 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about two things including how we weigh the galaxy and what is present at the center of our galaxy. So let's start off by talking about how do we weigh a galaxy? How do you figure out how much a galaxy weighs? Well, if you want to find out the mass of the galaxy, we need to look at a couple of different things. And one of the best ways is to look at the orbits of the stars. How the stars orbit will tell you how much material is inside that star's orbit. So if we look for a star near the visible edge of the galaxy, and then measure its rotation, we could then determine its orbital period, we could figure out its distance from the center of the galaxy, before we could use Kepler's third law to determine how much mass is inside the orbit of that star. We could then measure the mass inside that area. Now, this works fine as long as there is little matter beyond the star. So if we look for the visible edge, that would mean that we would be able to then determine the mass of the galaxy. However, one problem we've had is that we're now finding there is a large amount of dark matter in the halo. So even though we don't see material further out here, there is a lot of mass out beyond the visible edge of the galaxy. And that leads us to dark matter. So what is dark matter? Well, if we measure the rotation curve of a galaxy, so we look at how fast stars are moving at various distances. So we start at the center of the galaxy, work our way outward, then we can then determine where the mass is. So eventually, we expect the blue curve here. That means we've gotten outside most of the mass, and the velocities will drop off with distance, because all of the matter is interior to the star. However, what we actually find is the red curve. Velocities continue to increase. So, again, look at our scientific method. Something's wrong here. Either we don't understand how gravity works at these large distances from the center of the galaxy, or there has to be a lot more material out there. And how much more? It's a lot more. 20 times the amount of what we see in luminous matter. So for every star you see, there's 20 stars worth of material. For every gas cloud, there are 20 gas clouds worth of material out beyond the visible edge of the galaxy. That how much has to be there in order to account for the rotation curves that we see. Now, what could this be? Well, we can start eliminating things. We know it cannot be ordinary matter. We would detect it. It would give off radio emissions. It would give off absorption lines. If this was hydrogen gas or anything, we wouldn't be able to see that. Could it be black holes? They're hard to see. But we detect them by X-rays. And if there were that many black holes, that means we'd have a lot more heavy elements than we have. Brown dwarfs or planets, very small objects, we wouldn't be able to see them. But they're also so low mass that we would need a tremendous number of them. And they'd also be detectable by gravitational lensing. One of the other thoughts is exotic subatomic particles. Weird particles which have mass, but don't interact with the rest of matter. We talked about one of these with the neutrino, but the neutrino is extremely low mass. We need one of these similar, but with a much higher mass. And experiments continue to look for these kind of things. So what does dark matter tell us? What are the implications of this? It means that we see only a tiny fraction of the mass of our galaxy as well as other galaxies as we'll see. And that most of the mass of the universe, 96% of it, is something undetectable to us. We see dark matter halos around nearly every galaxy, so it's not just some galaxies that have it, but nearly every galaxy, the actual visible portion of the galaxy is a very small amount of the amount of material that makes up that galaxy. Now, how about our center of our galaxy? What do we know about the galactic center? First of all, it's invisible at visible wavelengths. What does that mean? We can't see it in visible light. It's blocked out. So if you look towards the constellation of Sagittarius, and here we have what is known as the teapot of Sagittarius, a set of some of the brighter stars there, up over above the edge of the teapot, we have the galactic center. So can we see anything there? No. However, if there were no dust there, this would be the brightest object in the sky. It would be incredibly bright because of all of the light, all of the stars, all of the energy coming from there. But the dust in our galaxy blocks it. We can see it at other wavelengths. It is the brightest and was the first detected radio source in the sky. Now, we have, if we want to look at this, we also note what we call Sagittarius A star, the supermassive black hole at the center of our galaxy. This is about four million times the mass of our sun. Sounds tremendous, but we're actually going to see that this is a very small compared to others. So Sagittarius A is the bright source at the center. Sagittarius A star is the very central black hole that would be present at the center. Here we have zoomed in looking at radio wavelengths of 90 centimeters to look at the area right around the, around the center of our galaxy. If a diameter of about the full moon would be around this area here at the galactic center. So again, it's stuff we cannot see in visible light. And in fact, we see plenty of supernova remnants. We see one there and there and there. We see a lot of supernova remnants that have occurred that were not even visible to us. These supernovae that occurred were never visible simply because there's so much dust in between us and the others. We also see a number of stars and star clusters, which will be important when we want to determine the mass of that object at the center of the galaxy. How do we figure out that it's four million solar masses? So when we look at these infrared observations, again, penetrate the dust better and allow us to look at the stars close to that central black hole. We find that they orbit close, quickly around that mass. And in fact, we can watch some of them and as we watch here, these stars moving, if you watch especially that one star right here, this bright star, it comes in, it whips in and it turns around and it just goes, turns almost immediately around as it goes there. So quick turn around there. What can take a star and very quickly flip it from going one direction to going the other? Well, you can use those orbits to determine the mass of that central object. And we find that they are consistent with a supermassive black hole about four million times the mass of our sun. Now, we've been able to learn a little bit more about this with modern instruments and we use now what is called the Event Horizon Telescope to be able to map this and we were able to map the black hole at the center of our galaxy in 2022 using the Event Horizon Telescope. And what we find is that there are four million solar masses compacted into an area less than 0.3 AUs. If you recall, Mercury is about 0.4 AUs from the sun. This is four million suns within the orbit of Mercury. Nothing else, nothing that exists could allow that much mass to exist in such a small space. So it can only be a supermassive black hole. And again, as we've looked at the Event Horizon Telescope, what do we see? Again, we're seeing the shadow of the Event Horizon and the material around it. That Event Horizon itself is where no material from which no material can escape. So that's why it appears dark against the brighter background. Now the black hole, what will happen to this? It will continue to grow. It may gain already about a solar mass a year and it can consume larger objects such as stars or dust clouds every 10,000 years or so. What this does is give us a significant burst in activity and we'll look at galaxies and active galaxies, but with the more material being fed into that central black hole, the more active it is. So the more times we see X-rays and radio emission from this area, the more material is being fed into that black hole. And when we look at galaxies, we'll look at some extremely active galaxies that are being fed a lot faster than this. So let's go ahead and finish up this section with our summary. And what we've looked at is that we can determine the mass of our galaxy by using the orbits of stars and going back and remembering Kepler's third law. We find that the majority of the mass of the galaxy is in a dark matter halo of an unknown composition. And we found that the center of our galaxy contains a supermassive black hole about four million times the mass of our sun. So that concludes this lecture on weighing the galaxy and the galactic center. 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.