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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.

Download MP3 | Show Notes | Transcript

Show Notes

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 to Make an Engine from a Battery, Wire and a Magnet (WikiHow)

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)

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Transcript

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.

Fraser:                         Right.  

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.