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Quasars. Created by Sal Khan.

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Video transcript

What I want to do in this video is talk a little bit about quasars. And that's a short form for quasi-stellar radio sources. And this name is just a byproduct of the first observations of quasars. Because all they looked like were these kind of point-like sources of electromagnetic radiation, mainly in the radio part of the spectrum. So that's why we called them quasi-stellar radio sources. Now, it turns out that they are neither stars, or even quasi-stellar. And actually, their main energy isn't even being released in the radio band of the electromagnetic spectrum. They're far more energetic than that. What they really are are the active nucleuses of galaxies. So let's think about that a little bit. So if we have a supermassive black hole at the center of a galaxy-- so let me draw that right over here. So that's our supermassive black hole. And maybe that's the surface of the event horizon of the supermassive black hole. The actual mass of the black hole is in the center of that event horizon. If there is material that's passing by this black hole, it's going to get attracted to it. And it's going to form an accretion disk around it. This material is going to start rotating around this black hole. And some of it, if it doesn't have enough velocity, is going to actually fall into the black hole. So you have all of this material going around the black hole. And some of it, if it doesn't have enough angular velocity and not enough to orbit around the black hole, it's actually going to fall in. Let me label this. This is the accretion disk. So as things are getting faster and faster as they fall closer and closer to this black hole, and bumping into each other more and more, that gravitational potential energy from things falling into it is being turned into actual energy, actual temperature. And so what you have is things start to get really, unbelievably hot near the surface. They get hotter and hotter as they fall closer and closer to that event horizon. And so near the event horizon itself, things are so intense that they're actually releasing high-frequency electromagnetic radiation, mainly in the X-ray part of the spectrum. Now, I want to been very clear. So there's two things here. One is when you learn about quasars, or when I first was exposed to quasars in like a Nova special, they make you think that the quasar, that the radiation is somehow being released by the black hole itself. And I would scratch my head because I was just told that nothing can escape the event horizon of a black hole, including electromagnetic radiation. So how could that be being emitted by the black hole? And the answer is, it's not being emitted by the black hole. It's being emitted by the matter in the accretion disk that hasn't quite gotten to the event horizon yet. Once it's inside of the event horizon, any electromagnetic radiation that it might emit will not be able to escape the black hole anymore, will not be able to escape the actual event horizon. So all of this is from the accretion disc around the supermassive black hole. And the other question that used to pop in my mind is why does it come out at these kind of perpendicular, orthogonal to the plane of the actual accretion disk? And at least my logic tells me, well, things aren't going to pop out-- they're not going to pop out along the direction of the accretion disk because then they're going to be absorbed by other things. In fact, that's what's going to cause other things to get heated up closer to the actual event horizon. So any energy that's going out in that direction is just going to be absorbed and make other things hotter. And only when you go roughly perpendicular to the plane of the accretion disk is that energy allowed to kind of go and transmit freely into space. Now, I want to be very clear. Quasars, these are the most luminous things that we know of in the universe. The brightest-- or actually, many quasars are on the order of a trillion suns in luminosity. So they can be brighter than an entire galaxy. And that's just coming from material around a fairly small region of space, much, much, much smaller than an actual galaxy. It's the very center. It's kind of just the galactic core. Now another interesting thing about quasars, and this kind of gives credence to this notion of a constantly changing universe, and even to some degree the Big Bang itself, is you have these supermassive black holes that may be formed shortly after the Big Bang. Now you can imagine, at an early stage in the universe's development there would have been a lot of mass that would have been near these black holes, that didn't have quite the velocities to be able to escape them or be able to orbit around them. And so these would actually start falling into the black hole. And then over time, all of the mass that had to fall into the black hole, into the supermassive black hole, will have fallen into the supermassive black hole. And if you imagine at some future period of time, you should still have the supermassive black hole. But all you should see is mostly things orbiting around it. Anything that had to fall into it, would have already fallen into it. So you're just going to see things orbiting around it. And this is actually what we see if we look around us. If we look at our Milky Way Galaxy, we don't observe a lot of things falling in. For example, the Milky Way Galaxy does not have an active nucleus, an active core. It is not currently a quasar, the center of the Milky Way Galaxy. The supermassive black hole there is not, I guess we could say, digesting, is not digesting or consuming material. But you could imagine at some point in the Milky Way's past, there might have been a lot of material that didn't have quite the velocity to be able to orbit. And so that was consumed. And as it was consumed, it would emit all of this X-ray radiation and could be observed as a quasar. And that's actually what we observe. The closest quasars-- and we've observed more than 200,000 quasars-- the closest quasars are on the order of 780 million light years away. So what does that mean? We don't observe quasars closer than 700 million light years. So what that tells us is, at least in our region of the universe, the most recent quasars were 780 million years in the past. When we look at closer parts of the universe-- let me draw, let's say this is the observable universe. This is us. So we only start to observe quasars at a certain distance away from us. And that distance is actually also a certain time in the past. Because it took the light 780 million years to get to us. And actually, most of the quasars are more than 3 billion light years away. Which tells us that they only existed more than 3 billion years in the past, at a younger stage of the actual universe, when there was actual material for these supermassive black holes to consume at the center of galaxies. You move closer in time to us, and most of that material has actually been consumed. And we just have material orbiting around these supermassive black holes, which we call galaxies. And so we don't observe quasars anymore. And just to give an idea. I mean these are, as everything we learn in cosmology, kind of these mind-bending concepts, unbelievable distances, unbelievable masses, unbelievable brightnesses, I guess you could think about it. But just to give a sense, the brightest quasars, the brightest known quasars, devour on the order of 1,000 solar masses per year. So that's on the order of 10 Earths, 10 Earths per second, if I did my math right. 10 Earths per second are being devoured by the brightest quasars. And it's that energy of that mass that's acreting around it that's generating all of that energy. And actually, I should say-- I shouldn't even talk about it in the present tense. This happened in the past. We're just observing it now. For all we know, the rest of the universe looks fairly similar to the way our universe does. And so there really aren't that many quasars around. Although the other side of the coin might be, even though most of the material has already been consumed, maybe even by our own supermassive black hole in the center of the Milky Way, at some point in the future, maybe it will be able to consume on some more stellar material, some more-- well, any type of material in the future. And that might happen about 4 or 5 billion years in the future when we actually collide with the Andromeda Galaxy. So anyway, hopefully that gave you some food for thought.