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Black Holes

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Black Holes

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In the last video, we saw that if we started with a massive star,
about nine to twenty times the mass of the Sun,
and when it finally matures, the remnant of the star is roughly...
or that remnant core of the star
is roughly one and a half to three times the solar mass
or the mass of the Sun, then this remnant right here,
and let me just be clear: this is nine to twenty times
is the mass of that star when it's in its main sequence.
This one and a half to three times is the mass once it has shed off
a lot of the, I guess outer material of the star,
and this is really the mass of the remnant of the star.
Kind of the core of the star.
But if that remnant, once its stop fusing
once its stops having outward pressure,
once it has enough density, this we saw in the last video,
will cause a supernova, it will cause a shockwave to move out
through the rest of the material
and essentially cause it blow up,
and this will condense into a neutron star.
Into a neutron star...
Now in this video, what I want to talk about is
what if we're starting with a star
that has a mass more than--and this is give or take, we don't
know the actual firm boundaries here,
but what if we have a star, what if we have a star
that is more than twenty times the mass of the Sun?
And this is kind of the original mass,
before the star burns itself out.
Or when that star has kind of reached this old age,
once it has that iron core,
it has more than... So I could say the remnant..., the remnant...
the dense remnant has more than three to four times
the mass of the Sun.
And remember,
its gonna have three to four times the mass of the Sun,
but it's going to be far denser. It's just gonna be a core.
It's gonna be an iron/nickel core that's no longer fusing.
So what happens to these stars?
It turns out that these are so massive
that even the neutron degeneracy pressure
will not be enough to keep the mass from imploding.
In these stars, all of the mass in these stars
will just keep imploding, so the neutron...
So we imagine in the first sun-like stars
things would collapse into white dwarfs.
Maybe i should draw that in white.
So they would collapse into white dwarfs.
No, thats not white either.
There you go. They would collapse into white dwarfs eventually.
So this is a white dwarf.
White dwarf.
And here the pressure that is keeping this
from collapsing further is electron degeneracy pressure.
The atoms are squeezed so much
that the electrons are essentially keeping them
from squeezing any more.
But if the pressure gets large enough then
you have the neutron star. So you have even
more mass in even a smaller...
And I'm not drawing this to scale.
Neutron stars are tiny.
White dwarf stars are on the scale
of an earth like planet. Neutron stars we learned in
the last video are on the scale of a city!
So they are super dense, super tiny and this has
more mass then this over here.
In fact, maybe I'll just draw this as a dot,
just so you have a sense how dense it is.
It's really just like one big atomic nucleus, or...
Well, it's still small.
It's like the size of a city.
It's like a nucleus the size of a city.
But this right here is a neutron star.
Neutron star.
And what's unintuitive about what I'm drawing is
each of these smaller things have more mass...
This overcame the electron degeneracy pressure
to collapse even further.
But if the mass is large enough,
and this is what we're talking about in this video,
even the neutron degeneracy pressure will not be able
to keep that mass from collapsing.
And there's even theoretical quark stars,
where the quark degeneracy pressure...
But even beyond that, all collapses into a single point,
and I'm simplifying here, but it collapses
into a single point of infinite density.
Infinite mass density.
And this is really the mass of a black hole.
And I'm calling it the mass of a black hole,
because there's different ways how you could view
where a black hole starts and ends
or what exactly is the black hole.
So this is all the mass,
all the mass of the black hole.
Or we could say, of the original star.
So when we're talking about that remnant
being three or four solar masses,
all of that mass is now being contained...
Well, not all of it.
Some of it was released as energy during the supernova,
and that was also true for the neutron star.
But most of that mass is now being contained
in this infinitely small point.
And you'll hear physicists and mathematicians talk about singularities.
Singularities.
And singularities are really points, even in mathematics,
where everything breaks down,
where nothing starts to make sense anymore,
where the mathematical equations don't give you a defined answer.
And this is a singularity,
because you have, you have a ton of mass in an infinitely small space.
You essentially have an infinite density right here.
And this is hard to visualize,
but you have kind of an infinite curvature in space-time right here.
And I can't visualize that.
So maybe we'll think about that in more videos.
But the reason why I said that it's...
There's different ways to think about what a black hole is,
about where it starts and ends.
This is where the mass is.
And if there was any other mass over here
it would obviously become attracted to this mass
and then become part of that singularity.
It would add to that mass, that already huge mass
that's in an infinitely small point in space.
But the reason why the boundary is hard to define is
because there is some point at which, there is some point in space
around that singularity, at which,
no matter what that thing is,
no matter how much energy that thing has,
it will not be able to escape the gravitational influence
of the black hole.
Of that ultra dense mass.
So even if it was electromagnetic radiation,
even if it was light,
and even if it was light that shone away from the mass,
it will eventually have to go back.
It will not be able to escape the gravitational influence.
And so the boundary where, if you're within that boundary,
that's really a sphere.
So that boundary around the singularity...
And that boundary around the singularity where
if you're within the boundary, no matter what you do,
no matter if you're electromagnetic radiation,
you're still going to...,
you're never going to be able to escape the black hole.
If you're beyond that boundary, you might be able to escape the black hole.
So this guy could escape.
This guy over here, no matter what he does,
is going to have to go back into the black hole.
This boundary right here is called the event horizon.
This right here is the event horizon.
Another word used in a lot of science fiction movies.
And for good reason,
because it's fascinating.
And we'll actually learn in future videos, hopefully, about Hawking radiation.
We'll see that it's not radiation from the black hole itself,
it's the byproduct of quantum effects
that occur at the event horizon.
But the event horizon is just this point in space
or the sphere in space, or this boundary in space.
Anything closer than or within the event horizon
has to eventually end up in the singularity,
contributing to that mass.
Anything on the outside has a chance of escaping.
So what does a black hole look like?
Well, not even light can escape from it,
so it will be black.
It will be black in the purest sense.
It will not emit any type of radiation from the black hole itself,
from that mass.
So here are some depictions I got from NASA of black holes.
And so just to be clear what's happening here,
what you're seeing here as black,
that is not... You can view that as the black hole.
And when people talk about the black hole,
that's often what they're talking about.
But there's a point of infinite density
at the center of this black sphere right here.
And what you see as that black sphere
that really is the boundary of the event horizon.
So this right here is the boundary of the event horizon.
And what we're seeing right here is the accretion disc around the black hole.
As all of this matter gets closer and closer to it
it's being squeezed more and more.
It's moving faster and faster
and getting hotter and hotter.
And that's why the way the artist depicted...
It looks like this stuff over here is redder and hotter than the stuff further out.
It's just accelerating as it approaches that event horizon.
Once it approaches that...
Once it's IN the event horizon
we cannot even see the light that it's emitting,
even though it would be starting to become unbelievably energetic.
Here's some other pictures.
This is a picture of a star being ripped apart.
Not a picture, it's actually an artist depiction.
All of these are artist depictions.
We would never be able to get such good pictures
of actual action occuring near black holes.
These are artist depictions.
This is a star being ripped apart by a black hole.
So this star is getting pretty close to this black hole.
Already out here various... where the star is
its very strong gravitational attraction,
so any mass that's being emitted from the star
in that direction is slowly being pulled into the black hole.
So this star is kind of being ripped apart by the black hole.
This is maybe a better depiction of it.
This is the star at first.
And once it gets under the influence
of the black hole's gravitation
it starts to kind of elongate
and it gets ripped apart,
and its matter starts spiraling in
closer and closer to the black hole.
And once it's IN the event horizon,
we won't even see it any more,
because even the light from that matter,
from that intensely hot matter that's entering into the black hole
cannot even escape the black hole itself.
Anyway, hopefully you found that interesting.
And I want to be clear: we still don't understand a lot about black holes.
In fact, this whole notion of a singularity,
the fact that all the math and all the theory breaks down at the singularity
is a pretty good sign that our theory isn't complete.
Because if our theory was complete,
we would maybe get something a little more sensible
than just all of our equations not making sense at that infinitely dense point.
Anyway, hopefully you found that interesting.

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Black Holes

Black Holes