 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about active galaxies. We've previously looked at ordinary galaxies. Now we want to see what is meant by an active galaxy. And we'll look at one specific type of those as well known as the quasar. So what is the difference between a normal galaxy and an active galaxy? Well, there are two primary differences between the normal galaxy and the active galaxy. One is that the active galaxies are more luminous. And the second is that active galaxies emit non-stellar radiation. What that means, it is radiation that is not produced by stars. Stars produce thermal or stellar radiation, which is radiation just based on their temperatures. And that will give off most of the light in the visible and some in the infrared and ultraviolet portions of the spectrum. However, active galaxies will also give off a lot of x-rays and radio waves, which are typically not given off in great quantities by stars. We will often refer to an active galactic nucleus as an AGN, or its abbreviation there. And it is the activity in the nucleus of the galaxy that makes it an active galaxy. So let's look at some of the different types of these galaxies. Well, we can start off with what we call a seaford galaxy. A seaford galaxy is a spiral galaxy where the nucleus is unusually bright. And that we see here, the nucleus is extremely bright. In fact, it's very hard to see the spiral arms because of the overwhelming brightness of the nucleus of the galaxy. So it has something going on at the center. It could have enhanced star formation within that inner ring of material. And it could have a supermassive black hole at the center, which is helping produce this excess radiation. Now we also have radio galaxies. A radio galaxy, as you might guess, has excess radio emission. They also often have jets of material streaming out. And here we see jets streaming away along with the radio lobes of material where this has struck the interstellar medium. Or in this case, I guess, where this has struck the intergalactic medium, medium between the galaxies. And we see this is a combination image looking at visible light for the background image with the galaxy itself, as well as the other background galaxies and stars, and the radio image showing the radio lobes of this material. Now we can also have blazars. Just like seafords were associated with spiral galaxies, blazars are associated with the compact centers of elliptical galaxies. And it may not look like anything prominent here, but an extreme amount of energy that is coming from the core. So it gives off much, much more energy in the radio and other forms than we would possibly expect from a typical elliptical galaxy. Now one we mentioned at the beginning would be the quasars. So let's look at an example of a quasar. And we see the image of it here. This is the quasar known as 3C273. And quasars are what we call quasi-stellar radio sources. They're often now called QSOs because we find some that are not specifically radio sources, but this is how they were first identified. And they are star-like objects. They just look like a star in the sky, but they're emitting radio waves, which most stars do not do. They were very mysterious because they had spectral lines that did not match with any known elements on Earth. So what were these things made up of? Well, early on we found a new element in the Sun that was not known on Earth. That was helium and named Helios for the Sun. However, at this point we knew the periodic table. There were not missing elements that we could assume that quasars were made up of. So it has to be something associated that we would have on Earth as well. And Martin Schmidt in 1963 noted that the emission lines in this quasar, 3C273, had the same spacing as the hydrogen lines. What it means is that we were seeing hydrogen that is extremely redshifted. And this quasar was receding from us at 15% the speed of light. Now remember, we've looked at Hubble's law. Hubble's law relates the distance and the velocity. If it is receding at that high of a speed, it must truly be extremely distant. So what are these quasars? What is a quasar? Well, they are only appear star-like because of their immense distance. What they really are is the nucleus of a distant galaxy. We just cannot see the rest of the galaxy because it is so far away. Millions of these are known. All, every single one of them shows a redshift. Everyone is moving away from Earth with velocities up to 97% the speed of light. In fact, the most distant quasar as of 2023 was one known as J0313-1806. If you're wondering about the naming, these are the coordinates. And this is the right ascension of the quasar and this is the declination of the quasar. And this one was formed when the universe was about 600 million years old. So it's a very, very ancient object looking back to the very early times in the universe. Now, what do we know about quasars? Quasars are a part of galaxies. Since galaxies obey Hubble's law, so do quasars. That means there are billions of light-years away from us and that none exist nearby. So whatever formed these only existed long ago. And we are seeing part of the early history of the universe. So if we want to see that surrounding galaxy, here we have the central quasar. And if we mask that out, then we can see the faint surrounding galaxy, which is otherwise invisible and overwhelmed by the brightness of the central core of the galaxy. Now, how big is this central core? What is powering this quasar? Well, we know that an object cannot vary in brightness faster than it takes the time it takes light to travel across it. If we look at an object such as this with things that are about ten years across, we would first see the increase in brightness from the near side, and five years later we would see the increase in brightness from the center, and five years after that we would see the increase in brightness from the far side. And what that means is that it's going to get blurred out. So if it were varying over the course of say a year that would get smoothed out because we can only see parts of it at one time. We don't get the light from all of it at one time. We will get one part first, then five years later, and then another five years later we will see the light from the other side. We find that quasars vary quickly in months or even weeks. The energy source must be only a little bit larger than a solar system. So these are incredibly small energy sources. And what is that energy source? Well, that would be a supermassive black hole. So this is the same for a quasar, and active galaxies all have the same energy source of that supermassive black hole. Now it's not the black hole itself, it is the accretion disk around the black hole that gives off a tremendous amount of energy. This is the only way to explain how to produce a large amount of energy coming from such a small volume of space. So it is a supermassive black hole accreting material. And in the early history of the universe, they were accreting a lot more material than they do today, giving rise to the extremely energetic quasars. So let's go ahead and finish up with our summary. And what we've looked at this time is that active galaxies differ from ordinary galaxies in two ways. They emit more energy and a different type of energy than an ordinary galaxy. Quasars, also called QSOs, are an important type of active galaxy, and we're determined to be the cores of distant galaxies. And the energy source of an active galaxy is a supermassive black hole at its center. 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.