 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to be talking about the interstellar medium. And what that is, very quickly, is just the material between the stars. Anything that we see between the stars. So what type of material is this? Well, let's take a look and see. So what is the interstellar medium? Well, it is any material that is found between the stars. So it doesn't matter what it is, we consider it any material. And it is broken down into two components. There is the gas, the interstellar gas, which is 99% of the interstellar material. And there is interstellar dust, which is a slightly larger particles, which makes up the remaining 1% of interstellar material. Now, this is extremely low density material. So even when we look at images, as we see here of a nebula, where we see gases that have been ionized and are being excited to glow, that's part of the material between the stars. And that's a very small part that actually was, that actually is, visible. However, what we look at very low density materials, the material that we see is actually very low density. In general terms, about one atom for every cubic centimeter. Now, what does that mean? Well, for comparison, the Earth's atmosphere here has 10 to the 19th atoms in every cubic centimeter. So that's a one followed by 19 zeros number of atoms in each cubic centimeter of space. So lots of atoms here and very few scattered around the universe. Of course, because there are lots of cubic centimeters out there in space, it still ends up being a lot of material, but is extremely low density and better vacuums than we can begin to create here on Earth. So let's look at these two components separately. And what we have, first of all, is interstellar gas. So gas between the clouds can be seen in a number of ways. And one of the ways that we can see these is in ionized hydrogen or what we call H2 regions. And that's H with the Roman numeral 2 after it. These are only visible near hot stars. And this is an example, this is what we call the Orion nebula in the image here. And this is an example of that. This is an area where we have a few hot stars located at the center here. So few hot stars located down at the center. And those stars are giving off ionizing ultraviolet radiation, so UV light. And what it does is it excites the atoms that are left over around it and causes them to glow. It would not be just hydrogen, but hydrogen is prominent because we see most of the material in the universe is made up of hydrogen. So this is an example where we can actually see the gas where it is excited to glow and we can see it glowing. But it is only a small portion of the interstellar gas. There is a lot more material out there that would be invisible to us. The only time you can see these ionized hydrogen regions is when there is a bright star giving off lots of ultraviolet light near it. A star like our sun wouldn't give off enough ultraviolet light to ionize a cloud like this. It takes some of the hottest stars to be able to give off this kind of amount of material. So how do we see most of the interstellar material then? And that would be it is neutral hydrogen, not ionized hydrogen, but neutral hydrogen clouds. Now these are much cooler and they do not emit visible light. However they can be seen in a couple of ways. They can be seen when light from a star passes through the cloud as we see here. So here is our star over on this side. Here is the interstellar cloud in the middle. And what happens is as the light from the star passes through this cloud we are going to see additional lines. This is the lines we would see here from the star itself. And then as it passes through this cloud we are going to see additional lines and in this case much narrower absorption lines that are telling us that there is an interstellar cloud in between us that would not be visible otherwise. It would be invisible to us at optical wavelengths because there is nothing there that is exciting those atoms and causing them to glow as there was in an H2 region. Now the other way we can see these is because they are so much cooler we can see them through radio waves. And what that means is that they will emit radio radiation because they are much cooler. They are not hot enough to give off visible light or even infrared light but they are hot enough to give off radio emission. And the most prominent of these is what is known as the 21 centimeter hydrogen line. And what happens in these is that a hydrogen atom is made up of a proton here at the center and an electron orbiting it. Now each of those has its own spin to it and you can see here in this case they are both spinning counter clockwise and in this case one is spinning clockwise and one is spinning counter clockwise. This is the stable region. This is the lowest energy so this is the state that the atom wants to be in. So if it gets excited a little bit however those spins could be aligned. And when the atom transitions from one to the other and when it goes from this level to this level it will give off a wavelength of 21 centimeters. Much the same as changing energy levels within the hydrogen atom also would give us off specific wavelengths. Changing the what we call the spin flip transition when the spin of the electron will flip from going parallel with the proton to going opposite to the proton will give off that wavelength of 21 centimeters. Now that can then be detected so a radio telescope could detect this. Now and it does not take as much energy because this is a very long wavelength radio wave it does not take near as much energy to be able to cause it to give off this radiation so even these cool dark clouds can give off this and allow us to be able to detect them even though they would otherwise be completely invisible so the 21 centimeter line allows us to map the hydrogen within our galaxy even though we can't see it directly with for example an optical telescope. Now the other things, some of the other ways we can see this in addition are through very hot gas so we can see very hot gas temperatures of millions of degrees extremely hot material this is very very hot gas and we see the example here of a supernova remnant so this is a supernova and that is what we are seeing here is those outer parts of a star being expelled out into space those are also part of the interstellar medium so not only do we have cool clouds but we have hot clouds these tend to give off x-rays because of their very very high temperatures and very high energy so we're getting a lot of x-ray emission here and that's another way that they can then be detected when you have these very high temperatures this is where a lot of the enriched material comes from after it has gone through a star's life and it has evolved from lower elements things like hydrogen and helium and created heavier elements now one other way we can see this so that's another type of interstellar gas is through molecular clouds molecular clouds are darker areas so we have some dark dense areas here here that are darker areas that are visible and those are areas where the dust and material has gotten so dense that it's not possible to see through it any longer so in this case normally in this case we can actually begin to form molecules so in these molecular clouds more complex molecules can form they don't normally occur outside this because of ultraviolet radiation ultraviolet light will normally rip those molecules apart but within this they are shielded so deep within these molecular clouds more complex molecules can form including certain types of organic or carbon type molecules so we can look at some of these that are actually able to form here and these are some examples of what we find within molecular clouds things like ammonia that's used in household cleaners but even more things that are present including acetic acids, vinegars different types of alcohols and antifreeze you can see how complex these begin to become we get very complicated formulae that are beginning to form here so lots of interesting molecules that are able to form within these molecular clouds and as you can see things like ethylene glycol and ethyl alcohol have very complex formulae in needing lots of atoms that means they are very easily broken apart in with ultraviolet light so it takes a lot of protection for them to be able to remain together now as we look at the interstellar gas the other thing that was part of the interstellar medium was the dust so interstellar dust here dust is not visible as well it does not emit visible light but it does block light and we can see the image here where we can see the interstellar dust we have this big dark patch here that does not show anything it shows hardly any stars there in reality there is just as many stars in this section at the middle of this as there are out here the problem is that there is dust in between us and that blocks out the light from behind so even those dust particles spread out at one per cubic centimeter however they are then spread out over so many light years that they are then able to be visible and able to be seen so they are now blocked out we see them by the absence of material not through the presence of material so what it does is a couple of things first of all it emits infrared light meaning that we can see it in the infrared and we can see it through what we call a reflection nebula so an example of this would be shown here and we can see that reflection nebula over here in the blue that is when it is reflecting the dust is reflecting the light from nearby stars and it is much the way that the sky works in terms of making the sky blue the atoms in the atmosphere are very good at scattering blue light and less efficient at scattering red while dust particles do the same thing they are very good at scattering the blue light so it comes from all over the place and we see this nebula around very hot young stars and less good at scattering the red it is much better able to make it through the dust now the dust can do a couple of things here and the dust actually does two things to the light that happens to pass through it and what the dust does is it is good at scattering light as we have talked about that the shorter wavelengths the blue are more easily scattered than the longer red wavelengths so it scatters the light it also reddens the light of the stars passing through the light through the dust because of this the light that is more likely to pass through is the light that is red so if we looked at this in the infrared we looked at the same image we would see something like this so now it is the same part of the sky nothing else has changed but we are now looking at it in the infrared infrared wavelengths are better able to penetrate the dust and allow us to see through it and allow us to actually study what is going on there so something that we could not do with an optical telescope those wavelengths do not penetrate the dust in this case we are able to see through the dust and to find out what is actually happening and now you can see better that really the number of stars does not depend on where you are in the sky here which part of this has roughly the same number of stars so the dust does tend to redden things and make stars look redder and as we saw in the previous slide actually if you look around the edge here you can see that many of these stars actually look very red in color so these stars right around the edge that we are barely seeing through the dust have been severely reddened that does not mean that they are really red stars they may actually be regular type stars that we are just seeing prominently red because all of the other wavelengths have been blocked out so we see them as red but we are not able to but that is only because all of the other wavelengths have been blocked out so what does the dust do well it actually does a couple of things and first of all it's going to make the stars appear fainter than they otherwise would and it's going to make them appear redder now when it makes them appear fainter they don't look as bright that is going to throw off our distance estimates it's going to make them look more distant than they otherwise would and that's because they look fainter so if we don't know about the dust and do not take that into account we are going to then underestimate their luminosity and make us think that they are further away than they actually are so before we understood dust that had an effect on our distance measurements they are also redder as we've looked at due to the scattering of light and the more dust that is present will increase both of these two so these will both become more and more prominent with more dust present so what does the dust grain look like now we can't actually go out and find these but we can make some models to try to understand what a dust grain might look like and what we see is something as pictured about here in our image it has a core which could be two types of things it could be either very carbon rich or very silicon rich just depending on the materials that would be the central core like we have the core of a planet but we would have the core of this tiny dust grain these are incredibly small we're only talking about tens of billions of meters so much smaller than household dust but they have that solid core which may be carbon, may be silicon or it may, and then around it we have a mantle of material that are going to be ices now the ices that we see are methane, ammonia, and water the very common materials that combine hydrogen with carbon, nitrogen, and oxygen respectively as the some of the most common elements in the universe so hydrogen being the most common helium next which doesn't combine with other things chemically and then carbon, nitrogen, and oxygen are three of the next most common elements so we see those in the mantle but again these are incredibly tiny much smaller than what we think of when I say dust in terms of thinking about household dust so let's finish up here as we usually do with our summary and what we find is that the interstellar medium is divided into two parts there's the gas and there's the dust so the gas can be seen in various ways it can be seen visibly as ionized hydrogen regions or we can see it through radio waves as it's 21 centimeter emission as a couple of examples we can also see that dust can block the light from the stars which makes them appear fainter and redder than they would normally appear and this has affected our distance determinations and the like in the past because before we understood the exact nature and the extent of the dust within our galaxy so that concludes our lecture on the interstellar medium we'll be back again next time for another topic in astronomy so until then have a great day everyone and I will see you in class