Ep. 332: Stellar Collisions

Out here in the Milky Way’s suburbs, stellar collisions are unheard of. But there are places in the galaxy where stars whiz past each other, and collisions can happen. When stars collide, it’s a catastrophic event, and the stellar wreckage is visible half a galaxy away.

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This episode is sponsored by: Swinburne Astronomy Online, 8th Light, Cleancoders.com

Show Notes

  • What Happens When Stars Collide? In this case a new kind of pulsating star — Universe Today
  • Blue stragglers  — Northwestern U
  • Binary Stars — Cornell
  • R Cornoae Borealis stars — AAVSO
  • Accretion Disks in Compact Stellar Systems — free ebook
  • Milkdromeda 
  • Andromeda is Coming Right At Us! Or is it? — Discovery Space
  • Short Gamma Ray Bursts Linked to Stellar Collisions — UC Berkeley
  • Transcript

    Transcription services provided by: GMR Transcription

    Female Announcer: This episode of Astronomy Cast, is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit astonomy.swin.edu.au, for more information.

    Fraser Cain: Astronomy Cast episode 332, Stellar Collisions. Welcome to Astronomy Cast, our weekly fact-based journey through the cosmos. We help you understand not only what we know, but how we know what we know. Month name is Fraser Cain, I’m the publisher of Universe Today and with me is Dr. Dr. Pamela Gay, a professor answer Southern Illinois University, Edwardsville and the director of Cosmos Quest. Pamela, how ya doing?

    Dr. Pamela Gay: I’m doing well, how are you doing Fraser?

    Fraser Cain: Good, how you feeling, you were a little sick last week?

    Dr. Pamela Gay: I was, I – today’s my first day without taking all the cough and sinus and everything medicine and so, I seem to have my brain back and my voice is mostly back. Unlike last week where I sounded like, I’d crossed bread with a garbage disposal unit. It wasn’t pretty.

    Fraser Cain: No, we were trying to catch out and we were like, can we use it? Like, there’s no way I’m gonna be able to record on the Monday. We were like, could we – maybe on the Wednesday and you were like, I think so. No, there’s no way we could record on the Wednesday. And on the Friday, I think – no, there’s no way we’re recording on Friday. Like, you literally were no even getting out of bed of the Wednesday and there was no way you were going to record without going into a coughing fit on the Friday. So we just sort of had to pull the plug and now, we’re gonna catch up this week, so.

    Dr. Pamela Gay: I think I slept as much last week as in the entire month of December.

    Fraser Cain: Good, well you needed to rest and now, as I mentioned in the preshow, fortunately someone came along and did all your work for you, so there’s to catch up.

    Dr. Pamela Gay: Yeah, no.

    Fraser Cain: So I just sort of want to thank everybody who has been emailing me and commenting on all of their favorite science fiction books. Like, going through the Arthur C. Clarke two parter was a chance for everyone to let us know what they loved to read and it was just terrific. It was so great to get all those recommendations that I literally now have my reading materials set for the next, I don’t know, forever. But, thanks to everybody for getting, you know – sending us your suggestions, I love it.

    Dr. Pamela Gay: And one of the things that I love and hate is how easy it is to get all of these books now through either Guttenberg for the really old ones, through Kindle for new things and the same time, it’s so frustrating because we don’t have bookshelves the same way we used to. So loaning books is more difficult, but –

    Fraser Cain: Yeah, or have time. Yeah, I know what you mean, I’m terrible. I’ll buy stuff and just never read them. And I don’t feel bad about them piling up in my house anymore because they’re just digits, they’re just bits. They sit in the computer somewhere. Anyway, so thank you for – keep them coming. What I’m saying is keep them coming because I need more. All right, let’s get rolling.

    Female Announcer: This episode of Astronomy Cast is brought to you by 8th Light, Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. 8th light provides disciplined software leadership on demand and shares its expertise to make your project better. For more information, visit them online at www.8thlight.com. Just remember, that’s www, dot, the digit eight, T-H-L-I-G-H-T, dot com. Drop them a note, 8th Light, software is their craft.

    Fraser Cain: So out here in the Milky Way suburbs, stellar collisions are unheard off. But there’s places in the galaxy where stars wiz past each other and collisions can happen. When star collide; it’s a catastrophic event and stellar wreckage is visible half a galaxy away. So is this going to happen, or is this thing going to collide with something?

    Dr. Pamela Gay: No.

    Fraser Cain: Okay, great. Because we’re in the suburbs, it’s boring out here, nothing really happens.

    Dr. Pamela Gay: That is entirely true, but yeah – no, we live in the suburbs and as near as we can tell, the statistical probability of us impacting anything is as close to zero as it can get.

    Fraser Cain: Yeah, it is literally, I don’t know, iron grains of sands from the opposite sides of the United States and them having to collide with each other. Like, it’s –

    Dr. Pamela Gay: It’s as if you managed to throw a thing of sand from Vancouver and get it in my eye in St. Louis, which is may be even more challenging because I’m inside a building.

    Fraser Cain: Right, well, the same odds. But there are places in the universe, in our Milky Way where this kind of mayhem does go done.

    Dr. Pamela Gay: Yeah, and this is one of those things where when we think about stellar collisions, there’s basically three different types. There’s the first thing that comes to your mind, which is a star kind of hanging out by itself and gets whacked for no oblivious reason. Then, there are binary stars, where you have two stars formed together, live together and then, became hateful towards one another on collided.

    And then, you have an intermediate situation, which is gal – not galaxy clusters, globular clusters where you have thousands to tens of thousands of stars all very tightly packed together and two stars that may not have formed together eventually passing close enough together, they form a new binary system later in life. And those systems, just like the naturally forming ones, can also collide and lead to stellar mergers.

    Fraser Cain: Well, let’s start with that first example then. So you’re talking about just one star hanging out, doing its thing and then, another star comes out of nowhere, wham, runs into it.

    Dr. Pamela Gay: Yes.

    Fraser Cain: So I think we can all imagine this in our mind, you know, perhaps me more – can really imagine this, but like, what’s the scenario? What would happen? What would set this up?

    Dr. Pamela Gay: You know, this is one of those fluke things that incredibly rare odds of happening. You either have just two orbits that have poor timing, so as two stars orbit around the galaxy, their paths cross at the same moment place in time. Same moment and same place, time is the same thing as the moment. Where you have galaxies colliding in the process, orbits can change, things end up crossing. This is not something that’s statistically supposed to be something we need to worry about, but it can happen.

    Fraser Cain: Well, it’s a big universe and so, something horrible is happening to someone, somewhere.

    Dr. Pamela Gay: All the time.

    Fraser Cain: All the time, an infinite amount of time, something horrible is happening. Stars are colliding all over the place if you look out billions of light years to the hundreds of billions of galaxies that are out there.

    Dr. Pamela Gay: So while this is the least likely to occur, it is the one most often imagined. You see it in science fiction books; you see it in all sorts of different things. Collisions between stars, not likely, but possible.

    Fraser Cain: So what would happen then, in this situation? So you’ve got just two, random stars, splash. So what would happen?

    Dr. Pamela Gay: So in the most improbable case of they have sufficient center of mass on center of mass action. That they don’t end up spiraling around each other, but they actually pretty much collide in one momentous, kablewey. It would literally go kablewey, in the form of some sort of a nova. You throw all of this mass together, it’s gonna radically change the type of thermal-nuclear reactions that can go on. You’re gonna have ran – radical changes in the pressure of the system and the temperature of the system and depending on the types of masses involved, you’re either going to get a system that completely annihilates itself or simply flairs out and leaves behind a new type of star as a remnant.

    The latter case, is what’s going to happen if you end up with a tiny star falling into a larger star or two smaller stars coming together. That’s less exciting, but how exciting it is will also depend on what is the difference in velocity. So you have to take into consideration, not just the mass of the two systems, but they could be coming together fairly quickly.

    Fraser Cain: Right, so I guess I just imagine just the huge velocities involved, but if you sort of very slowly pored one star into another star, you would end up with the combined mass of the two stars and then, whatever – I guess, as a baseline, you’d be – like, what would that turn into if you had that – and then, you add on top of that, all of the velocity and densities and things like that, right? So if you just, like carefully pored one star into another star –

    Dr. Pamela Gay: Which the universe does for us on a regular basis. So when it comes to the nice, slow, gradual, one star eating another star, we’ve got lots of examples when we think of that. When we look out at globular clusters, we see what we call, “blue stragglers”. This is stars that are unusually blue, which indicates unusually young, for the age of the system that they’re located in. we haven’t totally caught one in the act of becoming a blue straggler, so there’s a few different theories for how they came into existence.

    But one of the main theories for where blue stragglers come from, is you have a binary system and the stars in the binary aren’t that massive and they’re very close and over time, they slowly either suck matter, one from the other or they slowly get close enough that they merge into a single star. And that single star bloats up nice, happy, bluer than you would expect and so, you end up with this unusual composition, unusual color from the nice, gentle merger.

    Fraser Cain: Right, but I imagine the situation, you’ve got these stars that have been living out their life and the main sequence phase to a certain extent, for as long as they have, they’ve used up X amount of the hydrogen in the core. They’ve turned it into helium, you know, they’re – just the standard thing. I mean, if you bring this other star in and you collide them together, is it just literally reset the clock on the star? Is it gonna bring it back to it’s – just be freshly born star again, with nice hydrogen all mixed up again and it’s time to begin fusion again?

    Dr. Pamela Gay: Pretty much, yeah, it’s unclear what happens in the nucleus. This is one of the really neat things for people who are trying to model the insides of stars. You’re going to end up, essentially, with two stellar nuclei, two stellar cores orbiting one another within a shared envelope for a while. How quickly those two nuclei merge into a single nuclei, that’s gonna shake up the entire process. You’re also going to end up with the new nuclei, has some form of mixed up composition that will both include the nuclearly, enriched materials from the processes that have already been going on. But it’s impossible to imagine one of these situations where you don’t also mix in more virgin material from the envelopes of the stars. So yeah, it’s gonna reset the clock to a point, but you’re still gonna end up with the star with a really strange composition.

    Fraser Cain: And is this maybe what we’re seeing with these blue straggler star, right? That they’re gone, someone’s pressed the reset button on a star that should have been dead billions of years ago.

    Dr. Pamela Gay: Or at least more evolved, I mean, these aren’t big stars so dead is stretching it. But they definitely shouldn’t have been blue, they should have been significantly more red, significantly more evolved. And they’re sort of hanging out being elderly stars instead of being young and excitedly blue stars.

    Fraser Cain: So I’m guess interesting the outcome then, is really gonna depend on the amount of total mass that you’re dealing with. If you’ve got two solar mass stars, you’re gonna end up with, when everything’s said and done, twice the mass.

    Dr. Pamela Gay: And it’s also gonna depend on what are the things merging. So blue straggler, we’re pretty sure, are two run of the mill, fairly moderate sized stars that ended up combining. You end up with a large star, but not supernova candidate material. But everything that occurred was a star, when the merger took place. Other situations that we worry about are what about when red giant stars and white dwarfs end up merging together?

    Fraser Cain: What happens?

    Dr. Pamela Gay: Well, depending on how it occurs, you can either end up with the white dwarf sucking material off of the red giant star until it generates a giant thermo-nuclear reaction that we referred to as a type 1A, supernova or type 1B supernova. And another possible situation, and again, this is like the blue stragglers, we can’t catch the stars in the act, so this is strictly a theoretical, well maybe this explains what we’re seeing. There’s a class of variable star called R Coronae Borealis stars, these are stars that are hanging out, shining quite nicely and suddenly, plummet in brightness.

    They’re magnitude gets to be a larger number, which means they’ve become significantly fainter. This is tied to some sort of dust production in the envelope of the star. They’re really hard to understand and one possible explanation for them is that there’s a white dwarf star merging into the envelope of a physically larger, lower mass star and this is churning up all sorts of usual behaviors.

    Fraser Cain: Wow, when you think about these binary pairs, you get this white dwarf companion with a star and its syphoning material off this star, it’s happening over millions of years, right. It’s slowly building and it hits that 1.4 times the mass of the sun, hits that limit and then, detonates as a supernova, but if it’s going right into the star, it’s gonna be creating that material very quickly. Getting into that envelope, right, where it’s getting actually surrounded, like an all you can eat buffet.

    Dr. Pamela Gay: And depending on the mass ratios, it might not actually hit that supernova point. You can have a very small white dwarf star, you can have a low mass red giant star and they may not hit the kablewey mark. And in the process of falling into the red giant companion, that white dwarf may puff back out. And if it’s puffed back out, suddenly that electron degeneracy pressure that causes the supernova, well, if it’s puffed back out, the pressures gone down. So you no longer have those same constraints. Lots of things can play in, and it’s this whole perimeter space of just the white dwarf puffed back up, what’s the total mass, all of these things together, as these stars merge has these stars collide, determine what we end up in the outcome.

    Fraser Cain: Right, so maybe we’re pressing the reset button, maybe they’re detonating supernova, maybe all kinds of crazy mayhem. Okay, so now we talked about just the crazy stars just smashing into each other, talked a little bit about blue stragglers and we started talking a bit about binary pairs, like what kind of environment would we see for that to come together? Because I can kind of imagine, well, you’ve got two stars and they’re just orbiting each other or orbiting a common center of gravity. Unless something happens, they’re just gonna do that forever, right?

    Dr. Pamela Gay: Well, and that’s the thing, is it turns out that lots of binary stars have slowly evolving orbits. In some cases, the evolution is caused by the fact that pretty much all stars are undergoing mass-loss. Our sun is regularly losing mass. The bigger the star, the more mass it’s going to lose over time. As those two stars lose mass, gain mass from one another, how they orbit one another is also going to evolve. There’s also going to be moments of inertia that come into play, as you have a giant star contracts down to a little, tiny white dwarf. All of these things effect how they orbit one another, the drag that they experience from dust and gas in the system, how much material they absorb from their companion star. And so this means that even though in a nice simplistic view of binary stars, you’ve two little centers of mass that just orbit one another in the exact same way over time as the two stars evolve, no. this whole thing is going to be changing over time, as the separations between the stars evolve. And this means that it’s quite feasible to imagine that the stars slowly get closer to one another, that one star ends up creating drag on the other one by gravitationally sucking material off of it. And all of this can lead to the one star eventually consuming the other star.

    Fraser Cain: That’s what I’m imagining, right, is this red – you know, one goes first and turns into a red giant and maybe the two stars just formed really close together and they’ve been – they just haven’t collided yet. And then, the one goes, reaches the end of its life. If it puffs out its atmosphere, bigger than the orbit of Jupiter, right. And now, that star used to be completely separate and there was no drag on it and now, it’s inside or rubbing against or really close to that other star and now, things are gonna south.

    Dr. Pamela Gay: And they do, and this is where as I said, one entire family of supernova comes from. We see recurrent nova, which is where you have small, compact objects, neutron stars, white dwarfs that develop excretion disk material. They’re sucking off their companion, there’s excretion disks periodically gets dense enough that they flair up, give off lots of light and energy. That itself, isn’t a stellar collision, but over time it effects the orbits and it’s totally feasible for you to end up with shared envelope, where the two stars get close enough that they bloat each other up. Essentially heating one another, essentially with binary stars, if you can imagine it, it can probably happen.

    Fraser Cain: Right and I think of all kind of outcomes where, as you said, one goes off as a red giant and it interacts with the other one. And then, maybe they create this shared, as you mentioned, this shared envelope or one absorbs material from the other or the combined mass detonates as a supernova. I mean, it just all depends on the raw ingredients of the interaction, right?

    Dr. Pamela Gay: And we thought we had a good understanding of all the different ways that things can go wrong. And then, we saw V838 Mon. This was a star that back in 2002 went from being a completely boring star that no one paid any note to. To suddenly, it became one of the brightest stars in the entire galaxy and this flash of light that it gave off radiated through surrounding shelves of material. And in desperately trying to understand what was going on with this really weird star system, it was determined that this just might be a case where you had a run of the mill star hanging out, minding its own business and it didn’t go through a nice gradual merger.

    It simply went through a merger, like that merger, flash of light merged. It’s unclear if it consumed a star, if it consumed a planet, if it consumed multiple planets. But unless it undergoes a second nova-like event, one of the best explanations we have for why the heck this star did whatever it did, was that it was a stellar merger. So the universe is still finding new ways to surprise us.

    Fraser Cain: Now, we’ve talked about one extreme environment, which is these globular clusters. Oh, I said that wrong, globular clusters, you guys are infecting me. So what about the center of the Milky Way? The center of the galaxy is where you’ve got stars orbiting the super massive black hole, like planets going around the sun, it’s really dense.

    Dr. Pamela Gay: And so, in these situations, we do occasionally with things falling into the super massive black hole, that’s fairly rare as far as we can see in our own galaxy. But we also end up with a much, much denser stellar environment, so just like with globular clusters where you see the blue stragglers forming through various types of binary stars coming together. We can’t necessarily say what a blue straggler is in the center of the galaxy, but there’s no reason to think that the same physics doesn’t apply. With globular clusters, the blue stragglers stand out because in a globular cluster pretty much all of the stars formed at one time.

    And all of them formed with the same basic material. So when you look at them, you can say okay, all of these should be the same age, we shouldn’t be seeing these overly blue guys over here. When we look at the center of the Milky Way, we can’t say that all of the stars formed at the same time, we can’t say they all formed with the same material. So we can’t say, these are overly blue, but the densities are high enough that we’d expect there to be the same sorts of capture events. Where two stars pass near one another, gravitationally grab hold of each other become a binary star system and over time, evolved to consume one another and become a blue straggler that you can’t tell is a blue straggler.

    Fraser Cain: And then, that thing gets smashed into the super, massive black hole and its mayhem all around.

    Dr. Pamela Gay: Who knows, the black hole doesn’t eat everything, it just occasionally gnaws on some things that unfortunate orbital perimeters.

    Fraser Cain: Yeah, it’s always important to remember that it’s just a big, gravitational source. It’s not like a vacuum cleaner.

    Dr. Pamela Gay: No, so in order to fall in, you have to some series of unfortunate events that lead you to gain an orbit that sends you literally on a death spiral.

    Fraser Cain: So one event that’s coming up, it’s gonna test this probability is our merger with Andromeda in the next, I don’t know, five to seven billion years from now.

    Dr. Pamela Gay: The Milk-Dromeda.

    Fraser Cain: Yeah, the Milk-Dromeda event, so what’s gonna happen for the stars in both galaxies?

    Dr. Pamela Gay: You know, this is one of those things where that type of collision that you worry about is not something that you really actually need to worry about. For the most point, all of the stars are going to just move past one another. They’re going to collectively settle into new orbits and yes, in the center of the new resulting galaxy, you’re going to have higher densities, higher frequencies of binary stars.

    But overall, things aren’t going to hit each other. You’re not going end up with a whole bunch of new V838 Mons taking off, it’s just not a concern. Now, dust is gonna – dust clouds colliding left and right, massive amounts of star formation material getting driven into the core of the resulting galaxy. All of that stuff is gonna happen, active black nuclei, as the resultant black hole or binary black hole system in the center happily consumes dust. Yeah, but stellar collisions, very, very low probability.

    Fraser Cain: Yeah, so you’re gonna get all of this crazy formation and supernova after that, and you’re gonna get this gas and dust piling up. But the stars are just gonna go, phew.

    Dr. Pamela Gay: Exactly.

    Fraser Cain: All pass each other, okay. So we’ve talked about sort of normal, what we would all imagine as stars, but there’s a lot of exotic objects. Like neutron stars and white dwarfs and they can collide with other objects and with each other. And so, and those we do see a little more often, don’t we because they’re bright?

    Dr. Pamela Gay: And this is the binary merger situation, this is typically where you have two stars born together, grew up together, one became more aggressive, exploded and did bad things to its neighbor. You can end up depending on the mass ratios sometimes with a pair of neutron stars that end up eventually merging, colliding, supernova, black hole or just kablewey, depending on the mass ratios. You can end up with white dwarf neutron stars neutron black hole, black hole, black hole.

    If you can come up with a combination, nature has created it and over time, all of these things are going to have orbits that lead them to eventually merge. May be not every binary system, definitely not every binary system. But every possible combination will at some point, have an example of what happens when two stars merge.

    Fraser Cain: but like with neutron stars, don’t you have almost gravitational waves, are contributing to them spiraling inward on each other?

    Dr. Pamela Gay: You get that with all of the extraordinary high mass systems, so you get that with the neutron stars, with the black holes. We even start to see that with things where you have a neutron star and a white dwarf. So pulsars are one of the places where we’ve been able to spot these things. Pulsars are special types of neutron star that’s spinning quickly. The universe is filled with all sorts of exotic matter, trying to merge with other exotic matter.

    Fraser Cain: But I think the amazing thing with those is they’re probably even more rare. Like, two neutron stars coming together and detonating as a gamma ray burst, right? But the resultant explosion is so powerful that we see it in galaxies hundreds of millions or even billions of light years away. So we’re seeing these incredibly rare events, but they’re so energetic that we just see them from half way across the universe.

    Dr. Pamela Gay: Right, and that’s one of those things that I have to admit, that I sort of take for granted is, yeah, exotic matter, it explodes, kablewey. But yeah, when two neutron stars merge, the resultant supernova and gamma ray burst will make that merger brighter than the entire host galaxy that it’s sitting in.

    Fraser Cain: And wipe out a goodly portion of the everything in its region.

    Dr. Pamela Gay: Yeah, it’s the universe, it’s destructive, and it’s trying to kill us.

    Fraser Cain: Yeah, we know it’s trying to kill us and its showing, it’s demonstrating it by happily scouring a quarter of a galaxy with one explosion, and it’s crazy.

    Dr. Pamela Gay: No big deal.

    Fraser Cain: No big deal, yeah universe, I’ve got my eye on you, awesome. Well, thank you very nu Pamela, that was cool. Next week, we’re gonna talk about planets colliding, yikes, and then, I’m not sure. But actually, someone recommended we talk about galaxies colliding, I don’t know if we have. Anyway, I think we have, anyway, awesome. Thanks again Pamela and we’ll see you next week.

    Dr. Pamela Gay: Sounds good.

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