Astronomy Cast Ep. 784: Pulsar-Powered Science

It's the 365 days of astronomy podgang coming in three two one

It's the 365 days of astronomy podgang coming in three two

Astronomycast Episode 784

Pulsar-powered science

Welcome to Astronomycast, 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

I'm Fraser Cain, I'm the publisher of the University of A with me as always

This is Dr. Pamela Gay, senior scientist

For the Planetary Science Institute and the Director of CosmoQuest

Hey Pamela, how you doing?

I am well and I need to know what stage of spring you have encountered already

Snow and snow drops, so both

So winter is still here and yet we also are starting to get snow drops and other early spring flowers coming through

Excellent

Yeah, but I want to just mention briefly that many years ago we were gifted telescopes by Dustin Gibson

And both of us found the telescopes complicated, the software unusable, and we both use them for hanging clothes, I'm sure

We use the refractor that he gave us and a completely different mount using an eyepiece and not a camera system

Well, I finally was able to get the telescope operational, get the camera going, get the mount going, and make it all fully functional from my laptop

And so I'm able to sit

Mac laptop, that was the thing that foiled me is like

So I had to buy, I had a Raspberry Pi kicking around and I was able to install all the control software on the Raspberry Pi

And then I bolt that to the telescope mount, I put the thing outside and then I control the telescope from my Mac

Well, I'm just sitting inside watching the telescope move around and it's still herky jerky and I've got all kinds of problems to it

But it is kind of amazing to be able to use this telescope in sort of the what it was originally intended to do

And hopefully people very soon will get a chance to just see what I have come up with because I think you're all going to really love it

And in fact, I've ordered a color like a much fancier, newer, more powerful color camera for the telescopes so that I can do live streaming with it in this really cool environment

But it's kind of amazing, you've got these old projects that you thought you were going to work on and you just never had the time to do it

And now I'm working my way through some of these projects that were beyond my ability and hopefully people will get a chance to see this sometime soon

So stay tuned for that

Pulse Arts are dead stars and fascinating in their own right but astronomers can use their predictable rotation for exploring the cosmos in a series of amazing ways

We can detect gravitational waves, navigate the solar system, test general relativity and find exoplanets

So before we talk about how we can do the science with Pulse Arts, we should probably get to what are Pulse Arts, although we did a whole episode

So go listen to that one first, no, we're going to give you the short version of what Pulse Arts are

All right, short version

Take a massive star, something probably more than tensile or masses in size

Let it go through its life, at some point it runs out of fusible materials in its core

When this happens, core collapses, core is massive enough that the electrons and protons cannot hold each other apart

They combine, they become neutrons

We are left with a core of neutrons, a supernova explosion

Pushes all the outer layers of the star out wherever they want to go, Crab Nebula is a great one to go look at

Take a look at that and when these things are young, they are fast rotating, they have powerful magnetic fields

The magnetic fields are not perfectly aligned with the rotational axis

So what you end up with is as it goes round and round, the two poles, actually, of the pulsar go flashing by like a lighthouse

And it is the material coming out of the poles of the magnetic field that we see as pulses, these are super easy to find in radio

And they get their name because they are literally going beep, beep, beep, beep in radio, except sometimes in milliseconds

Yeah, but were they originally designated as like LGM, so was that, yeah, little green man, the first pulsars were found

Are these aliens trying to communicate with us?

So Jocelyn Bell Bernal discovered these initially

And it was part of her dissertation work, she did the engineering of the system

There is a fabulous recording of her with her British accent, her advisor with his Texas accent when they made this discovery

And they didn't think it was a little green man, they just didn't know what the heck it was at that time

And it eventually led to this revolution in how we understand the magnetic fields associated with these extremely dense little objects

So before we get on to how we can use her science, you sort of made a bunch of comments and I wanted to sort of get to the wise of these things

So they spin rapidly, why?

You take roughly more than two solar masses of material that was fairly big, you collapse it down to something roughly a diameter of Manhattan Island

And it is like an ice skater with arms the width of our planet pulling them in around her body

And that body then shrinks until it's the size of like spaghetti

Right

And so it's zipping around really really fast because conservation of angular momentum

They generate powerful magnetic fields, why?

So they have charged material inside of them and charged material that is rotating generates magnetic fields

It's not entirely clear how you end up with the rotational axis in the magnetic field axis

That was my next wise

Yeah, I'm not going to try and answer that, there are people who study magnetohydrodynamics which is fun to say, less fun to calculate

Those people, they're working on it, they're working on it

And so imagine this thing is spinning like a little sphere really fast

And when I see it fast like 700 times a second, like it's crazy, how fast these things are spinning

And then there is this magnetic field beams coming off of this thing that is also rotating

Not necessarily aligned with the axis of rotation that is sweeping past you like cones

As you said, like a lighthouse that you can use

Okay, so that's the, they are wondrous and you know can fill a lifetime's worth of science just to study them

But now we can use them for scientific experiments

So then how accurate are these things? Why can we use them for making these kinds of measurements?

If you put a atomic clock on a shelf next to a receiver for a radio telescope receiver pointed at a pulsar

The pulses, except for the rare instances where these things glitch because the magnetic field to rearrange themselves

Ignoring the occasional glitch, the overall accuracy of a pulsar is better than the atomic clock with the cesium oscillations

That's crazy

Uh-huh

Yeah, so you don't even need atomic clocks, you just need pulsars

Exactly, and this is what makes them so interesting for so many different kinds of science

And because they're doing their thing in the radio, we can look through a whole lot of gas and dust and see them even when we can't see them

Right, so then how do we use them as instruments to measure?

What is the kind of a core physics phenomenon that these all rely on?

I'm assuming like the movement of the pulsar in some way

It's the Doppler shifting, so when you have this fast rotating object that is ticking like a clock

When it's moving away from you, each pulse has to travel a little bit further than the one before it, so the pulses appear to spread out

When it's moving towards you, each pulse doesn't have to travel as far, so they're compressed, they're blue shifted

This change in the timing allows us to very, very precisely get a handle on changes in their motion

This is actually something where my senior year of high school, where being the nerd that I am, I was working at Haystack Observatory

My advisor came running into the computer next to the laser printer, which was where I worked, full ozone onto me

He came running in, closed the door, and he's like, okay, and he just proceeded to download into my brain the discovery of a pulsar planet that was actually real

And originally there'd been one that was found earlier that they forgot to correct for the Earth's motion

So they ended up discovering a planet that weirdly had the same period as the Earth does and then people realized, oh, we screwed up

The second time, the second time, they did everything correctly and it worked

And when you find structures around something that has undergone a supernova, that just makes it even cooler

Because these are literally the remnants of death star levels of destruction

And so like physically we've got this pulsar, this dead star, and it has planets, and they're not very massive, going around it

But the gravity of the planets are pulling the pulsar back and forth, and so you're measuring that doppler shift on the radio waves that are coming to you

Allowing you to tease out the masses of the planets that are going around the pulsar

That is really impressive, and it's also very frustrating for me as a science communicator

And I'm sure you go through this as well, which is you say the first planet to ever found was in 1995 with the Pegasite

And then we'll go, well, actually the first planets that were ever found were around a pulsar and you're like, yes, sorry

You always have to put in this disclaimer, the first planets ever found orbiting around a sunlight star, or a mean sequence star, was Pegasite blah blah blah

Right? If you won Pegasite B, but the first planet ever found orbiting around a pulsar, and that is just, it always drives me crazy

They just weren't planets in the sense that we're used to, and they weren't orbiting a star in any sense of the word

So a stellar remnant with asteroid-like things that came out of a supernova

We just sort of set that on a shelf and go, that's an exception, but yes, it's very, very cool

And think about the weirdness that it has whatever it is, like a Mars-sized, like several planets orbiting around it

This star exploded, and yet it has planets

Yes, it's awesome

Yeah, and yet weirdly, we haven't found many other examples of this, which you would think you would find lots more

You would think, but when you start to realize pulsars are very young, they're very hot

The heat is capable of destroying solid objects very effectively

There was a recent paper, probably six months old now, looking at white dwarfs and their ability to ionize planets

You have to have material that survived the heat, survived the explosion, or migrated in, and in the time scales, just it's going to be rare

Yeah, so that's just one example, and I think that's great because it gives you that sort of basic

The tool is always the same, which is that you're calculating that doppler shift to discover something about the environment that the pulsar is in

So let's pick another one

So Joseph Taylor and Russell Holsa, back in 1974, were studying binary systems containing pulsars

And they noticed that one of these systems that was showing the variations indicative of the pulsar being a binary system

And the companion was not visible

It was also showing a change in periodicity over time that appeared to be radiation of gravitational energy, which is something at that point was strictly theoretical

Predicted by Einstein

Predicted by Einstein, not yet seen

But ultimately what they were able to figure out was pulsar B1913 plus 16 was in orbit with a stellar mass black hole

And over time these two objects were radiating away gravitational energy

And Taylor and Holsa went on to get the Nobel Prize in 1993 for work that also was just a graduate student while doing

And I just love that they went, they proved something

And it literally took a generation before everyone was like, okay, we got you, we agree this was actually, yeah

Here's the Nobel Prize

It was just such cool work and we found other systems like that since then

And this was the first evidence that gravitational waves should be out there

And what I love is this Nobel Prize was given out at the same time that so much energy was going into building LIGO

So that we could start directly measuring gravitational waves instead of just seeing them from how their energy changed orbits

So it's almost the same thing, which is that whenever I say, oh yeah, the first detection of gravitational waves was from LIGO in 2015 and people go, well, actually

The first gravitational waves were confirmed by pulsars because we detected the loss of energy, the loss of orbital momentum

Caused by the pulsar and its companion, bleeding off energy into gravitational waves

Okay, yeah, you're right, you're right first, so now I have to always disclaim that the first directly detected gravitational waves came from LIGO

But the first gravitational waves, yeah, found by pulsars, incredible, what else you got?

So there's the classic idea that at some point in the future we're going to need to be able to navigate through the galaxy

At least one hopes and one way to do this is to have essentially radio eyes on the sky that monitor in all directions where the pulsars are

And what is, so you look for the pulsars, you measure the periodicity and you measure how they're shift

And the grid on the sky of three dimensionally of where pulsars are located is set by where these things are in their own orbits around the galaxy

The rate that they appear will be blue shifted or red shifted by the navigators motion through the galaxy

And this is a way to get a unique solution to how you're moving and where you're spatially located

And this is not just theoretical, this has been demonstrated, so there is a pulsar detection system on the International Space Station

And they were able to use its ability to track its position based on pulsars to within tens of meters

So it was able to accurately measure its movement in a way that is independent from the other methods that are used to navigate the International Space Station

That if you were dropped randomly in the Milky Way, if you found a bunch of pulsars, you would be able to find out where you are

If you were moving, you would be able to know the direction that you're moving purely based on the blue shift red shift from the various pulsars

On the, is it the voyagers or is it the pioneers? But there is a plaque, I feel like it's on the voyagers

It's on the voyagers?

It's on the voyagers that shows where the solar system is based on known pulsars in various directions

And so any alien civilization can come and destroy planet Earth and steal our resources because we gave them a map to our home

Thanks to pulsars, but and so there are people that are working on these essentially navigation boxes that you will put on all spacecraft

That will then just use pulsars to know where they are

And so the spacecraft could could go to sleep, wake up, look around, measure all the pulsars around it and go, oh I know where I am

Which is a level of accuracy that it can make deorbit burns and do the kinds of spacecraft maneuvering that would be required without depending on communication from Earth

Thanks pulsars

Okay, let's talk more about gravitational waves

I was hoping that was where you would go

And we'll talk about the background gravitational waves

So once you understand that the timing of these things can be affected by any change in distance

And you start realizing monitoring these things over time is actually super useful

You can start to imagine, alright, we're monitoring pulsars in every direction and large enough gravitational waves moving through the universe

We'll be able to stretching compact the distance between us and pulsars in a way that we will see as timing changes and we'll see those timing changes as the gravitational waves sweep through the galaxy

Now I'm going to give you a very simplified picture here

So you can imagine in the perfect setup there is a massive gravitational wave moving through the Milky Way galaxy

And we initially see changes in stars in that direction at great distance and then we see it from closer and closer and then we start seeing it from behind us

And so you literally see these timing differences propagate across the galaxy in a way that allows you to say, aha!

So a gravitational wave came from over there and it's headed in that direction

Now the problem is our universe is vast, it has interesting stuff going on all the time and all the different directions

And so what we see instead is the pulsar timing array is out there looking in all directions, looking at the noise in the pulsar timing

And using that noise to say, is this consistent with gravitational waves wrecking very, very minor havoc on the distances of these pulsars?

Right, so the description is always like it's buoys floating on the ocean and yes if a tsunami went by then the buoy would probably rise up and fall back down

But instead you're just watching the buoy from all of the collective wave action of everything is going around it and that statistically over 15 years looking at dozens of pulsars, astronomers have confirmed

The noise is consistent with gravitational wave detections

Right, from merging supermassive black holes

Yes, which is awesome

Awesome, yeah, that we know that supermassive black holes are merging, we can't detect them directly, it's beyond the capability of LIGO and other ground-based detectives

But in aggregate, their mergers are sending out gravitational waves that are causing the pulsars to bob around in a way that tells you that this is happening

There's a paper that just came out yesterday that we're reporting on, yes, so astronomers attempted to confirm if they could detect any single gravitational wave event from supermassive black holes and they failed

But as always when you fail you said constraints, so the longer the pulsar timing array operates for 25 years, 50 years, that we may get these individual events starting to get teased out if the event is strong enough, close enough, significant enough that we may actually eventually get individual colliding supermassive black holes from the pulsar timing unit

It hasn't happened, you only get it in aggregate, you don't get it in as a specific event, still amazing, amazing

I think we have one last thing to bring up

Right

And that is a recent candidate discovery from the breakthrough folks where they were out there looking for little green men

So this was research that was designed specifically to look for civilizations, technosignatures, and they were looking towards the center of our galaxy

So this is breakthrough listen, they're looking within 1.4 arc minutes of the galactic core

And this is a hairy region to look at because there's our own supermassive black holes magnetic field that is making a massive any radio signals that are coming from that direction

They found within that region in 1 hour of data a candidate pulsar that if confirmed could be down in the center of our galaxy where it would be under the influence of all the different things in the center of the galaxy

And it could show relativistic effects in how it's timing changes

Now there's a whole lot of caveats from what I just said because while they saw it in 1 hour of data they weren't able to confirm it in other data sets

They're going to go back, they're going to take more data

And the concern I saw expressed in the paper was there is the potential that interactions with other local magnetic fields

could cause this thing to go in and out of view which is just a big furry mess to imagine

And again magneto hydrodynamics is fun to say hard to do

Hard to do

And so it's unclear if this is real or not but we can use pulsars to measure relativistic situations to measure motions in small places to do all sorts of cool physics

They are literally sitting there as metronomes demanding our attention in ways that allow us to measure their motion extraordinarily precisely

So you actually did leave out a bunch

It's true

Which just shows how useful pulsars are so I want to give just a few more quickly

One is just the analysis of pulsars themselves so you know we've learned recently that pulsars do have these glitches

You mentioned like you got to you know ignoring the glitches

Well the glitches are important and that they tell us just by measuring the spin rate of the pulsars that you can detect when they go through these glitches

And it appears that even though they are balls of compressed material that is just like seems like can go no further

They actually can't and that they crunch and crumble and have a little mini earthquakes on them which is very impressive

They are the best way to measure the mass of a star because you've got a star and a pulsar that are in orbit around each other

That the that this you know this atomic clock level precision of the measurement of the orbit gives you a precise measurement of the mass of the star that the pulsar is orbiting and there is no better way to do that

It's a standard homework assignment

Yeah there you go yeah exactly you got a pulsar here's the orbit here's the change in the doppelgschift tell me the mass of its companion star

And in fact telling the mass of stars is actually really hard

So every now and then when you get a pulsar in orbit around one of these things you have this beautiful gift from the cosmos

And then the other thing is just that the radio waves that are coming off of the pulsars are going through whatever is the material that is between us and them

And so they've been used to probe the interstellar medium, the intergalactic medium

The be able to as you mentioned the detection of a pulsar close to the galactic center these are places that are hard to observe visually but radio waves can pierce through them

And that the more of this material that the radio waves are going through you get this probe of the intervening material and pulsars are very useful for this

So pulsars are just this incredible gift from the cosmos for astronomers to learn more about the cosmos and we are so grateful

And conservation of angular momentum is why we have them

Yeah

And they're slowing down over time and that allows us to like get it evolution and yeah they're just cool because they're weird but they're precise in their weirdness

Yes

So more of that please, more of that

Yep

Alright, thanks Bill

Thank you Fraser and thank you so much to all of our patrons out there on patreon.com slash astronomy cast

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