Ep. 679: High(per) Velocity Stars

Most stars in the Milky Way are trapped in here with us, doomed to orbit around and around and around. But a few have found a way out, an escape into the freedom of intergalactic space. How do stars reach escape velocity, never to return?


(This is an automatically generated transcript)

Fraser Cain [00:01:19] Astronomy cast. Episode 679 Hyper Velocity Stars. Welcome to Astronomy Cast your weekly facts space journey to the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, the publisher of Universe Today. With me is Doctor Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmic Quest. Hey, Pam. How are you doing? 

Pamela Gay [00:01:40] I am doing well. Although if you hear any sniffling, it’s because this is the worst pollen year of several years. All the trees are far too happy and far too fertile. 

Fraser Cain [00:01:53] Yeah, like my annual reminder. If there’s anything wrong with you right now, it’s because of spring. It’s allergies. 

Pamela Gay [00:02:02] Yeah. 

Fraser Cain [00:02:02] You can like. Like maybe if you not. If you live in the southern hemisphere and you’re moving towards winter. But whenever trees do their thing for you in your neighborhood, that is like. Are you feeling a little tired? It’s allergies. Are you feeling a little, out of it? Allergies, cold? Allergies. It’s it’s all allergies. Blame everything on allergies. 

Pamela Gay [00:02:29] But I’m hearing that for those who don’t have the cloudy, pollen filled skies that I have, there are northern lights right now under CNE. 

Fraser Cain [00:02:40] So we tried. So it was like really cloudy. And I’m like, oh, we’re doomed. But Cali like showed me a picture of the aurora activity here. And it’s just like off the charts. Yeah, yeah. And so but it was cloudy. I’m like, okay, so went to bed and then I woke up around two in the morning and it was clear skies. And I went and I stood at the window and I looked out to the north and I could see the green aurora glow. Wow. Far away. And I waited for like ten minutes and nothing really interesting happened and I went, now I’m going back to bed. 

Pamela Gay [00:03:07] So that’s fair. 

Fraser Cain [00:03:09] Yeah, yeah. So so in theory, it’s, this is it. This is our time. But hopefully the weather’s supposed to improve later on this this week, so we’ll get some more shots at it. 

Pamela Gay [00:03:18] And as we get closer and closer to solar Max, everyone needs to install their Aurora alerts on their phone because we’re going to get more and more opportunities. 

Fraser Cain [00:03:28] Don’t ask us for recommendations. We have no idea what’s a good piece of software install. So most stars in the Milky Way are trapped in here with us, doomed to orbit around and around and around. But a few have found a way out and escape into the freedom of intergalactic space. How do stars reaches keep velocity never to return? I think about like, the Watchmen. You know that classic line? I’m not. I’m not trapped in here with you. You’re trapped in here with me. So. High profile city stars. What’s going on? 

Pamela Gay [00:04:03] So basically, there are stars out there that are going 50km per second. 100km per second. More than that, in some cases faster around the galaxy than the stars around them. They have been accelerated in a variety of different ways. And some of them, actually end up looking like comets because of the combination of mass loss and motion and. I don’t know where to start. There’s so much awesomeness to look at. Myra, maybe. Is Myra a good place to start? 

Fraser Cain [00:04:38] Well, maybe. I mean, it’s not it’s not leaving, but it’s definitely moving fast. So. Yeah. So then I guess, like, just in general, what are the motions? What’s a normal motion of a star in the Milky Way? 

Pamela Gay [00:04:52] So. So this is where it gets tricky. It’s it’s you look at what is the average motion of all of the stars in a given area. And the high velocity ones are the ones that are going 50 to 100km per second faster than the average where they are. And so the velocity changes depending on where you are. And there’s a lot of complication. So basically these are the ones that are just going significantly faster than the things around them. But in the case of some of the most quickly moving ones, there are actually moving 10% to 30% the speed of light, right? 

Fraser Cain [00:05:27] So like like when we think about, say, the solar system, Earth is going 30km per second around the sun and is in orbit. And the closer you get to the sun, the faster you go. The farther you get from the sun, the slower you go. It doesn’t work that way. In the Milky Way, though, everything is roughly moving at the same speed in the hundreds of kilometers per second range. 30. 

Pamela Gay [00:05:49] 20, 50. Yeah, you get regional differences because you’ll have like this open clusters moving together, that group of things is moving together. And and so this is where you get the really cool dynamics that you can start to see. One of my favorite things about open clusters, which is not where we were going with Myra. We will return to Myra. 

Fraser Cain [00:06:11] Yeah. We’ll get we’ll get back to that. 

Pamela Gay [00:06:14] So with open clusters in particular, you have these groups of stars that formed together out of the same collapsing molecular cloud and star systems. When they’re small and compact. I end up having lots of multi star interactions. And wherever you get three stars interacting, you can often get one star getting flung away. And so when we find these hyper velocity stars, quite often you can figure out what is their three dimensional motion through the galaxy and work backwards hmhm and be like, oh, that open cluster got rid of you for. 

Fraser Cain [00:06:55] Each K dejected. Yeah, so so what is the method that astronomers use to measure the speed of stars? 

Pamela Gay [00:07:02] We look at two different things. One is what’s called proper motion. This is how the stars appear to be moving relative to one another in the plane of the sky. This is the kind of thing that Gaia was designed to do, like no other telescope has been able to do previously. We see most stars over the course of human lifetime won’t really appear to move relative to the background galaxies at all. But there are few stars out there that pretty much year to year we can see their slight motion as they simply flow through the galaxy. 

Fraser Cain [00:07:43] So they just like taking a picture of the sky and then seeing how that stars position changes from the previous year. 

Pamela Gay [00:07:50] Exactly. And one of the easiest ways to measure this is if you’re looking at out of the plane of the galaxy, you can see distant galaxies, quasars, bright, other things that are so far away that we can’t perceive their motion. So we can use those as signposts to measure other objects moving against. And, yeah. So there’s things like Barnard’s Star that we just look at. And with Bernard Star, we see it moving in the sky rather quickly because it’s rather close. So it’s that thing of when you’re driving down the highway, you don’t see the mountains really moving that much past your window. But there’s mile markers. They zip past because they’re nearby and the relative motions seem magnified. They’re not actually magnified. It’s just our perception changes. But yeah, there are stars out there. And the reason I brought up mirror. Can we go back to mirror? Are we ready for mirror? 

Fraser Cain [00:08:45] Yeah. I just wanted, like, if you’re about to say, you know, here’s how fast mirror’s moving. And then I was going to say, well, how do we know? Well, now we can just refer to the proper motion Doppler effect. 

Pamela Gay [00:08:55] So there is another half to it. So I guess I will get to the other half of it. And the other half of it is we can take spectra of stars and we can measure what wavelength does the hydrogen alpha line appear at what wavelength does the calcium doublet appear at what wavelength does all these different molecules? When we look at their wavelengths, they form a fingerprint that can get slid back and forth from red to blue. The spacing between the individual lines stays exactly the same. But what color? We see those lines that can change when an object is moving towards us. Those lines all get shifted towards the blue. When something is moving away from us, all those lines get shifted to the red. And so we measure the the motion in and out of the plane of the sky using spectra. We use. We measure the motion in the plane of the sky using proper motion. We do trigonometry, and thanks to trigonometry, we can figure out the three dimensional motion of these objects. 

Fraser Cain [00:10:05] Right? Right. All right. Let’s talk about mirror. Why? Why are you so obsessed about mirror? What’s this about? 

Pamela Gay [00:10:11] Because. Because so? So this is a star. It’s. It’s not totally weird. It’s going 130km per second, relative to what’s around it. But it is shedding its outer atmosphere as it goes. It’s a, asymptotic red giant branch star. It is a kind of variable star. It is working its way towards becoming a planetary nebula. Except instead of being nice and polite and sitting still and puffing the material around it, it is hauling itself through the galaxy, leaving a train of material behind it. And because we have the capacity to use things like the Galaxy satellite to look at that stream of material left behind, we can actually start to do things like what was the chemistry of the material it gave off 300 years ago. What was the chemistry of the material? It gave off 200 years ago. And so we can see the chemical evolution of the debris cloud left behind by this shooting star. And in addition to that, it’s shocking the interstellar media. So it’s basically plowing into the material in front of it. So we have this really cool kinematic picture because you have a fast it’s not outgassing. That’s the wrong word, but that’s the closest analogy to things you might be familiar with. We have we have a star flying through the galaxy, shedding material, crashing into material, and we can see all of this going on, and we can see the star with our own eyes and with our own eyes. It’s fairly boring. It’s it’s changing in brightness and all that, and it’s cool. But when you look at that little red point of light, it’s at the end of a three degree long stream of material, which means that that little star has six moon widths behind it of material trailed across the sky. 

Fraser Cain [00:12:12] That’s amazing. So like in instead of the star just sitting there and puffing out its outer layers and you sort of measure it like tree rings, it is hurtling through the cosmos. And so you can see, you can just measure it based on history. That’s that’s pretty crazy. Yeah. Mirror, mirror is amazing. And it’s I mean, it’s it’s a type of variable star, like. Yeah, it is the quintessential the because there are, there are mirror type variable stars. Right. That is the that is the OG. 

Pamela Gay [00:12:45] Yeah. It Omicron study is its fancy name. Everyone calls it mirror the wonderful. That’s what my remains. Yeah. And I love that being Canadian. You say it differently than me mirror in America. Yeah. It’s it’s. Yeah. 

Fraser Cain [00:12:58] I don’t know if that’s Canadian. Could just be West Coast, I don’t know. Oh. All right. But that is nothing. That is, that is peanuts compared to what stars can do. So let’s. 

Pamela Gay [00:13:12] Yeah. 

Fraser Cain [00:13:12] Go faster. 

Pamela Gay [00:13:15] So, so like I was saying, some of these stars, the the hyper velocity stars instead of just the high velocity stars, these things are getting to 10 to 30% the speed of light. Wow. And and these are stars that in general have gone through a very rough history. They they were probably part of a binary system and they got too close, we think in general to the supermassive black hole in the center of our galaxy and the two stars in the binary, plus the supermassive black hole in the center of our galaxy. That’s a three body problem. And one of the stars, basically the supermassive black hole said, no more galaxy for you. And when you get a star that’s moving 10 to 30% the speed of light, it’s not going to get held on to by our galaxy. It is going to leave and keep going. And. 

Fraser Cain [00:14:21] Like those numbers like 30,000 to 100,000km per second. That’s how fast these stars are going. That is fast. Yeah. And I think escape velocity of the Milky Way is something like a little over a thousand 1500, I think, kilometers per second. So we’re well beyond that. Yes. Now, you mentioned a possible source of this is getting too close to the supermassive black hole. 

Pamela Gay [00:14:48] That that is believed to be one of the sources of difficulty for these poor objects, right? Our belief, that this is the case comes from looking at the chemistry of a bunch of these systems. We we know of, over a thousand now, thanks to a variety of different, surveys, which which ten years ago, we thought there were only a thousand out there. Now we know of over a thousand. So it apparently is something that happens fairly regularly. Stars get accelerated. So. So there’s. Known populations of stars that get in close to the center of our galaxy, they have a given chemical signature. When you see things with that chemical signature on a high velocity escape orbit that traces back to the center of the galaxy, it’s a pretty good bet that that was a star that formed in the center, lived in the center, and was ejected from the center. 

Fraser Cain [00:15:49] But that’s not the only way that these stars can probably get this level of an escape velocity. 

Pamela Gay [00:15:55] Now that’s the so there’s a variety of different ways. Like I said. So there’s the standard three-body problem. There is we also think that some supernovae where you have a close in companion and the star that goes boom, that through a variety of different effects, ranging from conservation of angular momentum to just the general shockwave of the explosion. You were sending that companion away at a high velocity, right? 

Fraser Cain [00:16:30] So you I mean, you’ve got these two stars whirling around each other. Yes. And then one of them ceases to be. 

Pamela Gay [00:16:37] Yes. 

Fraser Cain [00:16:37] And the other star is now hurled into space. It has that same velocity that it was going around that other star, but now it no longer has the the gravitational anchor. It’s being thrown like a sling. 

Pamela Gay [00:16:52] And and what’s really cool is we see both the kinds of stars that get formed during supernovae moving at high velocities, and the kinds of things you would expect to be the companion, in the case of a supernova. So there are probably cases where you have both stars ending their their coexistence as they leave behind a now empty nebula of material. And I just kind of love that visualization. This is the ultimate stellar divorce. No one gets to keep the house and, off they go. 

Fraser Cain [00:17:31] And it’s kind of a grim analogy though, right. Because I’m. 

Pamela Gay [00:17:34] Sorry. Vaporizes well, one might turn into a neutron star while the other one flies away. 

Fraser Cain [00:17:42] Sure. But even that would be a bit of a gravitational pull. 

Pamela Gay [00:17:45] Would be a bad day. Yeah. Would be a bad day. 

Fraser Cain [00:17:48] Yeah. 

Pamela Gay [00:17:49] Yeah. It’s the kinematics of of all the different things that can happen is, is endless in its variety. 

Fraser Cain [00:18:00] All right. What is the other way that we can get these hyper velocity stars. 

Pamela Gay [00:18:04] So, so we also see stars that have been evicted from many things in the outskirts of our galaxy. So these are the situations, where. You have globular clusters with multi star interactions. You have dwarf galaxies that have merged together and things get flung out. And in a lot of cases you’ll see these stars that are on highly elliptical orbits moving at not quite escape velocity. They still appear in many cases, but not all cases to be stuck to us. And these are again systems where due to mergers, due to close passes, due to high densities of stars, you end up with that three body system or more. More body systems can have. 

Fraser Cain [00:18:57] More body. 

Pamela Gay [00:18:58] And something just got ejected. What’s really cool is, well, they don’t end up on these massive, sweeping orbits. When we look at the dynamics of globular clusters over time, they actually seem to beat like a heart. Heart? Yeah, that’s the right word. Beat like a heart as, basically the kinds of orbits that exist in them heat things up and then they settle back down, heat things back up, and then our back down. Kinematics is amazing. 

Fraser Cain [00:19:27] So but I mean, these kinds of stellar interactions, they can come from almost anywhere. You can have a star floating around inside a globular cluster. It gets into some kind of through body interaction and it gets the boot. You can have a young star cluster because I know that astronomers have recently seen that as well. They’ve seen stars coming. And the story that sort of helped trigger this was it was assumed that these hyper velocity stars were mostly coming from the supermassive black hole at the heart of the Milky Way, but there was a few fairly recently found that are nowhere near the black hole. And so, yeah, had to have come from some other source. I mean, I guess it could be another black hole, a small one. 

Pamela Gay [00:20:13] Well, yeah. So this is this is where there is a really interesting story that came out last week, maybe two weeks ago. Time has no meaning of a dwarf galaxy that was spotted with, for lack of a better term, a proboscis of forming stars coming off of it. And this long, narrow train of stars didn’t look like your standard jet formed stars. Because instead of being highly collimated, tightly held together close to the system, and then falling apart, the further away it got, like water coming out of a fountain, it was. The stars were more scattered near the galaxy and then got tighter and tighter focused. The further out and after follow up observations and modeling and looking closely, what is thought to happen is three dwarf galaxies or more came together. And at one point you had three supermassive black holes in the center of this dwarf galaxy. One black hole got flung in one direction, whereas it went through the intergalactic media. It triggered star formation, and the other two went in the other direction and triggered shockwaves, but not star formation. And so this is the full scale version of what we see happening at stellar scales, with globular clusters, with with open clusters, with any high density area of stars. We used to think stars couldn’t collide. We used to think it wasn’t common for stars to gravitationally grab onto each other. It turns out all those things we thought weren’t happening. Totally happening, totally happening. 

Fraser Cain [00:21:58] So there are also. So I guess black holes are a type of star. And that and that story about how about the black hole on escape trajectory? I love that, I love that idea that it’s maintaining. It’s like building a trail of star formation in its wake. Yeah. And you kind of wonder like what impact that has had on other galaxies. This is a common thing. Yeah. So the other thing is you can get high velocity pulsars. 

Pamela Gay [00:22:24] Yep. Yeah. So. So these are where you have that system of a binary, a binary system where one of the stars goes supernova and the neutron star. All pulsars are neutron stars. Not all neutron stars are pulsars, right? 

Fraser Cain [00:22:41] I guess you have high velocity neutron stars, right? 

Pamela Gay [00:22:44] Including pulsars. Yeah, exactly. And and so what’s really cool about high velocity pulsars is you have yet another way to get at their velocity. And that’s from the pulsing of their spinning. You can actually start to get the dynamics of where you have binary systems. It doesn’t help with the high velocity, but with binary systems of systems of planets, you can see in the the changes in their velocity over time that there’s an orbit going on. Back to the high velocity part. I’m easily distracted today, apparently. This this is what we were talking about earlier, where you have a supernova and and when it goes, you have an asymmetric blast, and and star goes and one way stellar remnant goes in other way, and you end up with this massive supernova remnant left behind. And it’s in trying to understand these non spherical stellar explosions that we’ve started to piece together. Well let’s go looking for the central star. Oh I can’t find the central star. Let it go. Yeah yeah. So expand your view. And often they find these runaway pulsars that again you trace back their their motion and you can follow them back to where they originated. 

Fraser Cain [00:24:09] That’s amazing that you’ve you’ve got this star and for some reason, like maybe it’s got a binary companion, some instability in the shape of the star, some, you know, it’s not perfectly symmetrical. 

Pamela Gay [00:24:21] And the disc. 

Fraser Cain [00:24:22] Explodes where one side explodes harder than the other side, that it experiences a thrust. Yeah. And so it’s turned into a little rocket in a way. It goes off into space and and. Yeah. So you’ve got this situation, you’ve got this, this beautiful supernova remnant nebula, like the Crab Nebula or something. But like, you look at this end of the Crab Nebula, you find the pulsar. But there are these, these remnants, if you look at the center and it’s gone. And like, one possibility is that star just exploded and disappeared, but they’re finding them receding away from the scenario. And one of the things that I really like about this is that we can use gravity in theory, we can use gravitational waves in the future, larger observatories to measure these, the formation of these hyper velocity stars as they’re forming, because it’ll be an instability. 

Pamela Gay [00:25:18] What I really love is the basic physics behind this is super simple. You have all the mass tied together and and if the cloud is going to go away asymmetrically, the, the momentum has to go somewhere. Right? And so this is purely a conservation of momentum problem. It’s a really complicated conservation of well conservation of energy and momentum problem. it’s just cool. 

Fraser Cain [00:25:46] Space is scary. 

Pamela Gay [00:25:47] Oh yeah. And awesome. 

Fraser Cain [00:25:49] Well sure. But I mean awesome in this sometimes in scary way too. Like in the, in the sort of the full meaning of the word. Awesome. Yeah. All right. Thanks, Pamela. 

Pamela Gay [00:26:00] Thank you, Fraser, and thank you to all of the folks out there in our audience who are keeping this show going by being part of our Patreon community. Those of you who join our Patreon get to get all of our episodes, advertisement free. So, this week I would like to thank planetary, Alex, rain, Karthik, Becca, trauma and Steven. Kathy, Andrew Stephenson, cameo rotation, Sean Matz, Gabriel Galvin, Glenn McDavid, Nat detweiler, Szymanski, Sam Brooks and his mom Bart Flaherty, the air Major Benjamin Carrier, Brian Kelby, Benjamin Davies, Arctic Fox, Lou Zealand, the lonely sand person, new LA dean, John Drake, Jordan Turner, Reuben McCarthy, Ian Abdullah, Jeff McDonald, Lee Ha born Andre Lev, soul Zero, chill Wanderer and 101 Felix Good Astro sets. Simon Parton, Papa Foxtrot and Christian Golding. Thank you all so very much. 

Fraser Cain [00:27:05] Thanks, everyone, and we’ll see you next week for bye. 

Pamela Gay [00:27:13] Astronomy cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy cast is released under a Creative Commons Attribution license. So love it, share it, and remix it, but please credit it to our hosts, Fraser Cain and Doctor Pamela Gay. You can get more information on today’s show topic on our website. Astronomy. Cars.com. This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep the show going, please consider joining our community at Patreon.com Slash Astronomy Cast. 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 are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.