it might go
into the black hole. But if it’s going
pass it fast enough, it’ll just start going in
orbit around the black hole. And so you could
imagine that this is how the early galaxies
or even our galaxy formed. And so you might
be wondering, well, what about the black hole at
the center of the Milky Way? And we think there is one. We think there is
one because we’ve observed stars orbiting very
quickly around something at the center of the
universe– sorry, at the center of our Milky Way. I want to be very clear, not
at the center of the universe. And the only
plausible explanation for it orbiting so
quickly around something is that it has to have a
density of either a black hole or something that
will eventually turn into a black hole. And when you do the
math for the middle of our galaxy, the
center of the Milky Way, our supermassive black hole is
on the order of 4 million times the mass of the Sun. So hopefully that gives you a
little bit of food for thought. There aren’t just only
stellar collapsed black holes. Or maybe there are
and somehow they grow into supermassive
black holes and that everything in
between we just can’t observe. Or that they really are a
different class of black holes. They’re actually
formed different ways. Maybe they formed
near the beginning of the actual universe. When the density of things
was a little uniform, things condensed
into each other. And what we’re going to
talk about in the next video is how these supermassive
black holes can help generate unbelievable sources
of radiation, even though the black
holes themselves aren’t emitting them. And those are going
to be quasars.

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