Pulsars are the rapidly spinning degenerate husks of dead stars, turning hundreds of times a second. But they’re also handy clocks, spinning with such certainty and accuracy that astronomers can use them for all kinds of stuff. We might even use them to navigate the cosmos.
Daily Space (CosmoQuest)
Dark Energy, Dark Matter (NASA)
Pulsar (Swinburne University)
What Are Pulsars? (Space.com)
What Is a Supernova? (NASA)
Neutron Star (Swinburne University)
PODCAST: Ep. 158: Pulsars (Astronomy Cast)
PODCAST: Ep. 38: Neutron Stars and their Exotic Cousins (Astronomy Cast)
Foundation (Apple TV)
Doppler Shift (Swinburne University)
PSR B1257+12 b (NASA)
What Is A Star? (Sky & Telescope)
Haystack Observatory (MIT)
The Nobel Prize in Physics 1974 (The Nobel Prize)
Catching frame dragging in action (Cosmos)
Gravitational Waves (LIGO)
Stochastic Gravitational Waves (LIGO Science Collaboration)
VIDEO: Using Pulsars to Detect Gravitational Waves: with Boris Goncharov (Fraser Cain)
The Interstellar Medium (University of New Hampshire)
Magnetar (Swinburne University)
Transcriptions provided by GMR Transcription Services
Fraser Cain: Astronomy Cast Episode 631, All the Uses of Pulsars. 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. Pamala Gay, a senior scientist for the Planetary Science Institute and the director of Cosmo Quest. Hey Pamala, how you doin’?
Dr. Pamala Gay: I am doing well. We’ve been working with Daily Space to get things going with a new format that we’re super proud of and we are now bringing a mix of rockets and science –
Fraser Cain: Nice.
Dr. Pamala Gay: – to our growing staff. So, if you’re not listening to Daily Space podcast, listen to the Daily Space podcast.
Fraser Cain: You really are creating a media empire. You’re creating a formidable competitor to Universe Today and I couldn’t be happier.
Dr. Pamala Gay: No, we are a collaborator.
Fraser Cain: Well yeah, no. Exactly, we’ve said this many times before that space is big and there’s room for all of this and so I’m so happy that you guys are doing this. More genuine, good space science news reporting please. Pulsars are the rapidly spinning degenerate husks of dead stars turning hundreds of times a second, but they’re also handy clocks. Spinning with such certainty and accuracy that astronomers can use them for all kinds of stuff. We might even use them to navigate the cosmos. All right, Pamela, this is another one of your episodes. You queued up this topic and I love it. I sort of have this saying that there are things in the universe that we don’t understand, but we use them as telescopes, right?
Like dark matter. We know what dark matter is, but we use it as a telescope.
Dr. Pamala Gay: Oh, yeah. Yeah.
Fraser Cain: Right? And pulsars, you know, we’re still not entirely sure everything that’s going on with a pulsar, but boy can we use them for all kinds of other science; mostly to prove Einstein right.
Dr. Pamala Gay: This is true.
Fraser Cain: So, before we get into how we use them, I guess we should give people a refresher on what is a pulsar.
Dr. Pamala Gay: Okay, so stars die in a variety of ways. So, they’re directly related to how much mass they have. Star like our sun that’s smaller is going to eventually have a white dwarf left behind and puff off its outer atmosphere. A white dwarf is massive enough that it collapses down to be a mixture of electrons and protons. It’s normal stuff but in a very special, highly dense version.
Now, if you dump too much matter onto that white dwarf or you start with an even larger amount of mass, when it gravitationally starts to collapse down because fusion’s no longer going on in the center of a star, electrons and protons can only push each other apart because the gravity is exceeding the push of the electrons and the protons and they smash together, release a whole bunch of energy, and become neutrons. And so, when stars that are the next size tier up from the sun died, they die of supernovae, their cores collapse down from neutron stars and their outer layers try to crash down, undergo massive nuclear reactions, explode back out and form a fabulous nebula around the system.
Fraser Cain: Right. And so, neutron stars are stars many time the mass of the sun, the big ones, explode a supernova, create this beautiful nebulae, and then collapse down forcing the protons and the electrons into the exact same space, turning them into neutrons. It’s a thing they don’t normally wanna do, but the gravity of the situation forces them to do it.
Dr. Pamala Gay: Yes.
Fraser Cain: Okay. All right.
Dr. Pamala Gay: Now, not all neutron stars are pulsars though.
Fraser Cain: Right, right. So, now we know what a neutron stars is and so what is a pulsar?
Dr. Pamala Gay: A pulsar is a special kind of neutron star that is rapidly spinning. The ionized material inside of it, when you moved charged particles in an ionized atom, one that’s missing some of its electrons, is a charged particle, you get that whole thing rotating fast enough and it generates a massive magnetic field. And for reasons we don’t completely understand, the magnetic field is not aligned with the rotation. So, you end up with an object that is spinning and as it spins its magnetic pulls will come in and out of view like the lights on a lighthouse or police car, ad these in and out of view beams from the magnetic field can carry radio waves our direction.
And if it’s spinning fast enough and is the right kind of pulsar, it even flashes x-rays in our direction.
Fraser Cain: Wow. Now, do we have to be staring down the beam? Are there pulsars out there that we can’t detect because they’re shooting in other directions, or are you sort of always seeing the blast of radio emissions as the pulsar’s turning?
Dr. Pamala Gay: So, it is very hard to get a geometry that is just right that you never see the pulse because pretty much the only way to have that happen is if somehow you have a pulsar that has the rotation and the magnetic field directly aligned and they are completely perpendicular to us. That is like the special case. Otherwise, because you have this tilted system that’s rotating, you may only catch one of the pulses. But it gets messy because if the system is rotated, you may have a swirling pulse that seems to be fairly constant because it’s beamed at you all the time, but you can tell something weird is definitely going on there and that weirdness is a pulsar.
Fraser Cain: Right. Okay, okay. So, if the pulsars are around us, they’re very bright, they’re spinning, we have the right equipment, we can detect them.
Dr. Pamala Gay: Yes.
Fraser Cain: And we’ve done whole episodes on pulsars, on neutron stars, on their twisted relatives, magnetars, so there’s lots of ways that a neutron star – and generally, right, pulsars are new neutron stars?
Dr. Pamala Gay: Yes. Over time they will slow down and stop being the awesomeness that they are.
Fraser Cain: Right. Well, different kind of awesomeness. All neutron stars are awesome. Okay, all right. So, we’re gonna now shift into talking about what they’re good for. Let’s focus in on this rotation and the blast of radiation. How do astronomers use this? This is the key to pulsars and so I’d like to talk a bit about just like why this is a thing, and then we can shift into what we actually use them for.
Dr. Pamala Gay: One of the first things that it was realized we could with them is use them as positional beacons essentially. Pulsars have fairly unique spin rates and as you look out across the sky, if you go okay, that one over there is going 112 times a second, that one over there is going 110 times a second, that one over there is going 342 times a second ‘cause it’s a crazy one –
Fraser Cain: A baby.
Dr. Pamala Gay: – you can – yeah baby. You can then use these to figure out your position and this was used to locate the sun within our galaxy on the voyager gold disk, and it was also used in the foundation series to allow one of the characters to figure out where the spacecraft she was on happened to be located.
Fraser Cain: All right, so how? I mean, I guess you know where these pulsars should be in the Milky Way, and then you see one off to the left and you see one off to the right and now you can kinda triangulate your position from these pulses. Cause I guess they give off a unique signature? There’s only one that’s gonna be blasting in exactly this way?
Dr. Pamala Gay: Well, we wish it was as precise as that. There are some that look exceedingly similar, but they’re varied enough where they are in the sky that it’s like when you’re in a large city and you navigate by skyscrapers. Sure, a couple of the skyscrapers might look kind of similar, but you know if it’s next to the Prudential building it’s gonna be this building, and if it’s next the Hancock it’s this building. So, you look around and you start from the this is the general distribution we know of them, and as you move through the galaxy, how they appear relative to one another in the sky is going to vary with our position relative to them.
So, you can get rid of the ambiguities by ignoring object that are similar enough and sticking to the ones that are dissimilar enough and then you can fine tune by going okay, these two may be similar but I know based on my rough estimate of where I am which ones they are. Let me fine tune everything.
Fraser Cain: And I guess to use like a – you talked about them being sorta similar to lighthouses. Like, to use an analogy, if you know that that lighthouse over there is turning twice a second and that one over there is turning once a second, then you know how far away you are along the coast just by measuring the angles to the lighthouses. You know you are, you know, 27 degrees south of the one that goes every two seconds and 31 degrees north of the one that’s going ever one second and so you can plot your position. And also get kind of a sense of then how far away you are. So, that’s helpful.
Dr. Pamala Gay: And you can also get your velocity out of these things relative to them.
Fraser Cain: Okay, so how does that work?
Dr. Pamala Gay: So, if you’re moving towards something that’s pulsing, you are encountering its pulses faster than they’re coming off the object, so you see the timing between them get shorter. If you’re running away from an object that is pulsing, each pulse has you travel a little bit farther to get to you.
Fraser Cain: Right.
Dr. Pamala Gay: So, you can use geometry and all of these different changes in the known pulse rates to get at your exact velocity through space.
Fraser Cain: And this is like the Doppler shift, like I guess same thing, if you were hearing a fire truck drive past you it sounds higher as it’s moving towards you and then it sounds lower as it’s moving away from you, but in this case you’re just measuring the red shift of the light. How precise is this?
Dr. Pamala Gay: It’s precise enough that the slight changes in pulsars – so, you can also move the pulsar. Don’t generally recommend that, but if you’re a planet it’s a good thing to do. And back in the early ‘90s, the very first star, admittedly a dead star ever found to have planets, which were admittedly not planets, that looked very planet like, was found orbiting a pulsar because the pulsar, as the planet went around, got tugged by the gravity of the planet just enough that we could see variations in its pulse that were cyclical.
Fraser Cain: You jumped to the next use of pulsars.
Dr. Pamala Gay: I’m sorry.
Fraser Cain: Which is fine. No, it’s fine, it’s exciting. I can’t wait to talk about it but something that maybe you might not know is that there’s an experiment onboard the International Space Station right now. They have the NICER project.
Dr. Pamala Gay: The NICER, yeah.
Fraser Cain: And they are detecting the Space Station’s position based purely on pulsars.
Dr. Pamala Gay: That I did not know. That is awesome.
Fraser Cain: Yeah. And it’s completely automated and the Chinese are doing a version of this as well. And so, you can envision a near future, and it won’t be long, where there will be a box that’s attached to ever single spacecraft that uses this pulsar navigation method to determine your location and position in the solar system no matter where you are and what you’re doing.
Dr. Pamala Gay: And this will make trying to figure out some of the enigmas that we eventually got to the bottom of, like what is the voyager anomaly, that much more interesting to study because we’ll have this external framework in which to judge what’s going on.
Fraser Cain: Yeah, and so incredibly, like with a good clock with the ability to detect pulsars, you will be able to know with enough precision – like, any spacecraft will never get lost ever again, which is absolutely incredible. Because right now they’ll communicate with a spacecraft that’s flying away with a radio telescope, and then they measure how long it takes for the signal to come back and that tells you have far away it is. And then they measure the angle to where the spacecraft is and that tells you where it is, but the spacecraft has no idea where it is. It’s only the ground-based observers who are telling it where it is.
But soon, they will know with that level of precision. It’s incredible. All right, you jumped into the next use, so let’s talk about it. We can use pulsars to find planets.
Dr. Pamala Gay: Very weird planets that are not really places that life’s going to be and are probably actually the remnants of supernova explosions and things that were destroyed. But nonetheless, we can find chunks of stuff that are large enough to move around a 2 solar mass star. Dead star, I have to keep correcting myself.
Fraser Cain: It’s a star.
Dr. Pamala Gay: With their gravity – well, no ‘cause the definition of a star is it has fusion going on in its core. This is where brown dwarfs –
Fraser Cain: Main sequence star, sure.
Dr. Pamala Gay: I said fusion.
Fraser Cain: Yeah, no I know, but a main sequence star is a kind of star. A bright dwarf is a kind of star, neutron star is a kind of star. It’s right there in the name.
Dr. Pamala Gay: All right, fine. Fine. Fine. So, we’re gonna call brown dwarf stars stars all the time now?
Fraser Cain: No, they’re not a kind of star. It’s not in the name.
Dr. Pamala Gay: All right. Anyways, anyways. So yeah, the way Doppler shifting can work we use with spectroscopy all the time to find planets. You look at the precise wavelength of a given spectral line and that exact color of the spectral line will get shifted redward and blueward as the star moves with respect to the planet going around it; they’re both going around a little tiny center of mass. Now, it’s a whole lot easier with a pulsar because you don’t have to do hyper high-resolution spectroscopy that requires you to have a really, really bright star.
Instead, you look at this object that is pulsing for you and you measure the timing of the pulses and the timing changes so that the pulses get closer together or further apart depending on if the object’s moving toward or away from you. This opens the possibility to find chunks of stuff orbiting very dead stars all over the galaxy if we just happen to look hard enough.
Fraser Cain: And the first planets were found around pulsars.
Dr. Pamala Gay: Were found this way. Do you remember where you were when this happened? I don’t know if you were an astronomy geek yet.
Fraser Cain: No, this was a little before my time as like doing this job. ‘Cause it was like in the ‘90s, right?
Dr. Pamala Gay: Well, we were in high school. You might have been a freshman in college when this happened.
Fraser Cain: Yeah, like early ‘90s, ’92?
Dr. Pamala Gay: It was ’91 I think. I’m pretty sure.
Fraser Cain: ’91, okay. Yeah, yeah, yeah. I was out of college, but just doing stuff, working.
Dr. Pamala Gay: Yeah, so I was a senior in high school and because I was a nerd, my high school job was reducing radio data working at Haystack Observatory and my advisor came in and he’s like, “Okay, I have to tell you this.” And he was so excited, and I didn’t know what was coming and it was a planet. And it’s always good to get a surprise planet in the afternoon.
Fraser Cain: I mean, that’s the famous one. And in fact, the pedants will catch me every time I talk about 51 Peg as being the first extra sort of planet discovered.
Dr. Pamala Gay: You have to always say a normal star.
Fraser Cain: ‘Cause I always have to say yes, I know some planets were found, were on a pulsar, but come on. Pulsars? Like that’s no way to live.
Dr. Pamala Gay: It’s true.
Fraser Cain: You want the hot Jupiter that’s heated up to thousands of degrees Celsius. Now that’s a planet.
Dr. Pamala Gay: Pick your death.
Fraser Cain: Yeah, exactly.
Dr. Pamala Gay: Pick your death.
Fraser Cain: Right, pick your death. But still, and so the point is like, yes, yes, the first planets ever discovered were around a pulsar. Have other planets been found around pulsars ‘cause I never have heard of others.
Dr. Pamala Gay: There’s not many that have been found this way. I wanna say there’s a handful of stars that have been found to have chunks of mass going around them. Now, the catch is the chunks of mass have to be really close and have a noticeable effect, just like looking for Doppler shifts on stars, you’re more likely to find those hot Jupiters. And in all likelihood, most stuff getting close to a pulsar’s going to get disrupted and destroyed.
Fraser Cain: Yeah, it’s a bad day.
Dr. Pamala Gay: So, it’s a small window of time in the lifetime of a short-lived object on galactic scales that you’re gonna be able to find these. So, the fact that we’ve found a few of them is pretty awesome.
Fraser Cain: All right. I’m assuming we use pulsars to prove Einstein right.
Dr. Pamala Gay: Yeah. Yeah, that’s a thing. It’s sort of like okay, homework assignment that has gotten so standard that it is literally something you give your students to do as a homework assignment. We have various systems where there are pulsars and other high mass objects. I believe there’s one pulsar pulsar system that has been found. That number may have increased since the last time I looked it up. And that pulsar pulsar system and the other pulsar heavy, high density object systems, you can actually see the radiation of gravity coming off of them.
Fraser Cain: Wow.
Dr. Pamala Gay: There was a Nobel Prize given for looking at the timing changes. And you can see the timing of the system change over time because the pulsar very precisely allows you to measure the timing of its orbit at the, in some cases, millisecond level. And if the orbit is evolving you can see that in the pulsar data.
Fraser Cain: And so, astronomers were able to measure how the orbit of the pulsars is changing over time in tiny little bits that perfectly match the loss of mass –
Dr. Pamala Gay: A loss of energy.
Fraser Cain: Loss of energy from gravitational waves. That’s crazy.
Dr. Pamala Gay: Yeah. And it’s just one more set of evidence where the hardest part is finding an example in the vastness of the sky of what you’re looking for, but once you find it is such a clean result that first you give someone a Nobel Prize and then you assign it as homework.
Fraser Cain: Right.
Dr. Pamala Gay: And I love these kinds of things where the prediction was there, the data was found, and it’s just clean and perfect.
Fraser Cain: And there’s been a bunch of these.
Dr. Pamala Gay: Yeah.
Fraser Cain: Like, they’ve been found doing frame dragging, other predictions made by it – so, they discovered gravitational waves, frame dragging, they’ve done a pile of ways, and in fact, the most recent one is using pulsars to detect the background gravitational waves of the Universe.
Dr. Pamala Gay: And this is where we have to look at two totally different things and get them clear in our head as two different things. The original Nobel Prize given for seeing the changes in orbits of degenerate objects, so this neutron star system, that was seeing gravitational energy that was lost through gravitational waves, but not directly seeing the gravitational waves.
Fraser Cain: Right.
Dr. Pamala Gay: There is now work that is looking for differences in the timing of pulses that are created by the expansion and contraction of spacetime due to gravitational waves propagating through the Universe. Where as we look out in all directions, we can see the change and pulses propagating through the sky. And this is hard research. The error bars are still squirrely but…
Fraser Cain: Yeah.
Dr. Pamala Gay: It’s the kind of thing where once we get the measurements precise enough we should be able to do this.
Fraser Cain: So, I did a fairly lengthy interview with one of the researchers working on this and it’s pretty amazing what they’ve discovered so far. It’s hard to tease out the specifics, but they think they’re detecting the mergers of super massive black holes.
Dr. Pamala Gay: Yeah.
Fraser Cain: It’s just they can’t – and it’s taken a long time. And the longer they do this the more they pick up these. And you can just imagine they’re like radio buoy floating on an ocean, they’re gently going up and down as the waves are passing them. And from our perspective, we see the pulsars shift a little towards us and then a little bit away from us in a way that’s detectable with lots of computers and large amounts of time. But you’re actually seeing just the ripples of the Universe. Incredible.
Dr. Pamala Gay: And I love that it’s like watching a tsunami move across the ocean. Yeah, that’s the perfect description.
Fraser Cain: Yeah, absolutely. All right, was there anything else that you think pulsars are good for using?
Dr. Pamala Gay: Well, so I am not deeply invested in the Interstellar medium other than as something that is annoying and gets in the way of understanding stars and stuff like that.
Fraser Cain: Right, it’s always dust.
Dr. Pamala Gay: Yeah. So, pulsars though can actually be used to study the Interstellar medium. As the pulse energy, which is spread out across a variety of different wavelengths, travels through the medium, the pulse will actually get dispersed and the amount that it is dispersed is directly related to how many electron interactions it has to deal with. So, we can measure what’s called column density of Interstellar medium using the dispersion of pulses from pulsars. So, this is another way of making the invisible visible and getting a better sense of how much that invisible stuff is disrupting our understanding of the more distant background Universe.
Fraser Cain: And so, you sort of assume how bright a pulsar should be and then you can just detect these interactions.
Dr. Pamala Gay: It’s literally the burst is getting dispersed in time, so your pulse in the different wavelengths is getting smeared out different amounts.
Fraser Cain: Oh, I see.
Dr. Pamala Gay: By looking at how the pulse gets smeared in different wavelengths you can get at the Interstellar medium.
Fraser Cain: Oh, that’s really interesting. So, there’s a way a pure pulse, that’s going to say vacuum, should look.
Dr. Pamala Gay: Yes.
Fraser Cain: And then, by looking at what the actual parts of the, you know, almost like the spectrum of the pulse as it comes at you tells you what could be getting in the way of it.
Dr. Pamala Gay: Exactly.
Fraser Cain: Incredible.
Dr. Pamala Gay: Exactly.
Fraser Cain: Anything else?
Dr. Pamala Gay: I mean, they’re also really good at like eating their companions and spinning faster as a result. Some pulsars are also magnetars, that when they rearrange their magnetic fields, can give off bursts of gamma rays and destroy spacecraft and murder the spacecraft. So, these things have a small murderer side. There’s a murdering spacecraft, they’re murdering their neighbors, the – yeah, yeah, so –
Fraser Cain: But are those things that we use them for? Not yet. That’s soon.
Dr. Pamala Gay: Well, people actually – and by people I mean astronomers, actually refer to pulsars that are consuming their neighbors and spitting up as either black widows or I wanna say redbacks. They’re too different kinds of poisonous spiders. So, when you start naming pulsar classes after poisonous spiders yeah, they’re getting used for science while they’re being murderous.
Fraser Cain: That’s awesome. All right, thank you, Pamala. We’ll see you next week.
Dr. Pamala Gay: See you next week. And I have named – so, thank you Fraser and thank you all of you out there watching this episode. We are here thanks to you, and we are able to have the group of humans around us that keep this show going.
Nancy, Beth, Ally, Rich, all of you, thank you. And thank you to Daniel Loosli, Scott Bieber, David, David Gates, Alex Cohen, Nicky Lynch, Marco Iarossi, Justin Proctor, Gregory Singleton, Jim Schooler, Paul L Hayden, Randa, Matthias Heyden, the Lonely Sand Person, Disasterina, Nial Bruce, Cooper, Jeff Willson, Tim McMackin, Eran Segev, Paul D Disney, Kenneth Ryan, Nate Detwiler, Allan Mohn, Alex Raine, Steven Shewalter, Dean McDaniel, Don Mundis, Omar Del Rivero, Benjamin Müller, Scott Briggs, Karthik Venkatraman, NinjaNick, Michael Regan, Janelle Duncan, Benjamin Carryer, Jeremy Kerwin, Matt Rucker, Mark H Widick, Michelle Cullen, Schercm, it’s a bunch of letters, you know who you are. Anitusar, Frode Tennebø, J AlexAnderson, Jim McGihon, Moose and Deer, Bruce Amazeen, Philip Grand, Dwight Illk, Abraham Cottrill, Father Prax, Jen Greenwald, Kimberly Rieck, and Mark Steven Rasnake. Thank you all so much for everything you do that makes our show possible.
Fraser Cain: Thanks everyone and we’ll see you all next week.
Dr. Pamala Gay: Bye-bye. Astronomy Cast is a join product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a creative comments attribution license, so love it, share it, and remix it, but please credit it to our hosts Fraser Cain and Dr. Pamala 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 Patrion. If you want to help keep this show going, please consider joining our community at patrion.com/astronomycast.
Not only do you help us pay our producers a fair wage, you will also get access to content right in your inbox and invites to online events. We’re so grateful to all of you who have joined our Patrion community already. Anyways, keep looking up. This has been Astronomy Cast.