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- [Voiceover] In the last video we learned that there are a class of stars called Cepheid variables. And these are the super giant stars, as much as 30000 times as bright as the sun. A mass, as much as 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 far away. 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 it's luminosity actually is if you were kind of at the star, because you know you can see how it's period of pulsation. And so if you know it's actual luminosity, and you know it's obviously apparent luminosity, you know how much it's gotten dimmed. 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 variables. Why they pulsate. And to do that, to do that, what we're gonna think about is doubly and singly ionized helium. And just to review, helium, so neutral helium, let me draw a neutral helium, neutral helium's got two protons, it's got two protons, two neutrons, two neutrons, and then two electrons 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 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, let me do this in a different color, this helium now has a net charge, we could write one plus here, but if you just write a plus you implicitly mean a positive charge of one. Now you can also double the ionized 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 then it's really just a helium nucleus. It's really just a helium nucleus like this. This right here is doubly, 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 these, 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, you know, it's hot, this is a hotter part of the star versus a cooler part of the star I guess is a way you think about this, it's still a very hot environment by our traditional, every day standards. Now the other thing about the doubly ionized helium is that it is more opaque. It is more opaque, which means it doesn't transmit, it doesn't allow light to go through it, it actually absorbs light. It is more opaque, it absorbs light. It absorbs light. Or in 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. This is more transparent. 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, heat up the ion. So let's think about how this might cause cepheid variable to pulsate. So assuming that cepheid variables have a large enough quantity, 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. It's dim state, and it's dim state just like this, you have a lot of the doubly ionized helium, you have a lot of doubly ionized helium in the star, at least kind of the outer surface of the star. Doubly ionized helium. 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 because this doubly ionized helium is opaque it is absorbing the light, it is getting heated. 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 the star will actually expand. 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 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 cooler, helium won't be doubly ionized anymore, it'll start to get a couple of, it'll get an electron from, each helium atom can now get an electron from the plasma, I guess you can say, to become singly ionized helium. So now we have singly ionized helium. 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. 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 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 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 heated they're going to become, they're going to become doubly ionized. So then we have doubly ionized helium, again. Doubly ionized, 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. And so this is the current best theory of why cepheid variable stars are variable to begin with. It's this whole notion of having the doubly ionized helium versus the singly ionized helium in kind of the outer layers of the star itself.