 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about supermassive black holes, which are the energy source of active galaxies that we previously talked about. So what is the energy source? What powers this? The power properties of the quasars and galaxies are what? Well, they are extremely luminous. That means they're brighter than the entire galaxy. Or how we could only see quasars as looking like a star because of how bright the central portion of the galaxy was. They're very small, only about the size of our solar system. They emit jets of material in a narrow beam at speeds very close to the speed of light. So what can we do? This much energy emitted from such a small region needs something much more powerful than nuclear fusion. So we also know that quasars were more common in the early history of the universe than they are today. So what is the observational evidence for a supermassive black hole? Well first we can look at indirect evidence. We already talked about how there is a large amount of mass in a small volume. How can we get so much mass into such a tiny volume? And we previously looked at the short time period variations in brightness, all telling us that there's a lot of material collapsed into a very small size. How do we determine the mass? Well we use Kepler's third law, recall. And we can do that with orbits of stars near the center of our galaxy, for example. So let's look at this here as we watch these two stars come near Sagittarius A star, which is the central portion of our galaxy. Now we watch this star here, and let's start it, here it goes, and starting in March and through April and May. So we're looking over at the period of months, and this star has gone from going one direction to being almost completely turned around and heading back out. So that takes a tremendous amount of mass, and measurements show that has to be about 4.1 million solar masses. The size is about out to the orbit of Neptune. Now it would be about 60 astronomical units in diameter. Now other evidence we have is looking at other galaxies. So let's look at the galaxy M87, a giant elliptical galaxy, and we look at one side of the central portion here. We're looking very zoomed in, and this has a very strong blue shift. This section has a very strong red shift. We can use the amount of those shifts to determine velocities, and we can use that velocity to determine how much mass must be present at the center. And again we find it is millions or a billion solar masses that are needed. We also now have direct imaging in a way of black holes from the Event Horizon Telescope. So we can use that to see the shadow of the Event Horizon against background material. So while we cannot actually see the black hole directly here, we see its shadow in the galaxy as well as in the galaxy M87. So how do we produce energy? If we know this is a black hole, how does a black hole produce energy? Remember that nothing escapes from a black hole, so we get nothing out of it. However, energy can be produced in an accretion disk around the black hole. And as that material is spiraled into the black hole from other materials, whether it be from another star or something else, it spirals around and accelerates to very high speeds and heats through friction to 100,000 Kelvin, giving off energy in the form of X-rays. In fact, 10% or more of the mass is converted to energy. So it isn't perfect efficiency, E equals MC squared would get you 100% conversion and be even more energetic. But compared to nuclear fusion in a star which is less than 1% efficiency, this gives off a lot more energy every single time. Now we also get jets of material. How can jets of material be produced by an active galactic nucleus? Well, large amounts of material are spiraling in. You get intense pressures that fling the material back outward. However you have all this material in a disk around it and that prevents material from heading out in the direction of the disk. So it is pushed out perpendicular to the disk. So the jet goes out this way and the thicker that disk is, the more well confined the jets of material will be. So the thicker the disk, the more it confines, the less material can try to go in this direction. It just can't move there because there is too much material. And looking at active galaxies show that this type of outflow is very common in material. Because all this material spirals into the black hole, it gets compressed and pushed out along perpendicular to the axis. Now we can actually see some of these jets, for example. We'll come back to M87 again. And here we have the central core of M87. And we can see one of the two jets heading outward. So jet of material heading out at very, very high speeds. And we see this not only in M87 that we've looked at previously, but in many other active galaxies and quasars as well. Now what can we learn from these quasars? Well, quasars are a way to look at the early universe. So we can probe the early universe. Why? The more distant an object, the longer ago we see it. So we look at a quasar that is 10 billion light years away. And it is seen not as it is today, but as it looked 10 billion years ago. What does it look like today? Well, that's a big question. And we'd have to wait for sure 10 billion years for it to reach us, for that light to be able to reach us, for us leaving it today. Now we do know that the number of quasars has decreased over time. So here's the number of quasars shortly after the Big Bang. We were talking about 10,000 or more. And now after we get through 10 billion years we're talking about only a handful. And now none. So what has happened to these? Well the thing is that long ago quasars had fuel. Where do they get their fuel? All galaxy collisions colliding together, throwing material into that accretion disk around the black hole. Those black holes don't go anywhere. They are still there at the centers of galaxies. So they are still here today. They're just dormant because they're not being fed. And as they are given more material, then they can sometimes be seen as active galaxies currently. So what is the source of fuel for these active galaxies? Well we can get the occasional passing star or gas cloud that can terribly torn apart. So it can be torn apart by the tidal forces. So the star or a gas cloud can be ripped apart and into that accretion disk around the black hole. Galaxy collisions, colliding galaxies can provide additional material for the black hole and energy. And these collisions were much more frequent in the past than they are today. So why were they so much more energetic? Well we see these collisions and we can look at a number of them here. Here's a whole bunch of colliding galaxies that we see looking through Hubble images and seeing a whole bunch of galaxies that are colliding. The further back we look earlier in time the more galaxies were distorted by these collisions. So while many galaxies nearby look like ordinary galaxies, here we see galaxies that are very much in the process of colliding, adding additional material into that black hole to energize it and have it give off that amount of energy that we associate with things like a quasar. Well let's go ahead and finish up with our summary and what we've looked at here is that active galaxies are powered by a supermassive black hole at their core. We infer their existence because of the amount of energy being produced from a small area and the motion of stars at the center. And studying quasars tells us something about the early history of the universe since this is when they were most active and most common and today the black holes are simply dormant. So that concludes this lecture on supermassive black holes. 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.