 Greetings and welcome to the Introduction to Astronomy. In this lecture, we are going to discuss the interstellar medium, the material we find between the stars. So, what is the interstellar medium? Well, the interstellar medium in simple is any material found between the stars. But what is it made up of? Well, there is gas between the stars, which accounts for 99% of interstellar material. There is dust, which accounts for about 1% of the interstellar material. Now, gas is harder to see, dust can be easier to see because of its dimming effects. This material is extremely low density, so even when we see nebulae such as the one pictured here, we are talking about densities in many areas on average of one atom for every cubic centimeter. So imagine a cube, one centimeter each side, there is one atom in there. How does that compare? In Earth's atmosphere, there are 10 to the 19th, the one followed by 19 zeros, atoms every cubic centimeter. So let's take a look at each of these. Let's look at the interstellar gas first. The gas can appear in a variety of ways. We see what we call H2 regions or ionized hydrogen regions. These are only visible near hot stars, so we see here the Orion nebula and ultraviolet light from the hot stars will ionize the hydrogen and excite it and cause it to glow. When the hydrogen has been ionized, we add electrons and protons then recombine and it gives very specific lines visible. Now of course we would see other elements as well, but always recall that most of the material that we see is hydrogen gas. Now when we look at the gas here, let's look at another version of this. Let's look at the neutral hydrogen clouds. These are a lot harder to see. These are sometimes called H1, H2 for ionized, H1 for unionized. So neutral hydrogen clouds do not emit visible light. These are too cold. How can we see them? Well, we can see them when the light from the star passes through the cloud and absorbs that material. So here we see the spectrum of the star and we then see the spectrum of the star and the cloud and see how there's now narrow absorption lines from the cloud which shows us the presence of hydrogen gas in between us and the star even if it is not normally visible. Cool hydrogen also emits radio radiation. Let's take a look at that and we can see that and it does that through the 21 centimeter hydrogen emission. So 21 centimeter radio wavelengths and this occurs during what we call the spin flip transition. Hydrogen has a proton and an electron and can have one of two states. Either they can be spinning in the same direction which is a little higher energy or they can be spinning in the opposite direction which is a little bit lower energy. The energy difference corresponds to a wavelength of 21 centimeters. So when clouds of gas and material bump against each other they can excite them and change them to the excited state and when they give off that energy they will then give it off as 21 centimeters and that allows radio astronomers to map the galaxy with this coal hydrogen gas. We also can have an extremely hot interstellar gas that is millions of degrees and this is material from supernovae that have exploded. Now remember millions of degrees we're talking about temperatures that means how fast things are moving they're very fast moving particles expelled from the supernovae. We can also have dark molecular clouds so molecular clouds molecules normally don't form in space because they're broken apart by ultraviolet light so ultraviolet light from all the stars would rip these molecules apart. However in molecular clouds more complex molecules can form. We see one of those clouds here that those dust grains then shield the molecules and allow them to form. And we can form some very complex molecules you'll see a few here as simple as ammonia which has nitrogen and hydrogen but we also have things such as the antifreeze ingredient ethylene glycol very complex ethyl alcohol and benzene so much more complicated much more complicated molecules that can form within these and these aren't the most complex you can actually form some of the fullerines which have 60 to 70 carbon atoms sometimes known as buckyballs where the carbon actually forms bonds and forms itself into a spherical little shape there. So very complex molecules can form within these dark molecular clouds. That's some of the gas that we see let's look at the interstellar dust. First of all dust does not emit visible light it does block light so we see here that the nebula is blocking out the light from stars behind it there are certainly just as many stars behind this dust cloud as there are in other areas however we can't see them because of the dust so what it does do is it emits infrared light so infrared would allow us to penetrate this and we will see that shortly it also can reflect light from nearby stars and we can get things like the blue reflection nebula that we see here a reflection nebula occurs when those light from the stars is scattered off of those dust particles they're much better at scattering the short wavelength blue light than the longer wavelength so that allows them to be to be scattered and gives it this distinct blue color now what else can the dust do it blocks out the light but that's not all it also causes what we call interstellar reddening and that's while dust is good at scattering light it is really good at scattering blue light and not so good at scattering the red light that means it reddens the light of the stars passing through the dust so if you look around the edge of this nebula here you'll see a lot of very red stars that does not mean that they are truly red it just may mean that a lot of their blue light is being absorbed by the dust in the cloud and only the longer wavelength red light is able to get through infrared and radio can penetrate the dust and if we look at this same nebula in the infrared we can then see that wow look at all of those stars that are there that were previously hidden so without the dust sorry without the invisible light and in infrared light so that infrared light is longer wavelength not very well scattered and easily makes it through the dust cloud and therefore we can see the material there now so we've looked at what the dust does let's summarize that here what the starlight is going to do is going to do two things one is going to make stars appear fainter because fainter than they actually are because of the absorption of light this means they look more distant if something appears fainter we're going to think that it's further away so if we don't take dust into account we are going to overestimate distances it also makes them redder than they would otherwise appear so the starlight emitting red and blue light the blue light gets scattered comes from all directions the red light tends to make it through making the star appear redder because the blue light is so much better scattered the more dust the more these effects will increase so the more dust there is the more we have to be able to take this into account when we are determining distances so what is an interstellar dust grain what do these dust grains look like this is not like household dust that you tend to think about when I say dust it consists of a core of material which may be one of two things it could be rich in carbon or silicon so it could be a sooty material or more of a sand like material in molecular clouds they end up covered by mantles of material so you think about it as a very mini planet that we've looked at a core and a mantle and the mantle would be covered by ices that would be water, methane and ammonia ice that would be present typical size is about 10 to 100 nanometers so very tiny compared to household dust but when you have these spread out over very large distances in great clouds they are sufficient to block out the light from stars behind it so let's go ahead and finish up this section with our summary and what we've looked at is the interstellar medium and we divided that into two parts we had the gas and we had the dust the gas can be seen visibly as H2 regions or in radio waves through 21 centimeter emission dust blocks the light from the stars which makes them appear fainter and redder than they would normally appear and again remember that causes us to have problems determining distances if the amount of dust is not taken into account so that concludes this 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