Pulsars 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. Pulsars are the time keepers of the sky, with their precise ticking allowing researchers to track gravitational waves, find exotic planets, and study weird relativistic effects. Come learn about how pulsars can be used to explore our universe.
Show Notes* What are Pulsars? * Formation of Pulsars (Neutron Star Remnants) * Pulsar Rotation and Magnetic Fields * Pulsars as Cosmic Clocks * Doppler Shifts in Pulsar Timing * Discovery of the First Exoplanets Around a Pulsar * Rarity of Pulsar Planet Systems * Binary Pulsars and Evidence for Gravitational Waves * Nobel Prize Discovery (Taylor & Hulse) * Pulsar Timing Arrays for Gravitational Wave Detection * Pulsars as Galactic Navigation Beacons * ISS Pulsar Navigation Experiment * Pulsar Glitches and Neutron Star “Starquakes” * Measuring Stellar Mass with Pulsar Binaries * Studying the Interstellar Medium with Pulsars * Pulsars as Tools for Fundamental Physics
Transcript'Fraser Cain:
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 Universe Today. With me, as always, is Dr. Pamela Gay, Senior Scientist for the Planetary Science Institute and the Director of CosmoQuest. Hey Pamela, how are you doing?
Dr. Pamela Gay:
I am well, and I need to know what stage of spring you have encountered already.
Fraser Cain:
Snow and snowdrops. So both. So winter is still here, and yet we also are starting to get, you know, snowdrops and other early spring flowers coming through.
Excellent. Yeah. But I wanted 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 used them for hanging clothes, I'm sure.
We used the refractor that he gave us on a completely different mount, using an eyepiece and not the 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.
Dr. Pamela Gay:
And so now I'm able to sit- A Mac laptop.
Fraser Cain:
That was the thing that foiled me, is like- So I had to buy a, 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 while 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 what it was originally intended to do.
And hopefully, people very soon will get a chance to just see what I've come up with, because I think you're all going to really love it. In fact, I've ordered a color, like a much fancier, newer, more powerful color camera for the telescope 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.
Pulsars 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 pulsars, we should probably get to what are pulsars, although, you know, we did a whole episode, so go listen to that one first.
No, we're going to give you the short version of what pulsars are.
Dr. Pamela Gay:
All right, short version. Take a massive star, something probably more than 10 solar 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 pole of 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're literally going beep, beep, beep, beep, beep in radio, except sometimes in milliseconds.
Fraser Cain:
Yeah, but weren't they originally designated as like LGMs or was that, yeah, little green men, the first pulsars were found, people thought, are these aliens trying to communicate with us?
Dr. Pamela Gay:
So Jocelyn Bell Burnell 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 men, 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.
Fraser Cain:
So before we get on to how we can use your science, you sort of made a bunch of comments and I wanted to sort of get to the whys of these things. So they spin rapidly, why?
Dr. Pamela Gay:
You take roughly more than two solar masses of material that was fairly big, you collapse it down to something roughly the 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.
And so it's zipping around really, really fast because conservation of angular momentum.
Fraser Cain:
They generate powerful magnetic fields, why?
Dr. Pamela Gay:
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 and the magnetic field axis.
Fraser Cain:
That was my next why.
Dr. Pamela Gay:
Yeah, 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.
Fraser Cain:
And so imagine this thing is spinning like a little sphere really fast. And when I say fast, like 700 times a second. Like it's crazy how fast these things are spinning.
Spinning really fast. And then there is this magnetic field beams coming off of this thing that is also rotating aligned with the axis of rotation that is sweeping past you like cones, as you said, like a lighthouse that you can then use. So they are wondrous and 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?
Dr. Pamela Gay:
If you put a atomic clock on a shelf next to a receiver for a radio, 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.
Fraser Cain:
Yeah. So, so you don't even need atomic clocks. You just need pulsars.
Dr. Pamela Gay:
Exactly. And, 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.
Fraser Cain:
Right. So then how do we use them as instruments to, to measure? What is the kind of the core physics phenomenon that, that these all rely on?
I'm assuming like the movement of the pulsar in some way.
Dr. Pamela Gay:
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, is actually something where my senior year of high school, where the 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 unto 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.
Fraser Cain:
Dr. Pamela Gay:
And, 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 like death star levels of destruction.
Fraser Cain:
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, 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, that is really impressive.
And, 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 planets ever found was in 1995 with the Pegasi, right? And then people go, well, actually the first planets that were ever found were around a pulsar.
And you're like, yes, sorry. So you always have to put in this disclaimer, the first planets ever found orbiting around a sun-like star or a main sequence star was Pegasi blah, blah, blah, right? 51 Pegasi b, but the first planets ever found were orbiting around a pulsar.
And that is just, it's always drives me crazy.
Dr. Pamela Gay:
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.
Fraser Cain:
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.
Dr. Pamela Gay:
Yes.
Fraser Cain:
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.
Dr. Pamela Gay:
You would think, but when you start to realize pulsars are very young, they're very hot, but 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 the time scales.
It's going to be rare.
Fraser Cain:
Yeah. So that's just one example and I think that's great because it gives you a 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.
Dr. Pamela Gay:
So Joseph Taylor and Russell Hulsa, 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 in a binary system and the companion was not visible, 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.
Fraser Cain:
Predicted by Einstein.
Dr. Pamela Gay:
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 Hulsa went on to get the Nobel Prize in 1993 for work that Hulsa 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, 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.
Fraser Cain:
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, pulsar, no, pulsars again, right? 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.
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?
Dr. Pamela Gay:
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 those shift and the grid on the sky 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 navigator's 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.
Fraser Cain:
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. 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 go to sleep, wake up, look around, measure all the pulsars around it and go, oh, I know where I am.
To 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, 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.
Dr. Pamela Gay:
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, all right, we're monitoring pulsars in every direction and large enough gravitational waves moving through the universe, we'll be able to stretch and 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, sweeps 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 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 in 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 to these pulsars?
Fraser Cain:
Right, right. So it's, you know, 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 that's going around it and that statistically over 15 years looking at dozens of pulsars, astronomers have confirmed what?
Dr. Pamela Gay:
Noise is consistent with gravitational wave detections.
Fraser Cain:
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 detectors. It's the wrong scale. 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. Oh, I haven't seen it yet. That we're reporting on.
Yeah, 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 set 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 array.
But so far, it hasn't happened. You only get it in aggregate. You don't get it as a specific event.
Still, amazing. Amazing.
Dr. Pamela Gay:
I think we have one last thing to bring up.
Fraser Cain:
Right.
Dr. Pamela Gay:
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 civilization's 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 mess of any radio signals that are coming from that direction. They found within that region in one 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 its timing changes. Now, there's a whole lot of caveats from what I just said because while they saw it in one 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. Again, magnetohydrodynamics is fun to say, hard to do.
Yeah, 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 because they are literally sitting there as metronomes demanding our attention in ways that allow us to measure their motion extraordinarily precisely.
Fraser Cain:
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. So, one is just the analysis of pulsars themselves. So, we've learned recently that pulsars do have these glitches.
You mentioned 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 then it appears that even though they are balls of compressed material that seems like it can go no further, they actually can and that they crunch and crumble and have little mini earthquakes on them, which is very impressive. They are the best way to measure the mass of a star because when you've got a star and a pulsar that are in orbit around each other, that 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's no better way to do that.
Dr. Pamela Gay:
It's a standard homework assignment.
Fraser Cain:
Yeah, yeah, there you go. Yeah, exactly. You've got a pulsar, here's the orbit, here's the change in the Doppler shift, tell me the mass of its companion star.
And in fact, telling the mass of stars is actually really hard. And 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, 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.
Dr. Pamela Gay:
And conservation of angular momentum is why we have them.
Fraser Cain:
Yeah.
Dr. Pamela Gay:
And they're slowing down over time. And that allows us to like get at evolution. And yeah, they're just cool because they're weird, but they're precise in their weirdness.
Fraser Cain:
Yes.
Dr. Pamela Gay:
So, more of that, please.
Fraser Cain:
Yes.
Dr. Pamela Gay:
More of that.
Fraser Cain:
Yep. All right. Thanks, Bella.
Dr. Pamela Gay:
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Fraser Cain:
Thanks everyone, and we will see you next week. Bye.
Dr. Pamela Gay:
Bye everyone
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