Welcome back, everyone. So we now have our units for describing large distances. Light years or parsecs. We can use them interchangeably. And it turns out that to measure larger and larger distances, we need to think about ways of Climbing what we'll call the cosmic distance ladder. That will have some way of measuring distances and we'll take a gotten measure a lot of stars and find their distances. And then if we want to go any further, that distance method, distance determine method, distance determination method gets exhausted. It can only go out so far. So we have to find a new Measure by, and we're going to talk about the methods that we can do this. And then what we have to do is basically ensure that the method worked, the new method that takes us even to larger distances works. And we do that by testing, testing it against our old method. So parallax, which we've talked about before. Parallax is a great method. It's very direct. We just look at how stars change their position in the sky, as the Earth goes around its orbit. And that's a great method but it only gets us out to about 500 parsecs, or about 1600 light years. So, if we want a different, if we want to go out further than that, we're going to have to find a new method, and then to test that method, we'll look at more nearby objects, where we can both use the new method and parallax to make sure that the new method is working. So what are those new methods? Well one of the most powerful mechanisms for determining distance in Astronomy is by finding something we call a Standard Candle. And we've talked about Standard Candles before. These are objects who's known. Oh who for some reason from the physics of the object we know their intrinsic brightness we know how much energy its putting out per second and then using the inverse square law for brightness the fact that brightness falls off as one over the distant squared. We can compare how bright it should be to how bright it actually appears on the sky, and from that, calculate its distance. So a great example of a standard candle, or one great example, are what are called Cepheid variables. And these are a very particular kind of star that pulsate, they change their brightness over time. And over time early in the last century people came to recognize that there was a direct relationship between how the star changed its brightness. The period over which it changed. Went from very bright to less bright. And it's intrinsic brightness so in some sense the period of the star's brightness variation became like the hundred watt light bulb label on a light bulb. So if you have a standard candle like a [UNKNOWN] all you have to do is go out and measure something say like it's period and you immediately know its intrinsic brightness and then you can compare how bright it appears Appears to be, and get a distance from that. Now the great thing about this is if your standard candle, like a Cepheid variable, is very bright, you may be able to see it across enormous distances. You may be able to measure a Cepheid variable and its period. Of brightness variation. Even in another galaxy and in that way get the distance to that other galaxy. So that's why we like standard candles so much. Now there are other methods if you don't have say a standard candle around. That are [INAUDIBLE] come more complicated. There's something called Spectroscopic Parallax and this nothing really has nothing to do with parallax but it was one of the first ways that astronomers realized how to make distance measurements. After parallax was Or after parallax. And basically ideas just looking taking a Spectra of a star and looking at it's the absorption lines in its spectrum. And it was found that by measuring certain properties of the absorption lines. You could actually tell how intrinsically bright the star was. And this didn't work perfectly for individual stars. But it worked very well for collections of stars, statistically essentially. So spectroactive parallax could is very helpful out to distances of even 10,000 parsecs, or so, 10 kiloparsecs, which is about 32,000 light years. So, standard kilo Spectroscopic parallax can be quite good for getting measurements within the galaxy of course if you have a se, sephiod variable you know, that's going to be great and you'll be able to get distance measurements from there. What the rest of the history of distance measurements in astronomy is going to be about, is going to be trying to find. Ever better standard candles. So as we've said, there's [UNKNOWN] variables going out to very large distances. We've even been able to find supernovas these super bright explosion of stars that we learned about in the last lecture and in some cases certain types of supernova, what are called supernova type 1A. The way that they dim after the explosion can actually be used as a way of getting their intrinsic brightness. Hence they become a standard candle. And since supernova are so bright, they can be seen literally across Cross the, the universe and those become some of our most powerful means of getting distances to very, very distant objects. [BLANK_AUDIO]
