Hello everyone, and welcome to week three of our course. So, our question for this week is a big one. How big is space and what fills it? So this question of, you know, sort of does space go on forever is sort of, you know, the one that we've all had as kids. The history of astronomy is the history of recognizing that the universe is is increasingly larger. I don't mean that the universe is expanding. We're going to see that it is, but just the recognition that the, the what we consider to be the observable universe is so much larger than our grandparents or great grandparents understood it to be. So, you know, it used to be that space was pretty considered, or the universe was considered, pretty much to be the solar system. With sort of a sphere of stars around it and then people recognize that no, no, no those stars actually fill three dimensional space but they you know, people were arguing about how big this collection of stars were that we would call a galaxy. And then at the beginning of the last century it was recognized that the universe is actually full of many different galaxies so it's been this ever expanding understanding of the of the, how much space there is, how much matter there is, and how this matter is distributed through space so that's our question for this next week. So the first thing to understand along these lines is that there is no such thing as empty space. All space has some matter in it. It could be just due to one hydrogen atom per cubic centimeter or even less than that, but you know, there is interstellar or intergalactic matter distributed everywhere, so you know, you can see this. If you get away from the cities and you get away from the light pollution of cities you can actually see that the sky is full of stars and if you get far enough away where the sky becomes very dark, you can see that these stars are not evenly distributed, but there is in fact, actually, a band of both light and dark, which stretches across the sky. And this is what we call the Milky Way, and it took a while for people to understand, the, the true structure of the Milky Way, but it was recognized that, that, that is actually a disk of, gas and dust that, a Frisbee in some sense, that we are in the middle of. Not in the, the geometric center of, but we are a part of. We're part in the the, the disk of the of the Milky Way. We're going to see, as we study galaxies, what that means. So we ourselves the solar system is part of a larger structure. A city of stars, which we call a galaxy. And then as we begin to study the sky in even more detail. We recognize that there were other galaxies as well. So, you know, here are some of the questions we want to answer during this week. What exists between stars, in, within a galaxy? And then when we start measuring the distances between galaxies, we're going to have enormous distances to think about. And how do we measure those enormous distances? We can't put a tap ruler between the stars or, or certainly not between the galaxies. So how do we measure these distances? What is a galaxy and what different types of galaxies have we found as we've explored this vastly enlarged space of the universe that we've come to understand in the last hundred years? What exists between galaxies? Just as we've talked about what exists between stars. What exists between galaxies? Are there large scale structures that are even bigger than galaxies? Do the galaxies themselves collect into even larger structures? And if so, what fills up those structures as well? So, these are our questions. This is what we're going to be answering for the next bunch of lectures or, or small sub-lectures, and let's get started.
