 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to talk about a couple of things. We are going to talk about active galaxies, and specifically one type of active galaxy known as a quasar. So these are some very distant objects, and what represents some of the very earliest stages of things in the history of the universe. What were galaxies like long ago? Well, many of them were actually active galaxies or quasars, and have since settled down to the ordinary galaxies we see today. So let's go ahead and get started here. And what we see, what are the differences between a normal and an active galaxy? And there are two primary differences that we see. First of all, active galaxies are more luminous. They are putting out more energy. Luminous does not just mean brighter in terms of visible light, but can mean all sorts of things. Many times it can be x-ray, it can be radio emission. So they are not only putting out the visible light, but they are more luminous across the spectrum. And they emit what we call non-stellar radiation, or radiation, not radiation that is not produced by thermal activity of stars. So for most galaxies, an ordinary galaxy, its spectrum is essentially the combined spectrum of all of the stars that make it up. For active galaxies, its overall spectrum looks much different. And then we also use the term AGN, or active galactic nuclei, to talk about the activity in the nucleus of the galaxy, which in most cases is where something is going on, down in the nucleus of the galaxy, near the center, near that supermassive black hole, that is what is causing a galaxy to become active. So let's look at some examples of types of active galaxies. So one example of an active galaxy would be what we call a seaford galaxy. These are essentially spiral galaxies with an unusually bright nucleus. So what we see here for the nucleus is a lot brighter than we would normally expect for a typical spiral galaxy. That can often be also be associated with extensive bursts of star formation. So you can have sometimes see intense regions of star formation around as well. So seafords are one type that are associated with a spiral galaxy. Another would be what we call a radio galaxy. Now here we see the radio galaxy. They have, as you might expect, excess radio emission and often emit jets of material. So we see that here. The galaxy itself is down at the center. So this would be a nice, looks like a nice elliptical galaxy at the center. And it is emitting a lot more radio emission than a regular elliptical galaxy. And it has jets of material beaming out in each direction. You can see how highly collimated the beams are here as they leave this area. And then as they reach and strike the intergalactic medium, they then spread out into these great radio lobes. So this is actually a combined image looking at a couple of things in that you can see the optical emission, but you can also see the radio emission from the jets plowing in to material in between the galaxies. And when we look at the centers of these, the other thing that we'll see is that we call what we call a blazar. Now a blazar is actually the compact center of the galaxy. So when we see them, they are considered the central portion, so the very compact material going on at the center of, in this case, an elliptical galaxy. So what we're beginning to see is that certain types of galaxies, certain types of active galaxies are associated with certain type of normal galaxies. Seaford galaxies are an example of a spiral galaxy, whereas the blazars are related to the elliptical galaxies. Now the one we really want to look at here and take some time on is the quasars. So quasars are what we call a quasi-stellar radio source. And they are essentially a star-like object, meaning they look like a star, they just look like a point of light, but they are emitting radio waves. Now that's not unusual. Our sun does emit radio waves, but we can only detect them because we are so close. A nor-ordinary star, we would never be able to detect it. Even within our galaxy, we would not be able to detect it if emitting radio waves. So this was something very mysterious as to why these stars were emitting radio waves. And the other issue was that the spectral lines did not match with any that were known here on Earth. What were these things made up of? Their spectral lines did not match with what we saw here on the Earth. And in 1963, Martin Schmidt noted that the emission lines in the spectrum of a quasar known as 3C273 had the same spacing as the hydrogen lines. But they were extremely redshifted, not just slight redshifts, but an extreme redshift and in fact enough to show that it was receding at 15% the speed of light. So that is extremely fast, much faster than any star is moving, and much faster than the galaxies that had been used at the time, it's seen at the time, were moving. So if we use this, we now have, if we can use Hubble's law, which says that the velocity is equal or is related to Hubble's constant times the distance. That means this object must be extremely far away to be receding at such a great speed. So what are these quasars? Well, let's take a look here. And they are star-like, and we see here, it looks much like a star. A galaxy normally has some kind of extended structure to it. We see a couple examples of galaxies here. And they do not show the diffraction pattern, the cross-like pattern that we see that goes through the center of a star caused by the telescope that is observing it. So that is normally associated with a star. So if we'd look at this and would say, well, this is a star, it is a point source of light. But they are only star-like because they are so far away. This is actually the nucleus of a distant galaxy blazing incredibly bright. We now know of millions of these. Every single one of them shows a red shift. And they now have velocities going up to 96% the speed of light, meaning that these are things that existed very early in the history of our universe. Let's take a look at what this means in terms of Hubble's law. And what we see is that quasars are parts of galaxies. So since galaxies obey Hubble's law, so should the quasars. And that means that based on the measurements that we make, every single one of these is at least 10 billion light-years away. Our universe is 14 billion light-years, 14 billion years old. So these things are really the very first couple of billion years of the universe. There are none of them nearby. There are other types of active galaxies that we looked at, but nothing that matches what we see for a quasar. So whatever form these had to have only existed long ago, and this is a way to look at the early history of our universe. We are actually seeing, then, the early history of our universe. So here is an example when we take an image of one of these quasars and look in more closely. And if we blot out the quasar light, we can actually see that there is a galaxy around there as well, just much too faint to normally be seen. So there is actually a galaxy. This is the core of the galaxy. But if the core of the galaxy is receding at such a speed, then so will the entire galaxy. Now what we have to learn, we have to look at what is this energy source? What could be powering something to be this bright and be seen over such tremendous distances? And one thing to look at is how big this can possibly be. And what we look at is that an object cannot vary in brightness faster than it takes the time, the time it takes light to travel across it. If we look at an example, an object ten light years across would then, first we would see the increase in brightness from things on the near side. So if this is a cluster of stars or an object and looking at each of these different parts of this object, if it were this big, we would get the near side, we would get an increase in brightness. Then five years later, we would get an increase in brightness from the middle, and ten years later we would get the increase in brightness from the far side. So the increase in brightness instead of just jumping up really fast would take a much longer time to increase its brightness, because it takes time. Even if the whole thing got brighter at once, we would not know about it for ten years. Five years, we'd learn out the first side, five years later the next side, five years later the end. And then it would continue to vary in brightness. But quasars can vary with time periods of months or weeks. That means these things cannot be light years across, they cannot be galaxy size, they must be closer to solar system size. And what that means is what is the energy source of these quasars, what is the only thing that could possibly produce this much energy in such a small space. And that is where we're going to come back to the idea of a supermassive black hole being the energy source for a quasar. And quasars and active galaxies both have the same energy source, which is this supermassive black hole. What it is, is the black hole here at the center accretes material, so that accretion disk is where all the energy is coming from. So as material spirals around and into the black hole, it heats up to extremely high temperatures and gives off lots of energy. We can convert a good fraction of the mass into energy in something like this, before it actually spirals into the black hole. This is the only way we have to produce such a large amount of energy, lots of energy from a small amount of space. So we have lots of energy coming from a very small space. The only thing we know that will do that is this supermassive black hole. So let's finish up as we do with our summary and what we've looked at in this section. First of all, we saw that active galaxies differ from ordinary galaxies in two ways. They emit more energy, so more energy coming from them. And it's a different type of energy. It is non-stellar energy. It is sometimes called QSOs, our important type of active galaxies, and have been determined to be the core of very distant galaxies. The energy source of that active galaxy is a supermassive black hole at its center, which is being fed by material and therefore giving off a lot of energy. So that concludes this lecture on active galaxies and quasars. 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.