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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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