 Greetings and welcome to the Introduction to Astronomy. In this lesson we are going to talk about the cycle of cosmic material and how material gets recycled and changed as it goes through various different processes in space. So let's start off looking at some of the regions here and in fact looking first of all what we've been talking about is the interstellar medium and what we find is that there are continual changes to the material in the interstellar medium. What we see as a nebula today was not the same type of nebula millions of years ago and will not be the same type of nebula millions of years from now. Material is consistently changing so as stars form we may see an emission nebula at the time when the stars are emitting enough ultraviolet energy to energize that nebula and to cause the hydrogen to glow. However after hundreds of thousands or millions of years those stars will be gone and now the nebula will no longer be visible. The material can still be there but may no longer be visible in the same way it was before. So this material will go through different stages during the lifetime of the galaxy and as an example for what was once part of a molecular cloud may then form into a star and eventually be ejected in a supernova explosion. So the same material that was in the cloud became the star, lived its life as a star and if that was a very massive star may have ended its life as a supernova sending that material back out into space but the material that goes in is not the same as the material that comes out. The material has been changed so we continue to repeat this cycle with each new generation of stars but the material now becomes enriched in heavier elements. So what we mean by that is that it now has more heavy elements, more things other than just hydrogen and helium. And let's take a little bit of a look at how that can work. So what we see for the stellar cycle is that stars will form from the cool gas and dust in space. This will form stars and planets. What the stars do is the stars convert light elements into heavier elements. So examples of things like hydrogen fusion converts hydrogen into helium, that's what our sun is doing right now. Other stars, more massive stars could be converting helium into carbon, our sun will do this towards the end of its life and other stars can then convert carbon into even heavier elements. And this enriched material is then ejected back out into the galaxy and will begin to form the next generation of stars. And how is it ejected out? Well supernovae are one example when a star explodes but even for smaller stars things like planetary nebulae can form around low mass stars and those can send some material back out into space and stellar winds are constantly putting material back into space. And what it means is that each generation of stars here will be more enriched in what astronomers call metals and that is anything that is not hydrogen or helium. So let's take a little bit about the stellar recycling in a picture form and get an idea of how it works. So we can see here various different ways and different to the life cycle of a star but how this kind of works, how we go from a nebula back out into a nebula once again. So we can start off with a molecular cloud, a dark molecular cloud, something like the Orion Nebula here which is in the process of forming stars. Eventually it'll form a cluster of stars and then what happens depends on the mass of the star. Brown dwarfs will form very low mass objects and those will essentially stay unchanged for the rest of the history of the universe. They are locking up all of that material. Low mass stars like our son will go through their lives and may form a red giant star at the end of their life, eject a planetary nebula again sending material back out into space and then end up as a white dwarf and finally a black dwarf star. Now the material that becomes here ends up being trapped so that is not ejected back out but things like the planetary nebula would be. Now if it is a low mass star in a binary system it takes a slightly different route in which depending on the masses it can form either a supernova explosion which again sends material back out into space or a nova explosion which will also send some material back out into space. All of these are enriched materials. They have higher concentrations of heavier elements than the material that originally formed the star. Now finally let's look at the most massive star here. A very massive star will again become a red giant and super giant and will explode as a supernova. So that supernova remnant is again material being expelled back out into space and just depending on the mass we may end up with things like a black hole or a neutron star here at the end and again this is material that is locked up like the white dwarfs and the black dwarfs these materials are locked up they will not be released back out into space but all of these continue the cycle the material from the supernova remnant the material from the type 2 1A supernova the material from a nova itself and the material from a planetary nebula these all go back and then form new molecular clouds which now have higher concentrations of these heavier elements. So if we clear this up and take a look a little bit more here again there's our still our image to the side but that material that we've looked at the supernova remnants here the nova and the planetary nebula these all go back at the end to form new stars the earliest stars the very first would have formed with only hydrogen and helium after the Big Bang so the first stars that formed would never have been able to form an earth-like planet because there was no carbon there was no silicon there was no oxygen things that make up our planet would not have been present there so it wasn't until later stars that formed that allowed earth-like planets and life so we would not have been able to have life at least not life as we know it around those very earliest generations of stars but with each cycle we go through we enrich more and more and eventually after billions of years we were able to build up high enough concentrations of heavy elements that stars planets like the earth would be able to form so let's look a little bit about some of the things here and one aside is to look at where the water came from on earth because at the temperature of the earth where it formed we shouldn't have much water very little the ice would have been able to condense so we shouldn't have a lot of water and there are a couple things that are thought about is that it could have formed in could it have formed in the interstellar dust grains could it have been part of those grains that became incorporated in the gas cloud and eventually formed the solar system some of it could have been trapped into other materials and then eventually released when so when that rocky material was melted with the trapped water could have been released and could give us the water that we have here on earth today or it another thought is that it could have come from further out in the solar system from asteroids and comets that it would have formed further out where there was more water where it was colder and water was able to condense and this could have been counted through impacts early in its earth's history and that could have brought the water so there are a couple different thoughts as to where the water on the earth came from because we do have at least on our surface a very significant a percentage of water here on earth so the last thing I wanted to look at in this section would be what we call the local bubble so the local bubble here here is our son and we see these when we look at the x-ray emissions from the galaxy we find that there are bubbles of hot gas around that have cleared out and that is regions of much lower density than normal in the interstellar medium so here is one example of the sun there the bright star Antares in Scorpius is another one of these and this would be examples of these areas that where material has been pushed away the cooler gas has been cleared out possibly through supernovae a supernova explosion could send a big shockwave from around this area and clear out a big bubble and our sun just happens to be located in or very near to one of those right now or very strong stellar winds from extremely massive stars that form could do something very similar so we do see this and it's again a part of the interstellar medium that we want to look at a little more closely so let's finish up this as we always do with our summary and what we find is that the interstellar medium is constantly changing over times of millions and billions of years the material in space that we see is being recycled with new generations of stars being formed from the remains of previous generations so each generation enriches a little bit more gives a little bit more metal content heavier element content to the material that forms the next generations of stars and then finally we do note that there are bubbles of hot gas that exist in our galaxy that are being studied by astronomers possibly created either by supernovae or by strong stellar winds which clear out regions of the interstellar medium making it much less dense than the overall interstellar medium that we see 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