well they were just talking about brightness of stars and of course one of the most fundamental properties of stars is temperature and we know now that it is closely related to color surface color of a star as well and it's all relates back to you know the first chapters that we started with which was on light as a particle and and light as a wave and we learned about davines law we talked about how you know black bodies seem to change their color in terms of changing their temperature simultaneously and that's all gonna relate to this topic and this egg quantity right here because stars are the closest things we have to perfect black bodies so let's just put down some basics here which is that you know in our night sky we see stars of varying colors all the time you're right to the naked eye when you glance up there very quickly they all pretty much look pretty white to us but and I'm a really dark night especially if you have some sort of photographic equipment it really captures the color well and if you're out there long enough and you have pretty good eyesight I'll bet you start to detect subtle differences in color as well all Ryan the wintertime constellation is a great example of stars at both ends of this visible spectrum Betelgeuse the shoulder starter Ryan otherwise known as alpha Orionis has this distinctly orange D red color to it and maybe you've heard of Betelgeuse in the news that last year it appears to be dimming and we think we have a handle on why that might be at this point due to intervening a cloud of the dust basically between the star and us but more needs to be figured out about that in the other corner of a Ryan we got the beta Orionis or the second brightest star although these days it's the brightest known as Rigel and looking at that photograph there you see a distinctly bluish white color to it if you look at the belt stars of Orion those three in a straight line right here they too have this very bluish white color to them yeah you're right you got a good eye that's a bluish white star to about tricks and that's a fairly bluish white star as well safe but Betelgeuse is just kind of the odd person out in in this color spectrum the Orion Nebula looking kind of reddish as well but that's because emission nebulae are regions of star birth appear to be distinctly reddish like that now down here in the second frame frame B you know you see a Deep Field photograph of a whole lot of stars nearby in our galaxy I see yellows I see white I see blue I see red lots and lots of red I even might see something approaching a kind of a yellowy green but it's just reminding us that stars come in all the colors of the rainbow so for our photographs at the right here Betelgeuse is the reddish one Rigel and Sirius Sirius is pointed to by Orion's belt it's down about here in the real sky also a bluish white star we know from means law that that tells us about the overall peak flux our greatest amount of energy being a bit of ivy stars I'll remind you that in the visible spectrum the colors are from red to violet ROYGBIV and four stars that have a distinctly reddish or orange color to them these are lower energy immittance are photons from the surface now that would be a lower frequency that also be a longer wavelength and a lower surface temperature overall four stars it appear to be a distinctly bluish and indigo and violet they of course have higher energies at their surfaces higher frequencies of light emitting from the surface shorter wavelengths for that peak wavelength and higher temperatures overall for that star yeah these are the things that are intuitive that have made sense for a few chapters now but here's some good ballpark numbers I'm gonna put this right up here but for you know bluish white stars they're surface temperatures are at least 10,000 kelvins if not hotter first stars that are yellowy at the surface like our Sun they're surface temperatures around there range of 5 to 6,000 kelvins and for these distinctly reddish stars which are in abundance their surface temperatures are around 3 to 4,000 kelvins remember to make these into Fahrenheit you basically double them don't you so you know bluish-white 20,000 degrees Fahrenheit or hotter reddish stars you know six or eight thousand degrees Fahrenheit [Music] now here we see two blackbody curves sorry it's the advance that I want to get myself all the way that's all I wanted to do but here we see two blackbody curves almost huh there we go and the scaling is very different this just says some relative flux on the left so it's kind of arbitrary what these units are but then it gives you the wavelength of the light being emitted and four stars that are very hot surface temperatures Rigel in Orion again at 11,500 kelvins up the surface and Sirius the brightest star no sky besides the Sun having about 10,000 kelvins at the surface where they both peak they put out the majority of their flux in the non visible part of the spectrum which is the ultraviolet when you're to the left violet on the skier which has wavelength on the on the horizontal so if you could actually see ultraviolet light you know these two stars be even brighter because you'd see their enormous amount of flux in that range but both of them appear to be distinctly bluish white because look at their long curves they give off much more blue visible light than they do red visible light yeah just from the general slope of that curve so both of these stars appear bluish white even though they're really both ultraviolet stars aren't they look way down here and they're very small homes down here we have stars that are like our Sun and like capella almost our sun's surface temperature both of these are yellow stars and they both peak them you know the kind of yellow greeny area and this on the scale but it's hard to see on such a small scale so we magnify our zoom in on that area over here on the right and now I can see that the Sun sure enough with a 1500 Kelvin surface temperature peaks in about the yellow-green tennis-ball green we always say for the sun's that most predominant flux and a star like capella only slightly cooler than our Sun at 5200 kelvins is really much more of a golden star not so much green in it at the surface but how much or kind of a yellowy pure yellow color for star like Betelgeuse which is you know looking red in that photograph just a second ago yes it's a reddish star but its peak flux if you look at the curve down here come on cursor is actually in the infrared you know it's even to the right of red there and at 3600 kelvins if you could actually you know see very short wavelength infrared light you'd see Betelgeuse looking even brighter in that spectrum right here but of the visible part of spectrum the red flux is definitely overwhelming the blue flux so that's why that star appears to be kind of a reddish orange you know things we've talked about before it kind of remember here is less busy not just one star this is about serious in surface temperature 10,000 kelvins and once again showing us the fact that to analyze all these different wavelengths would be you know hard to do on earth especially underneath our atmosphere you know most of our ultraviolet light is absorbed as is a lot of our infrared so how do you construct a curve like this underneath earth blanketed atmosphere it seems impossible but the good news is is that for a perfect black body these curves have a very mathematical shape to them and just by measuring the bloom intensity and visible lights and the red intensity in visible light we can get these two points and then we can mathematically fit them using a computer to perfect blackbody curves and still ascertain what the surface temperature of the star would be about 10,000 kelvins just based upon really two observations one on the blue and one on the red which saved us a lot of time not to actually analyze every single wavelength of light there is to construct the entire blackbody curve as you might imagine so this slide here talks about just that the problems with constructing your perfect clunker but well come up with some solutions here very quickly but of course stars amid over a large range of the spectrum from radio of course all of them do all the way up until ultra violet if you're a 10 or 12 thousand Kelvin star and our hottest stars of surface temperatures you know 40 to 50 thousand kelvins are really still technically in the ultraviolet their peak of Lux but you know for things that are hotter still which you're a very exotic objects they can of course even peek in the x-ray so okay yes it's a large range of possibilities in the electromagnetic spectrum so the problems are you know the range that we're able to measure is well it's limited isn't it by the fact that we have so much absorption going on in our atmosphere so it can oftentimes be impossible to locate the peak emission like it was for Sirius which peaks in the ultraviolet since ultraviolet light is absorbed by our atmosphere but never fear there's a way to figure it out still because you know stars do approximate perfect blackbody curves none of them are perfect black bodies but a perfect black body curve has this very mathematical shape to it so here's your solution as you already knew we can simply make two measurements at two very well-defined wavelengths typically in the visible part of the spectrum because that's what makes it dirt and therefore it's ideal for observing from you know our earth-based observatories and based upon bees you know relative intensities or brightnesses at these two very specific wavelengths again we can fit that set of points to a perfect black body curve and here's your reminder it's a good one you know the hotter object is the more intensely it emits at all the wavelengths including radio and infrared and even invisible that was in the visible part of the spectrum yes a star like Sirius with 10,000 kelvins puts out more yellow light than our Sun does but it's yellow light pales in comparison to its blue light and therefore it appears to be a bluish white star it's a very important detail so in the final construction much like a computer would do for you in a very quick order we have the your authors plot of blackbody curves remember he does it differently than that we typically do where it fluxes the vertical and instead of wavelength being the horizontal he uses frequency so these blackbody curves are sort of inverted and a star with a 3000 Kelvin surface temperature looks like this started with 10,000 Kelvin surface temperature like Sirius looks like this in a star with 30,000 kelvins if the surface temperature which are actually some of those belt stars in our ions they are that hot at the surface look at their primary flux for a star of the 30,000 kelvins for sure and even a star of the 10,000 kelvins you know just about Peaks almost in the violet or specifically in the 30 thousand Kelvin case the ultraviolet in a star which is more like a Betelgeuse 3,000 kelvins you know Peaks really in the red part of the visible spectrum and would peak even better still in the infrared part of the spectrum but we call the blue band visible light the B index and we call the yellow portion of the visible spectrum of the V which stands for simply for invisible so B for blue and V for visible or literally the yellow part of the spectrum because your son puts out mostly yellow light let me talk about these stars having a B - V index or simply a B the index if your stars that are cool only 3000 kelvins there be flux - their V flux is actually a negative value right this is a small number subtracting a larger number so that's becomes a negative value doesn't it so B minus V is negative where a cool star B - V for 10,000 Kelvin star is kind of right exactly in the in the equilibrium point it's about equal so you get basically 0 B - V is just about zero are equivalent to each other and for extremely hot stars there B - there V is going to be a pause number yeah they put an even more be light than to doovy light or more blue light than they do yellow light so if you ever go on into the science of astronomy you'll learn about you know be the indexes and see them quite often which is just a description of how the plot curve comes across these two very specific band widths talk about this on the next lecture [Music]