Most of the time stars hang around for billions of years. But the Universe is a big place, and anything that can go wrong, inevitably does. Today we talk about what happens when these stars come together. The outcome is violent, and fortunately for you, also interesting.
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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast episode 406: Stellar Cannibalism. Welcome to Astronomy Cast, 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. My name is Fraser Cain. I’m the publisher of Universe today. And with me is Dr. Pamela Gay, a professor at Southern Illinois University, Edwardsville, and the director of Cosmo Quest. Hey Pamela, how are you doing?
Dr. Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Doing great. I mentioned before the show, it’s the herring run season here on the west coast of Canada, and it’s just wildlife to an extent you can’t even imagine; hundreds of eagles, hundreds of thousands of seagulls, herring, row, piling up on the beaches, snow drifts, sea lions, whales – If you ever want to see the greatest accumulation of nature ever, check it out, west coast, in March.
Dr. Pamela Gay: The Mississippi estuary is awesome, but nowhere near that awesome. You truly have gotten the massive awesome birds and mammals.
Fraser Cain: Yeah, and we get two of them. We get one in the spring, and we get the one in the fall when the salmon do their run up the river. Again, the eagles all come back and feast. But yeah, it’s a great time. So I just want to give people a reminder that we actually do Astronomy Cast as a live show. And so the actually goes for an hour. Now, we only put out a half an hour of it on the podcast, but if you want to join us and hang out for the chat and talk to us and we’ll answer your questions, we do the show live every Monday –
Dr. Pamela Gay: With occasional exceptions –
Fraser Cain: – With occasional exceptions, at 12:00 p.m. And so the way you can join us is just go, search on YouTube for Astronomy Cast, you’ll find our channel, make sure you subscribe to it, and then give yourself notifications to be notified when we do our live show and we will be glad to have you join us. And then we’ll answer your questions, I’ll say your name, and it’s a good time.
Dr. Pamela Gay: And if you’re listening to this live on March 14th, this is one of those lucky weeks when we’re not recording on Monday because I’m actually going on my first vacation in years. So we’ll probably be recording on March 16th, which is why you subscribe on YouTube, so that you get these notifications for when we do these weird things.
Fraser Cain: That’ll be me on April 4th, FYI.
Dr. Pamela Gay: Yeah, we take turns.
Fraser Cain: Yeah.
Female Speaker: 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.8thlight.com . Drop them a note. 8th Light; software is their craft.
Dr. Pamela Gay: Astronomy Cast is proudly sponsored by Cleancoders.com, training videos with personality for software professionals.
Fraser Cain: Okay, so most of the time, stars hang around for billions of years, but the universe is a big place, and anything that can go wrong inevitably does. And today we’re going to talk about what happens when stars come together, and the outcome is violent but, fortunately for you, also interesting. Now let’s do a big disclaimer. The events that we’re about to talk about happen so rarely that you have to see across an entire universe and catch them occasionally happening. But when it does happen, kaboom.
Dr. Pamela Gay: For the most dramatic kinds of cannibalism, it turns out just like mosquitos are constantly out there doing their little happy vampire thing, it turns out cannibalism is actually fairly common throughout our universe if you look for the less dramatic forms where they’re just grabbing a bite full here and there and not eating the whole body.
Fraser Cain: So today we’re going to talk, I guess, about how stars come together; one star feasting on another star, stars colliding, and what happens and how you can even detect it. So let’s start with, as you said, sort of the more common – Let’s talk about some of the situations. And I guess to set this up is to talk about how common actual binary star systems are out there in the universe, right?
It’s not just solitary stars. Most stars are in multi-star systems.
Dr. Pamela Gay: Yeah, the running joke is four out of every three stars is in a binary.
Fraser Cain: I’ve never heard that before. That’s awesome.
Dr. Pamela Gay: And the way that works is every three stars that you look at with your eye on the sky, it turns out that two of those so-called single stars are probably actually two stars. So when you think you’re looking at three stars, you’re actually looking at two binaries, plus two binaries, plus a singleton, so five stars. It’s fairly common, and with all of these stars appearing in pairs, it turns out that sibling rivalry is a rather real thing. And gravitationally, they sure like to yank on one another’s outer atmosphere.
Fraser Cain: So set this up then. In the more common binary stars, they’re fairly far apart, right? They’re not actually transferring any matter. It’s really just down to they’re gravitationally connected and they’re orbiting one another, or orbiting a common center of gravity, but they’re far apart and so they’ll just go for their billions of years. One will die, the other is going to die –
Dr. Pamela Gay: They’re a nice, happy, wide binary star.
Fraser Cain: So how close do they need to be before things start to happen?
Dr. Pamela Gay: It depends on their state of evolution and how massive the two stars are. One of the most common situations is you have two stars that are fairly run-of-the-mill. One is maybe several times the mass of the sun. The other one is more like the mass of the sun. And the bigger one evolves faster. That’s just the way space works. We’ve talked about this in a million different episodes. But because it’s evolving faster, it hits that point in life where it goes from happily burning hydrogen in its very core to, well, bloating out into a red giant star, and it’s this bloating period that gets it into trouble.
If the two stars start off maybe a couple of solar systems apart, well, as the one bloats up it’s going to get bigger and bigger, and eventually its outer layers might get just close enough to that sibling that is sitting there being a main sequence star that the sibling is able to start yanking off that material and bloat up to become even bigger.
Fraser Cain: Well let’s take a look at, for example, our solar system, right? So the sun, right now, teeny tiny, a million kilometers across – Sorry, 1.4 million miles across. But when it hits that red giant phase, it’s going to be as big as the orbit of the earth maybe.
Dr. Pamela Gay: Probably a little bit bigger than that. Now luckily conservation of angular momentum and all of that, and mass loss, and factor, and all of these things – The Earth is going to migrate outwards. So in all likelihood, according to all modern models, well not all, but many modern models, the Earth survives being absolutely consumed because it’s moved. But where the earth is now, the sun will be later.
Fraser Cain: Right. But the Earth is whatever, 150 million kilometers, so we’re looking at, really, a factor of 100 bigger that the sun is going to get, right? Although I factor 50, sorry, because in one it’s a diameter and the other case it’s rigged. But anyway, it’s going to get way bigger, right? Dozens of times bigger. And so what could have been – If it was just two stars, these two stars orbiting some common point, and they were 150 million kilometers apart, they would – It’s that when one dies and it gets that envelope that’s so much bigger, that that’s when you get those interactions.
Okay, so you’ve got these two stars. One dies first, it expands out – Or, it hits the red giant phase.
Dr. Pamela Gay: It hits the ledge.
Fraser Cain: It starts to burn some helium and hits that red giant phase, and the other one starts to go through its outer atmosphere.
Dr. Pamela Gay: It starts to eat it. So we have what’s called a Roche lobe. This is if you have a star and you have a star next to it, they have this combined gravitational field they’re creating. And at a certain point between these two stars, a particle that just sort of set down, it’s going to be gravitationally attracted equally to both of these objects. Now you can trace out an entire shell where something at that shell is just on the equally potential surface, is what we call it.
And if it goes past that shell, it goes to the other star. If it stays inside of that shell, it stays with its parent star. Well when you get two objects close enough together and one of them decides to puff up, it can puff up so that matter starts spewing through that Roche lobe. It’s called filling the Roche lobe, overflowing the Roche lobe, and now you have matter streaming from one star off to the other. Conservation of momentum means that it usually goes through a disk on the way two the other star. And once it gets there, that other star does what it will with it.
Fraser Cain: So I’m trying to sort of imagine this situation, right? So we’ve got the one star. It’s puffed out into a red giant. There’s this stream coming off of the star that’s going almost like a river to the other star, and the other star is accumulating this matter into a disk, kind of like a black hole or kind of like a protoplanetary disk like a galaxy. And it’s accruing this material onto the star. Shenanigans?
Dr. Pamela Gay: All sorts of nifty, neat things happen now. So in the least violent of cases, you have, in the end, what we call a blue straggler, and it’s been a long road figuring out what is up with these blue stragglers.
So when you look at a globular cluster, a population of really old stars, even some of the oldest open-cluster stars, what we find is there are these weirdo, “Why haven’t you finished evolving?” bright blue, young appearing stars where they have no right to be. If you have a cluster of stars, all of the stars should have formed at the exact same time. They should all be evolving in lock-step according to their mass, bigger ones doing interesting things first, smaller things doing interesting things later.
But we keep finding these random blue ones, which we call blue stragglers because they’re straggling in their evolution. And for the longest time we didn’t know where they came from. Was it the merger of two stars in a binary? Was it cannibalism? Was it something else? And in recent years, people have looked at more and more of these blue stragglers with high resolution spectroscopy and seen that that blue star was doing the dance of a binary.
It’s the same thing that we see when we’re looking for exoplanets, that little tiny wobble that’s caused by getting gravitationally yanked to and fro. And in looking at these stars, it appears that mass was transferred from the more massive companion onto the less massive one when that more massive star became a giant. And it allowed the other one to just hang out on the main sequence all that much longer.
Fraser Cain: So when you say hang out on the main sequence that much longer, you mean it’s like it’s gathering fresh fuel that it can then use and continue to burn? And so when it would maybe have died, it now gets a second chance at life.
Dr. Pamela Gay: It ends up being able to support a larger core, have more matter down in that core, and just, yeah, get a second chance at life. These things look like young stars in almost every way. It’s their deficiency in lithium, but yeah.
Fraser Cain: But why do we call it a blue straggler?
Dr. Pamela Gay: Because all the other stars in the family have already evolved. So why is this star still straggling behind the crowd? Why hasn’t it evolved on like everybody else? And it’s because it got this second chance at life.
Fraser Cain: Okay. I see. I see. So you’ve got this cluster that you would assume has all formed together at the same time. And as we know, the most massive stars detonate a supernova first and they’re gone. So you shouldn’t see any of these young, hot, blue stars. They should all have died a long time ago. And yet, when you look in this cluster, you see these stars that have no place there.
So instead of actually being young stars, they are actually older stars that have been fed by a companion and they have become rejuvenated like some horror movie.
Dr. Pamela Gay: Yes. They’re zombie stars. We need to start calling them zombie stars.
Fraser Cain: Right. But I think if some older star feeds on the life force – Vampire stars, I think, is probably better. It feeds off the life force of its companion star and then gets a second chance at youth. It’s a science fiction/fantasy theme.
Dr. Pamela Gay: It comes right out of the DC universe. It’s the old evil killing off the younger one to have immortality.
Fraser Cain: Right. Okay. So this is if they are, I guess, if this happens in a reasonable period of time, what other kinds of – I asked for some shenanigans. So what else can happen in this kind of scenario?
Dr. Pamela Gay: So the blue stragglers are, like I said, they’re the boring ones. We also have the Algol variables that we’re actually catching them in the process of the mass transfer. So blue stragglers, they’ve finished. They’re done. They’ve ate everything there was to ate. The Algol variables are variables that we actually see them in the process of consuming their nearby neighbor. We call them this because Algol itself is one of these kinds of variable stars.
So again, in this case, it’s the one star became a sub-giant and it transferred its mass to the lower-mass star.
Fraser Cain: How much of the material will it consume? Will it kill the other star dead?
Dr. Pamela Gay: It turns out that if it’s the more massive star that’s transferring its mass to the lower mass companion, this is a fairly stable mass transfer. And it can go on and on and on until all that’s left behind is essentially the white dwarf core of the other star, so it just eats the whole atmosphere off.
Fraser Cain: Wow. But in the case of a star, like our own sun in the future, it’s going to blow off that outer atmosphere anyway and then turn itself into a white dwarf. So it’s almost like it’s going through that same process. But in this case, instead of that whole atmosphere being blasted off into space and making a pretty planetary nebula, it’s donating that outer atmosphere to this other star to give it a second chance.
Dr. Pamela Gay: It’s a bit of an accelerated process though. Like our sun, it could hang out and be a Mira variable for a great deal of time. But instead, you basically have the neighbor just gravitationally slurping off the life force of the other star and leaving behind – It’s like an aging drug.
Fraser Cain: I’m now thinking it’s more like organ donation. The bigger star is just kind of giving the material to the younger star a little sooner and perhaps a little more violently than it was planning, but still. Okay, so what if those stars are closer together, though? Right? Like, what if one star is literally in the atmosphere of the other star?
Dr. Pamela Gay: You can actually get shared envelope stars now and then. We’re still trying to understand them. There was one case a few years ago where we’re pretty sure we captured one in the act. It had the crazy name of Nasty One. That’s actually what we called it, because its classification was N-A-S-T 1, from the catalogue it was in. This was a Wolf-Rayet star that was in a binary system in a shared nebula with violent stellar winds, and it was impossible, and is impossible, to see all the way down into the core to fully understand what is going on.
But observations with Chandra led folks to believe that this probably, probably, is a case where they’re somewhat sharing this violent outer cloud of gas and dust.
Fraser Cain: If you get a star kind of within another star, is that going to act like an atmospheric drag? Are the stars going to spiral into each other?
Dr. Pamela Gay: Yeah. Well, it depends on if this is a permanent phase or a temporary phase. There are different cases where the possibility – For instance, in some of the carbon variables where you see a star just suddenly, essentially, wink out, it’s thought that perhaps this is a Conman envelope problem where you have a white dwarf spiral into a normal star, and this just messes with the outer layers of the star.
In cases like that, where it’s two stars in the process of merging, shared envelope, it’s probably exactly what you’re talking about. Nice drag, happily merge and everything goes well. There are cases where they have a temporary shared envelope. Things get blown away and then you end up with two naked cores later.
Fraser Cain: So what happens if the star spirals all the way in and hits the core? What happens if they actually merge in the middle?
Dr. Pamela Gay: That is fine. It’s a bit violent, but it’s fine.
Fraser Cain: Right, because you’re only going to get maybe two, three times the mass of the sun. Probably not a super nova. Okay. All right.
Dr. Pamela Gay: Yeah, it’s not a big deal. You’re going to get X-ray flares when they first mix. But it’s okay. We know who to deal with X-ray flares. We observe them. We stare at them. They’re fine.
Fraser Cain: In that we know how to recognize them in our telescope. We don’t know how to deal with them.
Dr. Pamela Gay: So the truly violent things actually don’t involve the cores merging. The truly violent things involve cannibalism where you have a small, compact object of high mass violently stealing the life force off of a lower-mass companion.
Fraser Cain: Right, but I mean, think about this scenario. You’ve got these two stars, you’ve got the bigger, hotter one dies first, feeds all this material off to the other star. And now it’s left, and it’s a white dwarf, and the stars keep orbiting one another. And then the second star dies, puts off its material, feeds the white dwarf, and now you’ve got a different situation happening.
Dr. Pamela Gay: And there are scenarios where people have projected, “Maybe you can go back and forth,” but because of the way the angular momentum works, it’s not a perpetual energy system where you constantly are bouncing an atmosphere back and forth between two different stars.
Fraser Cain: Well no, I was setting you up for what happens when a white dwarf is the object that’s stealing material from its companion object.
Dr. Pamela Gay: Oh, okay. Sorry, I failed to recognize that.
Fraser Cain: Do you need me to do this again?
Dr. Pamela Gay: No. So let me step back. So usually when you get a white dwarf, it’s not because it’s been stripped naked by its peer. Normally you just have one star that, again, it’s more massive, it evolves faster, but in this case it gets to hold onto its atmosphere. It goes through normal evolution. It lands as a white dwarf star, kind of like our sun will someday in the future. But for a variety of reasons, the stars may end up migrating closer together or, as its neighbor star, as the lower mass companion, much more likely, as it bloats up, as that lower mass companion decides to be a main sequence star and becomes a giant star of one form or another, it overflows its Roche lobe and material starts to stream onto the higher mass companion.
Now as it streams in, that white dwarf, it’s just going to crush the material violently onto its surface. And the cool thing is, as it crushes that material on its surface, when that material gets crushed to the point that it’s about 20 million degrees’ kelvin, it’s going to violently undergo nuclear fusion, which is another way of saying it acts like a lot of nuclear bombs. And via the carbon nitrogen oxygen cycle, it’s going to happily burn kind of like the core of a star, but on the outside of a star.
Fraser Cain: Right. So you’ve got, literally, the fusion like the core of the sun, but on the outside, on the surface, of the sun. So what happens then?
Dr. Pamela Gay: Well luckily it blows off a lot of this extra mass. So it flares up, does its, “Hi, I’m a nova,” thing, blows off all of that energy, and then goes back to sitting there. Now as its neighbor star, yet again, bloats itself up, it may transfer a little bit more mass, and this is where you can end up with a recurrent nova kind of situation. Now what’s cool is we get to see these things as x-ray binaries, and in some cases, we actually end up with what we call micro-quasars. This is where that accretion disk around the compact companion has all of the physical properties of a normal quasar, except it’s just around a stellar mass black hole or neutron star instead of around a super massive black hole.
Fraser Cain: But isn’t it something wonderful that happens when the white dwarf has consumed a certain amount of material from its partner?
Dr. Pamela Gay: It’s true. There are cases. See, I’m thinking all about the black holes because they’re cool, but if you have a white dwarf that’s happily being cannibalistic and nomming on its companion, eventually, just like all the rest of us that overeat, it gets fat. And once it exceeds about 1.44 solar masses, kapow, it kind of goes off as a type 1A supernova.
Fraser Cain: I actually just did some research on this, and there’s nothing left. There’s no black hole. It’s just gone. But these, of course, the type 1A supernova, these are those standard candles that astronomers use to measure distance in the universe, because you got that wonderful 1.4 times the mass of the sun every time. That’s when it happens, which is super cool.
Dr. Pamela Gay: And all of these different little compact binaries, they have amazing physics tied up with them. So we have the white dwarfs that consume too much. They go off as type 1A supernovae. They are our standard candles. But then we also have millisecond pulsars are actually spinning that fast because they are in low-mass X-ray binary systems, they stole material off of their partner, and that theft of material was kind of like an ice skater drawing in her arms. It spun them up so that they ended up going, well, hundreds of times a second around their happy little rotational axes.
Fraser Cain: Well, what if these things collide?
Dr. Pamela Gay: Well that’s where we get gravitational waves.
Fraser Cain: And things like gamma ray bursts and hyper novae.
Dr. Pamela Gay: Yeah. It’s thought that short gamma ray bursts are probably caused by the merger of neutron stars, and that’s just kind of awesome to think about.
Fraser Cain: So you’re not going to get collisions like two stars happening to collide with one another. You’re going to get a situation where you’ve got these two stars that are orbiting each other in this really close binary situation, and eventually they spiral inward, because of gravitational waves, and merge with each other.
Dr. Pamela Gay: And that gives us violent gamma rays and afterglow that is very short in duration. I think we’ve caught them two or three times at most. And yeah, explosions are cool. And what’s awesome is globular clusters are kind of the ultimate place to go looking for this kind of stuff because, while all of the stars in a globular cluster more or less formed at the exact same time, the binary stars that we see in globular clusters aren’t necessarily binaries where the stars formed together, because these stars are so close together that they periodically swing past one another and are like, “Oh, let’s co-orbit.”
So you have new binary systems forming, not on a regular basis, but statistically more frequently than anywhere else. So you have these situations where stars can end up close together, cannibalism can end up happening not just between siblings, but between cousins, and you get all of these neat X-ray binaries and millisecond pulsars and everything else.
Fraser Cain: And we’ve mentioned this many times before, which is that when we have some really great survey instruments, things like the large synoptic survey, we are going to see these kinds of events with more regularity, because we’re now scanning the whole sky. And we’re going to be looking at any brightenings, any movement, any explosions. It’s a great time to be in this field of science.
Dr. Pamela Gay: So it’s a neat future where we get to finally start learning the actual frequency of all of these different blue stragglers of current novae, and thank Chandra and Fermi for all of the millisecond and other X-ray binaries that are out there.
Fraser Cain: Awesome. Well thanks Pamela.
Dr. Pamela Gay: My pleasure.
Fraser Cain: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astro Sphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at firstname.lastname@example.org . Tweet us @astronomycast. Like us on Facebook, or circle us on Google Plus.
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