 Greetings and welcome to the Astronomy Picture of the Day podcast. Today's picture for January 1st of 2024 is titled NGC 1232, a grand design spiral galaxy. So, what do we see here? Well, we see a great spiral galaxy and called grand design for its beautiful spiral arms majestically sweeping out from the center of the galaxy. So, almost a prototype of what a spiral galaxy should look like. Now, when we look at galaxies, we see a lot of things here. And we see the central core of the galaxy. One thing we cannot see there is the central black hole. Every large galaxy like this has a supermassive black hole at its center. Some, like our own galaxy, is about four million times the mass of our sun. Other galaxies are much, much larger even than that. Now, as we look farther out from the center, we start to see a bluish color in the spiral arms. And that is caused by young blue star clusters which have formed recently. How do we know they've formed recently? Well, the types of stars that form like this and that are this blue color don't live a very long time. They might only live five or ten million years. And as we understand, if we can see something that only lives for, say, five million years, it must have formed within the last five million years, or it would not be present in our image. Now, that may seem like a very long time to us, but on the timescale of galaxies, five million years is very short compared to something like our own galaxy, which is ten billion years old. And we see those clusters scattered throughout, in fact, dense star-forming regions and clumps of those blue stars highlighting the spiral arms. And that is where the star formation is ongoing within the galaxy as the denser material compacts together, taking dense gas clouds and compacting them into stars, giving us those bright blue stars that we see at the center. Now, what do we not see farther out in the galaxy? Well, that is what is called dark matter. Now, why does dark matter exist? Well, we know that something is wrong with our understanding of this, and that's dark matter is postulated. We think that it exists in order to explain the rotation of the galaxies. Based on our understanding of gravity, and that's through Einstein's general relativity, we know how the galaxies should rotate at great distances from the center. Once we get outside most of the visible mass, we would expect the stars to slow down in their orbits, much as the planets do within the solar system, where star, where the planets are outside of the vast majority of the mass, which is within our own sun. But what we find is that those stars more distant outside the visible edge of this galaxy and gas clouds beyond that continue to rotate faster and faster. So either our understanding of gravity is incorrect, or there has to be far more mass outside of the galaxy than we see within the galaxy itself. And in fact, in many cases, it can be 10 or 20 times the amount of galaxies, amount of material that we see within the galaxy. So for every star and cluster of stars that we see here, you can imagine at least 10 of those in this invisible dark matter that we cannot see. And that's needed to explain the observations that are made of the galaxy and how it rotates. So that was our picture of the day for January 1st of 2024. It was titled NGC 1232, a grand design spiral galaxy. We'll be back again tomorrow for the next picture. So until then, have a great day everyone, and I will see you in class.