Cepheid Variables 1

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Cepheid Variables 1

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This right here is a picture of Henrietta Swan Leavitt.
And she made,
a little over a hundred years ago -- this is in the early 1900's --
while working for Edward Charles Pickering, who was a Harvard astronomer,
while working for his observatory,
she made what is arguably -- well, defintely
one of the most important discoveries in all of astronomy.
And probably, well, I would say it ranks in the top three,
because it really enabled people like Hubble to start
realizing that the universe is expanding.
Or even be able to think about how to
measure distances to objects in space well beyond
the reach of our tools of paralax.
We saw with paralax you have to have
extremely sensitive instruments
just to even measure distances to stars
relatively close to us,
very sensitive instruments to get to stars
maybe further out into our galaxy.
And we don't have the intstruments even today
to measure things beyond our galaxy.
But because of Henrietta Swan Leavitt,
we're able to approximate or get good senses
of the objects beyond our galaxy.
So let's just think about what she did.
So her job was literally to classify stars in the Large Magellanic
(I have trouble saying that, "Magellanic Cloud.")
and the Small Magellanic Clouds.
And this is what they look like from the Southern Hemisphere.
This is the large, right over here.
And this is the small, right over here.
And remember, this is before Hubble realized,
or showed the world,
that there are stars beyond our galaxy,
that there are galaxies beyond our galaxy.
So at this point in time people didn't even fully appreciate
that these were separate galaxies.
We just said, "Hey these are kind of these blobs,
or these clusters of stars that we see
in the Southern Hemisphere."
And just to get a sense of where they are relative to
our galaxy, the Milky Way galaxy,
this is obviously not an actual picture --
we can't take a picture from this vantage point.
This would have to be very, very far away,
but this is the milky way right here,
and this is the small magellanic cloud,
and this is the large magellanic cloud
(I'm getting better at saying it.)
So her job was just to classify the different stars
that she saw.
But while she was classifying,
she looked at these things called variables.
And it turns out what she was looking at were a
class of stars called Cepheid variable stars.
And what's interesting about them is two things:
They're super-dooper bright;
they're up to 30,000 times as luminous as the sun,
and they're five to twenty times more massive than the sun,
5 - 20 X the sun's mass.
But what makes them interesting is, one,
they're really bright so you can see them from really far way.
You can see these Cepheid variable
stars is other galaxies,
in fact we can see them well beyond even the Small Magellanic Cloud
or the Large Magellanic Cloud.
You can see these stars in other galaxies.
And what's even more interesting about them is that
their intensity is variable,
that they become brighter and dimmer
with a well-defined period.
So if you're looking at a Cepheid variable star
(and this is just kind of a simulation,
a very cheap simulation)
it might look like this [large circle],
and then over the course of the next three or four days,
it might reduce in intensity to something like this [small circle].
And then after three, four days again,
it will look like this [large].
And then it'll look like this again [small].
So it's actual intensity is going up and down
with a well-defined period.
So if this takes three days,
and this is another three days,
then the period --
one entire cycle
of its going from low-intensity back to high intensity --
is going to be six days.
So this is a six-day period.
And what Henrietta Leavitt saw --
this wasn't an obvious thing to do.
She plotted . . .
She assumed that everything in each of these clouds
were roughly the same distance away;
everything in the Large Magellenic Cloud is
roughly the same distance away.
And it's obviously not exact.
This is an entire galaxy so you have obviously things
futher away in the galaxy,
and things closer up.
You have stars here,
and here.
And their distance isn't going to be exactly the same to us.
We're sitting maybe over here someplace.
But it's going to be close.
It wasn't a bad approximation.
And by making that assumption, she saw something pretty neat.
If she plotted . . .
Let me plot this right over here.
So she plotted on the horizontal axis
the relative luminosity.
So, really,
the only way she could measure this is how bright did they look to her,
and she's assuming that they're the same distance.
So, obviously if you have a brighter star but it's much
much further away,
it's going to look dimmer.
So if you assume that they're all the distance
roughly the same distance
then how bright it is will tell you how bright it is
at the actual star.
So she plotted relative luminosity of the star
on one axis.
And on the other axis
she plotted the period of these variable stars.
And what I'm going to do is,
I'm going to do this on a logarithmic scale.
So let's say that this is in days.
So this is one day, this is ten days,
this is one-hundred days, right over here.
So a logarithmic scale 'cause I'm going up in powers of ten.
I could say that . . .
If we take the log of these,
this would be zero,
this would be 1, this would be two.
And so that's what I'm using as a scale.
So I'm using the log of the period.
Or I'm just marking them as 1, 10, 100
but I'm giving each of these factors of ten
an equal spacing.
When you plot it on this scale:
the relative luminosity vs. the period --
she got a plot that looks something like this.
And this is obviously not exact.
She got a plot that looks something like this.
It was a fairly linear relationship,
when you plot the relative luminosity against
the log of the period.
So this is obviously a logarithmic scale over here,
and so you could fit a line.
And why, I'd argue --
and I think most people would argue --
this is one of the most important discoveries in astronomy is
if you know . . .
'cause think about what the problem here is:
We can look at all these stars in space.
Let's say you look at a fraction of the sky,
and you look at something that looks like that,
so it's really bright.
And then you see something dim
that looks like that.
So if you had a very superficial understanding,
you'd say, "oh, this star is brighter."
You would say that this is a fundamentally brighter star.
But how do you know that?
Maybe instead of being brighter,
maybe it's just a dimmer, closer star.
Maybe this is a closer star.
Maybe this is an entire galaxy,
but it's so far away you can't even tell.
But all of a sudden,
by the work that Henrietta Leavitt did,
if you see one of these Cepheid variable stars in another galaxy,
you know it's relative brightness
compared to other Cepheid variable stars.
So if you can place just one of these Cepheid variable stars,
if you know exactly the distance to one of them,
and then you know its absolute luminosity,
you then know the absolute luminosity
of any other Cepheid variable stars.
So let's say using parallax,
which is our other tool,
we find . . .
Let's say there is some star in our galaxy
and let's say using parallax
we're able to come up with a pretty good measure
that it is -- I don't know --
let's say it's 100 lightyears away.
And this star is a Cepheid variable star.
And let's say it's period is one day.
So we know, we now know something interesting.
We know variable stars, with a period of one day,
at 100 lightyears away,
will look like this.
Will look like this drawing right over here.
So if we later on see a Cepheid variable star
with a period of one day --
so it get's brighter and dim over the course of one day
(and maybe it's red-shifted as well)
but maybe it looks a little bit dimmer;
it looks like this --
we now know that if it was 100 lightyears away,
it would have this luminosity.
So based on how much dimmer it is,
we can then figure out how much further away
this cepheid variable star is.
If that confuses you a little bit,
I'll do a little bit more details in the next few videos,
and we can get a closer sense of how the math worked.
But this was a big discovery.
Discovering this class of stars, this Cepheid variable class --
She wasn't the one who discovered them;
people knew before her that there were
these stars that got brighter and dimmer. --
But what her big discovery was is seeing this
linear relationship between the relative luminosity of these stars
and their period.
Because then, if we see Cepheid variable stars in
completely different galaxies
or galactic clusters,
by looking at their period,
we know what their real relative luminosity is.
And then we can guess how far those things really are.
No, we can ESTIMATE how far those things really are.