As astronomers look out across the Universe, they see various objects spewing jets of material light years into space. What causes these jets, and what impact do they have on the Universe.
PDF: The Role of Magnetic Fields on Astrophysical Jets (arxiv)
What is the Interstellar Medium? (University of New Hampshire)
Angular Momentum (Swinburne University)
Ideal Gas Law (Hyperphysics)
How Rail Guns Work (How Stuff Works)
Binary Star (Swinburne University)
Einstein’s theory of general relativity (Space.com)
Frame-dragging (Einstein Online)
T Tauri Stars (Swinburne University)
Herbig-Haro Object (Swinburne University)
What is a neutron star? (EarthSky)
What Is a Black Hole? (NASA)
IMAGE: A cosmic searchlight (ESA)
Stanford astrophysicists report first detection of light from behind a black hole (Stanford University)
Gamma Ray Burst (Swinburne University)
Quasar (Swinburne University)
A stellar vampire mimics a black hole (Physics Today)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, episode 635: Astronomical Jets. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos where we help you understand not only what we know but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela. How you doing?
Dr. Gay: I am doing well. How are you doing?
Fraser: Also well. Sort of dealing with the trials and tribulations of getting our house completed. We’re in the final stretch now and it’s all the little details that need to be done perfectly and each one can hold up the thing. But we’re getting there. We’re getting there. As astronomers look out across the universe, they see various objects spewing jets of material light years into space. What causes these jets and what impact do they have on the universe? All right, jets. Now, we’re not talking about aircraft jets. We’re talking about space jets. But they’re not aircraft jets in space. What is a jet?
Dr. Gay: So, at the most simplistic level, a jet is anytime you have something that sends out materials at high velocities. So, when we call an airplane a jet, what we’re actually referring to is its engines that are spewing material out at high velocities instead of just having a propeller at the front.
Fraser: And throwing an airplane in the opposite direction.
Dr. Gay: Exactly. Exactly.
Dr. Gay: Now, in astrophysical situations, we are talking about situations where you have some sort of an object that has a entire environment around it that is leading up to it sitting at the center of normally bipolar, going-out-in-two-different-direction jets that have been built up most typically by a magnetic field.
Fraser: So, I guess, think of a fountain throwing water high up in the air. The distance that the fountain goes is limited by the force of gravity, so it reaches sort of a maximum height. It’s running into the air, and so you’re getting both the jet getting a little bit slowed down but also sort of broken apart a little bit as well as other spray and material going off in all directions; except with the astronomical ones, they go in two directions, so you’ll get jets, as you say, in polar directions.
Dr. Gay: And the gravity of the object can, if it doesn’t make it very far away, cause the material to flow back at you. But most of the time, as the material’s getting kicked out at such high velocities, you don’t really see this – we refer to it as collimated – getting focused into a tight beam. We don’t see that focused beam start to diffuse out into a cloud of material like at the top of a fountain until it hits that interstellar media, until it hits all the stuff that is just filling space between galaxies or between stars.
Fraser: Right, right. I guess there’s not that much to run into.
Dr. Gay: Depends on the system.
Fraser: So, what can generate these jets? What is their cause?
Dr. Gay: The bulk of the time it is extremely complex physics where the correct is, we don’t entirely know.
Fraser: You can’t just say “extremely complex”. “The answer’s “extremely complex physics”.” That is unsatisfying to me.
Dr. Gay: I know. I know, and this is why in introductory textbooks, magazine articles, all that complicated physics gets hidden behind the phrase magnetic fields.
Fraser: Perfect. Now, we’re talking. It’s magnets.
Dr. Gay: And the truth is there are magnetic fields that are involved, where you have this compact object that has materials spiraling around it and it’s often getting either stolen off of another object or it’s flowing in the center of a galaxy. And as it comes in, it is getting denser and denser and denser because the material has to shed angular momentum in order to get in towards the center. So, it backs up while it’s going through this process of shedding angular momentum, and this built-up material gets denser and it gets hotter due to that PV nRT equation that we learned in high school. Chemistry, pressure, density, temperature, all related things.
And if it gets hot enough, it can even start having nuclear reactions. So, hot gas, it’s gonna eventually become, hot ionized gas. And hot, ionized gas means that you have atoms that have shed electrons away and these are now charged objects. Charged objects orbiting are charged objects going in a circle. Moving charged objects create magnetic fields. Moving charged objects going in a circle create basically electromagnetic fields like a battery and some wire can do, and you’ve now built a very small railgun that can fling things out of the magnetic field along these polar axes.
Fraser: All right, so you’ve got matter moving at high speed crushed together, heated up to enormous temperatures, starting to undergo more exotic processes like fusion. But ionization of these particles, they’re moving in circles around whatever object they’re orbiting. This is generating electricity, magnetic fields, and complex physics, that soup of madness, is now creating this, as you call it, a Gauss gun, a railgun, a magnetic accelerator that has been taking this raw material and hurling it away from the accretion disk. Right.
Dr. Gay: And what I love is, at the most simplistic level, this is something you could do with a sixth grader: wire batteries and a tiny magnet. So, go figure this out for yourselves, people.
Fraser: And feeding material from a binary companion star or generating nuclear fusion. Any sixth grader can do this. All right, so we got a sense of what the underlying complex physics, or at least the ingredients of the mess, that is happening. And I think you said it perfectly. Astronomers still don’t know. Some of the finest minds in nuclear physics, in particle physics, in magnetic field, in fluid dynamics, in all of these fields working together are still arguing and discussing.
Dr. Gay: Lemme give you one example.
Fraser: Also, relativity has effect as well.
Dr. Gay: Frame-dragging.
Dr. Gay: Frame-dragging is a thing. And to give you one example of why this is confusing. If you go back to your sixth-grader model. Literally you can do this. Go grab some wire that isn’t insulated the way this is, wrap it in a circle, attach both ends of the wire to a battery and now you have charge flowing and you can use this to fling small objects. It’s fun. If you get big and fancy, car battery, PVC wrapped in wire. Now, you have a small railgun. Now, when you build your small railgun, it is always going to fling magnets in the same orientation. With galaxies, you have electrons being thrown both directions out of this, and so you have to build up a system that allows symmetrical flinging of materials in both ways.
Now, electrons have both north and south magnetic orientations. There’s ways to get there, but just understanding it is flinging things in a symmetric manner. It’s kind of –
Dr. Gay: – where you get at the “This is hard”.
Fraser: But it’s perpendicular to the axis of –
Dr. Gay: Right.
Fraser: – rotation of the object. So, if the galaxy is –
Dr. Gay: Right-hand rule.
Fraser: – rotating, right-hand rule. You’re gonna see material blasting up out of the center of the galaxy and also down out of the center of the galaxy in completely opposite directions of the galaxy. All right. Again, I guess I say, “We know why this happening.” We don’t know why this is happening. All we know is that, in that mess of complicated physics this is happening. What kinds of objects are capable of generating these jets?
Dr. Gay: Okay, at the most pathetic level, we have very young stars called T Tauri stars. The bulk of a T Tauri star’s jets are due to outflows of just it’s not eating the material fast enough as it flows down onto the star and things escape. But, also, its magnetic fields will bloop out material in pockets now and then, so you have ineffective bloopy jets coming from T Tauri stars. Then, skip through most of the stars in between that are happily doing their main sequence and beyond life of nuclear fusion and their core, and then we get to the neutron stars.
So, as soon as you get to something as dense as a neutron star, these objects become capable of gravitationally pulling stuff off a companion and getting it going fast enough around the neutron star because it has enough gravity and it’s tiny enough and you need both, that you can start getting that jet that has the magnetic field again. So, for individual objects that have long-term lasting jets, you have T Tauri stars, Herbig-Haro objects at the very beginning; things forming stars. Stars are happy, stars are happy, stars are happy, stars die. And for dead stars, you have neutron stars and bigger. And when I say “and bigger” I mean, all black holes all the way up to super black holes are capable of having jets. So, you can have a full fledge galaxy that has the exact same, not-entirely-well-understood physics as the neutron star.
Fraser: It’s just a matter of scale?
Dr. Gay: Yes.
Fraser: Right. All right, so let’s talk about scale then. We talked about how a neutron star will generate some level of jets, and super massive black holes, the most massive objects in the universe, will generate these jets. What is the scale? How far out can these jets go from their source?
Dr. Gay: Light years and light years and light years and light years, by which we’re talking thousands and thousands and thousands of light years from the center of a galaxy. And the image of those of you who happen to be watching on YouTube or live while we’re recording on Twitch, there is a tiny speck that, on my particular monitor is the size of my pinky fingernail, that represents where the galaxy is. And about 6 inches away in this image is where the jet ends, and the jet is actually pointed a bit at us, so that’s hiding some of the full length of this jet. In the radio sky, galaxies are a very different size than they are in the optical sky because of how big these jets are.
Fraser: So, for a neutron star, we’re talking like dozens of light years?
Dr. Gay: It can be. Now, the neat thing about neutron stars and stuff is they exist in much more complicated parts of our galaxy, so their size is going to be beautifully dependent on how much stuff is around them. And so, we can end up with, they go out and now they’re pushing their way out, blowing a bubble around them. And it’s this blowing a bubble around them that, at a certain degree, allows us to figure out how long they’ve been doing things in some cases.
Fraser: Because you know how fast the material’s moving and then that lets you figure out when it started this process? Okay.
Dr. Gay: Yeah. And so, you’re looking at different kinds of environments, but what is particularly fascinating is when we look back at the center of our galaxy, even from what we are, in the correct wavelengths of light we see all these bubbles all over the place. Some of them are spherical bubbles that are created by supernova explosions. But in other cases, what we’re seeing is the material pushed out by jets.
Fraser: That’s really fascinating. That you look at the universe in these different wavelengths, and now you see this historical record of the events. You see bubbles blown out from ancient supernovae. You see the wakes of jets released by neutron stars while they were actively feeding on some companion star or black holes while they were feeding on some companion star. And I’m guessing we see something similar for galaxies with active nuclei that we can see how things have changed over time with them.
Dr. Gay: I’m not sure we’ve actually seen something like that, but it could be I’ve missed it in the literature.
Fraser: There was a paper that I recall where they were seeing the material falling back in or they were seeing light echoes from the jet.
Dr. Gay: So, light echoes we see.
Fraser: They were seeing light echoes from the jet that was allowed them to figure out when it was an active galaxy. And it’s not an active galaxy right now.
Dr. Gay: Light echoes are fairly common-ish in the grand scheme of, we’ve seen more than three now. I’ve been in astronomy long enough that, anytime you see more than three, that’s awesome; but in terms of just seeing a galaxy clear out the world around it and no longer be doing that, that’s what I’m not sure we’ve seen. So –
Fraser: Right, right.
Dr. Gay: – we’ve seen a light echoing, but what’s awesome to me, and what I can’t wait for us to see the relics of is supernova explosions in just the right format, specifically gamma ray bursts. They have the shockwave going out in a sphere that may be interrupted by planetary disks and things, but generally going on in a sphere or bipolar outflows that is caused by the star itself exploding. But what we’re now starting to think is, in some cases where you have a nearby companion, the material that’s getting shot off can quickly spin up in a disk as it gets the companion creating this gamma ray burst. That’s only one of several explanations of a gamma ray burst. It’s the one I like the best.
Dr. Gay: Whatever the source of these gamma ray bursts is, you end up with tremendous high-energy jets that last very briefly and are capable, we think, of destroying planets several hundred light years away.
Fraser: Several tens of thousands. It will cause damage.
Dr. Gay: Is it 7,000 or 700? It will cause damage.
Fraser: It will cause damage. If it’s a few hundred light years away, it will destroy planets –
Dr. Gay: You’ll have roasted rock left behind.
Fraser: – right. But even if you’re halfway across the galaxy –
Dr. Gay: Ionizing radiation.
Fraser: – the ionizing radiation will blast away your ozone layer and everybody will get cancer and die.
Dr. Gay: But the planet will recover. We believe we have experienced –
Dr. Gay: – that in our past, the world is still here. Within a few hundred, you basically have Mercury left behind.
Fraser: And good news, there are no stars capable of going gamma ray burst in our vicinity that we know of. All right, so you talk about like one impact of a gamma ray burst with jets, but what influence, what effect do these jets have on their galaxies and the areas surrounding them.
Dr. Gay: You clearly don’t wanna get into one of them because they’re a source of ionizing radiation. They are giving off loud radio signals. So, communications near them is certainly going to be difficult, but in general it’s that disk that is generating them that I’d be afraid of because that is giving off a truly amazing amount of light in the center of a galaxy. The jet is going out of the galaxy, so you can sit there and go, “Oh, beautiful”. But that accretion disk in the galaxy you’re living in that is generating this disk, for super massive black hole ones, there’s another word for that disk. That’s a quasi-stellar object.
Dr. Gay: And so, the light from that has the potential to be dangerous to fairly large distances within the galaxy it’s in.
Fraser: So, I did research into this. If the super massive black hole at the middle of the Milky Way turned into a quasar, you wouldn’t see it with the unaided eye. You would need a very powerful telescope to even detect it. Because you wouldn’t staring down the throat of it the way that we were with –
Dr. Gay: Right.
Fraser: – a lot of other ones, right? To see it from the side, it actually wouldn’t be significantly brighter. But I guess what I’m getting at is you’ve got this jet of material that’s flying out kind of like a fountain, like that original analogy, and then this material is falling back down into the galaxy in some cases. What influence does that have on the galaxy?
Dr. Gay: So, the material coming back, just like anytime you get a shockwave, it can trigger nice, happy, sitting around molecular clouds to collapse down and possibly form stars. And I just wanna go back to what you said. With your eye, all the dust in our Milky Way’s gonna hide that accretion disk from us. The trickstery bit gets to be, there are other wavelengths of light that you’re gonna have blinding light for other instruments coming out and –
Dr. Gay: – this is where the sky again is so different radio versus optical.
Fraser: And there was another piece of research that I saw, and I don’t know if you had seen this one, that a galaxy had gotten hit with the jet of another nearby galaxy?
Dr. Gay: No.
Fraser: And they think that it might in some cases maybe cause an increase in star formation, but maybe even quenching star formation, which was quite fascinating.
Dr. Gay: If you blast apart areas that are forming dust, then you’ve added enough energy in that, instead of the material collapsing, the material’s gonna dissipate in the face of this wind.
Dr. Gay: You shouldn’t smoke, people. You should not smoke.
Dr. Gay: But if you imagine exhaling smoke, you have a nice little cloud here, but on a windy day, you never get the cloud of smoke.
Dr. Gay: Let’s go with a cold day. You exhale your breath. If there’s no wind you can see your breath, if there’s a lot of wind you have no chance to see your breath.
Fraser: And you need a nice, cold cloud of gas to form a star.
Dr. Gay: Exactly.
Fraser: If energy is getting pumped into it and it’s heating up –
Dr. Gay: It can’t collapse.
Fraser: – then it can’t collapse. You may have the otherproblem. But the other thing that’s interesting as well is that because you’ve got fusion going on in some of the cases of these, you’ve got heavier elements that are actually being generated and thrown out into space. And so, it might actually be another source for heavier –
Dr. Gay: Hemorrhage.
Fraser: – elements that could contribute and enrich to some of these solar nebulae. So, it’s a field that is really wide open for discovery, both their underlying cause as well as their influence and their effect. It could be everywhere from no effect to life might not exist without these jets. So, more research is needed.
Dr. Gay: And just you can track the history of stellar vampirism. So, while we may not have vampires on our planet, there are vampires among the stars. And if they aren’t continually sucking at their neighbors, they will have puffy jets. So, I’m just gonna leave you with that thought, that there are puffy jets of non-continuous feeders out there.
Fraser: Very cool. All right. Thanks, Pamela.
Dr. Gay: Okay, thank you, Fraser. And it’s time for me to do that thing where we thank all the amazing humans out there that make this possible. Every time I say, “I’m sorry, Rich. You’re gonna have to edit,” well, it’s because of these people that we can pay Rich and Ally and Beth and the whole team of people. Oh my goodness, Nancy, thank you for keeping us lined up.
This week, I wanna thank Jeremy Kerwin, Rob Cuffe, Harald Bardenhagen, Alex Cohen, Rando, David Gates, marco iarossi, Nicky Lynch, Daniel Loosli, Phillip Walker, Scott Bieber, Brian P. Cox, David, Matthew Horstman, Matthias Heyden, Justin Proctor, Disasterina, Gregory Singleton, Nial Bruce, The Lonely Sand Person, Jim Schooler, Jeff Willson, Tim McMackin, Paul Hayden, Cooper, Nate Detwiler, Eran Segev, Allan Mohn, Benjamin Müller, Steven Shewalter, Omar Del Rivero, Kenneth Ryan, Karthik Venkatraman, Alex Raine, Paul D. Disney, Micheal Regan, Don Mundis and Dean McDaniel.
And Rob Cuffe, thank you for giving me a pronunciation guide for your name. And thank you so much to all of you. If you too wish to have me vaguely pronounce your name correctly in one of our episodes – I’m so sorry – please join us on Patreon. If you would like your name pronounced correctly, add a pronunciation guide.
Dr. Gay: Thank you.
Fraser: All right. Thanks, Pamela. We’ll see you next week.
Dr. Gay: Buh-bye. Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a Creative Commons Attribution’s License. So, love it, share it, and remix it, but please credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, astronomycast.com. This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep this show going, please consider joining our community at patreon.com/astronomycast. Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. We’re so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.