hello and welcome to general astronomy lecture 19 learning from light matter leaves its fingerprints wherever it interacts with light examining the color of an object is a crude way of understanding the clues that are left behind from the matter that it contains for example a red shirt absorbs all visible photons except those in the red part of the spectrum so we know that it must contain a dye with these special light absorbing characteristics if we take lights and disperse it into a spectrum we can see the spectral fingerprints more clearly the image here on the right shows the sun's visible light spectrum in great detail with the rainbow of color stretching in horizontal rows from the upper left to the lower right of the photograph we see similar dark or bright lines when we look at almost any spectrum whether it is the spectrum of the flame from a gas grill in somebody's backyard or the spectrum of a distant galaxies whose light we collect with a gigantic telescope as long as we collect enough lights to see details in the spectrum we can learn many fundamental properties of the object we are viewing no matter how far away the object is located the process of obtaining a spectrum and reading the information it contains is called spectroscopy if you project a spectrum produced by a prism onto a wall it will look like a rainbow at least for visible light however it's often more useful to display spectra as graphs that show the amount or intensity of the light at each wavelength so although it's nice to look at this rainbow image here it doesn't tell us as much information as we might be able to get so instead we're going to take this and put it into a graph an x and y graph so let's take a look at how this works so we're talking about the spectrum of light coming from an object well laboratory studies show that spectra come in three basic types the first is that spectrum of a traditional or incandescent lightbulb which contains a heated wire filament it gives a rainbow of color because a rainbow spans a broad range of wavelengths without interruption we call it a continuous spectrum so if you just take off its standard light bulb pass that light through a prism well you've seen this before I'm sure you get a rainbow of color so the intensity is pretty much just a nice flat curve so you have some amount or intensity of light at all of the wavelengths you get the full rainbow so because it's a full rainbow without any interruption it's called a continuous spectrum the second type is when you have a thin or low density cloud of gas that emits lights only at specific wavelengths that depends on its composition and temperature the spectrum therefore consists of bright emission lines against a black background and therefore is called an emission line spectra so if you have a gas that's giving off its own light at some specific wavelength and then you take that and pass it through a prism you'll see only specific bands of color not the full rainbow anymore just whatever light is being given off by this gas so when you look at the intensity or the amount of light at each wavelength you'll see nothing for a lot of it and then sudden jumps or peaks in its intensity because you're getting more light at that color so as a result this is called an emission line spectrum because the cloud of gas is emitting light at a certain wavelength so the third and final type is the absorption line spectrum if the cloud of gas lies between us and a light bulb we still see most of the continuous spectrum of the light bulb however the cloud will absorb light of specific wavelengths so the spectrum will show dark bands of lines over the background rainbow making it what we call an absorption line spectrum note that when the spectra are shown as graphs absorption lines appear as dips on a background of relatively high intensity light while emission lines look like spikes on a background with little or no intensity so here we can see what happens in this situation so you take the standard light bulb like we saw a moment ago let me go back and you got a full rainbow and a nice constant graph here of intensity but now we're putting a little cloud of gas in between the prism and the light source so this gas is going to absorb some of that light and then we're going to see that full rainbow but with little pieces of it missing because some of that was absorbed so our intensity drops off at these little regions and this is kind of like how our atmosphere can scatter some light we have some gas in our atmosphere that in this case of scattering lights so we don't see everything but the point is and there are three scenarios in which we will get some type of spectrum continuous if you just have a light source giving off light directly toward us in emission line spectrum if you have a cloud of gas giving off a certain light and here we see the absorption line spectrum if you have something blocking some of the light from behind well let's return to the Sun spectrum for a second we can apply these ideas to the solar spectrum which shows numerous absorption lines over a background of rainbow color this tells us that we are essentially looking at a hot light source through gas that is absorbing some of the colors right so here's our full rainbow but little pieces are missing so something is in between the Sun where the light comes from and us looking at it for the solar spectrum the hot light source is the hot interior of the Sun while the cloud of gas that we were talking about is the relatively cool and low density layers of gas at the top of the sun's visible surface which we call the photosphere we'll learn a lot about the Sun a couple lectures from now so this just gives you an idea so suddenly we now know a little bit about the Sun just because of this image we know it's giving off light at a full spectrum but we know there's some gold some gases in the upper atmosphere absorbing some of its light so already we know that much about the Sun just because of this image in studying its light chemists discovered that they could produce spectral lines in the laboratory and use these spectral lines to analyze what kind of atoms different substances are made of chemists had long known that many substances emit distinctive colors when sprinkled into a flame to facilitate studies of these colors around 1857 the German chemist Roberts Bunsen invented a gas burner which is called the Bunsen burner today you've probably used one in highschool or some chemistry classes in college that produces a clean flame with no color of its own so you can see that here on the right so what they would do is they would take a Bunsen burner and then sprinkle substances on it and it would change color so you can figure out what the substance is made of because of those color changes so let's go into this a little bit further Bunsen's colleague the Prussian Barnett physicist Gustav Kirchhoff suggested that the colored light produced when substances were added to the flame might best be studied by passing the resulting light through a prism the two scientists promptly discovered that the spectrum from the flame consists a pattern of thin bright spectral lines against a dark background in other words you should now know that that would be an emission line spectrum Kirchhoff and Bunsen then found that each chemical element produces its own unique pattern of spectral lines thus was born in 1859 the technique of spectral analysis which is the identification of atoms and molecules just by the unique patterns of spectral lines you can eat you can easily see that each substance produces a unique pattern of spectral lines each pattern can be thought of as a spectral fingerprint for identification this is enormous leader but important in astronomy because it allows us to determine the detailed compositions of distant planets and stars these photographs here on the right show the spectra of different types of gases as measured in a laboratory here on earth each type of gas has a unique spectrum that is the same wherever in the universe the gas is found so if you look at this each element here has its own unique spectrum that's what we call it a fingerprint so if you were to have some random cook like bottle of gas let's just say and you pat pour you take take it and you pass light through it and you see these different bands of lights so you don't know what the element is but you know there's just some gas in there well you can look at the lights and then compare it to these and figure out what it is now if this was a face-to-face class I would be able to show you this we actually have little tubes of gas that we run at an electric current through to heat them up and give off light and we can actually compare them to these different Spectras here so you can actually see this firsthand and it's really neat demonstration but it's a really cool thing here I mean and especially in astronomy because everything we see from a star is light so if we can take that light and figure out what these little bands look like we can suddenly figure out what those objects are made out of and that's a big deal at least at the surface all right we have seen how emission and absorption line spectra form and how we and how we can use them to determine the composition of a cloud or gas now we are ready to turn our attention to continuous spectra although continuous spectra can be produced in more than one way that is light bulbs planets and stars they all produce kinds of continuous spectrum that can help us determine their temperatures so not only can we figure out the respect their chemical composition like we just discussed we can also figure out their temperatures in a cloud of gas that produces a simple emission or absorption line spectrum the individual atoms or molecules are essentially independent of one another most photons pass easily through such a gas however the atoms and molecules within most of the objects we encounter in everyday life such as rocks light bulb filaments and people cannot be considered independent these objects tend to absorb light across a broad range of wavelengths which means that light cannot easily pass through them right we don't see light passing through our human bodies very easily so you can't easily see light pass through them and light emitted inside them cannot escape easily the same is true of almost any large or dense object including planets and stars in order to understand the spectra of such objects let's consider an idealized case in which an object absorbs all photons that strike it and does not allow photons inside it to escape easily photons tend to bounce randomly around on the insides of such an object constantly exchanging their energy with its atoms and molecules by the time the photons finally escape the object their radiative energies have become randomized so that they are spread over a wide range of wavelengths the wide range of wavelength of the photons explains why the spectrum of lights from such an object is smooth or continuous like a pure rainbow without any absorption or emission lines most importance the spectrum from such an object depends on only one thing the object temperature to understand why remember that temperature represents the average energy of the atoms or molecules in an object because the randomly bouncing photons of light interact so many times that those atoms and molecules they end up with energies that match the energy of the objects atoms or molecules which means that photon energies depend only on the object temperature regardless of what the object is made of the temperature dependence of this light explains what we call explains why we call it thermal radiation or what we call blackbody radiation and why it's spectrum is called a blackbody radiation spectrum any object that absorbs all radiation falling upon it is known as a blackbody no real object emits a perfect black body spectrum but almost all familiar objects including the Sun the planets rocks and even you emit light that approximates blackbody radiation the figure here on the right shows graphs of the idealized blackbody radiation spectra of three stars and a human each with its temperature given in the Kelvin scale be sure to notice that these spectra show the intensity of Lights per unit surface area not the total amount of light emitted by the objects for example a very large 3,000 degree Kelvin star given by the red line or curve can emit more total lights than a small 1500 degree Kelvin star even though the hotter star emits much more light per unit area so these to show you again a blackbody curve it's a continuous spectrum and it shows you for example the hottest star here is giving off more intensity and at all wavelengths and I need to get two cooler and cooler objects they're not giving off as much energy so you can see that a human doesn't give off anywhere near as much as a 1500 degree Kelvin star so these curves whenever you see a curve like this is what we call a blackbody spectrum and you get you have a blackbody whenever it's an object that absorbs all radiation falling upon it spectra obey two laws of blackbody radiation and these are very important law one the stefan-boltzmann law each square meter of a hotter objects surface emits more lights at all wavelengths for example each square meter of the surface of a 1500 degrees star emits a lot more lights at every single wavelength than each square meter of the 3,000 degree Kelvin star and the hotter star emits light at some ultraviolet wavelengths that the cooler star does not emit at all so the stefan-boltzmann law just says that it's emitting more energy at all wavelengths right it's more lights at all wavelengths compared to this red one right the blue is higher than the red at all points but it also says that this can give off ultraviolet lights whereas this red star doesn't give off any red light there are any ultraviolet light so it's very important so this equation here relates these two things this is the the energy given off the flux is equal to some constant it's just a number times the temperature raised to the fourth power so the intensity of light that you get only depends on temperature this is just a number so it doesn't matter there's a very important concept the second law is Venus law hotter objects emits photons with a higher average energy which means a shorter average wavelength that is why the peak of the spectra called the wavelength of maximum emission so here's the wavelength of maximum emission the peak here peak here and so on they are shorter at wavelengths for hotter objects so as the increase in temperature notice that the peak is moving further to the left where it is more energetic so hotter objects see their peak at smaller wavelengths right we're getting smaller as we go to the left so the hotter you get the smaller the wavelength gets that means it's more energetic for example the peak of the 1500 degree Kelvin star is in the ultraviolet light but the peak of the 5800 degree Sun is in visible light and the peak for the 3,000 degree star is in the infrared so this can be modeled mathematically it depends again only on temperature but that that wavelength of maximum emission or the peak is equal to this number divided by the temperature of your object that's it so temperature plays a huge role here so just become slightly familiar with these equations at least a little bit you won't have to use them very much if at all you might use this bottom one more but just know that this is where it's coming from basically just showing how important temperature is on these things so again you have more intensity with temperature right so the hotter your star the more intensity of lights and the hotter your star the smaller the wavelength so the decreases because thermal radiation spectra depend only on temperature we can use them to measure the temperatures of distant objects in many cases we can estimate temperatures simply from the objects color notice that while hotter objects emits more lights at all wavelengths the biggest difference appears at the shorter wavelengths at human body temperatures of about 310 degrees Kelvin people emit mostly in the infrared and emits no visible light at all which explains why we don't glow in the dark unless you're an alien or something a relatively cool star with a 3,000 degree Kelvin surface temperature emits mostly red light that is why some bright stars in our sky such as Betelgeuse in the Orion Nebula and Antares in Scorpius appear reddish in color the sun's 5800 degree Kelvin surface emits mostly in green lights around 500 nanometers but the Sun looks yellow or white to our eyes because it also emits other colors through the visible spectrum hotter stars emits mostly in the ultraviolet but appear bluish white in color because their eyes cannot see their ultraviolet light if an object were heated to a temperature of millions of degrees it would radiate mostly in x-rays some astronomical objects are indeed hot enough to admit x-rays such as discs of gas and circling exotic objects like neutron stars and black holes something we'll talk about far in the future of our course hotter stars emit no I said that are in excuse me so this is really important so now we can see that there is a connection between temperature and color so people radiate in up and infrared light so we don't see it a cool star may emits in the red the hottest stars emits in blue and white so this is just an example here showing a fire poker if you heat this up originally on its own without being heated it's just black but as you heat it up it begins to glow and it gets brighter as you glow I'm sorry it gets brighter as you go and it changes color from red to white so this shows both of those two laws that we talked about it's getting brighter because of our stefan-boltzmann law right I get it emits more lights with a hotter object so it's getting brighter because of that and then the second law it's changing from red to white because it's wavelength is decreasing as you go so it's going from red to yellow to blue is to white so very important connections to make and this is just another example of this so here we have an example just showing heating up a piece of glass again the hotter it gets the more goes from red to blue --is-- white so it's going from red to orange to yellow and you can see you get this compared to stars here's a star with that's red in orange star and a yellow star so just because of these colors we can tell that this star is the hottest this is the middle in temperature in this red star is the coolest so just by looking at color we can figure out what temperatures this figure here on the right shows the blackbody curve for a temperature of 5,800 degrees Kelvin as well as the intensity curve for light from the Sun so this dashed line is what it would look like if it's a perfect black body where it doesn't absorb any radiation for fifty eight hundred degrees Kelvin and then the squiggly line is the actual one that we see for the Sun and notice that they look very similar other than some of the squiggles well the peak of both curves is at a wavelength of about 500 nanometers near the middle of the visible spectrum note how closely the observed intensity curve for the Sun matches the blackbody curve this is a strong indication that the temperature of the Suns glowing surface is roughly 5,800 degrees Kelvin a temperature that we can measure across a distance of 150 kilometers here on earth so the spectrum of the Sun very closely matches the spectrum of something that is 5,800 degrees Kelvin so we know just from this that the sun's temperature at the surface is 5,800 degrees Kelvin it's that simple we just look at the lights put it onto a graph and we pretty much almost instantly can figure out the temperature of that object it's really quite amazing and then based on these little dips and bumps the emission absorption you can figure out what the sun's made out of at the surface as well so these are very powerful tools in astronomy blackbody radiation depends only on the temperature of the object emitting the radiation not on the chemical composition the light emitted by molten gold at 2,000 degrees Kelvin is very nearly the same as the emitted lights by molten lead at 2,000 degrees as well therefore it might seem that analyzing the light from the Sun or from a star can tell astronomers the object temperature but not what the star is made of the intensity curve for the Sun which is a pretty typical star is not precisely that of a black body the difference is between a star's spectrum and that of a black body allow us to determine that chemical composition of the star so the peak of each can help us determine the temperature and then the difference is because of these dips or bumps that is again emission or absorption the differences of those from the curve tell us what the object is made out of we can learn about the motion of objects relative to us from changes in their spectrum caused by what we call the Doppler effect now again I have a video here for you so if you're watching this video it's a good point to stop this video and go to the youtube description where I will have a link to this video and I highly recommend you watching it real quick if I recall it's just a car that drives by and you can hear this Doppler effect you might want to watch it at the end of this slide but at any points in this slide I would recommend going over to that and watching it so you probably noticed the Doppler effect on the sound of a train whistle near train tracks if the train is stationary the pitch of the whistle sounds the same no matter where you stand but if the train is moving the pitch sounds higher when the train is coming toward you and lower when it's moving away from you just as the train passes by you can hear the dramatic change from a high to low pitch to understand we have to think about what happens to the sound waves coming from the Train when the train is moving toward you each pulse of a sound wave is emitted a little bit closer to you so here's an example of this so say you're in front of the train not on the tracks hopefully but the trains coming toward you well every single time it releases a sound wave each sound wave is getting closer and closer to you so the result is that waves are bunched up between you and the train giving them a shorter wavelength and therefore a higher frequency or pitch so as it's coming toward you the waves are really close together so the wavelength the up and down motions are really small which means the frequency is high and you get a really high pitch so it sounds like a higher sound but as you are saying let's say the train is now passed by you as it passes by you each pulse comes from farther and farther away stretching out the wavelength and giving the sound a lower frequency in the lower pitch so that's why you hear this Doppler effect for the sound of that or car horn as it goes by will change its pitch well the Doppler effect causes similar shifts in wavelengths of Lights in of Lights excuse me if an object is moving toward us the light waves bunch up between us and the object so it's entire spectrum is shifted to shorter wavelengths because shorter wavelengths of visible light are bluer the Doppler shift of an oncoming I'm sorry the Doppler shift of an object coming toward us it's called a blue shift so now instead of sound we just use light so say maybe a star is moving toward us a little bit well it's going to bunch off those little light waves so it's got a higher frequency and therefore it will be a little bit bluer it has more energy and it shifts it to the blue and the opposite is true if it's moving away from us and it's light will be shifted to longer wavelengths then so we call this Doppler shift a red shift because we know that longer wavelengths of light are redder in color spectral lines provide the reference points we use to identify and measure Doppler shifts for example suppose we recognize a pattern of hydrogen lines in the spectrum of a different of a distant object we know the the rest wavelengths of the hydrogen lines that is their wavelengths if it was stationary from laboratory experiments in which a two of hydrogen gas is heated so that it's wavelength of the spectral lines can be measured so let's just say that's that we have here here is that example well if the hydrogen lines from the object appear at longer wavelengths then we know they are red shifted excuse me I lost my place then we know that they are red shifted and the object is moving away from us so lines are red shifted here so maybe it's an object moving away from us if we get this pattern here but we know this blue line should be to the left a little bit the yellow should be the left a little bit red to the left a little bit well it's shifted we know this now so just because it was shifted toward the red well now we suddenly know that this object is moving away from us so that's a really important tool so the opposite can be true as well so suppose we look at spectral lines of a planet or star that happens to be rotating as the object rotates light from the parts of the object rotating rotating toward us will be blue-shifted whereas light from the part rotating away from us will be red shifted and light from the center of the object won't be shifted at all the net effect if we look at the whole object at once is to make each spectral line appear wider than it would be if the object were not rotating the faster the object is rotating the broader in wavelength the spectral lines become so if we have a rotating object we will see thicker lines we can therefore determine the rotation rate of distant objects as well by measuring the width of its lines so this is amazing so I mean we can figure out not only composition and temperature of an object just by its light but now we can see if it's moving away from us or toward us because it'll be either red shifted or blue shifted and we can also figure out if it's rotating and how fast it's rotating because the lines will get thicker if it's rotating so it's just an incredible amount of tools that we can use just by looking at lights so light is a cosmic messenger it tells us everything well maybe not everything that tells us a lot about the universe so I leave you now with this image this is not an actual test but I'll test you a little bit here so this is a spectrum of n objects I won't tell you what it is yet but we'll get into that there's a lot of different things going on here so if this is a test I'd ask you what is this right here so I'm circling this so what are you seeing here so what you're seeing here are some dips in the spectrum so that probably tells you there's some absorption going on here over here we see some Peaks this should tell you that there's some emission at these wavelengths so you can also see this as extra color in the bands over here and some darkness in the bands over here so anyway let's get into this a little bit first of all you'll notice that there is relatively little blue light and a lot of red light all right so here's blue is not much of it but there's a lot of red so what does this tell us the visible light we see from Mars which is what this is a spectrum from is actually reflected sunlight Mars absorbs most of the blue lights but it reflects and scatters most of its red light this is why Mars is red here we see thermal radiation Peaks so there's some peak here in the thermal or in the infrared right so we have a big peak over here in infrared this is thermal radiation this is telling us that the objects emits a continuous spectrum of thermal radiation that peaks at this wavelength so we can figure out its temperature just by its peak the peak indicates a surface temperature of 225 degrees Kelvin so just by looking at this so far we know the object is mostly red and because it's Mars we know that to be true and just because of this peak here and the infrared we know that it's surface temperature is about 225 degrees Kelvin [Music] next we see these emission lines in ultraviolet in the ultraviolet region right these Peaks what does this tell us well this tells us that the thin atmosphere of Mars contains a hot gas at high altitudes right so remember that emission lines come from a hot gas giving off its own light so there's gases in the atmosphere of Mars even though it's a thin atmosphere that are giving off its own also violet lights so that's what we're seeing here next we see these absorption lines these reveal the presence of carbon dioxide in the Martian atmosphere so they are observe designs at a very specific wavelength and we can compare that to what the curve would look like without carbon dioxide in the atmosphere and we'll see that those dips wouldn't be there so we know based on where these dips are that this is a showing the presence of carbon dioxide last one this one's kind of hard to see but the wavelengths of these spectral lines are slightly shifted they are shifted by an amount that depends on the velocity of Mars toward or away from us as it moves so just by looking at this spectra of light so a second ago there was no information at all just by looking at this one graph that's a seemingly random we know the color of the object we know the temperature at the surface of this object we know there is an atmosphere with some hot gases in it we know that there's carbon dioxide in the atmosphere because of the desorption lines and we can figure out its speed relative to us because of these lines being moved back and forth due to the redshift or blueshift of the object so again light tells us a lot and it does go into far more detail than this as you can probably imagine but this is the basics of everything you might need to know for now so what we're going to do now is start to move into topics of our Sun so we're going to talk about our Sun next now that we have a little bit of information about light and then from there we're going to talk about the nature of all stars that is all stars in the universe not like all star or factors or anything so we learned from light now we're going to apply light to the Sun and then apply what we know about light and the Sun to all stars in the universe so that's the next several lectures from now I look forward to talking to you some more I'll see in the next video take care and have a good day