Welcome back everybody. So just for this one sub lecture, we're going to sort of focus on what occurs between stars. We're going to be spending most of our time thinking about galaxies and what what goes on between galaxies. But for this one next ten minutes or so, we want to talk about what happens between stars. And we're focusing on what is called the interstellar medium. And this for me is a very exciting subject. But sometimes some students taking this call subject the interstellar tedium. But it actually is a very interesting course because or interesting idea because the interstellar medium, first of all, most of the beautiful images you see of space are the interstellar medium. Those beautiful luminous gas clouds you see are the material in between stars. So in between stars, as we've said, it's not empty, that there is actually gas and dust out there. In fact there's even though this material is a hundred trillion times less dense than the earth's atmosphere. Say the densest regions of the interstellar medium are say 10 to the 6th, 10 to the 7th particles in a cubic centimeter as composed 10 to the 19th particles per cubic centimeter in the earth's atmosphere. There's still enough material out there to make billions of suns or billions of stars like the sun. In fact this the interstellar medium is dense enough to support sound waves and shock waves. So anybody who's seen the movie Alien, when that movie first came out the tag line for it was that in space no one can hear you scream. But in fact, actually in space somebody could hear you scream if you just happened to be able to hear with the low enough frequency sound waves. so, you know, the wavelengths of these waves that occur in the interstellar medium are actually ginormous and they the periods are, you know, take forever. But still there are sound waves. You can support sound waves in space. Now there are four different types of clouds in the interstellar medium and we want to go through these cause they each plays a role in the evolution of galaxies. The interstellar medium is an important part in the history or the story of galaxies. So first there are molecular clouds and these are the densest regions of the interstellar medium. And they're very low temperature, ten degrees above absolute zero. But they have the highest densities. Perhaps you know thousands, millions, tens of millions of particles per cubic centimeters. And these things are very important. We've already visited them, cause this is where stars form. All star formation activity occurs in these dense clouds. So molecular clouds are one type of intercellular medium cloud. The other type of the next is what we would call H1 cloud. And these tends to be sort of diffused clouds. They are atomic hydrogen. They are hydrogen gas where the electron is still a part is still whizzing around the proton. So this material is quite diffuse. It's, you know, all around the galaxy. The next type of cloud that we're interested in are what are called HII regions. And what these are really are, these are ionized clouds. These are clouds where some form of ionizing radiation has appeared, usually a young massive star, or perhaps an old star, like the ones that, generate the planetary nebula, as we saw in the last set of lectures. And the ionized, what happens when there's a lot of ultraviolet radiation is that electrons will be stripped off of their protons, by the ultraviolet radiation. The collision between the ultraviolet radiation the photon and the electron turns liberates the electrons. So now you have a cloud of ionized gas, so protons and electrons that are not bound to each other, and these are really some of the most beautiful objects we can see in the sky, these are what are called H two regions. And then finally there is what is called coronal gas and this is gas that is at millions of degrees above zero. H Two regions, for example, are typically around 10,000 degrees above absolute zero, but coronal gases, millions of degrees. And we, the coronal gas stretches through the interstellar medium sort of like, almost like tubes. Like almost a, a, a set of worm paths. And these we think come from supernova blast waves. This is gas that's been heated up by these enormous explosions from supernova. And in fact it's the the, this coronal gas is what's left over material that's been heated up to very high temperatures and has not been able to cool yet. So it's called coronal gas because it's at the same temperature, essentially, as the, what we see at the corona around the sun. So all four of these cloud, these different types of clouds, have roughly the same pressure. So, we think that they're sort of co-existing. The interstellar medium is a, bunch of different phases of gas that are roughly in equilibrium, but they're not static. There's a lot of movement to them, and in fact, they're turbulent. And you, anybody who has watched smoke coming off of a off of a cigarette say, I mean you've watched it go, you know it's, first it's a nice smooth column and then it breaks up and then forms those whirls, or watched boiling water have seen what turbulence looks like. And we think the interstellar medium is turbulent, that there's a lot of motion in it, probably coming from things like either the rotation of the galaxy or the supernova that occur. Now we don't want to forget about dust. Dust is very important. And dust are really what we call interstellar dust, sometimes you might want to, would, some people say would be better called smoke. These are the smallest particles of matter or sorry, of solid matter as opposed to just individual molecules. These are literally, you know, the smallest bits of solid. And they range in sizes from 0.1 to ten microns, where a micron is a millionth of a meter. And they're just the right size to absorb and scatter optical light the light that our eyes are sensitive to. And so it took a long time for people to understand the importance of dust in observing the universe because dust makes things redder and it makes things dimmer. So it was until we really understood the presence of dust we couldn't accurately gauge how far away things were and how bright they were. Dust is also very important because it is the beginning of planets as we've talked about dust. And it's the first bits of solid matter. Eventually we're going to get coagulation of the dust to be able to form things like planets. So it's important in, in that way as well. And there's about one gas dust particle for every 100 gas particles. The dust also kind of acts like fly paper. It's very good at attracting, any molecules that, or atoms that stick to it, or even molecules. And so, dust as we've seen is often the site of chemistry. Most of the chemistry that happens to make complex molecules in the interstellar medium happens actually on the surface of the dust, and eventually those molecules are ejected back into the interstellar medium. Okay, so that is our view of the interstellar medium, the gas and dust between the stars. So, we see that really, you know, the material between the stars, there's a lot of material between the stars. It's very interesting. It has an important role to play in the formation of stars, certainly. And you know we also have to understand, when stars die, whether through supernova or the stellar winds that accompany the death of stars, that that is material being ejected back into the interstellar medium. So there is a ecological cycle, so to speak, of material forming star or you know that interstellar medium material collapses to form stars and then the stars give that material back into the interstellar medium. So it is this constant recycling process. So Interstellar medium is very important for the life cycle of galaxies or understanding of galaxies. And we're going to see that when we, as we study galaxies, there, different types of galaxies will have different kinds, or different amounts of interstellar medium. And the interstellar medium will have different properties. [BLANK_AUDIO]
