What powers a quasar? Just how strong is a blazar? What’s the connection to giant black holes? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
One of my favorite parts of observing the night sky, of going sky watching, is the depth. That you're not just looking at things in our own atmosphere or even our own solar system. These objects there sending their light to you are thousands, tens of thousands of light years away. These incredibly, almost impossibly distant objects. And you get to see them. So there's this incredible richness and depth to the night sky that's almost overpowering, to me at least, sense of depth. And the things you look at, if you look in any one particular direction in the night sky, you are looking at something incredible. For example... If you look in the direction of the constellation Sagittarius, you're looking in the direction of the center of our own Milky Way galaxy. In the center of the Milky Way is a dense cluster, a bulge of stars, of nebula, of gas. It's the bustling downtown as viewed from way out here where our solar system is out here in the suburbs. And it's a beautiful, if you get a chance to see the center of the Milky Way galaxy through binoculars or a telescope, it's just flooded with stars.
But deep in the center of the Milky Way galaxy, there is a black heart. black hole four million times more massive than the Sun the closest example that we have of a class of giant black holes called supermassive appropriately named supermassive black holes this one particular black hole has a name Sagittarius a star and This object, like I said, four million times more massive than the sun, is a monster. It's a beast. But it's well hidden. It's obscured by the countless stars surrounding it and the clouds of gas and dust enveloping it. And right now, it's slumbering. It's sleeping. It's not active. It's sitting there quietly. But material falls onto that black hole, gets caught up in its gravity and funneled into it, crosses through the event horizon. As it does so, as that material compresses and falls onto that black hole, it screams. And the first person to notice this screaming was Carl Jansky, one of the founders of radio astronomy. He saw this in the 1930s when he first developed a steerable radio array so he could turn it around and he could use it to pinpoint positions on the sky.
And this particular radio array was sensitive to a range of frequencies. And it turns out those frequencies were loudest. The loudest thing emitting those frequencies were thunderstorms. And he was able to pick out nearby thunderstorms. He was able to pick out very distant thunderstorms. But once he isolated those sources, there was a faint hiss. a signal that wasn't connected to anything he could find in the atmosphere. In fact, it wasn't connected to anything on Earth. And at first he thought maybe it was the sun or maybe Jupiter, but it wasn't connected to those either. After months of observation, Carl Jansky found figured out that this strange radio signal, this background hiss, was strongest from the constellation Sagittarius, from the center of the Milky Way galaxy. That result, while interesting, was relatively ignored. He couldn't get his bosses, who were paying for all this work, for him to do continued astronomical research. So he went on to do other things. And it wasn't until decades later that...
Radio astronomy got into serious high gear and began really mapping the skies, doing surveys of the sky in the radio spectrum. And these later radio astronomers found hundreds of bright radio sources. And at first, it was a total mystery. These were not connected to anything that they could obviously see. It wasn't like the Andromeda Galaxy or a globular cluster or comets or planets. It was just all over the sky, appeared to be outside the Milky Way galaxy. Eventually, after a lot of dedicated work and some lucky coincidences, one of the first radio sources to be detected, and one of the loudest ever to be detected, for the curious the name of this source is 3C273, after enough work, this radio source was matched to an optical image. And this matching didn't solve any problems. It raised like a thousand questions because this optical image looked like a star. It was incredibly bright and incredibly small, just exactly like you'd See a star. If you look at a star through a telescope, it's small and it's bright.
If you look at something else like a galaxy, it's dim and diffuse because it's so far away, but it's so big. You look at a nebula, it's dim and diffuse because it's not very bright and it's very big. You look at a star, it's small and bright. Whatever this thing was, this object 3C273 was emitting serious radio emission and it looked like a star. And it gets worse. Eventually, we're able to take a spectrum of it, get its elemental fingerprints to see what it's made of. And all the usual suspects were there. Hydrogen, helium, carbon, oxygen, all the usual elements that you typically associate with stuff in outer space. But it was all wrong for a star. It had all the right elements in the right places, but everything was shifted to the red. Everything was moved over. And the only way to make sense of that, that this is a typical spectrum that we expect, that we associate with things like stars or nebulas or galaxies, but incredibly redshifted, is for it to be incredibly far away. Specifically, 2.4 billion light years away.
This one source, 3C273. 2.4 billion light years away. For some perspective, the width of the Milky Way galaxy is 100,000 light years. 100,000, this thing is 2.4 billion light years away. And it's so bright that it looks like a star. It's so intense. It's so intense that if you were to take this object, whatever this object is, 3C273, and put it, say, I don't know, a few dozen light years away, two or three dozen light years away, within range of some familiar nighttime stars, At that distance, this object would be brighter than the sun. That's how bright it is. Four trillion times more luminous than our sun. We didn't really know what to make of these at the time. It was a radio source, a source of radios, that kind of sort of looked like a star, hence the name Quasi-Stellar Radio Source. Shortened to Quasar because who couldn't resist coining an awesome name like that? I know I couldn't. If I see Quasi-Stellar Radio Source, I would want to shorten that to something cool like Quasar. Hence the name Quasar was born.
Over time, we discovered more quasars. It turns out most of these, if not all of these bright radio sources, were indeed quasars. Very, very distant. The closest were hundreds of millions of light years away. And most were incredibly far away, billions of light years away. The source of this emission of both the radio and the light is small. It has to be small because we would observe changes in the brightness over time. And let's say you make one observation, you measure the brightness at one time, and then say a year later you go back and look at it again, and its brightness has changed. That means over the course of a year, it's changed its brightness, which means the object must be smaller than one light year across. Otherwise, there'd be no way for everyone in the object to get all coordinated and communicate with each other and say, hey, everybody, we're going to lower our brightness. Make sure you set your clocks so we can all do it in time. Because of the speed of light, you have to be smaller than one light year across so that over the course of one year, you could change your brightness.
Well, we see changes in these objects over the course of a week. That means whatever this object is, whatever these quasars are, have to be smaller than a light week across, which is, I don't know, that's not much bigger than a solar system. which is not large. Considering this thing is four trillion times brighter than the sun, that's a small thing. What could these monsters be? The second side of the coin, they need to be small. The second thing is they need to be massive. to power the raw intensities that we observe. Something like a backyard star or a nebula doesn't have enough gravitational potential energy. It doesn't have enough oomph to drive something like that. It has to be massive. It has to be big just because of the raw energetic output that we're observing. So it has to be small, but it has to be massive. What could these quasars be? And that's when a big leap was made in about the 1970s because we have the case of Sagittarius A star, the giant black hole in the center of the Milky Way galaxy.
This giant black hole is emitting strong radio emission. Not the black hole itself, but the material falling into the black hole is emitting strong radios. Obviously not as strong as one of these quasars, but still pretty impressive. Loud enough for Carl Jansky to notice it in the 1930s. So if we have a giant black hole and it's emitting radio waves, and we see something super far away that is insanely bright and very small... Maybe quasars are powered by giant black holes. It's estimated. It's estimated that 3C273, the prototypical quasar, the first quasar, is powered by a black hole with a mass 800 million times greater than the sun. And it gets bigger from that. We see black holes 10 billion times more massive than the sun. True monsters sitting at the center of these galaxies. And now it's relatively commonplace. It's realized that almost every galaxy in our universe hosts a giant black hole in its core. We're not alone. So what's going on? How does a black hole emit bright and loud radiation? Well, you need two basic ingredients.
You need a giant black hole and you need a blob of gas. Pretty easy recipe. Gravity does all the work. The black hole's sitting there doing its thing, being massive, hanging out, not bothering anybody. The blob of gas is attracted to it gravitationally because that's what gravity does. The material falls in. If you take an enormous amount of material, like a giant blob of gas, and send it screaming towards a black hole, it's going to compress because everybody's trying to cram in through the relatively narrow area of the event horizon, the surface of the black hole, which is relatively small compared to the size of the stuff that's falling into it. So if you take a bunch of stuff and you squeeze it down, what's going to happen? It's going to heat up. That's what gases do. It's like a bunch of people crowding into a crammed subway car. It's going to get hot. It's going to get sweaty. It's going to get awkward. That's just the way things are. The material falls into a black hole. As it falls in, it gets hotter and hotter and hotter, releases all that gravitational potential energy.
Due to friction, it gets converted into heat and it glows. It glows. And it's hard to put a superlative on the amount of brightness of this gas falling into a supermassive black hole. To give you a picture, to give you a picture, you know supernovas, right? Good old supernovas, really, really bright. A single supernova detonation, the death of a massive star, can outshine an entire galaxy for a few weeks. An entire galaxy, that's hundreds of millions of stars, can be outshone by a single supernova detonation for a few weeks. When supernova occur in our own galaxy and the conditions are just right, we can see them during the day. The last time this happened was about 500 years ago. We can see supernova during the day. That's crazy, crazy intense bright. outshining a galaxy for a couple weeks. A quasar. A quasar, material falling into a black hole, can outshine tens of thousands of galaxies for millions of years. I'll say it again. I'll say it again. It deserves being said again. A quasar can outshine tens of thousands of galaxies for millions of years.
They are the single most luminous objects in the universe next to the cosmic microwave background itself. Next to the afterglow of the Big Bang itself. They are the most luminous objects. By far the most powerful engines. The energies released in a quasar rival the energies released in galaxy collisions. That is a lot of energy. That is raw output. Speaking of galaxy mergers, that's how we think quasars might ignite. How does so much gas get to a center of a galaxy? It doesn't happen a lot, but when two galaxies crash into each other, or more accurately, as we've explored, when the swarms of stars merge together, the black holes that each galaxy carries find each other in the center, orbit around each other, decay, and then merge, and you get a single, much, much more massive black hole. Wow. And lots of gas gets tossed around. Insert your own crude joke here if you want. The gas swirls into the center of the galaxy, settles into the center of the newly merged galaxy, falls into the black hole, and the show begins.
This relationship, though, is tough to tease out because it's not like we get to see this happen in real time. We only have before, during, and after snapshots. And so we have to rely on statistics to figure it out. But that seems to be it. It seems to be that when galaxies merge, we get an ignition of a quasar. And this might explain why the quasars are so far away. Farther away means you're looking at earlier and earlier epochs in the universe. And in the more distant universe, which equals the more earlier universe, mergers were more common because the universe was smaller and structure formation was really getting going. Galaxies were still building up. And as they build up, they would have a blast of quasar activity. And then the quasar would settle down. And then, oh, no, here comes another galaxy. And then a new round of quasar. Most modern day galaxies are quiet now. Structure formation has ceased about 5 billion years ago. Galaxies aren't growing the way they used to. They don't make them like they used to.
And so there aren't as many quasars. In fact, we don't see any nearby quasars. We have to go out hundreds of millions of light years before we see our first quasar. galaxies though are do still occasionally merge the andromeda and milky way are on the path to merger they will merge in about five billion years will that ignite a new quasar in our newly merged black hole most likely yes but that's not for five billion years from now but so far i've been talking about quasars what the heck is a blazar That's because if you want answers, you need to pay. That's right. You need to go to patreon.com slash pmsutter to learn how you can contribute to keep this show going. I truly appreciate it. It's your contributions that keep all my education and outreach activities going. I can't thank you enough. And if you want to unlock the rest of this episode, you need to make a donation right now. Of course, I'm just kidding. I'm just going to keep talking. The difference between quasars and blazars is that there's more to the story of just junk falling into a giant black hole.
And the story involves magnetic fields. Your favorite. You knew it. You knew they would come up, didn't you? That's right. Magnetic fields are here again to power quasars and blazars. Here's what happens. Here's what happens. This is so cool. You take a giant blob of gas. It's going to fall into the black hole. It's going to accrete onto the surface of the black hole. As that giant blob of gas falls in, it compresses, heats up, glows. We see it. Boom. Quasar. But there's more. If that giant blob of gas has just a little bit of spin, which it will just by random chance, then as it compresses, because of conservation of angular momentum, it's going to spin faster and faster and faster and faster. And as it spins, as it compresses, it's going to turn from a blob into a disk. Nature doesn't make a lot of shapes, right? Nature knows how to make blobs. Nature knows how to make balls. And nature knows how to make disks. And in this case, if you're spinning and you're compressing, you get a disk.
That's the same reason why the solar system is a disk. That's the same reason why the galaxy is a disk. That's why there are these accretion disks around these giant black holes. You have a swirling, whirling mass of high energy plasma swirling around a black hole, doing its thing. These are charged particles. They're gonna create electrical currents. Electrical currents are gonna generate magnetic fields. That disk contains a very strong magnetic field. And then dynamo actions kick in to amplify the magnetic fields to be much, much, much, much stronger than you would normally expect. This is the same kind of dynamo physics that happens in the Earth's core that gives us our magnetic field. The same kind of dynamo physics that happened in the sun that gives it its magnetic field. Now it's happening in these accretion disks around these giant black holes. These magnetic fields do something really cool. Relatively poorly understood because the physics is so complex here, but it still happens.
The magnetic fields twist and warp. They wrap themselves around the black hole. And due to their complex geometry, gas is responsive to that magnetic field. It will start following the lines of magnetic field. So most of the gas still falls through the event horizon into the black hole, never to be seen again. But some get caught on the magnetic field lines. Just like some of the solar wind gets caught on the magnetic field lines of the Earth and get funneled into the poles to create the aurora. This is like that in reverse. Some of the gas, as it's swirling into the black hole, gets caught up in the magnetic field lines. The magnetic field lines funnel them to the poles. Instead of going in, go out. and launch a jet, a relativistic jet, a jet of material traveling at close to the speed of light. The launching mechanism of the jet, like I said, is relatively poorly understood. It might be connected to the spin of the black hole itself. It might be tapping into that spin, extracting energy from the black hole.
It may be purely astrophysical, nothing to do with the black hole itself, just the physics of the accretion disk. We're not 100% sure, but we know it happens because we see it. These jets are enormous. And they're columnate. Columnate means the magnetic fields wrap around the jet like a straw, and they keep it nice and tight for tens of thousands of light years. That is bigger than a galaxy, folks. So this tiny little accretion disk, no bigger than, say, a solar system, as it collapses onto that black hole, gets spun up to a jet, and some of that jet leaves the entire galaxy. That's how powerful it is. Well outside the host galaxy before it finally dissipates. It does something else cool there. Once it goes beyond the galaxy, it can actually blow bubbles in the plasma surrounding galaxies. And I would love to talk about that in another show. So if you want to know about bubbles blown beyond galaxies, just ask the question. But this is crazy, crazy, crazy physics. Material jets jetting from the pole of a black hole getting shot out tens of thousands of light years at close to the speed of light.
Now, of course, most of these jets are going to point away from us. But every once in a while, the point right at us will be in the barrel of the blast. And then when that happens, we get an extra strong emission. We get a quasar that looks extra bright. One, because we have a giant ray of light pointed at us like a lighthouse. Because all this material traveling at close to the speed of light is hot, crazy hot. It's emitting radiation like there's no tomorrow. And it's blasting us full on in the face. So we're getting it. And there's a cool effect called relativistic beaming. If you take a light bulb, it's emitting equally in all directions. If you throw the light bulb at close to the speed of light, there's this relativistic effect where most of the radiation doesn't get emitted in all directions equally, but gets concentrated into a forward-facing cone. So at this material jetting away at close to the speed of light, if it's blasting us in the face, not only do we get the blast of the radiation, but all the other radiation that would have missed us, that would have gone off in all sorts of other directions, get focused onto us.
And because the gas is traveling at close to the speed of light, all the emitted radiation gets blue shifted into higher frequencies. So it's even more energetic and it's just nuts. And that's a blazar. A blazing quasar. A blazar. There's one blazar in particular that is powered by a black hole that is 40 billion times the mass of the sun. So think of the energy available with a system like that. 40 billion times the mass of the sun concentrated into a relatively small volume, like, I don't know, the orbit of Jupiter. That's a lot of energy available to do some interesting things. Add in magnetic fields, and you're good to go. gas falling in from a merger event so there's tons of gas available falls in compresses glows magnetic fields twist it up launch a jet and we see a blazar of course there's more to quasars and blazars they are two examples in a generic category of very bright and or loud galaxies When we do more detailed surveys in the radio or in the optical, we see all sorts of combinations.
Some galaxies have jets, some don't. Some are very loud in radio, some are quiet. Some are highly variable, some aren't. Some also spit out x-rays, some don't. All sorts of random names that seem like they've been pulled out of a hat to describe them, like Seifert or BL Lactase or Liners, etc., etc., etc., all grouped together under a common label called active galactic nuclei. Active because they're active, galactic because they come from galaxies, and nuclei because they come from the cores of those galaxies. Remember, this emission, this crazy hot emission, doesn't come from the whole galaxy itself, just the tiny little core. They're all powered by supermassive black holes. And the current guess is that it's the same physical scenario. It just depends on the direction we're seeing it, the angle we're seeing this object, or if it's in different parts of its life cycle. So maybe in some cases the jet is blasting us in the face and it's a blazar. Maybe it's not and it's just a quasar. Maybe the jet is very weak.
Maybe it shut off for some reason because there was a recent cooling event or heating event or vice versa. Maybe there's a disk of cloudy molecular gas surrounding the accretion disk that sometimes prevents us from seeing it very clearly. And so that gives it a different signature. That is in itself a completely different episode. on active galactic nuclei and all the varieties of active galactic nuclei, or AGN if you want to be cool. And of course, supermassive black holes are usually abbreviated SMBHs. Simbas? I don't know. Putting all the jargon together gives us the phrase of the day. Feel free to say this out loud with me. A blazar is a kind of quasar is a kind of AGN. Thank you so much for listening, especially the questions that led to today's show. We have at Ruth Kieran on Twitter asking, what is a blazar? At ChiliDog64 on Twitter, what is a quasar? TlockR on Facebook, supernova versus quasar, who wins? At KDA Welch on Twitter, is there a correlation between quasars and merging galaxies? Keith I on YouTube, how do black holes make jets? And Richard C on YouTube, how are all active galactic nuclei different? I'd also like to thank my Patreon contributors, my top ones this month, Justin G, Matthew K, Kevin O, Justin R, Chrissy, and Helgen B.
Thank you so much for your contributions. You too can contribute. Go to patreon.com slash pmsutter. Yes, I know I changed up the intro to this episode. Let me know if you like it. Thought I'd give it a shot. It's been about three years that Space, not Space Radio, that's the other show I do. It's been about three years since Ask a Spaceman has been on. So let me know. I thought I'd just change it up because sometimes change is nice. Speaking of Space Radio, you need to go to spaceradioshow.com. It is such a fun show. I do it every single week. It's live. You can call me. We have questions. We have discussions. Sometimes we even laugh a little. And you need to go to astrotouring.com. There are still some tickets available for the cruise, but it is booking up fast and we're launching new trips every single month. Uh, sneak preview. We're kind of going to Northern Chile in December of 2018 to the Atacama desert. And it's going to be super awesome. And you need to come with me, go to astro touring.com.
Thanks again. You can follow me on Twitter and Facebook. My name is at Paul, Matt Sutter. Keep those questions coming to hashtag ask a spaceman, or you can email, ask a spaceman at gmail.com or go, uh, go to ask a spaceman.com the website, uh, and go to iTunes to review. You know what to do. You know how this all works. Just please help me so that I can keep helping you. See you next time for more Complete Knowledge of Time and Space.