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Current time:0:00Total duration:7:10

Video transcript

in the last video we learned that there are a class of stars called Cepheid Cepheid variables and these are these supergiant stars as much as 30,000 times as bright as the Sun is a mass xx 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 you can see how its period of pulsation and so if you know it's actual luminosity and then you can you know obviously it's apparent luminosity you know how much it's gotten dim and the more dim its 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 to do that what we're gonna think about is 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 ionized helium you you knock out 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 I 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 doubly ionized helium if the environment is hot enough you can doubly ionized helium and W ionizing helium is essentially knocking off of both of the electrons so then it's really just a helium nucleus it's really just a 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 that it has to it has to be a hotter environment in order to be able to knock off both deselect this electron really doesn't want to leave I'm 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 the way you think about this still a very hot environment by our traditional everyday 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 doesn't allow light to go through it it actually absorbs light it is more opaque it absorbs light 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 this is more transparent more transparent it allows the light to pass through it so it doesn't it doesn't get heated as much by photons that are 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 a Cepheid variable to pulsate so assuming that Cepheid variables have large or large enough quantities I should say of these ions we can imagine that when a Cepheid variable is dim so let me draw a stim Cepheid variable so I'll draw that 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 in its dim state just like this you have a lot of 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 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 the the 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 it'll start to get a couple of it'll get an electron from each helium atom can now get an electron from from the plasma I guess we 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 healy 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 coat 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 their because they're closer now of 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 is now opaque it will now absorb more energy that'll cause it to have more kinetic energy to expand once it's 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 in kind of the outer the outer the outer layers of the star itself