- [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.
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.