Stars often come in groups of two or more. And if they’re orbiting close enough to each other, one star can feast on the other. And when that happens, well, mayhem ensues.
Red Dwarf (Swinburne)
Stellar Evolution (AAVSO)
What Are Multiple Star Systems? (Universe Today)
Stellar nucleosynthesis (Wikipedia)
What Is a Supernova? (NASA)
Carbon Stars (Universe Today)
What will happen when our sun dies? (EarthSky)
Planetary Nebulae (Center for Astrophysics | Harvard & Smithsonian)
Stellar cannibalism transforms star into brown dwarf (Science Daily)
PODCAST: Ep. 563: White Dwarf Mergers (Astronomy Cast)
Pulsating Variable Stars (CSIRO)
Cataclysmic variable star (Wikipedia)
Type Ia Supernova (Swinburne)
Electron Degeneracy Pressure (Swinburne)
What is a neutron star? (EarthSky)
Standard Candle (Swinburne)
Supernova ‘standard candles’ not so standard after all (Cosmos Magazine)
What is the Cosmic Microwave Background? (Universe Today)
PODCAST: Ep. 541: Weird Issues: The Expansion Rate of the Universe (Astronomy Cast)
What is a globular cluster? (EarthSky)
Blue Stragglers (Swinburne)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast Episode 593: Stellar Parasites. 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. I’m Fraser Cain, publisher of Universe Today, and with me as always is Dr. Pamela Gay, a Senior Scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela! How you doing?
Dr. Pamela Gay: I’m doing well. I think I’m one of the few people in the US who didn’t get snow today, and I’m kind of okay with that.
Fraser: Yeah. We were talking before the show that I just got all the weather. I collected the whole set. I got all the Pokémons of the weather. Sleet, freezing rain, hail, snow, sunshine, rain, everything. Yeah, so don’t worry. I got you covered.
Dr. Pamela Gay: It works. I just have grey. I have lack of anything.
Fraser: Yeah. That’s standard West Coast Canada weather. All right, well we’re going to get on with the show in a second. But first, let’s have a commercial break.
And we’re back. All right, so stars often come in groups of two or more. And if they’re orbiting close enough to each other, one star can feast on the other. And when that happens, well, mayhem ensues. Now, I actually had a little bit of a difficult time writing this introduction, because in the olden days, you would say the vast majority of stars are in binary, or multiple groups. But that’s no longer true. The vast majority of stars are singletons, because the vast majority of stars are red dwarfs, and red dwarfs are largely singletons. The stars like our Sun, or especially the heavy stars, they come in multiple partners. So, I’m trying to think of a way to say this, but I just couldn’t.
Dr. Pamela Gay: Yeah, it’s trickster-y, because –
Dr. Pamela Gay: – the kind of thing that we’re talking about today only happens when you have stars that are big enough to evolve.
Dr. Pamela Gay: And a red dwarf star that formed at the beginning of the universe would still be out there happily doing its red dwarf star thing.
Dr. Pamela Gay: For better or worse, the first generation of stars only came in extra-large. So, we don’t have any of that first-generation red dwarf, but there’s lots of really aged red dwarfs out there. And they’re really polite, and they tend to be all alone –
Dr. Pamela Gay: – and not eat one another.
Fraser: Right. And so, if someone tells you that, “Hey, did you know that most stars are actually in multiple star systems?,” you could say, “Mmm, no. That’s not true anymore. Sorry.”
Dr. Pamela Gay: Most large stars.
Fraser: Most large stars. Stars like the sun and bigger are in multiple groups. And most of the time, they’re fine. They’re –
Dr. Pamela Gay: Yeah.
Fraser: – far apart, and everything is fine. But every now and then, they get close to each other, and then we’ve got some problems. So, set the scene. When do we get problems?
Dr. Pamela Gay: We get problems when one star finishes doing all your standard nuclear reactions in its core, runs out of life, and becomes a small hungry beast. Now, to put all of this in more scientific terms, stars of different sizes go through their fuel at different rates. The bigger the star, the faster it burns through the fuel in its core. And a star can only fuse so many things in its core before it hits the point that it has an iron core. And when you try and fuse two atoms of iron, the iron is like, “Nuh-uh. I’m not releasing any energy. You can’t do that to me.”
Dr. Pamela Gay: And when fusion no longer generates power, when it no longer generates energy to support the outside of the star, the star collapses, you get a supernova if it’s really big. Or, if it’s a smaller star that simply hits the point of, “I’ve fused my core up to carbon, and I don’t have enough gravity to go any higher up the periodic table.” In that case, it just exhales its outer envelope, and you’re left with only the core of the star behind.
Fraser: Sorry, I just wanna stop on that for one second. I know this isn’t what this show is about, but that idea of a star like our Sun –
Dr. Pamela Gay: Yeah.
Fraser: – bloating up as a red giant, and then just puffing away those outer layers. When you think about the sun, it’s got that core where all the fusion is happening. You’ve got that radiative layer where light is bouncing around inside, and you’ve got that outer convective layer. And that radiative layer and the convective layer go. They just puff away. And what you’re left is the fusion engine at the heart of the sun. Just laid bare to the universe. It’s such a crazy idea. And that’s a white dwarf.
Dr. Pamela Gay: And one of the awesome things about this is because our solar system is one with planets, it’s essentially going to form an hourglass-like structure. So, these systems that I’m talking about that have this white dwarf core next to another star, all have or had at some point in their history – ‘cause planetary nebula aren’t forever – had this planetary nebula that was some extraordinarily complex shape associated with it. So, you have two stars. One is a little bit higher mass, so it’s evolving faster. Or a lot higher mass, it doesn’t matter. It’s bigger. Bigger star evolves faster. By hook or by crook, it gets rid of its outer layers, and you’re left with this extraordinarily dense leftover.
Fraser: Right. So, in all of those cases, you’re left with in the white dwarf core of a star, the case of a neutron star, you’re just left with a degenerate tiny little ball that’s been around –
Dr. Pamela Gay: Manhattan-sized blob.
Fraser: Right, exactly. And once again, if it’s all by itself, then that’s just what it is until it dies. Turns into whatever else it’s going to turn into. So, what kind of situation will it be in to have a companion that you’re gonna run into some kind of situation?
Dr. Pamela Gay: Well, for some reason probably having to do with all of that material that it’s given off in the past, it has a chance that it could spiral closer to its companion. Or much more likely, its companion as it gets older is going to expand out in size. Until one day as it’s expanding in size, becoming a red giant star, its outer atmosphere gets within gravitational reach of that jealous companion.
Fraser: Right. And we know that for example with the supergiant stars like Betelgeuse and things like that, they extend out beyond your bit of Jupiter, all the way up to Saturn. Some go beyond Saturn. And so, you can imagine if there is a binary companion and stars are close together, you could easily have one star orbiting within another star.
Dr. Pamela Gay: And you don’t even have to have one star within a star to talk about the things we’re going to talk about today. All you need is the dense object, the other object, and gravity.
Dr. Pamela Gay: And what happens is that companion star, as its atmosphere begins to expand out, it hits this point first where the atmosphere is exactly balanced between the little companion and that bigger star. The lower mass star, and the core of that lower mass star. This is called, “Filling the Roche-lobe,” because it’s lobe-shaped and –
Dr. Pamela Gay: – the person who figured it out was named Roche. Now, if they get even a snert with the surface of the bigger star closer to the compact star, matter will start flowing from the one star to the other. And spiraling in, there’s no direct fall. It’s a spiral in. And, depending on how the stars are working, lots of different horrible things that are glorious scientifically can now take place.
Fraser: All right. Well, we will talk about that in a second. But first, let’s go to a break. And we’re back. All right, you promised all kinds of glorious things, which in your disaster-addled brain, makes me a little nervous. So, what you think are glorious, some others might see as the apocalypse. So, what happens?
Dr. Pamela Gay: So, one of my favorite examples of this is there was recently since 2015, discovered a situation where there was a brown dwarf star and a white dwarf star side-by-side. And it was realized that brown dwarf, this is a star order of 80 times the mass of Jupiter –
Dr. Pamela Gay: – no longer capable of having sustained fusion in its core. It didn’t start out that way. Instead, it started out as a quite happy regular everyday star capable of undergoing regular everyday nuclear fusion as stars do. But as that white dwarf got a little too close, it just started stripping and stripping and stripping its companion until its companion’s like, “Whoa! I don’t have enough mass left. I’m brown dwarf.”
Fraser: So, a stellar companion turned a main sequence star? It downgraded it?
Dr. Pamela Gay: It downgraded it.
Fraser: To a brown dwarf?
Dr. Pamela Gay: Yes.
Fraser: That’s crazy.
Dr. Pamela Gay: Yes.
Fraser: That’s amazing.
Dr. Pamela Gay: And the universe likes to surprise us with all the crazy combinations of things that are possible. So, we have cases where that white dwarf can completely strip down its companion. We also have cases where that white dwarf will as you’ve alluded to earlier pass into its companion. One of my favorite cases is one that we discussed earlier this year, where while I said that the two stars evolving next to each other, the higher mass one evolves down into a compact object first, and the lower mass one evolves slower. That assumes these two objects formed together. And –
Dr. Pamela Gay: – in high-density regions, you can have two stars that formed completely separately, gravitationally come together and become a binary star late in their lives, which means you can end up ever so rarely in situations where you have a white dwarf star that evolved who knows where. And then you have a massive young companion star.
Fraser: Okay. Right, because the massive companion stars, you’re gonna expect them to blow up. So, it’s very rare to see one with a partner that should be billions of years old. That seems weird.
Dr. Pamela Gay: Now the cool thing is that if you have that little, tiny white dwarf star, and that big old massive companion star, that white dwarf can strip so much material off while going into that other star.
Dr. Pamela Gay: So, it’s spiral into its companion. It’s consuming its companion. And they can both end up exploding.
Fraser: Right. And I think we did an episode about this, didn’t we?
Dr. Pamela Gay: We did. They –
Dr. Pamela Gay: – are awesome. These are called, “Double Degenerate Stars.”
Fraser: Double Degenerate. Yeah.
Dr. Pamela Gay: It goes boom in two ways at once.
Fraser: Twice the magnus. Yeah. But that’s gotta be really rare, that you’ve got a supergiant star happening to pick up a white dwarf at exactly the time – ‘cause they only live for a few million years. So, you’ve gotta have –
Dr. Pamela Gay: Yes.
Fraser: – some pretty weird circumstances. And that’s why they’re incredibly rare. I think we’ve only seen one, maybe two of these events in all of Astronomy.
Dr. Pamela Gay: One that we knew, and… yeah. So, this now paints our two extremes.
Dr. Pamela Gay: We have the brown dwarf getting formed because the white dwarf ate everything that made it into a star. We have the double kablooey of the white dwarf moves inside of its massive young companion.
Dr. Pamela Gay: Now, the bulk of the systems are somewhere in-between.
Dr. Pamela Gay: In the majority of cases, you have a star that is bigger than our Sun by not a lot, a star that’s the size of our Sun or a little bit smaller, 2 to 5 solar masses, basically between these two systems.
Dr. Pamela Gay: And the one becomes a white dwarf, the other when it evolves into a red giant fills its Roche-lobe, and just starts tearing material off. And it can do this in a cyclic way. A lot of these stars when they evolve will become what’s called, “Pulsating Variable Stars.” And so, when that star gets bigger, it will sporadically dump material –
Dr. Pamela Gay: – onto its companion.
Fraser: So, it’s filling up the feeding trough for its parasite, and then it shrinks back down, and then the companion star digests, and then the variable bloats up again, sloughs off new material into the Roche-lobe, shrinks back down over and over again.
Dr. Pamela Gay: And so, this is exciting. You have mass transfer, it’s periodic, this leads to all sorts of cool lighting things up as the mass transfer –
Dr. Pamela Gay: – happens. But it’s not as dramatic as periodic things can come. One of the other cool things that can happen when you have these two systems is this one that has filled up its Roche limit will spiral material onto a disc next to its companion. And one of two things will occur. Either that disc will transfer matter onto the star until enough transfers onto the star that it blows up temporarily.
Fraser: Right, yeah.
Dr. Pamela Gay: As one does.
Fraser: Yeah, and again, we’ve talked about that. That’s your straight up classical nova.
Dr. Pamela Gay: And the other case is where the disc periodically goes kaboom, –
Dr. Pamela Gay: – because the disc can actually get dense enough along that center compression area of the disc, that it can support nuclear reactions. And so, if the disc gets too hot and dense, it can go boom, –
Dr. Pamela Gay: – and essentially clear itself out.
Fraser: So, what causes which? If it’s a trickle of material, it gets wound up onto the white dwarf like string going around the white dwarf, piling up on the surface until there’s enough material there that it detonates as a nova. But if there’s just way too much material, it flattens out into this great big accretion disk. And if it’s too much material, then that accretion disk starts to just explode with activity as well, because there’s just too much material and it’s starting to turn into little mini-stars. All right, that’s pretty crazy. Now, there is a more cataclysmic ending to this process, which we will talk about in a second. But first, we should stop for another break.
And we’re back. All right, so we’ve got the situation where you’ve got this one star feasting on this other star, gathering material, it’s detonating, but it’s getting heavier over time. This can’t be good. Where does this go?
Dr. Pamela Gay: So, the other case is what’s called a Type 1A Supernova that is normal.
Fraser: I love that there are abnormal ones too.
Dr. Pamela Gay: There’s plenty of –
Dr. Pamela Gay: – abnormal ones. The case of a star falling into another star that we’ve discussed earlier –
Dr. Pamela Gay: – was technically a Type 1A Supernova, but not a normal one.
Fraser: Yeah. From the inside, yeah.
Dr. Pamela Gay: Exactly. So, with a Type 1A Supernova, it turns out thanks to quantum mechanics that electrons and protons can only get so close to one another before all of the forces that caused them to be separate particles give up, and the two combine, become a neutron, and give off energy and particles in the process. And if you pile too much material onto a white dwarf star, the pressure of all that electron degenerate gas pushing out and those protons all existing, it’s no longer able to support the star against the inward push of gravity. And when protons and electrons can no longer stay separate, when that electron-degenerate gas can no longer support the mass of the star, it collapses, –
Dr. Pamela Gay: – becomes a neutron star, things go boom along the way –
Dr. Pamela Gay: Yes. It’s awesome.
Frasier: Utterly. We talk about when other kinds of stars explode, you’re like, “What kinds of remnant is left behind?” In some cases, it’s a neutron star, in other cases it’s a black hole. But in the case of a Type 1A Supernova, it’s just gone.
Dr. Pamela Gay: Yeah.
Fraser: It’s just 100% energy. Again, think about that process, that it is tearing itself apart at an atomic level, and just detonating. The whole thing just becomes one huge bomb.
Dr. Pamela Gay: And this happens at a very precise point that is based on how much mass is in the system. And there’s some variability that is related to how fast the star is rotating and things on that order. But the dominant factor is how big is that star? And just like if you have two sticks of dynamite that contain the exact same amount of explosive, the explosion they make might vary a little bit depending on temperature and humidity, but they’re basically gonna be the same size explosion.
Dr. Pamela Gay: And because Type 1A Supernovae have this consistently-sized explosion, we use them as standard –
Dr. Pamela Gay: – candles to measure distances throughout the universe.
Fraser: Or so we thought, because – and again, we’ve talked about this fairly recently –
Dr. Pamela Gay: We don’t necessarily accurately measure distances throughout the universe.
Dr. Pamela Gay: We are still measuring distances.
Fraser: Yeah. That up until probably the last year or so, astronomers were very confident that Type 1A Supernova were very standard candles blowing up when the star hits 1.4 times the mass of the sun. But now, maybe it’s a little different? Maybe depending on the kinds of material that were made from ones blew up a little earlier? Later? Depending on when they formed in the universe, which could possibly be contributing to the measurements of dark energy. It got weird, is all I’m saying. Yeah.
Dr. Pamela Gay: There’s a lot going on, and –
Dr. Pamela Gay: – different episode.
Dr. Pamela Gay: Complex story, all of the details short. There’s multiple ways to measure the age of our universe. One of them includes supernova and a bunch of other stuff, the other includes the cosmic microwave background. And these two sets of observations do not match.
Dr. Pamela Gay: Thus, we have confusion.
Fraser: Yeah. The crisis in cosmology, which is a whole other episode that we’ve done. Okay. But these are all extremes. But can we just get a situation where you just got two stars, regular old stars –
Dr. Pamela Gay: Hanging out swapping matter?
Dr. Pamela Gay: Yeah!
Fraser: Okay. Tell me about that. Now, kiss.
Dr. Pamela Gay: There are cases out there where stars as they evolve take turns swapping material back and forth, where essentially you can have what was a dead star reignite if you feed material to it just right. And when we look at globular clusters, we see groups of thousands to millions of stars that all formed out of these same original cloud of material in the same period of time. So, they should be the same age. Except in these globular clusters which are old, –
Dr. Pamela Gay: – we see these weird young blue stars that for ages, we couldn’t figure out. And around 2000, we started to build up this picture where it appears that blue stragglers are stars that simply stole enough material or combined with their neighbor, and in the process, ended up with a new young star that still has a full life ahead –
Dr. Pamela Gay: – of it out of the cinders of dead stars.
Fraser: And so, I think it’s important to say we know that these globular clusters formed billions of years ago, pretty much at the –
Dr. Pamela Gay: Yes.
Fraser: – edge of the universe. And so, there should only be old stars. And when you think about the sun – and our Sun’s only been around for 4.5 billion years, so it wouldn’t have formed in a globular cluster. And yet they see blue stars. And blue stars only last for a couple million years. So, that’s super weird. Shouldn’t see that. And yet, there’s so much mayhem going on in a cluster like that, that you’re gonna get these collisions happening. But still, is there a situation where you just got two stars forming, and one star like our Sun is feeding off the material from another star? What if one star goes red giant, and it has just a regular stellar companion beside it? Can you have a situation where those stars will feed off each other?
Dr. Pamela Gay: So, that’s gonna be a lot more rare, because they’re not going to tend to form close enough to allow that kind of a situation to happen with the degenerate stars, the old stars. All the things that you’d go through in the aging process if you’re a star can help you move around. If you start out with a star like our Sun and a significantly bigger companion that isn’t yet that cinder but has simply bloated out to the red giant stage, it’s possible that that red giant can transfer matter to its companion.
And we see this in the atmospheres of stars occasionally, where they’ve essentially been blasted with evolved material from their companion star. But that whole filling the Roche-lobe and sucking material onto the main sequence star will be more rare. But rare doesn’t mean it doesn’t happen.
Fraser: Mm-hmm. Yeah, it’s a big universe. Every experiment you could imagine has been tried by the universe.
Dr. Pamela Gay: And our universe keeps proving itself far more creative than theorists are. Admittedly, theorists are humans, and our computers can’t run that many experiments yet. But the universe keeps proving itself far more creative. And so, we do see things that when we think hard about them, we’re like, “Huh. Yeah, this probably ate its companion. This was a vampire. This was a cannibal.” These are words that we –
Fraser: Right, yeah.
Dr. Pamela Gay: – use in describing how stars keep finding new and innovative ways. And so far, we’ve largely blamed white dwarf stars –
Dr. Pamela Gay: – for doing the –
Dr. Pamela Gay: – vampirism. But with the situation we were talking about earlier where you can either have the disc exploding or material build up on the surface of the star exploding, well neutron stars, these are stars that are more massive than white dwarfs, which are about the size of the moon and the mass of the sun. Neutron stars are more like 2.5 times –
Dr. Pamela Gay: – or more of the mass of the sun. And they’re 20 miles across. That’s 30 kilometers or so. Inaccurate unit translations do not at me. And these little, tiny dense objects are perfectly capable of pulling material off of a companion, of building up that disc that goes boom, and completely stripping down a neighbor. And because the next size up from neutron star in terms of mass density is black hole, you can put a lot of stuff on them before they explode as a supernova, because they are trying to compress down into a black hole.
Dr. Pamela Gay: And this is where you end up with even more discs that are likely to go boom.
Fraser: Right. And again, imagine when you see a huge star with a couple times the mass of the sun, and it’s orbiting with a neutron star. The neutron star is gonna be bright, but it’s gonna be this tiny little spot. But it’s gonna be the dominant gravitational force in the system between a star like our Sun. I sort of imagine… I don’t know, one of those hammer throwers, right? That’s just yanking the star around and around in these circles. And then it’s siphoning material off and into an accretion disk around the neutron star until the neutron star has had its fill and detonates as a supernova.
Dr. Pamela Gay: And what’s amazing is when we look at these things, we will see that disk of material periodically going around and blocking the light from the larger star. And so, we can see and measure accretion disks by how they interfere with our ability to see that much larger companion.
Fraser: There’s one other kind that I wanna talk about before we wrap up. You can have a situation where two stars are so close together that it’s almost like there’s a connection between them. A bridge between the two stars. They’ve filled each other’s Roche-lobes. Let’s talk about that.
Dr. Pamela Gay: And these are common envelope stars. And technically, anytime two stars share an atmosphere, it’s called a common envelope. So, that case of a white dwarf plunging into its neighbor? Technically, a common envelope.
Dr. Pamela Gay: Technically.
Dr. Pamela Gay: But the more interesting case is where you have regular everyday stars. So, two very similar-massed red giant stars that expand in size. And, they might’ve started out with a separation from core to core of twice the distance to Jupiter. But then as they expand out, and their physical size starts to be so that each of them are roughly the same distances from the center of the sun to Mars, that puts their outer envelopes close enough to really start pulling on each other. And they can dynamically affect one another’s orbit this way. Tidally lock, migrate closer and closer, and eventually end up with a shared envelope as their cores continue to spiral together.
Dr. Pamela Gay: Now, we can’t see inside stars. We don’t’ always know when this is happening. We have interesting suspicions. There are a few papers out there. This is not a leading idea; it is a cool idea. There are a few papers out there saying that some of these really dusty stars that we see like our Corona Borealis, that periodically shed massive amounts of dust and appear to disappear behind their own dust. There’s some theories that these might be some sort of a common envelope effect. We really don’t know. The universe will do anything given the opportunity and gravity.
Fraser: Yeah, totally. Absolutely fascinating. Thank you so much, Pamela. Do you have some names for us this week?
Dr. Pamela Gay: I do. So, as always, we are supported thanks to the generous contributions of people like you. We know that 2020 and now 2021 are not the easiest of years. And the fact that you’re out there supporting our show means that we can pay Rich and Ally to put together the best show possible. And that really means a lot to us. You also allowed us to pay Hexacosichoron, Noel Ruppenthal, to do a new TV opener for us. So, those of you watching this on YouTube have seen that new opener. You’re allowing us to help people –
Fraser: I haven’t seen it.
Dr. Pamela Gay: Oh! It’s awesome!
Fraser: Okay, I’ll check it out. I didn’t know it existed.
Dr. Pamela Gay: Sorry.
Dr. Pamela Gay: I was just trying to get the show out the door. This is why we’re changing our –
Dr. Pamela Gay: – production schedule so that there’s fewer things on fire while we try and get things out the door. Anyways, you’re allowing us to make sure that there are people out there who are taken care of and allow us to keep producing science.
So, this week I wanna thank Frode Tennebø, Justin Proctor, David Gates, Claudia Mastroianni, Alex Raine, Abraham, Cottrill, Joe Wilkinson, Jean-François Rajotte, Eran Segev, Paul L Hayden, Matthew Horstman, John, Jeremy Kerwin, Brent Kreinop, Omar Del Rivero, Arthur Latz-Hall, Michelle Cullen, Dustin A Ruoff, William Lauer, Tim Gerrish, marco iarossi, Mark Steven Rasnake, J. AlexAnderson, Brian Kilby, Gregory Singleton, planetar, Corinne Dmitruk, Jason Thomas, Ruben McCarthy, Geoff MacDonald, Iggy Hammock, and Wayne Johnson. Thank you all so much for allowing us to do everything we do. Thank you.
Fraser: Thank you, everybody. And we’ll see all of you next week.
Dr. Pamela Gay: Buh-bye!
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