the Bombers thermometer is a concept that relates to stellar temperature and specifically a de marrón what the bomber series of lines related to it was for hydrogen hydrogen atoms inside of stars in case you don't remember well let's go over some details here the Balmer series of lines in spectra relate to transitions in the hydrogen atom either to down to or from starting at two and going up the second energy level of the hydrogen atom itself in D excitation we're talking about jumps are Falls that come down to the second energy level which gives rise to emission lines and for photons of just the right energies that are entering a hydrogen atom and the electrons already in the second energy state it can be excited right which gives rise to an absorption line so in the hydrogen atom we have these four lines in the visible part of the spectrum the jump from three to energy level three to energy level two gives rise to the red emission line here h alpha is what we call that the line that is in the tito part of the spectrum is a jump from level 4 to level 2 in the hydrogen atom and it's called hydrogen beta and 5 to 2 is the violet line here hydrogen gamma and 6 to 2 right here's the other violet line it if you have excellent eyesight you might be able to see called a hydrogen Delta line tank just reminder that there's these four very distinctive hydrogen lines in the visible part of the spectrum and that's the significance of the Balmer series that they're all in the visible part of the spectrum now imagine if your star is very very cool if it's too cool it says here then it's hard to get your electrons excited our out of the ground state and into their first excited state which is otherwise known as the second energy level if you don't have many electrons in that second energy state and it's impossible really to have them d excites down to energy level two they have to be an even higher level wooden thing so if the star is too cool you have very few electrons in this excited state or you know above that and therefore we're going to very few emission lines which are caused by dxi tation to the second energy level so what kind of stars are too cool to do this a star on the order of a surface temperature of around 3000 Kelvin soar you know about 6000 degrees Fahrenheit but the same argument holds if your star is too hot if your star is extremely hot when your electrons are at a much higher state than the second energy level and they might cascade down from mg level sixed and is level 4 but get even become all the way down to energy level 2 is kind of hard if the general activity in your plasma you're very hot gas has all your electrons excited to a higher energy state than number 2 so if you're too cool or if you're too hot there's not a whole lot of electron electrons transitioning to the second energy level so guess what if you're a very hot star like 20000 kelvins or hotter then you also have very weak or very absent Balmer lines and the visible part of the spectrum so we call this concept the Balmer thermometer you know because of the Balmer series and how it interacts with the second energy level of the hydrogen atom then you get very weak bomber lines for very hot stars and for grade cool stars so how you say all right well what's this have to do with anything else going on well in the original classification scheme this is in the early part of the last century there was a group of women at Harvard they called them calculators because that's what they are doing they were trying to figure out the spectra of stars and the very first condition they used for classifying stars what to say that any star which has strong bomber lines you know even this is all part of the spectrum caused by the excitation of the second energy level when I would be emission lines wouldn't it their stars give primarily absorption line spectrum so those would be excitations from the energy level to to higher energy levels any stars that have the strongest bomber lines mean thickest the boldest meaning that that transition is predominant will be given a classification of spectral class a so a stars have the strongest are thickest are boldest Balmer series lines and you're right most of stars have absorption lines the second-tier stars the ones that weren't quite as bold quite as thick would be called Spectre Class B and it starts with the third most bolded lines and the Balmer series would be Spectre Class C you get the idea right on down to Z and that was the original classification scheme now these days we have simplified things a little bit it's mostly because we come upon knowledge that they didn't have way back in the early you know nineteen teens in the 1920s but originally the ladies at Harvard categorized the stars based upon their Balmer lines strengths or thicknesses or boldness and there was no understanding in science much as at Harvard as how this even related temperature they didn't understand that there is electron energy levels in an atom people like Rutherford and Bohr had not come along and hypothesized what an atom might look like and why in fact light is emitted and absorbed nope that was all still really unknown and they used all 26 letters originally yeah from a the strongest mama lines to Z the weakest bomber lines and therefore there was 26 classifications of stars which is you know a lot but now we know based upon this concept known as the Ballmer's thermometer that both hot and cool stars can have weak Balmer lines if you'd like to reorder your classification from hot to cold or from cold to hot then you really can't use bomber lines anymore because that one's about the strongest bomber lines are kind of middle temperature stars right they're not the hottest normal not the coldest either one so the modern spectral sequence is based upon temperature and they had t choices at this point you know either alright let's go back and redo all the classifications we've already done all a hundred thousand stars that would be classified by I painstakingly by these women redo the whole thing or let's just take their existing letters that they used to classify the stars and rearrange them from hottest to coolest well what kind of starts would have you know the hottest temperatures it's ones that had very weak bomber lines which would be way down the alphabet like you know m n o P way down the alphabet would be you know really hot stars but then again really cool stars would also be way down the alphabet to when they may be in the queue and the R and the esses and the T's if you were cool so using the existing Balmer series sequence but rearranging it from hot to cold meant to rearrange the alphabet essentially so these days we've taken the original 26 classifications and reduced them down to just seven like you see here and the hottest stars were the invention of classification of oh because oh stars again had very weak bomber lines and now we know a thing about the bombers thermometer that makes a bit more sense the second hottest stars had the pretty strong bomber lines but not the strongest so they were classifications of be originally sorry I hit the wrong button and then the Stars with the strongest bomber lines of them all the original a stars would be kind of the middle zone of temperatures so sure enough they could come up third in this listing from hottest all the way down to coolest and an F and G you know very close and what the sixth and seventh letters in the alphabet you know were you know not strong bomber lines but they were still evident and they would be stars that we're well in the middle temperatures as well and she probably makes some sense to us now that the very coolest stars would have had very weak bomber lines and therefore would have been way down the alphabet maybe even in the you know the use and the B's and the double use but we've reduced the number of letters as well so I know it looks a bit crazy now but it kind of comes from the original classification system and it was just rearranged from hottest of coolest and they got rid of lot of letters try to make life simpler so nowadays we talked about the specter classification scheme going from OB a FG K and M and I try to color code them for you o and B stars are in fact you know bluish white and the a stars are really just kind of a pure white with it maybe that's to hit the blue to them F stars again are mostly white with maybe a hint of yellow to them and I look at the black out here but of course you can't make white on a white background but G stars like our Sun are definitely you know yellow surface color and then the case stars cooler still are orange and the coolest of them all are the nice you know dull red kind of blood-red stars and this is just something that's more intuitive now yes blue lights to white to yellow white to orange to red is the sequence of colors from hottest to coolest so if they're very bottom you you can't read it it's being cut off by the screen there but it says that this is also a color scale yeah right OB AF g/km and how do we remember this in the modern context well you come up with a mnemonic or a saying that helps you to remember this spectral classification scheme and it's a mnemonic was invented in the 1950s so it sounds very much like that but here's how most of us learned it when we were kids like me it's o be a fine girl or guy kiss me again o be a fine girl kiss me was the original way to remember the order of these letters from hottest to coolest and the next homework I'll make you come up with your own you get to invent your own mnemonic and I've heard some pretty good ones over the years here's a good one I heard oh brother astronomy formula is gonna kill me that's a good one and you can get very ingenious with all these letters appear yeah the first letters of words to make up a sentence a mnemonic to help you remember this sequence okay but knowing the modern classification sequence is important and even though we've reduced the number of letters from 26 down to just seven of course we always have a need for finer detail their resolution when it comes to science so all of these classifications are further subdivided into ten subdivisions called subclass but here you see the spectra of the many of these major varieties yeah OB a FG k and M and the O star has very few spectral lines very few absorption lines in it and the ones that are there are primarily due to hydrogen there's the Alpha line there's the beta line there's the gamma and there's the Delta and the epsilon line when you get to cooler stars B and a and F and G and K and M what do you notice more and more lines start showing up don't they yeah they have heavier elements in them and the different states of these elements ionization states can give rise to different spectral lines so it's very much true that you know by the time you get to you know 30,000 kelvins it's pretty a charge for any atom to exist and it all gets pretty much destroyed just by the thermal energy so you know oh stars are typically in all very very sparse in terms of their elemental abundance it's pretty much hydrogen it may be a little bit of helium and by the time you get down to M stars at 3,000 kelvins they can you know host molecules and even heavier elements like iron and magnesium and sodium so that's a common generalization that the spectra get more and more complex you should get two cooler and cooler stars all right we were just talking about a second ago each of these letters gets further divided up into ten classes called subclasses and you just assign a a number after the letter so you'll have an O one and O here was the first but Oh 100 200 300 400 500 600 700 800 900 B a FG km so then it's b0 b1 b2 b3 you get the idea and that's just to give us an even further nice resolution of subtle differences in both temperatures at the surface and even in elemental abundances in the specter themselves so what kind of stars our Sun our Sun is a g2 star this one right here yeah we're in the spectra class of G with our surface temperature right around 1500 kelvins and if we were a slightly cooler still we'd be a g3 so have a coolers do would be a g4 if you were slightly hotter would be a g1 but we are a g2 star and any other star that's a g2 has a similar surface temperature to ours right around 1500 kelvins so now we know that the strengths of Balmer lines you know suggest these differences in both stellar spectra and in surface temperatures both and that's really a good thing because a modern computer knowing this information can easily identify a star's surface temperature just by looking at the spectrum so there's no need to make this painstaking plot curve measuring wavelengths that are measuring flux at all different wavelengths to create this plonker bits it's actually much simpler than that and much quicker than that using this modern knowledge and a modern computer alright so again you know the hotter that's the star's temperature the more collisions occur with the particles the more ionization takes place and that really is what causes these and differences in spectral lines there's a gorgeous shot of all the different spectral classes on the left ear o to M and again what you notice is from all over down to M here is that the amounts of spectral lines get greater and greater as you get to cooler and cooler stars and the a zero stars there have the strongest Balmer lines of them all the H alpha line over here on the far right yes and there's the beta line gamma [Music]