all right a little help with question 19 on chapter 17 s homework this is the big graph this is the big HR diagram that is the last major problem on the homework and the very last page of the homework has a list of data that you'll need to plot sixteen brightest stars on this graph here first of all a bit about what you're looking at is an HR diagram so the vertical axis is luminosity or power and the horizontal axis is surface temperature where temperature is increasing to the left that's the bizarreness of the HR diagram yes temperature increases to the left these are units of thousands so this is 5000 Kelvin this is 10,000 Kelvin 20,000 kelvins 25,000 kelvins and he's utterly strange units here are the luminosity compared to the solar luminosity so if you know how powerful a star is in solar luminosities like it's a hundred times there are a thousand times the sun's luminosity which is what the units are on this data sheet let's say you're able to plot this relatively easily on this diagram so okay let's start looking at the data sheet on the back you know the first object you got on there is Sirius the brightest star in our sky besides the Sun find the column for luminosity it says the Sun is equal to one unit so seriously this luminosity is 38 solar luminosities that's how you say that and its surface temperature is 9700 kelvins just about 10,000 kelvins so all right looking at the graph these units over here that'd be one so a luminosity of the sun's power that's ten so luminosities to the second is a hundred of course that's 100 to the luminosities a thousand ten thousand hundred thousand and a million yeah one extra magnitude or exponent value as you go up the ladder here between ten and a hundred I cap this up into increments for you there's ten divisions but what do you notice about these little red marks on the left here they start to get closer and closer to each other he approached the next value this is a logarithmic scale so when you try to plot logarithms it's kind of a strange concept but what as they start getting bumped up to each other's getting near and near the next magnitude of the next exponent value so believe it or not this first to take mark here's four that's ten this is 20 that's 30 40 50 60 70 80 90 and then 5e 100 when you make it to the next major mark but again all those numbers are all those increments got closer and closer to each others you got further and further up toward the next major mark so back to the issue it was serious with 38 solar luminosities so where is it on this diagram well that's 20 that's a 30 that's 40 so 38 is about right here and of course I think of a straight into take this now you start over like this and intersect with we hope the surface temperature which is given as 9700 Kelvin or you know I'm very close to being 10,000 degrees Kelvin kelvins so I was about right here isn't it just a smidge to the right of the 10 you've still got a lot of it from that value like this they where those two intersect is right here and you simply make a dot there and put the weirdness series next to it from our superimposed guy here this is the area where the main sequence stars hang out what do you think is serious more likely a main sequence star yeah it is and how many the stars in the sky our main sequence stars at least 90% now next up we're gonna do we can do Arcturus next year what the heck Arcturus is 200 times the sun's luminosity and 4600 kelvins so 200 solar luminosity is now 100 is right here at 10 to the 2 and 200 again is you know a bigger jump than you might think it's more like about here if I take a horizontal I'm straight to the right and I find the value for its temperature which was 4600 at the bottom there's five thousand five thousand forty six hundreds about looking at my yeah that's right about here that Dayton I straighted stood up from there and at this intersection here is where our tourist is located well what did you just figure out about our tourists is our tourists a main-sequence star it is not yes it is evolved off of the main sequence that's near the last 10% of its life up in the giant range of stars which is also it makes it much larger surface area which makes it a much larger luminosity because of that increased surface area so it's becoming more luminous in its later part of life which most stars do and that's just an interesting note to make I'll do one bar with you um how about Betelgeuse cuz this kind of extreme Betelgeuse is number seven on that sheet and it's luminosity is a hundred and ten thousand times the power of our Sun and its surface temperature is only thirty one hundred kelvins well you know thirty three thousand Rather's about here thirty one hundred is you know about here you take a straight edge and go straight up from there that's definitely not straight is it I can fix that that's all being slightly better but they're looking at the luminosity again was about a hundred and ten thousand times the sun's luminosity well a hundred thousand is ten to the fifth a hundred and ten thousand is right about here yet remember they're bigger increments at the bottom they get closer closer as you approach the next magnitude so take a value our straight edge straight to the right from here and you find that Betelgeuse is located at this location and if this is an HR diagram will you recall them up stars and hang out in this area up here yeah those are luminosity class 1-a those are supergiant stars which be induced definitely is if you thought our torus was getting near the end of its life Betelgeuse is truly at the very very last a few days of its life in astronomical terms and Betelgeuse is a plot of gear definitely not a main sequence star okay what are three at the sixteen I'll bet you could do the rest and have some fun with it and the final question on the homework is you know adventure your mnemonic for the aspect of classification sequence will be AF dkm you'll have some fun with that too