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This right here is a picture of Henrietta Swan Leavitt.

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And she made,

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a little over a hundred years ago -- this is in the early 1900's --

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while working for Edward Charles Pickering, who was a Harvard astronomer,

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while working for his observatory,

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she made what is arguably -- well, defintely

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one of the most important discoveries in all of astronomy.

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And probably, well, I would say it ranks in the top three,

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because it really enabled people like Hubble to start

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realizing that the universe is expanding.

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Or even be able to think about how to

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measure distances to objects in space well beyond

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the reach of our tools of paralax.

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We saw with paralax you have to have

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extremely sensitive instruments

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just to even measure distances to stars

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relatively close to us,

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very sensitive instruments to get to stars

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maybe further out into our galaxy.

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And we don't have the intstruments even today

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to measure things beyond our galaxy.

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But because of Henrietta Swan Leavitt,

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we're able to approximate or get good senses

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of the objects beyond our galaxy.

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So let's just think about what she did.

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So her job was literally to classify stars in the Large Magellanic

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(I have trouble saying that, "Magellanic Cloud.")

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and the Small Magellanic Clouds.

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And this is what they look like from the Southern Hemisphere.

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This is the large, right over here.

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And this is the small, right over here.

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And remember, this is before Hubble realized,

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or showed the world,

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that there are stars beyond our galaxy,

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that there are galaxies beyond our galaxy.

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So at this point in time people didn't even fully appreciate

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that these were separate galaxies.

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We just said, "Hey these are kind of these blobs,

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or these clusters of stars that we see

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in the Southern Hemisphere."

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And just to get a sense of where they are relative to

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our galaxy, the Milky Way galaxy,

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this is obviously not an actual picture --

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we can't take a picture from this vantage point.

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This would have to be very, very far away,

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but this is the milky way right here,

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and this is the small magellanic cloud,

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and this is the large magellanic cloud

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(I'm getting better at saying it.)

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So her job was just to classify the different stars

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that she saw.

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But while she was classifying,

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she looked at these things called variables.

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And it turns out what she was looking at were a

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class of stars called Cepheid variable stars.

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And what's interesting about them is two things:

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They're super-dooper bright;

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they're up to 30,000 times as luminous as the sun,

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and they're five to twenty times more massive than the sun,

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5 - 20 X the sun's mass.

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But what makes them interesting is, one,

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they're really bright so you can see them from really far way.

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You can see these Cepheid variable

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stars is other galaxies,

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in fact we can see them well beyond even the Small Magellanic Cloud

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or the Large Magellanic Cloud.

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You can see these stars in other galaxies.

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And what's even more interesting about them is that

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their intensity is variable,

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that they become brighter and dimmer

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with a well-defined period.

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So if you're looking at a Cepheid variable star

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(and this is just kind of a simulation,

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a very cheap simulation)

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it might look like this [large circle],

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and then over the course of the next three or four days,

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it might reduce in intensity to something like this [small circle].

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And then after three, four days again,

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it will look like this [large].

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And then it'll look like this again [small].

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So it's actual intensity is going up and down

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with a well-defined period.

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So if this takes three days,

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and this is another three days,

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then the period --

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one entire cycle

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of its going from low-intensity back to high intensity --

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is going to be six days.

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So this is a six-day period.

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And what Henrietta Leavitt saw --

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this wasn't an obvious thing to do.

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She plotted . . .

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She assumed that everything in each of these clouds

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were roughly the same distance away;

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everything in the Large Magellenic Cloud is

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roughly the same distance away.

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And it's obviously not exact.

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This is an entire galaxy so you have obviously things

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futher away in the galaxy,

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and things closer up.

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You have stars here,

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and here.

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And their distance isn't going to be exactly the same to us.

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We're sitting maybe over here someplace.

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But it's going to be close.

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It wasn't a bad approximation.

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And by making that assumption, she saw something pretty neat.

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If she plotted . . .

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Let me plot this right over here.

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So she plotted on the horizontal axis

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the relative luminosity.

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So, really,

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the only way she could measure this is how bright did they look to her,

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and she's assuming that they're the same distance.

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So, obviously if you have a brighter star but it's much

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much further away,

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it's going to look dimmer.

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So if you assume that they're all the distance

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roughly the same distance

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then how bright it is will tell you how bright it is

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at the actual star.

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So she plotted relative luminosity of the star

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on one axis.

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And on the other axis

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she plotted the period of these variable stars.

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And what I'm going to do is,

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I'm going to do this on a logarithmic scale.

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So let's say that this is in days.

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So this is one day, this is ten days,

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this is one-hundred days, right over here.

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So a logarithmic scale 'cause I'm going up in powers of ten.

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I could say that . . .

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If we take the log of these,

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this would be zero,

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this would be 1, this would be two.

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And so that's what I'm using as a scale.

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So I'm using the log of the period.

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Or I'm just marking them as 1, 10, 100

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but I'm giving each of these factors of ten

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an equal spacing.

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When you plot it on this scale:

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the relative luminosity vs. the period --

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she got a plot that looks something like this.

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And this is obviously not exact.

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She got a plot that looks something like this.

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It was a fairly linear relationship,

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when you plot the relative luminosity against

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the log of the period.

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So this is obviously a logarithmic scale over here,

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and so you could fit a line.

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And why, I'd argue --

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and I think most people would argue --

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this is one of the most important discoveries in astronomy is

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if you know . . .

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'cause think about what the problem here is:

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We can look at all these stars in space.

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Let's say you look at a fraction of the sky,

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and you look at something that looks like that,

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so it's really bright.

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And then you see something dim

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that looks like that.

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So if you had a very superficial understanding,

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you'd say, "oh, this star is brighter."

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You would say that this is a fundamentally brighter star.

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But how do you know that?

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Maybe instead of being brighter,

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maybe it's just a dimmer, closer star.

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Maybe this is a closer star.

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Maybe this is an entire galaxy,

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but it's so far away you can't even tell.

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But all of a sudden,

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by the work that Henrietta Leavitt did,

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if you see one of these Cepheid variable stars in another galaxy,

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you know it's relative brightness

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compared to other Cepheid variable stars.

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So if you can place just one of these Cepheid variable stars,

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if you know exactly the distance to one of them,

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and then you know its absolute luminosity,

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you then know the absolute luminosity

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of any other Cepheid variable stars.

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So let's say using parallax,

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which is our other tool,

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we find . . .

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Let's say there is some star in our galaxy

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and let's say using parallax

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we're able to come up with a pretty good measure

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that it is -- I don't know --

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let's say it's 100 lightyears away.

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And this star is a Cepheid variable star.

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And let's say it's period is one day.

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So we know, we now know something interesting.

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We know variable stars, with a period of one day,

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at 100 lightyears away,

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will look like this.

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Will look like this drawing right over here.

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So if we later on see a Cepheid variable star

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with a period of one day --

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so it get's brighter and dim over the course of one day

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(and maybe it's red-shifted as well)

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but maybe it looks a little bit dimmer;

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it looks like this --

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we now know that if it was 100 lightyears away,

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it would have this luminosity.

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So based on how much dimmer it is,

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we can then figure out how much further away

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this cepheid variable star is.

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If that confuses you a little bit,

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I'll do a little bit more details in the next few videos,

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and we can get a closer sense of how the math worked.

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But this was a big discovery.

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Discovering this class of stars, this Cepheid variable class --

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She wasn't the one who discovered them;

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people knew before her that there were

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these stars that got brighter and dimmer. --

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But what her big discovery was is seeing this

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linear relationship between the relative luminosity of these stars

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and their period.

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Because then, if we see Cepheid variable stars in

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completely different galaxies

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or galactic clusters,

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by looking at their period,

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we know what their real relative luminosity is.

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And then we can guess how far those things really are.

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No, we can ESTIMATE how far those things really are.