Cepheid Variables 1 Cepheid Variables 1
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.
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