造父变星（一）

this right here is a picture of Henrietta and rieta Swan Leavitt and she made a little over a hundred years ago this is in the early 1900s while working for Edward Charles Pickering who was a Harvard astronomer while I was working for his observatory she made what is arguably well definitely one of the most important discoveries in all of astronomy and probably and I would say it's it ranks their top three because it really enabled people like Hubble to start realizing that the universe is expanding or even being able to think about how to measure distances to objects in space well beyond the reach of our tools with parallax we saw with parallax you have to be 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 you we don't have the instruments even today to measure things beyond our galaxy but because of Henrietta Swan Leavitt we can we were able to approximate or get good census of the distance to 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 Magellanic Cloud and the small Majella Magellanic Clouds and this is what they look like from the southern hemisphere this is the large right over here and that this is the small right over here and remember this is before this is before Hubble realize that there or showed the world that there are stars beyond our Gaucher 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 getting better saying it so her job was literally just to classify just to classify the the the different stars that she saw but while she was classifying she looked at these things called variables it turns out what she was looking at where class of stars called Cepheid or c fiood c fiood variable c fiood variable stars and what's interesting about them is two things they're SuperDuper bright they're up to 30,000 times as luminous as their son and they're 5 to 20 times more massive than the Sun five to twenty times the sun's mass but what makes them interesting is one they're really bright so you can see them from really far away you can see these C fiood variable stars in other galaxies in fact we can see it well beyond even the small Magellanic Cloud or the Large Magellanic Cloud but you can see these stars and other galaxies and what's even more interesting about them is that their their intensity is variable that they become brighter and dimmer with a 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 and then over the course of the next 3-4 days it might reduce an intensity to something like this and then after 3 4 days again it will look like this and then it will look like this again so it's in its actual intensity is going up and down with a well-defined period so if this takes three days and then this is another three days then the period one entire cycle of it's going from low intensity back to high intensity is going to be six days so this is a six day period and what Henrietta what Henrietta Leavitt saw and this wasn't an obvious thing to do she plotted she assumed she assumed that things everything in each of these clouds are roughly the same distance away everything the large Mountain Magellanic Cloud is roughly the same distance way and it you know it's obviously not exact this is an entire galaxy so you have obviously things further away in that galaxy and things closer up you have stars here and here and their distance is going to be exactly the same to us that we're sitting maybe over here someplace but it's going to be close it wasn't a bad approximation and I'm making that assumption she saw something pretty neat if she plotted so let me plot this right over here so she plotted on the horizontal axis if she applauded the relative relative luminosity so really the only way that she could measure this is just how did they how bright did they look to her and she's assuming that they're 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 same roughly the same distance than how bright it is will tell you how how bright it is at the actual star so she prodded she plotted relative luminosity of a star on one axis and on the other axis she plotted she prodded the period of these variable stars she plotted the period and what I'm gonna do is I'm gonna 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 100 days right over here it's a logarithmic scale cuz I'm going up in powers of ten I could say that if we say these are if we take the log of these this would be zero this would be 1 this would be 2 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 10 an equal spacing but when you plot it on this scale the relative luminosity versus 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 this is 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 cuz think about what the problem here is we can look at all of these stars in space we can look at let's say you look at the 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're a very superficial understand you say oh this this star is brighter you would say that this is a fundamentally brighter star but how do you know that maybe maybe instead of being brighter maybe it's just a dim or closer star maybe this is a closer star maybe this is an entire galaxy but it's so far away that you can't even tell but all of a sudden with you if you buy by the work that that that Henrietta Leavitt did if you know if you see one of these Cepheid variable stars in another galaxy you know its relative brightness compared to other Cepheid variable stars and 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 value Manasa T of any other Cepheid variable stars so let's say that lets say using parallax let's say using parallax which is our other tool we find let's say there's some star in our galaxy and let's say using parallax 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 a hundred light years away light years and this star is a Cepheid this is a this is a Cepheid variable star and let's say it's period let's say its period is one day it's one day so we know we now know something interesting we know sips we know variable stars with a period of one day at a hundred light years away will look like this will look like this drawing right over here so if we later on if we later on see if we later on see a Cepheid variable star with a with eight with a period of one day so it goes gets 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 we now know that it had that if it was a hundred light years 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 and that confuses a little bit I'll do a little bit more details in the next few videos so we can get a closer sense of how the math would work but this was a big discovery just discovering this class of stars this Fe 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 could guess how far those things really are not we could estimate how far those things really are