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So these are summer. So that's the summer in the southern hemisphere, which occurs during the winter in the northern hemisphere, and vice versa. Summer in the northern hemisphere occurs during winter in the southern hemisphere. And so the question might be rising in your head. And I did see a few comments on that first... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
And so the question might be rising in your head. And I did see a few comments on that first video asking this question. And it's a good one. If we are closer to the sun in January, or we are really closest to the sun in January, and so we're getting more solar radiation in January, does that moderate the winter in the... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
If we are closer to the sun in January, or we are really closest to the sun in January, and so we're getting more solar radiation in January, does that moderate the winter in the northern hemisphere? Or I guess another way to think about it, does it make the summer in the southern hemisphere when we are closer to the s... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
Because it's hot up there, but hey, we're a little bit further from the sun. And does it make the southern hemisphere winter colder? So once again, does it make this more extreme? Because it's already winter and we're further from the sun, so maybe we're also getting less radiation. And so there's a couple ways to thin... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
Because it's already winter and we're further from the sun, so maybe we're also getting less radiation. And so there's a couple ways to think about it. One, it is true that when we're further, we are getting a little bit less radiation from the sun, or we're getting heated up a little bit less. But the one reality is t... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
But the one reality is that the southern hemisphere climate as a whole is not more extreme, despite getting heated up, getting more solar energy in the summer, and getting less solar energy in the winter. And the reason why it is not as extreme, let me draw the equator here, just so that we can separate our hemispheres... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
So just if you look at the surface of the southern hemisphere, you're looking at a lot more water than the surface of the northern hemisphere. This is, of course, a Mercator projection, and so it distorts things so that things near the poles get really kind of built up to look really huge, even though they really aren'... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
It just spreads them out so that you can flatten out the map, so to speak. But the southern hemisphere has more water, and as you may have learned in chemistry class, water has a higher specific heat, higher specific energy. It takes more energy, more heat, to raise water a degree than it does to raise, say, land a deg... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
And so water can absorb more energy. Or when there's less energy, water will release more energy without dropping as much of a temperature. So water has a moderating influence on the climate. So even though the summers in the southern hemisphere actually are getting more radiation than the summers in the northern hemis... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
So even though the summers in the southern hemisphere actually are getting more radiation than the summers in the northern hemisphere, it's moderated on the actual temperature because the water has the ability to absorb more of that heat without changing the temperature as dramatically. Now with that said, it is true t... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
And so the altitude for most of Antarctica is around 8,000 feet. So it's kind of like an alpine altitude. So the main reason why it's colder is possibly being further away from the sun in winter might play some role there. But the main reason why it's colder is that it's just at a much higher altitude. And it's to some... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
But the main reason why it's colder is that it's just at a much higher altitude. And it's to some degree insulated from the water. Or I guess you could say it's on the land. So especially during the long winters, it's going to get that much colder. But I'll leave you there. And to some degree, and this is the other asp... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
So especially during the long winters, it's going to get that much colder. But I'll leave you there. And to some degree, and this is the other aspect of it, during the summers, and all of this stuff is super complicated, so you can't just throw out one rule of thumb and say this is the reason. But these are all influen... | Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3 |
The whole point of this video is really just to look at what, in my mind, is one of the coolest pictures ever taken by anything. And this was actually taken by the Hubble telescope. Let me get my pen tool in place. And what they did is they pointed the telescope at this area of our night sky. And obviously, the Hubble ... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
And what they did is they pointed the telescope at this area of our night sky. And obviously, the Hubble telescope, it's out in orbit, so it doesn't have to worry about all of the interference from our actual atmosphere. So it gets a nice, good look at things. But it's right over here, relative to the moon. Obviously, ... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
But it's right over here, relative to the moon. Obviously, the moon's moving around. But on that day, it was here, relative to the moon. And they picked this location right over here because there weren't that many nearby stars there. So it really allowed the telescope, because if there were nearby stars, that light wo... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
And they picked this location right over here because there weren't that many nearby stars there. So it really allowed the telescope, because if there were nearby stars, that light would have outshone things that are behind it, further away, maybe perhaps galaxies. So just keep in mind, everything you're going to see i... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
And I think the main point for showing the moon here, obviously, the moon's moving around. I'm not telling you which exact patch of sky this is. But to really give you an idea of how small of a patch of sky that really is. But you really could have done this with any patch of sky. But in other patches of sky, there wou... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
But you really could have done this with any patch of sky. But in other patches of sky, there would have been other nearby stars that would have blocked things. But the galaxies are there, beyond that, in the clusters of galaxies, in the super clusters of galaxies. So with that said, just remember, everything we're tal... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
So with that said, just remember, everything we're talking about in this video is in this little patch of sky right over here. And the whole point of this, once again, like all of these videos, is really to kind of just blow your mind. So this right here is what the Hubble telescope saw in that patch. Everything. Every... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
Everything. Everything that I'm showing you right here. I just want to be clear. Some of these things are nearby stars. But most of these things are galaxies. That's a galaxy. That is a galaxy. | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
Some of these things are nearby stars. But most of these things are galaxies. That's a galaxy. That is a galaxy. That is a galaxy. That's a galaxy. And that's a galaxy. | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
That is a galaxy. That is a galaxy. That's a galaxy. And that's a galaxy. And the reason I wanted to do this, obviously, in the last video, I showed you our local group. I showed you the Virgo super cluster. I showed you the kind of clusters of clusters. | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
And that's a galaxy. And the reason I wanted to do this, obviously, in the last video, I showed you our local group. I showed you the Virgo super cluster. I showed you the kind of clusters of clusters. And I even showed you a depiction of the observable universe. But what just is amazing about this, this is an actual p... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
I showed you the kind of clusters of clusters. And I even showed you a depiction of the observable universe. But what just is amazing about this, this is an actual picture. This is actually an image of a galaxy. Hey, there's another galaxy. Oh look, there's another galaxy up here. So you could keep doing that forever. | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
This is actually an image of a galaxy. Hey, there's another galaxy. Oh look, there's another galaxy up here. So you could keep doing that forever. And this is just in that little patch of sky. And this is obviously not all of the galaxies in the universe. These are just the ones that we could see. | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
So you could keep doing that forever. And this is just in that little patch of sky. And this is obviously not all of the galaxies in the universe. These are just the ones that we could see. There are ones that might be even further. Or there definitely are ones that are further back. And their light is just even more. | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
These are just the ones that we could see. There are ones that might be even further. Or there definitely are ones that are further back. And their light is just even more. We could probably even focus even on a patch of sky like that and see that many galaxies again. So you could kind of keep, keep zooming in. But I j... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
And their light is just even more. We could probably even focus even on a patch of sky like that and see that many galaxies again. So you could kind of keep, keep zooming in. But I just, this thing, and I encourage you. I mean, there's so many unbelievable images. You could look up, they're all on the NASA website. A l... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
But I just, this thing, and I encourage you. I mean, there's so many unbelievable images. You could look up, they're all on the NASA website. A lot of these are on Wikipedia. But these images are just, I mean, unbelievable. I mean, you see a galaxy, another galaxy, another galaxy, another galaxy. I suspect some of this... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
A lot of these are on Wikipedia. But these images are just, I mean, unbelievable. I mean, you see a galaxy, another galaxy, another galaxy, another galaxy. I suspect some of this stuff might actually be clusters of galaxies. A galaxy, another galaxy. And remember, where I'm just circling these galaxies just left and ri... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
I suspect some of this stuff might actually be clusters of galaxies. A galaxy, another galaxy. And remember, where I'm just circling these galaxies just left and right, you kind of lose sight of what each galaxy actually is. These galaxies have hundreds of billions of stars. That even from one pixel on these galaxies i... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
These galaxies have hundreds of billions of stars. That even from one pixel on these galaxies is an unimaginable distance. Something that we could never, based with current technology or even current science, we could ever hope to traverse in the lifetime of humanity. Much less the lifetime of one individual. So these ... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
Much less the lifetime of one individual. So these are just enormous things. And they're just an infinite number of them. So I just wanted to highlight this with you. And really just, I mean, just look at it. I mean, it's kind of breathtaking. There's a galaxy right over there. | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
So I just wanted to highlight this with you. And really just, I mean, just look at it. I mean, it's kind of breathtaking. There's a galaxy right over there. Another galaxy. I mean, I could keep doing it all day. But it's really, and I feel bad about drawing on it. | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
There's a galaxy right over there. Another galaxy. I mean, I could keep doing it all day. But it's really, and I feel bad about drawing on it. It kind of ruins the picture. But look at that. That's a nice looking galaxy right there. | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
But it's really, and I feel bad about drawing on it. It kind of ruins the picture. But look at that. That's a nice looking galaxy right there. You got a bunch right over here. So I mean, it really is just, you know. This video, it's probably the least dense with actual instruction. | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
That's a nice looking galaxy right there. You got a bunch right over here. So I mean, it really is just, you know. This video, it's probably the least dense with actual instruction. But hopefully it's one of the most dense with inspiration. Anyway, hopefully you enjoyed that. Let me just finish looking at the entire pi... | Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
In the last video, I gave a little bit of a hand-wavy explanation about why S waves don't travel in liquid or air. And what I want to do in this video is give you a little bit more intuitive understanding of that and really go down to the molecular level. So let's draw a solid. And it has nice covalent bonds, strong bo... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
And it has nice covalent bonds, strong bonds between the different molecules. And the bonds are drawn by these lines in between. So if I were to hit this solid, I have this really small hammer where I just hit it at a molecular level. But if I were to hit these molecules hard enough that they move, but not so hard enou... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
But if I were to hit these molecules hard enough that they move, but not so hard enough that it breaks the bonds, then essentially what it's going to look like is this kind of row of molecules are going to move to the left. So you're going to have that row of molecules moving to the left. And then the row above it won'... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
So let me just draw all of the bonds. I'm just drawing all of the same bonds. Because these are strong bonds that we have in a solid, actually they could be ionic bonds as well. Because they are strong bonds that we have in this solid, they'll essentially be pulled. They'll be pulled in the direction, the top row will ... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
Because they are strong bonds that we have in this solid, they'll essentially be pulled. They'll be pulled in the direction, the top row will be pulled in the direction of the bottom row. And so they'll start kind of moving in that direction. And then the bottom row will essentially recoil back. And then you fast forwa... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
And then the bottom row will essentially recoil back. And then you fast forward a little bit. And so then the top row will have moved to the left. And now the bottom row will start to move back. And then the bottom row will start to kind of move back. Especially because remember, it's bonded to other things down here. ... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
And now the bottom row will start to move back. And then the bottom row will start to kind of move back. Especially because remember, it's bonded to other things down here. It's bonded to more of the solid down here. So it'll move back. And you can see this transverse wave. You can see this S wave propagating. | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
It's bonded to more of the solid down here. So it'll move back. And you can see this transverse wave. You can see this S wave propagating. Essentially right over here, the kind of peak of the S wave is here. Now it has moved up. Now let's think about the exact same situation with the liquids. | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
You can see this S wave propagating. Essentially right over here, the kind of peak of the S wave is here. Now it has moved up. Now let's think about the exact same situation with the liquids. In liquids, you don't have these strong ionic or covalent bonds between the different molecules. You just have these weak kind o... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
Now let's think about the exact same situation with the liquids. In liquids, you don't have these strong ionic or covalent bonds between the different molecules. You just have these weak kind of bonds, usually formed due to polarity. So in a liquid, water is a good example. You just have these kind of weaker bonds form... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
So in a liquid, water is a good example. You just have these kind of weaker bonds formed because water is a polar molecule. So the kind of halfway polar sides or the halfway positive sides are somewhat attracted to the halfway negative sides. So they kind of flow past each other. But if I were to hit these water molecu... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
So they kind of flow past each other. But if I were to hit these water molecules right here with my hammer, what would happen? Well, they're going to start moving to the left. And this one's going to bump into that one, which is going to bump into that one, which is going to bump into that one. And they're going to mov... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
And this one's going to bump into that one, which is going to bump into that one, which is going to bump into that one. And they're going to move to the left. So they're going to move to the left. But these molecules aren't going to move with them. You could view it as going to break that very weak bond due to polarity... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
But these molecules aren't going to move with them. You could view it as going to break that very weak bond due to polarity. They're going to move away from each other. Let me draw these top molecules in green. They're essentially just going to flow past each other. And this guy might have had also weak bonds with stuf... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
Let me draw these top molecules in green. They're essentially just going to flow past each other. And this guy might have had also weak bonds with stuff below it, too. I should draw just dotted lines with stuff below it, too. But because of the impact here, these guys are just going to flow. They're actually going to c... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
I should draw just dotted lines with stuff below it, too. But because of the impact here, these guys are just going to flow. They're actually going to compress in this direction. You're going to have a P wave, a compression wave, go in this direction, where this one bumps into that one and then goes back. And then this... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
You're going to have a P wave, a compression wave, go in this direction, where this one bumps into that one and then goes back. And then this one bumps into that one and goes back. And then this one bumps into that one. But the bonds aren't strong enough. And it's even more the case with air. But the bonds aren't stron... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
But the bonds aren't strong enough. And it's even more the case with air. But the bonds aren't strong enough for these blue guys to take these green guys for a ride. And the bonds are also not strong enough for the adjacent molecules to kind of help these blue guys to retract to their original position. So when I talke... | Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3 |
In the last video, we learned that there are a class of stars called Cepheid variables. And these are these super giant stars, as much as 30,000 times as bright as the sun, as a mass 20 times the mass of the sun. And what's neat about them is, one, because they're so large and so bright, you can see them really, really... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
And what's even neater about them is that they're variable, that they pulsate. And because their pulsations are related to their actual luminosity, you know if you see a Cepheid variable star in some distant galaxy, you know what its luminosity actually is if you were kind of at the star, because you can see how its pe... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
And the more dim it's gotten from its actual state, you know the farther away it is. So that's the real value of them. What I want to do in this video is to try to explain why they're variable, why they pulsate. And to do that, what we're going to think about is doubly and singly ionized helium. And just to review, hel... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
And to do that, what we're going to think about is doubly and singly ionized helium. And just to review, helium, so neutral helium, let me draw neutral helium. Neutral helium's got two protons, two neutrons, and then two electrons. And obviously, this is not drawn to scale. So this is neutral helium right over here. No... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
And obviously, this is not drawn to scale. So this is neutral helium right over here. Now, if you singly ionize helium, you knock off one of these electrons. And these type of things happen in stars, when you have a lot of heat, easier to ionize things. So singly ionized helium will look like this. It'll have the same ... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
And these type of things happen in stars, when you have a lot of heat, easier to ionize things. So singly ionized helium will look like this. It'll have the same nucleus, two protons, two neutrons. One of the electrons gets knocked off, so now you only have one electron. And now you have a net positive charge. So here,... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
One of the electrons gets knocked off, so now you only have one electron. And now you have a net positive charge. So here, let me do this in a different color. This helium now has a net charge. We could write 1 plus here, but if you just write a plus, you implicitly mean a positive charge of 1. Now, you can also doubly... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
This helium now has a net charge. We could write 1 plus here, but if you just write a plus, you implicitly mean a positive charge of 1. Now, you can also doubly ionize helium if the environment is hot enough. You can doubly ionize helium. And doubly ionizing helium is essentially knocking off both of the electrons. So ... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
You can doubly ionize helium. And doubly ionizing helium is essentially knocking off both of the electrons. So then it's really just a helium nucleus like this. This right here is doubly ionized helium. Now, I just said, in order to do this, you have to have a hotter environment. There has to be a hotter environment in... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
This right here is doubly ionized helium. Now, I just said, in order to do this, you have to have a hotter environment. There has to be a hotter environment in order to be able to knock off both. This electron really doesn't want to leave. To take an electron off of something that's already positive is difficult. You h... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
This electron really doesn't want to leave. To take an electron off of something that's already positive is difficult. You have to have a lot of, really, pressure and temperature. This is cooler. And this is all relative. We're talking about the insides of stars. So this is a hotter part of the star versus a cooler par... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
This is cooler. And this is all relative. We're talking about the insides of stars. So this is a hotter part of the star versus a cooler part of the star, I guess, is the way you think about it. It's a very hot environment by our traditional everyday standards. Now, the other thing about the doubly ionized helium is th... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
So this is a hotter part of the star versus a cooler part of the star, I guess, is the way you think about it. It's a very hot environment by our traditional everyday standards. Now, the other thing about the doubly ionized helium is that it is more opaque, which means it doesn't allow light to go through it. It actual... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
It actually absorbs light. It is more opaque. It absorbs light. Or another way, it absorbs that light energy. That energy will make it even hotter. So that's just something to think about. Now, the singly ionized helium is more transparent. | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
Or another way, it absorbs that light energy. That energy will make it even hotter. So that's just something to think about. Now, the singly ionized helium is more transparent. It allows the light to pass through it. So it doesn't get heated as much by photons that are kind of going near it or through it or whatever. I... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
Now, the singly ionized helium is more transparent. It allows the light to pass through it. So it doesn't get heated as much by photons that are kind of going near it or through it or whatever. It allows them to go through it. Here, the photons are going to actually heat up the ion. So let's think about how this might ... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
It allows them to go through it. Here, the photons are going to actually heat up the ion. So let's think about how this might cause a Cepheid variable to pulsate. So assuming that Cepheid variables have large enough quantities, I should say, of these ions, we can imagine that when a Cepheid variable is dim. So let me d... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
So assuming that Cepheid variables have large enough quantities, I should say, of these ions, we can imagine that when a Cepheid variable is dim. So let me draw a dim Cepheid variable. So I'll draw that like I'll draw this in a dim color. So this is a dim Cepheid variable right here. In its dim state, just like this, y... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
So this is a dim Cepheid variable right here. In its dim state, just like this, you have a lot of the doubly ionized helium in the star, at least kind of the outer surface of the star. And so this does not allow a lot of light to pass through. So this is the dim part of the pulsation of the Cepheid variable. Now, becau... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
So this is the dim part of the pulsation of the Cepheid variable. Now, because this doubly ionized helium is opaque, it is absorbing the light. It is getting heated. It is getting heated. And because it's getting heated, it'll cause the star to expand. So because it's getting heated, it'll become more energetic, and th... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
It is getting heated. And because it's getting heated, it'll cause the star to expand. So because it's getting heated, it'll become more energetic, and the star will actually expand. Now, as the star expands, because this doubly ionized helium is getting heated, what's going to happen? The further away you are from the... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
Now, as the star expands, because this doubly ionized helium is getting heated, what's going to happen? The further away you are from the core of the star, the cooler it gets. So this expanded because it was getting heated, but then because it expanded, the outer layers of the star become cooler. And since they're cool... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
And since they're cooler, helium won't be doubly ionized anymore. It'll get an electron from each helium atom. It can now get an electron from the plasma, I guess we can say, to become singly ionized helium. So now we have singly ionized helium. And now the star is going to be more transparent. It's going to allow more... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
So now we have singly ionized helium. And now the star is going to be more transparent. It's going to allow more light to pass through it. So now this is the bright part of the pulsation. It's going to allow more light through. So now it is bright. The star is bright. | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
So now this is the bright part of the pulsation. It's going to allow more light through. So now it is bright. The star is bright. But what's happening now? Because the light is no longer, or it's not being absorbed as well by the helium when it was a doubly ionized helium, now it's letting most of the light, or a lot m... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
The star is bright. But what's happening now? Because the light is no longer, or it's not being absorbed as well by the helium when it was a doubly ionized helium, now it's letting most of the light, or a lot more of the light, get through. It's not going to get heated as much. And so it won't have the kinetic energy t... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
It's not going to get heated as much. And so it won't have the kinetic energy to kind of keep pushing out, to keep moving outward. And so it'll collapse back into the star. And so then this will cool down and collapse back in. And when it collapses back in, what's going to happen? When it collapses back in, when these ... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
And so then this will cool down and collapse back in. And when it collapses back in, what's going to happen? When it collapses back in, when these helium atoms get closer to the center of the star, to the core of the star, they're going to be heated again, because they're closer now to the core. And when they get heate... | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
And when they get heated, they're going to become doubly ionized. So then we have doubly ionized helium again. And then the cycle will go again. It is now opaque. It will now absorb more energy. That'll cause it to have more kinetic energy to expand. Once it expands, it'll get cool again, and transparent, and bright. | Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
What I want to do in this video is kind of introduce you to the idea of, one, how carbon-14 comes about and how it gets into all living things. And then, either later in this video or in future videos, we'll talk about how it's actually used to date things. How we use it to actually figure out that that bone is 12,000 ... | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
So let me draw the Earth. So let me just draw the surface of the Earth like that. It's just a little section of the surface of the Earth. And then we have the atmosphere of the Earth. I'll draw that in yellow. So then you have the Earth's atmosphere right over here. Let me write that down. | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And then we have the atmosphere of the Earth. I'll draw that in yellow. So then you have the Earth's atmosphere right over here. Let me write that down. Atmosphere. And 78% the most abundant element in our atmosphere is nitrogen. It is 78% nitrogen. | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
Let me write that down. Atmosphere. And 78% the most abundant element in our atmosphere is nitrogen. It is 78% nitrogen. And if I write nitrogen, its symbol is just N. And it has 7 protons and it also has 7 neutrons. So it has an atomic mass of roughly 14. And this is the most typical isotope of nitrogen. | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
It is 78% nitrogen. And if I write nitrogen, its symbol is just N. And it has 7 protons and it also has 7 neutrons. So it has an atomic mass of roughly 14. And this is the most typical isotope of nitrogen. And we talk about the word isotope in the chemistry playlist. An isotope, the protons define what element it is. B... | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And this is the most typical isotope of nitrogen. And we talk about the word isotope in the chemistry playlist. An isotope, the protons define what element it is. But this number up here can change depending on the number of neutrons you have. So the different versions of a given element, those are each called isotopes... | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
But this number up here can change depending on the number of neutrons you have. So the different versions of a given element, those are each called isotopes. I just view them in my head as versions of an element. So anyway, we have our atmosphere. And then coming from our sun, we have what's commonly called cosmic ray... | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
So anyway, we have our atmosphere. And then coming from our sun, we have what's commonly called cosmic rays. But they're actually not rays. They're cosmic particles. They're mainly, you can view them as just single protons, which is the same thing as a hydrogen nucleus. They can also be alpha particles, which is the sa... | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
They're cosmic particles. They're mainly, you can view them as just single protons, which is the same thing as a hydrogen nucleus. They can also be alpha particles, which is the same thing as a helium nucleus. And there's even a few electrons. And they're going to come in and they're going to bump into things in our at... | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
And there's even a few electrons. And they're going to come in and they're going to bump into things in our atmosphere. And they're actually going to form neutrons. So they're actually going to form neutrons. And we'll show a neutron with a lowercase n and a 1 for its mass number. And we don't write anything because it... | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
So they're actually going to form neutrons. And we'll show a neutron with a lowercase n and a 1 for its mass number. And we don't write anything because it has no protons down here. Like we had for nitrogen, we had 7 protons. So it's not really an element. It is a subatomic particle. But you have these neutrons form. | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
Like we had for nitrogen, we had 7 protons. So it's not really an element. It is a subatomic particle. But you have these neutrons form. And every now and then, and let's just be clear, this isn't like a typical reaction. But every now and then, one of those neutrons will bump into one of the nitrogen-14s in just the r... | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
But you have these neutrons form. And every now and then, and let's just be clear, this isn't like a typical reaction. But every now and then, one of those neutrons will bump into one of the nitrogen-14s in just the right way. So that it bumps off one of the protons in the nitrogen. And essentially replaces that proton... | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
So that it bumps off one of the protons in the nitrogen. And essentially replaces that proton with itself. So let me make it clear. So it bumps off one of the protons. So instead of 7 protons, we now have 6 protons. But this number 14 doesn't go down to 13 because it replaces it with itself. So this still stays at 14. | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
So it bumps off one of the protons. So instead of 7 protons, we now have 6 protons. But this number 14 doesn't go down to 13 because it replaces it with itself. So this still stays at 14. And now since it now only has 6 protons, this is no longer nitrogen by definition. This is now carbon. And that proton that was bump... | Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3 |
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