Welcome back everyone. So, now that we have covered, sort of, supernova in terms of the cosmic distance ladder, there's another way in which we want to think about supernova that are very important for galaxy. And that is something we call metallicity. As we've talked about before, supernova are, are mechanisms, are the mechanism that generate the heavy elements, the, the very heavy elements. Everything a-, above iron in the universe. And in general, stellar evolution, without stellar evolution there would be no processed elements. We've other than hydrogen and helium and a little bit of some of the, the very light elements which are born in the birth of the universe, as we'll discuss later. All the processed elements, carbon, nitrogen, oxygen, etcetera, all have to come from nuclear burning inside of stars. And, as we've just said, the heavier elements above iron all come from super nova. So, that means there's a direct relationship between how many, how much metal you'll find and we, when we say metals, we astronomers, for us, metals are everything above helium. How many so there's a relationship between the amount of processed elements we find, and the age of the system. And particularly age of the stars in that system. So let's just think a little bit about how metals are created and how they get pushed around the universe. Or particularly in a galaxy. So we know that stars are created from clouds that are mostly hydrogen and a little bit of helium. And then inside those stars, nuclear fusion takes that material and fuses it into heavier elements. When in particular supernova explosions occur, that material, that processed material, the quote unquote metals, now gets blown out into the galaxy. And as time goes on, that material will spread around the galaxy and form and be become part of the next generation of stars. Where even more metals will then be created via nuclear fusion, so what we expect is, that over time, we will expect to see more and more metals forming, if we have successive generations. If we're allowed to have successive generations of stars forming. And this allows us to actually look at stars, look at the amount of processed elements or metals in them, and distinguish between different populations of stars. So what astronomers call Population I stars are, they are relatively, newly-formed stars, stars that have formed relatively recently in the history of the galaxy and the history of the universe. And so, these tend to have very high abundance of metals because the material that they formed out of already had been processed by stars beforehand. Population II stars are stars that are older and formed further back in time and therefore, do not have as many metals, as many processed elements, as the Population I stars. Now this Population I, Population II categorization, was, is quite old actually. Astronomers recognized this quite a while ago. You know, as a far back as, you know, the, the last century. However, recently as we've been able to probe to farther and farther distances, we've been actually looking further and further back in time to the first generation of stars. The stars, this generation of stars that formed right after the big bang. And, of course, these stars have almost no metals in them, because they formed out of just pure hydrogen and helium gas. So, we call those, this is a new designation, Population III stars. They are literally the first generation of stars, and they are born out of, or they are born from material that is really almost all hydrogen and helium, with just tiny of amounts of light elements. So, okay, with that, let's remember that as we begin to think more about the structure of galaxies. [BLANK_AUDIO]
