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In the last video, we saw that if we started with a massive star, about 9 to 20 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 1.5 to 3 times the solar mass, or the mass of the sun. Then this remnant right here, let me just be clear, this 9 to 20 times is the mass of that star when it's in its main sequence. This 1.5 to 3 times is the mass once it's 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 that remnant, once it stops fusing, once it 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 to blow up, and this will condense 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, this is give or take, we don't know the actual firm boundaries here, but what if we have a star that is more than 20 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 dense remnant has more than 3 to 4 times the mass of the sun. And remember, it's going to have 3 to 4 times the mass of the sun, but it's going to be far denser, it's just going to be a core, it's going to be an iron-nickel core that's no longer fusing.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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But that remnant, once it stops fusing, once it 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 to blow up, and this will condense 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, this is give or take, we don't know the actual firm boundaries here, but what if we have a star that is more than 20 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 dense remnant has more than 3 to 4 times the mass of the sun. And remember, it's going to have 3 to 4 times the mass of the sun, but it's going to be far denser, it's just going to be a core, it's going to be an iron-nickel core that's no longer fusing. So what happens to these stars? So it turns out that these are so massive that even the neutron degeneracy pressure will not be enough to keep the mass from imploding. And these stars, all of the mass in these stars, will just keep imploding.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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And remember, it's going to have 3 to 4 times the mass of the sun, but it's going to be far denser, it's just going to be a core, it's going to be an iron-nickel core that's no longer fusing. So what happens to these stars? So it turns out that these are so massive that even the neutron degeneracy pressure will not be enough to keep the mass from imploding. And these stars, all of the mass in these stars, will just keep imploding. So in the neutron, so we imagine the first, in kind of sun-like stars, things would collapse into white dwarfs. So maybe I should draw that in white. So they would collapse into white dwarfs.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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And these stars, all of the mass in these stars, will just keep imploding. So in the neutron, so we imagine the first, in kind of sun-like stars, things would collapse into white dwarfs. So maybe I should draw that in white. So they would collapse into white dwarfs. No, that's not white either. There you go. They would collapse into white dwarfs eventually.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So they would collapse into white dwarfs. No, that's not white either. There you go. They would collapse into white dwarfs eventually. So this is a white dwarf. And here, the pressure that's keeping this from collapsing further is electron degeneracy pressure. The atoms are squeezed so much that the electrons are essentially keeping them from squeezing anymore.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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They would collapse into white dwarfs eventually. So this is a white dwarf. And here, the pressure that's keeping this from collapsing further is electron degeneracy pressure. The atoms are squeezed so much that the electrons are essentially keeping them from squeezing anymore. But if the pressure gets large enough, then you have the neutron star. So you have even more mass and 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.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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The atoms are squeezed so much that the electrons are essentially keeping them from squeezing anymore. But if the pressure gets large enough, then you have the neutron star. So you have even more mass and 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 these are super dense, super tiny, and this has more mass than this over here. In fact, maybe I should just draw it as a dot, just so you have a sense of how dense it is.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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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 these are super dense, super tiny, and this has more mass than this over here. In fact, maybe I should just draw it as a dot, just so you have a sense of how dense it is. It's really just like one big atomic nucleus. Well, it's still small, but it's the size of a city. It's like a nucleus the size of a city.
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In fact, maybe I should just draw it as a dot, just so you have a sense of how dense it is. It's really just like one big atomic nucleus. Well, it's still small, but it's the size of a city. It's like a nucleus the size of a city. But this right here is a 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.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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It's like a nucleus the size of a city. But this right here is a 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 if you get even beyond that, it all collapses into a single point, and I'm simplifying here, but it collapses into a single point of infinite density. Infinite mass density.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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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 if you get even beyond that, it 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 can view where a black hole starts and ends or what exactly is the black hole. So this is all the mass of the black hole.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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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 can view where a black hole starts and ends or what exactly is the black hole. So this is all the mass of the black hole. Or we could say of the original star. So when we're talking about that remnant being 3 to 4 solar masses, all of that mass is now being contained. Well, not all of it.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So this is all the mass of the black hole. Or we could say of the original star. So when we're talking about that remnant being 3 to 4 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 of 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.
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Well, not all of it. Some of it was released as energy during the supernova, and that was also true of 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. 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 a ton of mass in an infinitely small space. You essentially have an infinite density right here.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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And you'll hear physicists and mathematicians talk about 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 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. Maybe we'll think about that in more videos. But the reason why I said that there's different ways to think about where a black hole is or where it starts and ends is that this is where the mass is.
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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. Maybe we'll think about that in more videos. But the reason why I said that there's different ways to think about where a black hole is or where it starts and ends is that this is where the mass is. And if there was any other mass that was over here, it would obviously be 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's 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.
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But the reason why I said that there's different ways to think about where a black hole is or where it starts and ends is that this is where the mass is. And if there was any other mass that was over here, it would obviously be 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's 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's light that's shown 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, 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.
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But the reason why the boundary is hard to define is because there's 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's light that's shown 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, 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 are 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.
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And so the boundary, where if you're within that boundary, that's really a sphere, so 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 are 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. Another word used in a lot of science fiction movies, and for good reason, because it's fascinating. We'll actually learn in future videos, hopefully about Hawking radiation, we'll see that that is not radiation from the black hole itself.
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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. Another word used in a lot of science fiction movies, and for good reason, because it's fascinating. We'll actually learn in future videos, hopefully about Hawking radiation, we'll see that that is not radiation from the black hole itself. It's the byproduct of quantum effects that are occurring at the event horizon. But the event horizon is just this kind of point in space, or this 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.
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We'll actually learn in future videos, hopefully about Hawking radiation, we'll see that that is not radiation from the black hole itself. It's the byproduct of quantum effects that are occurring at the event horizon. But the event horizon is just this kind of point in space, or this 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 would a black hole look like? Well, not even light can escape from it, so it will be black.
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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 would 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.
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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. So just to be clear, what's happening here, what you're seeing here is black. You can view that as the black hole. When people talk about the black hole, that's often what they're talking about.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So here are some depictions I got from NASA of black holes. So just to be clear, what's happening here, what you're seeing here is black. You can view that as the black hole. 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 is that black sphere, that really is the boundary of the event horizon. So this right here is the boundary of the event horizon.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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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 is 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 disk 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 this art is depicted, it looks like this stuff over here is redder and hotter than the stuff further out.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So this right here is the boundary of the event horizon. And what we're seeing right here is the accretion disk 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 this art is 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's in the event horizon, we cannot even see the light that it's emitting, even though it's starting to become unbelievably energetic. Here's some other pictures.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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And that's why the way this art is 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's in the event horizon, we cannot even see the light that it's emitting, even though it's starting to become unbelievably energetic. Here's some other pictures. This is a picture of a star being ripped apart. Not a picture, this is actually an artist's depiction. We never were able to get such good pictures of actual action occurring near black holes.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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Here's some other pictures. This is a picture of a star being ripped apart. Not a picture, this is actually an artist's depiction. We never were able to get such good pictures of actual action occurring near black holes. These are artist's depictions. But this is a star being ripped apart by a black hole. So this star is getting pretty close to this black hole.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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We never were able to get such good pictures of actual action occurring near black holes. These are artist's depictions. But 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, where the star is, it's 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 the star is kind of being ripped apart by the black hole.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So this star is getting pretty close to this black hole. Already out here, where the star is, it's 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 the 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 becomes under the influence of the black hole's gravitation, it starts to kind of elongate and gets ripped apart, and its matter starts spiraling in closer and closer to that black hole. Once it's in the event horizon, we won't even see it anymore, because even the light from that matter, that intensely hot matter that's entering into the black hole, cannot even escape the black hole itself.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So the 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 becomes under the influence of the black hole's gravitation, it starts to kind of elongate and gets ripped apart, and its matter starts spiraling in closer and closer to that black hole. Once it's in the event horizon, we won't even see it anymore, because even the light from that matter, 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.
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Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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Once it's in the event horizon, we won't even see it anymore, because even the light from that matter, 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 is complete, we would maybe get something a little bit more sensical than just all of our equations not making sense at that infinitely dense point. Anyway, hopefully you found that interesting.
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Probably, and I would say it ranks their top three, because it really enabled people like Hubble to start realizing that the universe is expanding. Or even being able to think about how to measure distances to objects in space well beyond the reach of our tools with parallax. We saw with parallax, you have to have extremely sensitive instruments just to even measure distances to stars relatively close to us. Very sensitive instruments to get to stars maybe further out into our galaxy. And we don't have the instruments even today to measure things beyond our galaxy. But because of Henrietta Swan Leavitt, we were able to approximate or get good senses of the distance to objects beyond our galaxy. So let's just think about what she did.
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Very sensitive instruments to get to stars maybe further out into our galaxy. And we don't have the instruments even today to measure things beyond our galaxy. But because of Henrietta Swan Leavitt, we were able to approximate or get good senses of the distance to objects beyond our galaxy. So let's just think about what she did. So her job was literally to classify stars in the large Magellanic, I have trouble saying that, Magellanic cloud and the small Magellanic clouds. And this is what they look like from the southern hemisphere. This is the large right over here.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So let's just think about what she did. So her job was literally to classify stars in the large Magellanic, I have trouble saying that, Magellanic cloud and the small Magellanic clouds. And this is what they look like from the southern hemisphere. This is the large right over here. And this is the small right over here. And remember, this is before Hubble realized or showed the world that there are stars beyond our galaxy, that there are galaxies beyond our galaxy. So at this point in time, people didn't even fully appreciate that these were separate galaxies.
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This is the large right over here. And this is the small right over here. And remember, this is before Hubble realized or showed the world that there are stars beyond our galaxy, that there are galaxies beyond our galaxy. So at this point in time, people didn't even fully appreciate that these were separate galaxies. We just said, hey, these are kind of these blobs or these clusters of stars that we see in the southern hemisphere. And just to get a sense of where they are relative to our galaxy, the Milky Way galaxy, this is obviously not an actual picture. We can't take a picture from this vantage point.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So at this point in time, people didn't even fully appreciate that these were separate galaxies. We just said, hey, these are kind of these blobs or these clusters of stars that we see in the southern hemisphere. And just to get a sense of where they are relative to our galaxy, the Milky Way galaxy, this is obviously not an actual picture. We can't take a picture from this vantage point. This would have to be very, very far away. But this is the Milky Way right here. And this is the small Magellanic cloud.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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We can't take a picture from this vantage point. This would have to be very, very far away. But this is the Milky Way right here. And this is the small Magellanic cloud. And this is the large Magellanic cloud. I'm getting better at saying it. So her job was literally just to classify the different stars that she saw.
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And this is the small Magellanic cloud. And this is the large Magellanic cloud. I'm getting better at saying it. So her job was literally just to classify the different stars that she saw. But while she was classifying, she looked at these things called variables. It turns out what she was looking at were a class of stars called Cepheid or Cepheid variable stars. And what's interesting about them is two things.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So her job was literally just to classify the different stars that she saw. But while she was classifying, she looked at these things called variables. It turns out what she was looking at were a class of stars called Cepheid or Cepheid variable stars. And what's interesting about them is two things. They're super duper bright. They're up to 30,000 times as luminous as the sun. And they're 5 to 20 times more massive than the sun.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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And what's interesting about them is two things. They're super duper bright. They're up to 30,000 times as luminous as the sun. And they're 5 to 20 times more massive than the sun. 5 to 20 times the sun's mass. But what makes them interesting is one, they're really bright. So you can see them from really far away.
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And they're 5 to 20 times more massive than the sun. 5 to 20 times the sun's mass. But what makes them interesting is one, they're really bright. So you can see them from really far away. You can see these Cepheid variable stars in other galaxies. In fact, we can see it well beyond even the small Magellanic cloud or the large Magellanic cloud. But you can see these stars in other galaxies.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So you can see them from really far away. You can see these Cepheid variable stars in other galaxies. In fact, we can see it well beyond even the small Magellanic cloud or the large Magellanic cloud. But you can see these stars in other galaxies. And what's even more interesting about them is that their intensity is variable, that they become brighter and dimmer with a well-defined period. So if you're looking at a Cepheid variable star, and this is just kind of a simulation, a very cheap simulation, it might look like this. And then over the course of the next three, four days, it might reduce in intensity to something like this.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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But you can see these stars in other galaxies. And what's even more interesting about them is that their intensity is variable, that they become brighter and dimmer with a well-defined period. So if you're looking at a Cepheid variable star, and this is just kind of a simulation, a very cheap simulation, it might look like this. And then over the course of the next three, four days, it might reduce in intensity to something like this. And then after three, four days again, it will look like this. And then it'll look like this again. So its actual intensity is going up and down with a well-defined period.
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And then over the course of the next three, four days, it might reduce in intensity to something like this. And then after three, four days again, it will look like this. And then it'll look like this again. So its actual intensity is going up and down with a well-defined period. So if this takes three days and this is another three days, then the period, one entire cycle of its going from low intensity back to high intensity is going to be six days. So this is a six-day period. And what Henrietta Leavitt saw, and this wasn't an obvious thing to do, she plotted, she assumed that things, everything in each of these clouds are roughly the same distance away.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So its actual intensity is going up and down with a well-defined period. So if this takes three days and this is another three days, then the period, one entire cycle of its going from low intensity back to high intensity is going to be six days. So this is a six-day period. And what Henrietta Leavitt saw, and this wasn't an obvious thing to do, she plotted, she assumed that things, everything in each of these clouds are roughly the same distance away. Everything in the large Magellanic cloud is roughly the same distance away. And it's obviously not exact. This is an entire galaxy, so you have obviously things further away in that galaxy and things closer up.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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And what Henrietta Leavitt saw, and this wasn't an obvious thing to do, she plotted, she assumed that things, everything in each of these clouds are roughly the same distance away. Everything in the large Magellanic cloud is roughly the same distance away. And it's obviously not exact. This is an entire galaxy, so you have obviously things further away in that galaxy and things closer up. You have stars here and here, and their distance isn't going to be exactly the same to us, that we're sitting maybe over here someplace. But it's going to be close. It wasn't a bad approximation.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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This is an entire galaxy, so you have obviously things further away in that galaxy and things closer up. You have stars here and here, and their distance isn't going to be exactly the same to us, that we're sitting maybe over here someplace. But it's going to be close. It wasn't a bad approximation. And by making that assumption, she saw something pretty neat. So let me plot this right over here. So she plotted on the horizontal axis, she plotted the relative luminosity.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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It wasn't a bad approximation. And by making that assumption, she saw something pretty neat. So let me plot this right over here. So she plotted on the horizontal axis, she plotted the relative luminosity. So really, the only way that she could measure this is just how bright did they look to her? And she's assuming that they're same distance. So obviously, if you have a brighter star, but it's much, much further away, it's going to look dimmer.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So she plotted on the horizontal axis, she plotted the relative luminosity. So really, the only way that she could measure this is just how bright did they look to her? And she's assuming that they're same distance. So obviously, if you have a brighter star, but it's much, much further away, it's going to look dimmer. So if you assume that they're all roughly the same distance, then how bright it is will tell you how bright it is at the actual star. So she plotted relative luminosity of a star on one axis, and on the other axis, she plotted the period of these variable stars. And what I'm going to do is I'm going to do this on a logarithmic scale.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So obviously, if you have a brighter star, but it's much, much further away, it's going to look dimmer. So if you assume that they're all roughly the same distance, then how bright it is will tell you how bright it is at the actual star. So she plotted relative luminosity of a star on one axis, and on the other axis, she plotted the period of these variable stars. And what I'm going to do is I'm going to do this on a logarithmic scale. So let's say that this is in days. So this is one day, this is 10 days, this is 100 days, right over here. It's a logarithmic scale because I'm going up in powers of 10.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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And what I'm going to do is I'm going to do this on a logarithmic scale. So let's say that this is in days. So this is one day, this is 10 days, this is 100 days, right over here. It's a logarithmic scale because I'm going up in powers of 10. I could say that if we take the log of these, this would be 0, this would be 1, this would be 2. And so that's what I'm using as a scale. So I'm using the log of the period, or I'm just marking them as 1, 10, 100, but I'm giving each of these factors of 10 an equal spacing.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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It's a logarithmic scale because I'm going up in powers of 10. I could say that if we take the log of these, this would be 0, this would be 1, this would be 2. And so that's what I'm using as a scale. So I'm using the log of the period, or I'm just marking them as 1, 10, 100, but I'm giving each of these factors of 10 an equal spacing. When you plot it on this scale, the relative luminosity versus the period, she got a plot that looks something like this. This is obviously not exact. She got a plot that looks something like this.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So I'm using the log of the period, or I'm just marking them as 1, 10, 100, but I'm giving each of these factors of 10 an equal spacing. When you plot it on this scale, the relative luminosity versus the period, she got a plot that looks something like this. This is obviously not exact. She got a plot that looks something like this. It was a fairly linear relationship when you plot the relative luminosity against the log of the period. So this is obviously a logarithmic scale over here. And so you could fit a line.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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She got a plot that looks something like this. It was a fairly linear relationship when you plot the relative luminosity against the log of the period. So this is obviously a logarithmic scale over here. And so you could fit a line. And why I'd argue, and I think most people would argue, this is one of the most important discoveries in astronomy, is if you know, because think about what the problem here is. We can look at all of these stars in space. Let's say you look at a fraction of the sky and you look at something that looks like that.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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And so you could fit a line. And why I'd argue, and I think most people would argue, this is one of the most important discoveries in astronomy, is if you know, because think about what the problem here is. We can look at all of these stars in space. Let's say you look at a fraction of the sky and you look at something that looks like that. So it's really bright. And then you see something dim that looks like that. So if you have a very superficial understanding, you say, oh, this star is brighter.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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Let's say you look at a fraction of the sky and you look at something that looks like that. So it's really bright. And then you see something dim that looks like that. So if you have a very superficial understanding, you say, oh, this star is brighter. You would say that this is a fundamentally brighter star. But how do you know that? Maybe instead of being brighter, maybe it's just a dimmer, closer star.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So if you have a very superficial understanding, you say, oh, this star is brighter. You would say that this is a fundamentally brighter star. But how do you know that? Maybe instead of being brighter, maybe it's just a dimmer, closer star. Maybe this is an entire galaxy, but it's so far away that you can't even tell. But all of a sudden, by the work that Henrietta Leavitt did, if you see one of these Cepheid variable stars in another galaxy, you know its relative brightness compared to other Cepheid variable stars. And so if you can place just one of these Cepheid variable stars, if you know exactly the distance to one of them, and then you know its absolute luminosity, you then know the absolute luminosity of any other Cepheid variable stars.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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Maybe instead of being brighter, maybe it's just a dimmer, closer star. Maybe this is an entire galaxy, but it's so far away that you can't even tell. But all of a sudden, by the work that Henrietta Leavitt did, if you see one of these Cepheid variable stars in another galaxy, you know its relative brightness compared to other Cepheid variable stars. And so if you can place just one of these Cepheid variable stars, if you know exactly the distance to one of them, and then you know its absolute luminosity, you then know the absolute luminosity of any other Cepheid variable stars. So let's say using parallax, which is our other tool, we find, let's say there's some star in our galaxy. And let's say using parallax, we're able to come up with a pretty good measure that it is, I don't know, let's say it's 100 light years away. And this star is a Cepheid variable star.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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And so if you can place just one of these Cepheid variable stars, if you know exactly the distance to one of them, and then you know its absolute luminosity, you then know the absolute luminosity of any other Cepheid variable stars. So let's say using parallax, which is our other tool, we find, let's say there's some star in our galaxy. And let's say using parallax, we're able to come up with a pretty good measure that it is, I don't know, let's say it's 100 light years away. And this star is a Cepheid variable star. And let's say its period is one day. So we now know something interesting. We know variable stars with a period of one day at 100 light years away will look like this.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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And this star is a Cepheid variable star. And let's say its period is one day. So we now know something interesting. We know variable stars with a period of one day at 100 light years away will look like this. Will look like this drawing right over here. So if we later on see a Cepheid variable star with a period of one day, so it gets brighter and dim over the course of one day, and maybe it's redshifted as well, but maybe it looks a little bit dimmer, it looks like this. We now know that if it was 100 light years away, it would have this luminosity.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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We know variable stars with a period of one day at 100 light years away will look like this. Will look like this drawing right over here. So if we later on see a Cepheid variable star with a period of one day, so it gets brighter and dim over the course of one day, and maybe it's redshifted as well, but maybe it looks a little bit dimmer, it looks like this. We now know that if it was 100 light years away, it would have this luminosity. So based on how much dimmer it is, we can then figure out how much further away this Cepheid variable star is. If that confuses you a little bit, I'll do a little bit more details in the next few videos so we can get a closer sense of how the math would work. But this was a big discovery, just discovering this class of stars, this Cepheid variable class.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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We now know that if it was 100 light years away, it would have this luminosity. So based on how much dimmer it is, we can then figure out how much further away this Cepheid variable star is. If that confuses you a little bit, I'll do a little bit more details in the next few videos so we can get a closer sense of how the math would work. But this was a big discovery, just discovering this class of stars, this Cepheid variable class. She wasn't the one who discovered them. People knew before her that there were these stars that got brighter and dimmer. But what her big discovery was is seeing this linear relationship between the relative luminosity of these stars and their period.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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But this was a big discovery, just discovering this class of stars, this Cepheid variable class. She wasn't the one who discovered them. People knew before her that there were these stars that got brighter and dimmer. But what her big discovery was is seeing this linear relationship between the relative luminosity of these stars and their period. Because then, if we see Cepheid variable stars in completely different galaxies or galactic clusters, by looking at their period, we know what their real relative luminosity is. And then we could guess how far those things really are. Or we could estimate how far those things really are.
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Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3
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So let's actually do that here. So let's just assume that there are 100 billion stars. So that's my first term right over there. Let's say that one-fourth will develop planets. And let's say of the solar systems that develop planets, on average, let's say that they develop an average of 0.1 planets capable of sustaining life. Or really, that you'll have one planet for every 10 of these solar systems with planets. That's just my assumption there.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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Let's say that one-fourth will develop planets. And let's say of the solar systems that develop planets, on average, let's say that they develop an average of 0.1 planets capable of sustaining life. Or really, that you'll have one planet for every 10 of these solar systems with planets. That's just my assumption there. I don't know if that's right. Now let's multiply that times the fraction of these planets capable of sustaining life that actually will get life. And I don't know what that is, but I hinted in previous videos that life is one of those things that it seems like if you have all the right ingredients, it's so robust that you have life at these underwater volcanoes.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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That's just my assumption there. I don't know if that's right. Now let's multiply that times the fraction of these planets capable of sustaining life that actually will get life. And I don't know what that is, but I hinted in previous videos that life is one of those things that it seems like if you have all the right ingredients, it's so robust that you have life at these underwater volcanoes. You have bacteria that can process all sorts of weird things. So let's say that that probability is pretty high. Let's say that that is 50% or half of the planets that are capable of getting life actually do have life.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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And I don't know what that is, but I hinted in previous videos that life is one of those things that it seems like if you have all the right ingredients, it's so robust that you have life at these underwater volcanoes. You have bacteria that can process all sorts of weird things. So let's say that that probability is pretty high. Let's say that that is 50% or half of the planets that are capable of getting life actually do have life. I would guess that that might even be higher. But once again, just a guess. Now we have to think about of the life, what fraction becomes intelligent?
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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Let's say that that is 50% or half of the planets that are capable of getting life actually do have life. I would guess that that might even be higher. But once again, just a guess. Now we have to think about of the life, what fraction becomes intelligent? What becomes intelligent over some point in the history? Well, I'll say it's a tenth. Maybe if the asteroids didn't kill the dinosaurs, it wouldn't have happened on Earth.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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Now we have to think about of the life, what fraction becomes intelligent? What becomes intelligent over some point in the history? Well, I'll say it's a tenth. Maybe if the asteroids didn't kill the dinosaurs, it wouldn't have happened on Earth. Who knows? Or maybe we just have some very intelligent dinosaurs around. We don't know.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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Maybe if the asteroids didn't kill the dinosaurs, it wouldn't have happened on Earth. Who knows? Or maybe we just have some very intelligent dinosaurs around. We don't know. And maybe there's other forms of intelligent life even on our own planet that we haven't fully appreciated. Dolphins are a good candidate. Some people believe that octopuses, because they have such flexible arms, there's a theory that they could develop eventually the ability to kind of one day, if their brains mature and all of the rest make tools the same way primitive primates eventually were able to have larger brain sizes and actually manipulate things to make tools.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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We don't know. And maybe there's other forms of intelligent life even on our own planet that we haven't fully appreciated. Dolphins are a good candidate. Some people believe that octopuses, because they have such flexible arms, there's a theory that they could develop eventually the ability to kind of one day, if their brains mature and all of the rest make tools the same way primitive primates eventually were able to have larger brain sizes and actually manipulate things to make tools. So who knows? I don't want to get into all of that. So there's a 1 in 10 chance that you get intelligent life.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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Some people believe that octopuses, because they have such flexible arms, there's a theory that they could develop eventually the ability to kind of one day, if their brains mature and all of the rest make tools the same way primitive primates eventually were able to have larger brain sizes and actually manipulate things to make tools. So who knows? I don't want to get into all of that. So there's a 1 in 10 chance that you get intelligent life. And then assuming that intelligent life shows up, what fraction is going to become detectable? I don't know. I don't know whether dolphins will ever communicate via radio or not.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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So there's a 1 in 10 chance that you get intelligent life. And then assuming that intelligent life shows up, what fraction is going to become detectable? I don't know. I don't know whether dolphins will ever communicate via radio or not. So let's just say that that is, I don't know, let's say that that is another 1 in 10 chance or I'll say 0.1. And then we have to multiply it times the detectable life of the civilization on average. Once again, huge assumptions being here.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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I don't know whether dolphins will ever communicate via radio or not. So let's just say that that is, I don't know, let's say that that is another 1 in 10 chance or I'll say 0.1. And then we have to multiply it times the detectable life of the civilization on average. Once again, huge assumptions being here. But the detectable life of a civilization, let me just put it at 10,000 years. Either they destroy themselves or they get beyond that type of radio-type communication, electromagnetic-type communication. Maybe they start doing all sorts of weird, wacky things.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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Once again, huge assumptions being here. But the detectable life of a civilization, let me just put it at 10,000 years. Either they destroy themselves or they get beyond that type of radio-type communication, electromagnetic-type communication. Maybe they start doing all sorts of weird, wacky things. Probably won't take you 10,000 years to even progress it. That might take you less time. But let's just do this just for the sake of fun.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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Maybe they start doing all sorts of weird, wacky things. Probably won't take you 10,000 years to even progress it. That might take you less time. But let's just do this just for the sake of fun. And then the lifespan of your average star, that's probably one of the things that we have the best sense of. So on average, let's put it at 10 billion years. So let's calculate all of this.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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But let's just do this just for the sake of fun. And then the lifespan of your average star, that's probably one of the things that we have the best sense of. So on average, let's put it at 10 billion years. So let's calculate all of this. Let's get my handy TI-85 out. And so we're going to have 100 billion. That's 1 times 10 to the 9th.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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So let's calculate all of this. Let's get my handy TI-85 out. And so we're going to have 100 billion. That's 1 times 10 to the 9th. Sorry, that's 100 times 10 to the 9th. So let me clear it. Or you could have 1 times 10 to the 11th.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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That's 1 times 10 to the 9th. Sorry, that's 100 times 10 to the 9th. So let me clear it. Or you could have 1 times 10 to the 11th. That is 100 billion times 0.25 times 0.1 times 0.5 times 0.5 times 0.1 again times 0.1 times 0.1 again times 0.1 times 10,000 divided by 10 billion. So times 10,000 divided by 10 billion. So that's 1E10.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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Or you could have 1 times 10 to the 11th. That is 100 billion times 0.25 times 0.1 times 0.5 times 0.5 times 0.1 again times 0.1 times 0.1 again times 0.1 times 10,000 divided by 10 billion. So times 10,000 divided by 10 billion. So that's 1E10. 1 times 10 to the 10th power. 1 with 10 zeros. So let's see what we get.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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So that's 1E10. 1 times 10 to the 10th power. 1 with 10 zeros. So let's see what we get. We get 12.5, which is kind of a neat number. But these are heavily dependent on this. So we're saying, given these assumptions, there should be 12.5 detectable civilizations in our galaxy right now.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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So let's see what we get. We get 12.5, which is kind of a neat number. But these are heavily dependent on this. So we're saying, given these assumptions, there should be 12.5 detectable civilizations in our galaxy right now. So the question is, why aren't we detecting it? Maybe their radio signals, maybe their electromagnetic waves are getting to us. But we can't differentiate it from noise right now.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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So we're saying, given these assumptions, there should be 12.5 detectable civilizations in our galaxy right now. So the question is, why aren't we detecting it? Maybe their radio signals, maybe their electromagnetic waves are getting to us. But we can't differentiate it from noise right now. And that's what the whole SETI project is all about, trying to keep track of all of this information, all of these radio waves and electromagnetic waves that are coming from outer space towards Earth. And seeing if any of them actually have any non-noise signal that actually look like they're being generated by some type of intelligent civilization. So maybe we're getting them and we're just not detecting them.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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But we can't differentiate it from noise right now. And that's what the whole SETI project is all about, trying to keep track of all of this information, all of these radio waves and electromagnetic waves that are coming from outer space towards Earth. And seeing if any of them actually have any non-noise signal that actually look like they're being generated by some type of intelligent civilization. So maybe we're getting them and we're just not detecting them. Or maybe something else is at play. Maybe we've overestimated one of these. Maybe there is a lot of life, but maybe they're not using electromagnetic waves to communicate.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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So maybe we're getting them and we're just not detecting them. Or maybe something else is at play. Maybe we've overestimated one of these. Maybe there is a lot of life, but maybe they're not using electromagnetic waves to communicate. Maybe that's some type of primitive way of communicating. Maybe they start doing telepathy or something crazy. Or they start using some type of quantum thing that allows them to communicate more directly without having to wait for the speed of light.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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Maybe there is a lot of life, but maybe they're not using electromagnetic waves to communicate. Maybe that's some type of primitive way of communicating. Maybe they start doing telepathy or something crazy. Or they start using some type of quantum thing that allows them to communicate more directly without having to wait for the speed of light. Maybe that is a very slow way to communicate. And it is a slow way, frankly, if you're trying to communicate across solar systems and stars and planets or even across galaxies, one could imagine. So maybe we're just kind of in a transition state of communication.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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Or they start using some type of quantum thing that allows them to communicate more directly without having to wait for the speed of light. Maybe that is a very slow way to communicate. And it is a slow way, frankly, if you're trying to communicate across solar systems and stars and planets or even across galaxies, one could imagine. So maybe we're just kind of in a transition state of communication. That electromagnetic waves, radio, and all the rest is just a transition state. Maybe in 100 years we'll figure out another better way that's not detectable in our traditional ways. Maybe we're being bombarded with another type of communication mechanism that we're just not ready to perceive yet.
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Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3
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What I want to do in this video is give ourselves a basic introduction to the phenomenon of light. And light is, at least to me, it is mysterious. Because on one level, it really defines our reality. It's maybe the most defining characteristic of our reality. Everything we see, how we perceive reality, is based on light bouncing off of objects, or bending around objects, or diffracting around objects, and then being sensed by our eyes, and then sending signals into our brain that create models of the world we see around us. So it really is almost the defining characteristic of our reality. But at the same time, when you really go down to experiment and observe with light, it starts to have a bunch of mysterious properties, and to a large degree, it is not fully understood yet.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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It's maybe the most defining characteristic of our reality. Everything we see, how we perceive reality, is based on light bouncing off of objects, or bending around objects, or diffracting around objects, and then being sensed by our eyes, and then sending signals into our brain that create models of the world we see around us. So it really is almost the defining characteristic of our reality. But at the same time, when you really go down to experiment and observe with light, it starts to have a bunch of mysterious properties, and to a large degree, it is not fully understood yet. And probably the most amazing thing about light, well, actually there's tons of amazing things about light, but one of the mysterious things is when you really get down to it, and this is actually not just true of light, this is actually true of almost anything once you get onto a small enough quantum mechanical level, but light behaves as both a wave and a particle. And this is probably not that intuitive to you, because it's not that intuitive to me. In my life, I'm used to certain things behaving as waves, like sound waves, or the waves of an ocean, and I'm used to certain things behaving like particles, like basketballs, or my coffee cup.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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But at the same time, when you really go down to experiment and observe with light, it starts to have a bunch of mysterious properties, and to a large degree, it is not fully understood yet. And probably the most amazing thing about light, well, actually there's tons of amazing things about light, but one of the mysterious things is when you really get down to it, and this is actually not just true of light, this is actually true of almost anything once you get onto a small enough quantum mechanical level, but light behaves as both a wave and a particle. And this is probably not that intuitive to you, because it's not that intuitive to me. In my life, I'm used to certain things behaving as waves, like sound waves, or the waves of an ocean, and I'm used to certain things behaving like particles, like basketballs, or my coffee cup. I'm not used to things behaving as both. And it really depends on what experiment you run and how you observe the light. So when you observe it as a particle, and this comes out of Einstein's work with the photoelectric effect, and I won't go into the details here, maybe in a future video when we start thinking about quantum mechanics, you can view light as a train of particles moving at the speed of light, which I'll talk about in a second.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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In my life, I'm used to certain things behaving as waves, like sound waves, or the waves of an ocean, and I'm used to certain things behaving like particles, like basketballs, or my coffee cup. I'm not used to things behaving as both. And it really depends on what experiment you run and how you observe the light. So when you observe it as a particle, and this comes out of Einstein's work with the photoelectric effect, and I won't go into the details here, maybe in a future video when we start thinking about quantum mechanics, you can view light as a train of particles moving at the speed of light, which I'll talk about in a second. We call these particles photons. If you view light in other ways, and you see it even when you see light being refracted by a prism here, it looks like it is a wave. And it has the properties of a wave.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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So when you observe it as a particle, and this comes out of Einstein's work with the photoelectric effect, and I won't go into the details here, maybe in a future video when we start thinking about quantum mechanics, you can view light as a train of particles moving at the speed of light, which I'll talk about in a second. We call these particles photons. If you view light in other ways, and you see it even when you see light being refracted by a prism here, it looks like it is a wave. And it has the properties of a wave. It has a frequency, and it has a wavelength. And like other waves, the velocity of that wave is the frequency times its wavelength. Now, even if you ignore this particle aspect of light, if you just look at the wave aspect of light, it's still fascinating, because most waves require a medium to travel through.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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And it has the properties of a wave. It has a frequency, and it has a wavelength. And like other waves, the velocity of that wave is the frequency times its wavelength. Now, even if you ignore this particle aspect of light, if you just look at the wave aspect of light, it's still fascinating, because most waves require a medium to travel through. So for example, if I think about how sound travels through air, so let me draw a bunch of air particles. I'll draw a sound wave traveling through the air particles. What happens in a sound wave is you compress some of the air particles, and those compress the ones next to them, and so you have points in the air that have higher pressure and points that have lower pressure.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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Now, even if you ignore this particle aspect of light, if you just look at the wave aspect of light, it's still fascinating, because most waves require a medium to travel through. So for example, if I think about how sound travels through air, so let me draw a bunch of air particles. I'll draw a sound wave traveling through the air particles. What happens in a sound wave is you compress some of the air particles, and those compress the ones next to them, and so you have points in the air that have higher pressure and points that have lower pressure. And you can plot that. So we have high pressure over here, high pressure, low pressure, high pressure, low pressure. And as these things bump into each other, and this wave essentially travels to the right, and if you were to plot that, you would see this waveform traveling to the right.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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What happens in a sound wave is you compress some of the air particles, and those compress the ones next to them, and so you have points in the air that have higher pressure and points that have lower pressure. And you can plot that. So we have high pressure over here, high pressure, low pressure, high pressure, low pressure. And as these things bump into each other, and this wave essentially travels to the right, and if you were to plot that, you would see this waveform traveling to the right. But this is all predicated, or this is all based on, this energy traveling through a medium. And I'm used to visualizing waves in that way. But light needs no medium.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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And as these things bump into each other, and this wave essentially travels to the right, and if you were to plot that, you would see this waveform traveling to the right. But this is all predicated, or this is all based on, this energy traveling through a medium. And I'm used to visualizing waves in that way. But light needs no medium. Light will actually travel fastest through nothing, through a vacuum. And it will travel at an unimaginably fast speed, 3 times 10 to the 8th meters per second. And just to give you a sense of this, this is 300 million meters per second.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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But light needs no medium. Light will actually travel fastest through nothing, through a vacuum. And it will travel at an unimaginably fast speed, 3 times 10 to the 8th meters per second. And just to give you a sense of this, this is 300 million meters per second. Or another way of thinking about it is it would take light less than a seventh of a second to travel around the Earth, or it would travel around the Earth more than seven times in one second. So unimaginably fast. And not only is this just a super fast rate, or a super fast speed, but once again, it tells us that light is something fundamental to our universe.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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And just to give you a sense of this, this is 300 million meters per second. Or another way of thinking about it is it would take light less than a seventh of a second to travel around the Earth, or it would travel around the Earth more than seven times in one second. So unimaginably fast. And not only is this just a super fast rate, or a super fast speed, but once again, it tells us that light is something fundamental to our universe. Because it's not just an unimaginable fast speed, it is the fastest speed not just known to physics, but possible in physics. So once again, something very unintuitive to us in our everyday realm. We always imagine that, okay, if something is going at some speed, maybe if there was an ant riding on top of that something, and it was moving in the same direction, it would be going even faster.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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And not only is this just a super fast rate, or a super fast speed, but once again, it tells us that light is something fundamental to our universe. Because it's not just an unimaginable fast speed, it is the fastest speed not just known to physics, but possible in physics. So once again, something very unintuitive to us in our everyday realm. We always imagine that, okay, if something is going at some speed, maybe if there was an ant riding on top of that something, and it was moving in the same direction, it would be going even faster. But nothing can go faster than the speed of light. It's absolutely impossible based on our current understanding of physics. So it's not just a fast speed, it is the fastest speed.
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Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3
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