 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the cycle of cosmic material and how the interstellar medium is changed over time. Now let's take a look at this. And what we find is that there are continual changes in the material in the interstellar medium. Nebulae can change significantly over millions of years. And what we see is that the content and the composition of the interstellar medium would not be exactly the same. So material may go through many different stages during the lifetime of the galaxy. What was once a molecular cloud can form into a star. And then the material from that star could be ejected in a supernova explosion back out into the interstellar medium. Now when this happens the composition changes. So that will change the composition of the material. So this cycle is repeated with each new generation and giving us new generations of stars that form from material that has gone through these various stages. Now let's look at that cycle of gas that we have. And what we have is that stars will form from cool gas and dust in space. This forms stars and planets. What do those stars do? Well they convert light elements into heavier ones through nuclear fusion, as we discussed for our sun. So hydrogen into helium, then helium into carbon, and so on depending on the mass of the star. This enriched material is then ejected back out into the galaxy. Some of this material will help form the next generation of stars. So how do we get this material out? Well supernova explosions is one way, but less massive stars form planetary nebulae which will also expel material out into space. And stellar winds will push material out into space as well. What it means is that each generation of stars will become more enriched in metallic metals. Now remember what a metal means. A metal is anything other than hydrogen or helium. So each generation, the very first generations of stars after the Big Bang, would have been hydrogen and helium only. The later generations would have started to have things like carbon and oxygen and silicon and iron that would not have been present in that very earliest generation. Now let's take a look at this in a schematic form. And what we see is, again those earliest stars were only hydrogen and helium. And depending on the stars, as stars form from molecular clouds here, so a molecular cloud, forming material and starts to condense in down into clusters of stars. Now what happens to those stars depends on their mass. So some are very low mass, brown dwarfs, and they kind of just sit there and don't do much. We have low mass stars like our Sun here. And we have high mass stars much greater than the mass of the Sun. The star like our Sun can become a red giant and then a planetary nebula. Here is one way we expel material back out into space. The rest remains trapped as a white dwarf and a black dwarf. Now if it happens to be in a binary system you can actually get a nova or a supernova depending on the conditions. We will look at that in more detail later. But again these are ways of expelling that material back out into space. For a more massive star they are definitely going to expel a lot of material out into space. They will become super giants and then a type 2 supernova which expels that material out into space. And all of this material whether it be from a supernova remnant or if it be from a nova from this type of star or if it be from a planetary nebula it all goes back into forming the next generation of molecular clouds. So that material will slowly become part of the interstellar medium. But now it has a enriched material. It has higher levels of things like the carbons and the oxygens and the silicons and irons that are needed to form things like earth like planets and life. So as an example and a side on this let's take a look where did earth's water come from? Well did it come from interstellar dust grains? So it did form which means that it formed elsewhere and became incorporated into that gas cloud which eventually formed the solar system. Some of this material would have been formed the earth and the water could have been released from that rocky material that had water embedded in it. Or it could have come from asteroids or comets. Now asteroids or comets again would have formed from these interstellar dust grains but did it come directly to earth? Or did it come through these comets which formed further out in the solar system where there was a lot more water? And this impacted the earth in its early history which brought water to earth. So it's a good question as to what of these is responsible for the water that we need for life here on earth. In either case the oxygen in water came from the stars. Oxygen was not formed in the Big Bang. Now let's look at the solar neighborhood here and what kind of material we find around in the solar neighborhood. And we see here that there is our sun and the sun is moving in one direction through that. We do see X-ray emission from the galaxy gives us bubbles of hot gas and material that we see. This is a much lower density than the interstellar medium in general. So when we look this we see this hot gas it's a very very low density material. And it clears out the cooler gas. Now this could be from things like supernovae or strong stellar winds but it gives us things like bubbles of material as we look at our local region around our sun. There are great areas of material where there is far less material, far less than the interstellar medium than we would expect in others. So supernovae could have cleared out this material or extremely strong stellar winds could have helped to clear out this material and clear out that cooler gas. So let's go ahead and finish up with our summary. And what we've looked at this time are that the interstellar medium is constantly changing. Material in space is being recycled and that gives us to new generations of stars being formed from the remains of previous generations. And we saw that bubbles of hot gas exist in the galaxy possibly created by supernovae or strong stellar winds. So that concludes this lecture on the cycle of cosmic material. 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.