all right so getting ready for exam number two our second midterm in astro 120 it's over the uh three chapters that came after the uh unit one material so it's chapter 16 17 and 18. that's on the sun properties of stars and the interstellar medium the format to this exam is going to be very similar to the format on the first exam which is mostly free response you know mostly writing response questions there will certainly be conceptual questions that we'll call you know essay style but it's really just a large paragraph and there will be a number of shorter ones as well with more of a fill in the blank terms so i don't think there's going to be multiple choice in fact let's say no there isn't okay but you'll get a similar amount of time like about an hour and 20 minutes like you had on the first exam to complete it so what do you do well you should of course you know review all the lecture material so some of us choose to read the book and kind of neglect the lecture tutorials that's fine but i think that for most of us going through the powerpoints and listening to me talk through the concepts is probably the most beneficial way to really have that material stick although there's a lot of it i know there is there's a lot of it but it's time to start it's time to start reviewing now and uh doing all the homeworks of course for the three chapters and looking at the solutions to those homeworks which are posted up on the canvas page are also very helpful see how somebody writes in a more robust and explanatory manner when they're talking about these different concepts just like on first exam there will be three chapters worth of review questions from a previous author zylic and they just help you to quiz you on the concepts do you really understand what's going on and do you really have a firm uh grasp of the explanations okay so those will be available as well um so you know it's a matter of uh reviewing everything and just kind of being familiar with any concept that we might pull out and cherry pick as being the thing you're going to talk about on exam day it'll be fairly evenly distributed though over the three chapters although the first two chapters have more meat than the third chapter which is 18. okay all right let's talk about it chapter 16 the sun so you know you break this down into major categories and the first thing you learned about the sun was um its physical properties how far away is it how big is it how hot is it how much power does it put out luminosity and all these of course were detailed as exam or sorry homework questions that we answered in this chapter's homework assignment so review that as to how we got the answers okay it's the method as opposed to the mathematical solutions we're not going to make you do any math explicitly long term at least on this exam okay but knowing if you formulae is helpful for answering other questions so in the sun's case you had you know the parameters or the properties as detailed mostly in the homework then we talked about um energy transport how does the sun move its energy from the core out to the photosphere definitely know something about energy transport the three major types then we uh talked about the layers of the sun didn't we talked about the three atmospheric layers talked about the solar wind it just kind of grouped the interior into the interior at that point although we know there's a radiative zone and there's a convection zone so if i go core radiative zone convection zone three interior layers and then uh photosphere chromosphere corona for three atmospheric layers and the solar wind you know there's lots going on there and being able to describe the the properties of those different layers and why we call them distinctly this layer as opposed to some other is a really good thing to know okay so uh the sun has layers and know all of them if they ask you how we determine the temperature of these different layers uh that's also a good thing to know because after all nobody's going and dipping a thermometer into the sun to figure this stuff out and then we talked about um the active sun yes the violence if you will and most of the activity of course is visible to us at the photosphere or higher layers of the sun that's when you came up with uh you know talking about sunspots and uh prominences and solar flares and the magnetic cycle and all these things that are intertwined so as soon as i talk about concepts being intertwined you realize that makes a great essay question yeah where i can relate all those different things together yes indeed it does know about the sun's two sunspot cycles the 11-year cycle between two maxima and the 22-year cycle between not just getting back to the same frequency of sunspots but also the same magnetic polarity okay yes be able to talk about magnetic polarity and then finally you know we talk about the sun's energy production don't we so we talk about nuclear fusion and since one more time nobody's going into the sun to examine what's really happening there it has to be conclusive based upon the evidence and what do we get from the sun we get a cascade of neutrinos which are very hard to detect and now we understand why we're only receiving about a third as many as we thought we should have you know 10 years ago you should be able to explain that too there is no more neutrino dilemma we understand what's going on and you should be able to talk about the proton proton chain the primary uh chain or our mechanism by which our sun uses nuclear fusion to produce energy so that right there is enough isn't it that's a lot of stuff going on in just chapter 16. and are there any you know formulas or or mathematical properties that were kind of new um well all that trigonometric stuff about determining you know the sun's angular size and its distance i'm not going to quiz you or or examine you upon the trigonometry involved but again we should know that trigonometry was used and if you know a bit about trigonometry then of course it makes a lot more sense doesn't it but you know the old formulae from the first unit still are important you should know your luminosity flux equation right luminosity is just equal to what yeah flux times the surface area of a sphere at the distance that you are detecting the uh the flux so you know we say four pi d squared don't we right so you know your luminosity stays constant because what happens to the flux flux diminishes as d becomes larger so you know they uh they they cancel each other out to keep this value constant don't they for a non-variable star we should also know about how the surface temperature is detected for a black body surface temperature is equal to a constant k divided by the lambda max yeah an algebraically manipulated version of veeam's law got to know veeam's law so you know inverse square law for light algebraically manipulated dean's law are always important no matter what chapter you're talking about okay then in chapter 17 all about stars there's just as much going on with those wasn't there lots of stuff to determine so we let off with the solar neighborhood and distances to the nearest stars and the most accurate method of detecting distances to stars was something called trigonometric parallax so for parallax there is a fairly simple and therefore easily memorized formula for for parallax trigonometric parallax and that's that the distance to a star is equal to 1 divided by the parallax angle of that star but this only works well if the units are correct so what are the units of distance in this case units of distance are something called a parsec pc parsec remember what a parsec is it's the distance to a star that has a parallax angle of one arc second so in this case the units of parallax have to be in units of arc seconds to make this formula work but expect to be a question out of many it talks about the relationship between distances in parsecs relative to their parallax angle of arc seconds parallax angle is the angular shift yes of the star over how long yeah over the course of six months right as we orbit halfway around the sun so by taking two measurements of a nearby star at six month intervals we can detect the parallax angle and thus its distance but stars have to be near enough to us for actually uh seeing an angular shift and that is again only the nearest stars to us okay so of course we talked about distances to nearby stars we talked about the solar neighborhood um then we talked about uh you know color and temperature and flux and luminosity which are really review items at this point and then we talked about um the balmer thermometer which goes along with temperature and we talked about spectral classification didn't we so when i say spectral classification hope you think of letters because that is what it's reduced to in the end isn't it the modern spectre classification sequence from hottest to coolest gotta know it you have that saying in your head don't you right or be a fine girl or a guy kiss me the modern specter classification system that's from hottest to coolest and as soon as you talk about spectrum classification then you also talk about the hr diagram yes the hertz sprung russell diagram and you all did a version of it on the homework assignment for this chapter got to know what it tells you right it's a plot of what versus what absolute power or luminosity graphed against its surface temperature or we can do it inspect classification from hottest to coolest and it's so vital in telling us the relationship between the stars mass and its luminosity if you're a main sequence star so know the different sections of an hr diagram supergiant's way on top there white dwarfs way down low there red dwarfs on the other side way down low in the main sequence the s shaped curved in the middle okay um then we talked about um binary star systems there are three major categories to that right visual spectroscopic eclipsing and we talked about what we can ascertain based upon observing those systems and the most important thing was the correlation that we determined between its mass and the luminosity so the mass luminosity equation i should put binary stars there that was an important category but the mass luminosity equi our relationship our ml relationship four and this is key only main sequence stars that's an m-a-i-n made sequence stars if you had to quote it what is the mass luminosity relationship for main sequence stars look like it looks like this luminosity of a star compared to the luminosity of the sun which is just another star is equal to the mass of that star compared to the sun's mass all raised to the you know your author chooses the third or the fourth power but more accurately it's the three and a half power which is hard to do in your head i know but follow me here if a star has twice the mass of the sun then its luminosity compared to the sun's luminosity is at least how much greater we'll take it to the third power first it's twice as much mass two to the third power two times two is four times two is eight it's at least eight times as luminous if you took it to the fourth power two times eight is sixteen so it's between eight and sixteen times as luminous as our sun would be sure it's got twice the mass it's gas tank is twice the size of our sun it's got twice the fuel but if you're burning it at 10 times the rate then of course you're going to burn through that fuel much faster aren't you so what's the implication of the mass luminosity relationship yeah stars live much shorter lives don't they when they have greater amounts of mass so the biggest stars are bright and intense and awesome and they also have very very short lives as a result of that okay and i'm sure i looked out a few things right because it's a dense chapter there's lots of stuff going on with the stars but you know these are the true highlights of the chapter and then with chapter 18 you know the interstellar medium or the ism for short we talk about the two main components of the ism which is dust and gas yeah don't forget that the ism is made up of dust and gas and then talk about the different types of gas that we find there don't forget that the majority of our universe is made up of what gas hydrogen and hydrogen can come in different forms neutral or you know cold hydrogen h1 ionized hydrogen h2 which you find in emission nebulae and then there's also h sub 2 or molecular hydrogen which is also the most common molecule that is in our universe so being able to tell me how we detect those different varieties dust included how we detect dust you say it doesn't dust wipe out light yes but it also reddens light doesn't it and it also has a partial transparency depending on how thick it is so knowing all those different things about gas and dust and the components that make up the interstellar medium you know is the short summary isn't it of chapter 18. so know about molecular clouds and giant molecular clouds and ultimately this will lead into the next chapter which is how stars are born out of these large molecular clouds okay so you're right i kind of downplay chapter 18 there but with chapter 18 it's just a thinner chapter that's all but it's got important stuff to it so ism for interstellar medium you know gas and dust note that dust has these two um main decision points on how we detect it which is reddening and extinction right or the elimination of light know about the different components of the gas again we talked about hydrogen talked about the importance of detecting h1 neutral hydrogen because that's the majority of the universe which is the 21 centimeter line and how that's produced um h2 emission nebulae why do they become h2 regions it's because of a strong ionization flux within that cloud you have to have a star put got large amounts of ultraviolet light that allow the electrons to be ionized recaptured and then the bound bound transition the d excitation from level three to level two giving off that very prominent red line in the hydrogen spectrum yes so yeah reviewing all of that the bomber thermometer and then again in the end we talk about molecules we know that we need a nice cold environment for molecules that's why they don't really exist much in the envelopes of stars unless you have an extremely cool star a very very red star a little bit can be detected in the outermost atmosphere and then molecular clouds and ultimately the biggest ones of them all are the gmc's yeah the giant molecular clouds so the nice thing about chapter 18 is it has a little summary slide at the end which is all the components and how we detect them so that's an important slide to look at okay but are there any new mathematical equations that go along with the chapter 18 not really but you know knowing the two primary tools that an astronomer has for determining temperature and determining uh overall luminosity are you know key always okay all right i hope you found that useful and it's time to get going it's time to start reviewing and i hope you find the um review questions from xylic up on the canvas page a bit helpful just to kind of quiz you on the different concepts as well okay best of luck to you you