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Why cepheids pulsate

Why Cepheids Pulsate. Created by Sal Khan.

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  • blobby green style avatar for user tooth1
    How do Cepheid variables form in the first place? Why should there be this class of star?
    (18 votes)
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  • purple pi purple style avatar for user Smitesh Patil
    For a star to be Cepheid variable star the helium will have to be on the outer layers of the star to block light. But in previous videos we learnt that helium settles near to core and hydrogen is in the outer most layer as helium is heavier than hydrogen.
    Could somebody explain why the star is still Cepheid variable star?
    (12 votes)
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  • male robot donald style avatar for user Siddharth Narain
    How to recognise cepheid variable stars in the night sky?
    (7 votes)
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    • piceratops ultimate style avatar for user Nick Hall-16-1998
      Cepheid variable stars are found everywhere in the night sky. Polaris (the north star) is one of them. Another one is Delta Scuti found in the constellation Scutum. Eta Aquilae in the constellation Aquila, Zeta Geminorum in the constellation Gemini, and Beta Doradus in the constellation Dorado are three of the most visible Cepheid variable stars to the naked eye.Basically Cepheid variable stars are extremely luminous (or bright) stars that astronomers use to locate other constellations and much much more in the sky.To locate Polaris, (if you don't know how to already,) find "The Big Dipper" (this is part of the constellation Ursa Major.) Find its two pointer stars, Merak and Dubhe. Merak is the lower star and Dubhe is the upper star. They are the two stars on the "cup" part of the dipper, furthest away from its handle. After you locate those, go in a straight line (most people say about 5X the distance between Merak and Dubhe) and you will find Polaris, the star that is the last star in the handle of the "Little" Dipper or Ursa Minor. Or you can find Cassiopeia. (The M or W shaped constellation) I've found that this is helpful in the city or when Ursa Major dips below the horizon. Ursa Major and Cassiopeia are always across from each other. If Cassiopeia is at a position, the Little Dipper is at a position. Draw an imaginary line through Cassiopeia's star, Caph to Polaris and then to Dubhe and Merak.Then, congratulations! You have found a Cepheid variable star!I hope this helps, and good luck! Dont listen to the timestamps. they are degree angles
      (10 votes)
  • leaf green style avatar for user fkohn
    It seems to me that this mechanism should cause all stars of sufficient mass to be cepheids. Are all supermassive stars cepheids, and why isn't the sun a cepheid?
    (9 votes)
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  • starky ultimate style avatar for user Dionysius of Thrace
    So why aren't all stars cepheid variables?
    (3 votes)
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    • male robot hal style avatar for user Andrew M
      I posed this question to Professor Michael Merrifield, who is a professor of astrophyiscs at University of Nottingham in England. He frequently appears also in great science videos on the "Sixty Symbols" channel of Youtube (https://www.youtube.com/user/sixtysymbols). He very graciously provided a reply which I will paraphrase here since I can't link directly to it and I don't know what your background knowledge is:

      There are certain combinations of temperature and size that inherently lead to pulsation. Stars change temperature and size throughout their lifetimes, as Sal details in his videos about the life cycles of stars. When a star enters a phase during which it has the right combination of temperature and size, it will pulsate. In other words, many many stars will be Cepheids at some point during their life cycle. The ones we see now are just the stars that happen to be in the right part of the life cycle now.

      If you are familiar with the Hertzsprung-Russell diagram, then you will understand Prof Merrifield's exact reply, which was:

      "There’s a region of the Hertzsprung-Russell diagram where that mechanism kicks in, so whenever a star’s evolution takes it through that region, it starts to pulsate."

      He kindly provided a link to this Wikipedia page, where you can find a H-R diagram that shows where the Cepheids lie on it
      (10 votes)
  • piceratops tree style avatar for user apeezer
    I thought that atoms absorb light by raising the energy levels of their electrons. So how does an atom without electrons absorb light?
    (5 votes)
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  • male robot donald style avatar for user OM
    Why don't all stars pulsate like this?
    (7 votes)
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  • blobby green style avatar for user grant.eagon
    If light photons from our sun take 100,000 years to reach the surface, do light photons emitted by cephiads get to the surface much more rapidly? Does this account for their increased luminosity and rapid interval brightness?
    (6 votes)
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  • male robot hal style avatar for user smokenjoe11
    Does a sun "spin" the way a planet does?
    (5 votes)
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  • starky ultimate style avatar for user z
    In the process that Sal says about cepheids, I got confused. Can't all stars sometimes do this, perhaps having helium near the surface, and causing this variable to happen? And if the do, then aren't all stars cepheids?
    (3 votes)
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Video transcript

- [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.