Cepheid variables
Why Cepheids Pulsate Why Cepheids Pulsate
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- 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 thirty thousand times as bright as the sun,
- with a mass as much as twenty 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 can see its period of pulsation.
- And so if you know its actual luminosity
- and then you know, obviously, its apparent luminosity,
- you know how much it's gotten dim.
- And the more dim it's gotten from it's actual state,
- you know the farther away it is.
- So that's the actual 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 heliums.
- And just to review helium.
- So neutral helium (let me draw neutral helium) . . .
- Neutral helium's got two protons.
- It'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.
- 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 get 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+" here, but if you just write a "+"
- you implicitly mean a positive charge of one.
- Now, you can also doubly ionize helium if the environment is hot enough.
- 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 order to knock off both.
- This electron really doesn't want to leave.
- I mean, to take an electron off of something
- that's already positive is difficult.
- So you have to have a lot of,
- really pressure and temperature.
- This is cooler.
- Now, this is all relative.
- We're talking about the insides of stars.
- So, you know, this is a hotter part of the star vs. a cooler part of the star,
- I guess is the way to think about it.
- This is 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,
- which means it doesn't allow light to go through it;
- 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.
- 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 cause a Cepheid variable
- to pulsate.
- So assuming that Cepheid variables have a large enough quantity
- 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 this in a dim color. So this is a dim Cepheid variable right here.)
- In 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,
- or 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.
- 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 anyomre.
- Each helium atom can now get an electron from the plasma, I guess we could 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 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
- as 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 that 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 doubly ionized,
- so that we have doubly ionized helium again.
- And then the cycle will go again:
- It is now opaque; it will now absorb more energy;
- that will cause it to have more kinetic energy to expand;
- once it expands it'll get cool again, and transparent, and bright.
- So this is the current best theory of why
- Cepheid variable stars are vaiable to begin with.
- It's this whole notion of having the doubly ionized helium
- vs. the singly ionized helium
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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