Ep. 306: Accretion Discs

When too much material tries to come together, everything starts to spin and flatten out. You get an accretion disc. Astronomers find them around newly forming stars, supermassive black holes and many other places in the Universe. Today we’ll talk about what it takes to get an accretion disc, and how they help us understand the objects inside.

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This episode is sponsored by: 8th Light, Swinburne Astronomy Online

Show Notes

  • Accretion discs — NASA
  • Roche Limit — Universe Today
  • Conservation of Angular Momentum — UTK
  • Galactic Jets — COSMOS
  • Paper: Protoplanetary Disks and their Evolution
  • Beginner’s Guide to Cataclysmic Variable stars
  • Several scientific papers do actually refer to discoseismology!
  • Transcript

    Transcription services provided by: GMR Transcription

    Transcript: Accretion Disks

    Fraser: Astronomy Cast episode 306 for Monday, May 13, 2013 – Accretion Disks
    Welcome to Astronomy Cast, our weekly facts based journey through the cosmos. Where we help you understand not only what we know, but how we know what we know.
    My name is Fraser Cain, I’m the publisher of Universe Today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the director of CosmoQuest.
    Fraser: Hi Pamela, how are you doing?
    Pamela: I’m doing well how are you doing Fraser?
    Fraser: Doing great; the weather is improving. I went out last night with my new lens for my camera. I got a 14mm 2.8 lens and I captured some wide field astrophotography and it’s awesome.
    Pamela: Is it a lens with a rectangular field of view still?
    Fraser: It’s a little fish eye is that what you mean?
    Pamela: No, with my fish eye lens it’s actually a round part of the detector that it uses, does yours fill the detector?
    Fraser: I think it’s cropping a little bit. It’s so fast. I kept turning it down and kept getting a beautiful shot of the big dipper so I haven’t really explored it. Tonight there is going to be a potential for a chance of meteor showers so I’m going to go out tonight and see if I can get some meteors on camera.
    Pamela: That’s awesome.
    Fraser: By the time people hear this we’ve probably already done the 24 hour hangout-a-thon for CosmoQuest, but if we haven’t June 15th and 16th will be 24 hours of fund raising space madness on Google+.
    Pamela: It’s never too late to donate. We actually really need your donations to keep our programs going. We’re facing a lot of funding cuts and we’ve seen a radical drop in donations for Astrosphere new media this year. If you go to cosmoquest.org/donate you can see the links to donate for both citizen science and to keep media like Astronomy Cast going into the future.
    Fraser: Perfect. Enough talk, let’s talk space.
    Fraser: When too much material tries to come together, everything starts to spin and flatten out. You get an accretion disk. Astronomers find them around newly forming stars, supermassive black holes and many other places in the Universe. Today we’ll talk about what it takes to get an accretion disk and how they help us understand the objects inside. What do you think is the classic example of an accretion disk? I’ve always use an analogy: my bathtub drain.
    Pamela: Really?
    Fraser: Does that make sense? You have the bathtub with tons of water in it then the water tries to go down the drain and it backs up and starts to spin… and I guess flatten out?
    Pamela: I never would have gotten there from here. Sorry.
    Fraser: Too much water to get down the drain so you get spinning.
    Pamela: It’s a perfectly good analogy it’s just not one that my brain went to and that’s cool.
    Fraser: So what’s your analogy?
    Pamela: I have to admit I’m enough of a geek that for me it’s flattened like a spinning pizza except in this case it’s usually a hungry object like a black hole that is stripping material off a nearby neighbor. It’s a cannibalistic spinning piece of pizza.
    Fraser: So what are the forces involved? What are the environmental conditions that we require for us to get an accretion disk and what is going on?
    Pamela: In reality anytime you have an accretion disk, what you have is some sort of a hole, whether it be the hole that is quite literally in your bathtub or the gravitational low point that is in the three dimensional map of space using gravity, that things are trying to fall into. So as things fall down the gravitational well or as things fall down the drain in your bathtub, conservation of angular momentum prevents them from actually falling straight down. If you have even the slightest velocity to the left, right, above or below, that potential well that things are trying to fall down will end up spiraling in instead of falling straight in.
    Fraser: You can what seems like a perfectly content cloud of cold gas that doesn’t seem to have any momentum in it but it’s when it comes together that it gets that rotation happening right?
    Pamela: What you’re talking about is the type of disk that forms in the solar nebula model when you’re forming a solar system. In this case you have a giant molecular cloud of stuff. If you’re able to somehow destabilize that giant cloud so that you only affect its center of mass, you hit it neither to the left, right, up, down or anything else other than its center of mass, you simply provide the force to the center of the mass, you might be able to get it collapsing straight in. There is really no way to do that in the real universe that we live in. The reality is is however you bump or destabilize that giant molecular cloud, you might start imparting rotation so that as it starts collapsing it also starts rotating. That collapsing and rotating system ends up flattening into a disk the same way pizza dough flattens into a disk when you throw it up and set it spinning.
    Fraser: What are the constraints or the environmental factors that are going to define… lets go to the black hole example. So you got your black hole, it’s destroying some cloud of gas or some star or something else and the material is falling into it, what is going to define the size of the disk, the speed of its rotation and the temperature of it?
    Pamela: Well let’s break these down one factor at a time. Let’s start with the case of having a single black hole and there is a single small measly red dwarf star that is about to seek death on the surface of the black hole for some reason.
    Fraser: Uh oh
    Pamela: It happens. So we have this star falling toward the black hole and this is something that you’ve actually probably seen a model for at a science museum where you have that funnel shaped curving thing at the front of the museum that you’re encouraged to put coins in to raise money. As the coins roll around they get faster and faster as they get closer to the center. With that black hole that has the red dwarf falling towards it, as it comes in it’s going to end up going on ever-tightening spirals until eventually the red dwarf first gets close enough to the black hole that it hits its Roche limit, it hits the point that it is no long able to gravitationally maintain itself as a sphere and as it gets closer and closer it’s eventually going to get shredded apart or spaghetti-fied just like a human being would as it falls in, turning into a long stream of atoms. As those atoms now wrap around and around, you can imagine them forming a doughnut around that black hole. In this case the disk that is very tiny of this one stars material may actually end up forming inside the event horizon as the material works its way in towards that inner singularity. Here the key is that the object falling in can’t fall straight in because it has this angular momentum and so it ends up spiraling in getting shredded along the way and eventually death. Spaghettification and death.
    Fraser: Right but why do you get that disk forming around the black hole. Why doesn’t it just “gulp”, and then that’s it. Star goes in and that’s that.
    Pamela: It can’t just go in, that’s the thing. If the star just happens to be that its velocity has it just perfectly so that its center of mass is perfectly aligned with the center of mass of the black hole…
    Fraser: Boom, direct hit.
    Pamela: Right. Any other case, conservation of angular momentum says that some of its velocity is going to try and put it into orbit instead but the orbit is going to be a decaying orbit, in most cases when we have black holes to deal with, if the velocity is trying to carry it past the black hole initially it will probably end up in a death spiral instead. As the material gets stretched out you can imagine that it initially forms a comet then it ends up forming a single ring. As the material spirals in it’s getting to be a longer and longer spiral that essentially forms a disk. Think about coming in with a highlighter. You’re drawing around and as you spiral in some of the material is disappearing as you draw so it’s disappearing anchor something. But as you draw faster and faster in smaller and smaller circles that material gets distributed out into a band around the black hole. Am I making sense?
    Fraser: Yeah you are making sense. I guess the thing is is with a lot of these quasars and active black holes, these accretion disks can get rather large.
    Pamela: Those are not a single red dwarf star falling in.
    Fraser: No, no, of course not. I’ve switched from a stellar black hole to a supermassive black hole but even around a stellar black hole, if it’s in an all-you-can-eat star cluster, that material is going to pile up right?
    Pamela: Right, it’s a matter of as the stuff comes in… well you just threw all of these different things together that do not fit together so I’m going to try and pull all of these different variables apart. We can have an accretion disk formed when a normal everyday star is forming. In this case you have a giant molecular cloud that is in the process of collapse and as it collapses the center heats up forming a star and there is a disk of material around it that tries to spiral all the way in to die. Eventually the radiation pressure of the forming star will stop and start blasting things outward instead. That’s a very simple accretion disk. In this process all of the material involved came from that initial molecular cloud. You can also end up with an accretion disk when you have a compact object, a white dwarf, a neutron star, a stellar mass black hole that is next to another normal star, a red giant, or a main-sequence star. Just everyday star happily burning something into something new in its core. In these cases if these two objects get too close you end up with a cannibalistic white dwarf or a cannibalistic neutron star that is sucking material off of that nearby star. It’s said to fill its Roche low and the material is able to gravitationally escape and get pulled on to the other star but it can’t get there directly so it instead spirals in. In this case you have the gravity of that compact object pulling and expanding stream of material off and as it continues to eat the material you end up with a disk that is getting larger and larger. It’s getting denser as more material packs in and as it gets denser it’s actually able to reach the point where nuclear processes can start happening in that disk, in which case it may explode in a fury of reactions. So you end up with accretion disks in the situation of binary systems; these are called cataclysmic variables in general. Then of course you have the supermassive black holes and they are eating everything from stars to planets to massive amounts of dust and gas that are falling in. That material generally gets gravitationally flung in through a process of galaxies colliding.
    Fraser: Is the material that is piling up around the black hole for example… you said that it’s the environment of star formation. It’s a ferocious environment right?
    Pamela: It’s worse than the conditions for star formation; it’s actually like the conditions inside of a star. So when you’re looking at a supermassive black hole’s accretion disk in something like a quasar, an active galactic nuclei, in these cases you have an accretion disk that has many many stars worth of mass in it. This vast accumulation of matter is gravitationally bound together by a supermassive black hole so once nuclear reactions start happening in that giant disk, the disk doesn’t explode and fall apart like it does in a cataclysmic variable. The accretion disk around a cataclysmic variable can actually, essentially go “poof” and then has to strip more material off of that nearby star so it can build again and explode again. That’s where the repeating aspect of some classic novae come up.
    Fraser: I want to talk more about that but we’ll get to cataclysmic variables…
    Pamela: (Laughing) You’re jumping all over
    Fraser: I’m not, I’m not! You brought up cataclysmic variables, not me. Just with the material becoming the inside of a star surrounding a supermassive black hole, that’s crazy.
    Pamela: Yeah it’s density, it’s not crazy. These are completely logical relativistic objects so they are hard to understand but they make perfect sense if you understand general relativity.
    Fraser: Aren’t there like two people in the… wait no that’s quantum physics.
    Pamela: That’s string theory. This is completely straight forward. This is something they teach in a normal first or second year graduate course. It’s a matter of stripping the mass off, it’s bound together gravitationally, it gets sufficiently dense, nuclear reactions start going, nuclear burning starts going and this is why when you look at quasars they have an extremely hot disk of material that is radiating its own light. That radiating it’s own light is coming from, in some cases, nuclear reactions going on inside the disk.
    Fraser: And now one of the other factors you get with the supermassive black holes is you get these jets with the accretion disk. So what’s going on with the jets?
    Pamela: The jets are a byproduct of having extremely hot material, in this case hot gas, gets it’s electrons, it gets ionized and it is no longer neutral. Any time you have charged particles, so not neutral, that are moving in a circle they generate a magnetic field. This rotating disk of highly charged particles, highly hot particles, is going to create a magnetic field. The strength of that magnetic field is related to how fast the material is spinning and how fast the entire disk is spinning on individual atoms. Take that and then also how much stuff is in that accretion disk. When you have a massive accretion disk going around something like a supermassive black hole that will have massive amounts of gravity, accelerate it into massive orbital velocities and you will end up with massive magnetic fields. So you have these extremely powerful magnetic fields that are flinging charged particles that get into that core, out at relativistic speeds.
    Fraser: Relativistic speeds… these things can push all the way across entire galaxies right?
    Pamela: That’s an understatement.
    Fraser: Yeah, between galaxies.
    Pamela: Yeah you end up with jets of material, radio jets, that are significantly larger than the galaxy itself. When you look at the radio jets and you fit them into your entire field of view or even making them the background on your computer screen, the little galaxy in the center almost disappears in a lot of these systems.
    Fraser: Wow. One of the theories, and I know you don’t like some of the more fringy theories, is that some of these galactic jets may be responsible for periods of star formation in completely different galaxies.
    Pamela: Yeah, I don’t have a problem with that one. That is simply gravitational interactions versus getting thwomped by another galaxy’s field. In some cases when the jet is flying off you can actually see them compacting material as they interact with the inner galaxy medium, the inner cluster medium. This compacting of material can lead to star formation and if another galaxy makes the mistake of passing through one of these jets that could regularly… not a big deal to form star formation and also ionize stuff as it goes.
    Fraser: Just added distance. Just shooting another galaxy with your big laser beam, your big jet. That’s really cool. So you started to go to cataclysmic variables so let’s talk about kinds of examples and where we’re going to get these accretion disks. We’ve already talked about black holes and supermassive black holes and you get the situation where gravity of the black hole is tearing apart these stars and turning them into spirals and building up these disks around them. Wherever you get gravity you can get these kinds of situations. What are some other examples of where we get accretion disks in astronomy.
    Pamela: Well to go from smallest to largest it’s thought that when the Jovian planets were forming they probably had some sort of an accretion disk around them as they sucked material at the protoplanetary disk that was around the sun. You can have giant planets as they form actually have accretion disks of material that is feeding them the hydrogen that ended up becoming the bulk of the gaseous planets. You can have any old star that is in the process of forming having an accretion disk around it until it gets hot enough to start blasting material away.
    Fraser: Well what about a situation like with Mars with Phobos, because it’s below the Roche limit, is going to be torn apart in the next million years or so and turn into a disk of material around Mars until it all crashes into the planet. Is that sort of the same effect going on?
    Pamela: This is one of those things where it’s hard to think of that as much as an accretion disk since it’s not something that got captured from far far away necessarily as it is to think of it as an unstable planetary ring. At a certain point that just becomes semantics. Unstable planetary disk and accretion, if you look at the physics of how they die, are about the same thing.
    Fraser: Right in that you’ve got these gravitational tidal forces tearing something apart, putting it into a ring and then consuming it. You talked about cataclysmic variables and that is such a fascinating process that I want to spend some time just talking about those. So what is a cataclysmic variable?
    Pamela: It is a compact object like a white dwarf, a neutron star, a black hole or something along those lines that is capable of capturing material off of a neighboring star like a regular main sequence star or a giant star. As the material gets streamed off of its neighbor it forms a disk that periodically explodes.
    Fraser: …and then what? You say periodically explodes so it’s building up and then it detonates and what does the detonation do to reset the system?
    Pamela: It just causes all of the material that was in the accretion disk to go up in essentially in radioactive processes, not flames. That clears out the system to start all over again in some cases.
    Fraser: Is there some sort of end point to it? Will it go on forever?
    Pamela: Well it won’t go on forever because eventually it will use up all the material in the neighboring star but it will keep going as long as their is material that can be stolen.
    Fraser: So what happens when it does use up all that material? It just finishes off its meal and just shuts down?
    Pamela: Yeah it just shuts down. It can actually go a rather catastrophic way where you can have a white dwarf that rather than politely blowing up the disk of material around it now and then will simply consume it. If you feed it just right the material will build up on the surface of the white dwarf and eventually if the white dwarf gets too large it will explode as a type 1a supernova in which case it ends forever in a rather catastrophic way.
    Fraser: Right and helps us understand the size of the universe.
    Pamela: Yes.
    Fraser: Now what about protoplanetary disks? We talked a bit about how Jupiter and stuff were forming but this is the whole method of formation for our planets right?
    Pamela: As I was saying earlier it’s just as simple as you having a large molecular cloud, it becomes gravitationally unstable for some reason, begins spinning as it collapses, in the very center you begin to form a star and when that star lights up instead of continuing to consume material around it gravitationally, that radiation pressure stops the accretion process and just leaves a protoplanetary disk behind.
    Fraser: Oh little piece of trivia here, do you know what the study of accretion disks is called?
    Pamela: No
    Fraser: It’s called disco seismology.
    Pamela: I have never heard that despite working with many people who study accretion disks.
    Fraser: This is just what Wikipedia says. Now it could be that someone has hacked it.
    Pamela: Or that’s what they call it one place.
    Fraser: Yeah, disco seismology and QPO’s confront black hole spin. I just want to go back to black holes for a minute because I just love black holes and I know people love hearing about black holes. Supermassive black holes are rotating in many cases at the very limits as predicted by Einstein, relativistic speeds. Does that have an impact on the accretion disk when you reach the final speed limits of the laws of physics?
    Pamela: I don’t think we have enough evidence yet to say. We’re just starting to be able to use the observations that we have to use the inner edges of accretion disks to start to prove that we are actually seeing evidence of black hole rotation. It’s one thing to have theories that predict that but we are still working on gathering up the evidence to see if these theories are true and start to figure out if there are effects. One of the problems with studying accretion disks is luckily there aren’t any that are near by. When we are studying them we can’t get the fine grained measurements that we might want.
    Fraser: Wow. It’s an amazing process that we see in a lot of directions. It just comes down to that if you have something that can be torn apart by some other center of gravity, you’re going to get a disk.
    Pamela: What I really like about working with accretion disks is it’s the exact same physics across this huge parameter space of different masses. It’s one of those time where nature has allowed you to see the experiment run at the planetary scale, to see it run at the small star scale, to see it run at the large star scale and to see it run at all these different supermassive black holes sized scales. Over and over it’s the exact same physics just played out with a different twist.
    Fraser: So I can just use the same formula, whatever formula I learned the first time around I could just apply it to the object?
    Pamela: Yeah
    Fraser: That is really cool. Awesome, well thank you very much Pamela.
    Pamela: It’s been my pleasure Fraser
    This transcript is not an exact match to the audio file. It has been edited for clarity.

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