You think the Sun is a terrifying ball of fire and fury? Wait until you learn about today’s topic: Wolf-Rayet stars! These are massive, dying stars hurling their outer layers out into space before detonating as supernovae. Big stars live brief lives, and Wolf-Rayet stars are the punctuation mark we see before things go supernova.
Show Notes* What are Wolf-Rayet stars? Massive, short-lived stars near the end of their lives * Extreme properties: + Temperatures: 30,000–200,000 K + Strong stellar winds and intense mass loss * Historical discovery and early confusion with helium spectral lines * Powerful convection brings carbon, nitrogen, and oxygen to the surface * Massive outflows enrich space with carbon (dust/“soot”), seeding future stars and molecules * Formation of temporary planetary nebulae around massive stars End-of-life scenarios: + Core-collapse supernova + Pair-instability supernova + Direct collapse into black holes (“failed supernova”) * Binary systems create complex structures (spirals, shells) via colliding stellar winds * Connection to gamma-ray bursts (GRBs) and hypernovae * Example systems: + Gamma Velorum (naked-eye Wolf-Rayet star) + WR104 (potential GRB candidate, not Earth-threatening) * Role in cosmic chemistry: key sources of organic-building elements * Rare, rapidly evolving objects—understanding continues to evolve with new observations * Future discoveries driven by surveys and AI-powered data analysis
TranscriptFraser Cain:
AstronomyCast, episode 786, Wolf, Rye, Stars. Welcome to AstronomyCast, 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, I'm the publisher of Universe Today.
With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hello, Pamela.
Dr. Pamela Gay:
You didn't ask me how I'm doing, and I don't know what to say.
Fraser Cain:
I know.
Dr. Pamela Gay:
Okay, other than hello, have you seen a bear yet?
Fraser Cain:
No. No bears yet. No.
Oh, well.
Dr. Pamela Gay:
But you still have daffodils?
Fraser Cain:
Pine martin. No, no daffodils yet. Pine martin crocuses.
Dr. Pamela Gay:
Okay.
Fraser Cain:
And our various flowering fruit trees are about to flower. So we're about to have the flowering cherry festival here on my property. So apart from that, no, it's good.
But we're going through all of the things that we have on my spring calendar. I think I mentioned this years ago, that we put down a calendar of all of the momentous events that happened. So you don't just have four seasons, you actually have about 50.
And a bunch of them are showing up a lot earlier. So we had the frogs going berserk about a week early. The crocuses are up a little early, but it is sort of interesting.
I mean to keep track of them, and so we can see how the dates change from various events happening. But yeah, yeah. And the big thing of course is the herring are spawning, which is like just search for herring spawn Vancouver Island, and it will blow your mind.
It is the largest biomass movement on earth. Bigger than, will the beast, it's bigger than anything. It is out of this world.
And when it happens, it changes our landscape, changes the beaches, changes the oceans, the wildlife everywhere. It's next level.
Dr. Pamela Gay:
That's kind of amazing and also very gross simultaneously.
Fraser Cain:
Yes. Yeah. Yeah.
But let's just say that spawning herring means that everything on the beach is covered, in some cases, multiple feet of material.
Dr. Pamela Gay:
Yeah, yeah. That's the correct euphemism.
Fraser Cain:
Yeah. Yeah. You got to wear boots to walk through what's on the beaches.
So yeah.
Dr. Pamela Gay:
Wellies. Everyone needs wellies.
Fraser Cain:
Yes. You think the sun is a terrifying ball of fire and fury? Wait until you learn about today's topic.
Wolf-rayet stars. These are massive dying stars hurtling their outer layers into space before detonating as supernovae. All right.
Wolf-rayet.
Dr. Pamela Gay:
I love the fact that we both pronounce it very differently because I always said Wolf-rayet I don't speak French though.
Fraser Cain:
Yeah. Yeah. I do believe I'm going to go with you.
I'm pronouncing it in a way that a French person might want to pronounce it. Yeah. But definitely not rye-yet.
Dr. Pamela Gay:
No. No. That is, that is completely wrong.
That T just pretend it's not even there.
Fraser Cain:
Yeah. Yeah. I've heard people say it.
So yeah.
Dr. Pamela Gay:
But anyway, so what are they? They are giant stars that are contemplating going boom. These are one of the early massive astrophysics confusions.
One of my favorite details about these stars. So, so basic details we didn't know initially. They are 10 to 35 solar radii.
They are 30,000 to 200,000 Kelvin in temperature. They have- Hold on.
Fraser Cain:
Hold on. Hold on. Like, wait a second.
Like the radii, I think people can wrap their heads around it. That's big.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
It's not Beetlejuice big, but they're hot. And so they are hot and, and big, but, but actually fairly dense, but, but that temperature.
Dr. Pamela Gay:
Uh huh. I can't wrap my head around it.
Fraser Cain:
Yeah. The surface temperature of the sun is about 5,800 Kelvin, 5,800 Kelvin. And say the temperature again.
Dr. Pamela Gay:
Uh, 30,000 to 200,000 Kelvin.
Fraser Cain:
200,000 Kelvin on their surface.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
That's bonkers how hot these things are. Yeah.
Dr. Pamela Gay:
And so they're massive. They don't live very long because big stars have brief lives. But when they were first found, we didn't know any of that.
When they were first found in the middle of the 1800s, we didn't even know helium existed. This is, first of all, one of the things that's deeply confusing to me. It took a long time to discover helium because helium does not work and play well with other things.
So helium is like, I'm off here being helium. You shall not bond with me. And because of this, we didn't have spectra for it as early as we had spectra for many other elements.
So when these stars were first found and people took spectrum of them because we were learning how to do that, it was a cool thing to do. They saw these sets of spectral lines that were reminiscent of hydrogen but were not hydrogen. And it would take tens of years for helium to be discovered to figure out what was going on.
Oh, yeah.
Fraser Cain:
Isn't that wild? That's interesting. Yeah.
Yeah. So they knew that there was another chemical that was next in the number of protons, but not necessarily next in the order of finding them because actually helium is a pretty tricky non-reactive gas to find.
Dr. Pamela Gay:
And they hadn't gotten to that. They knew there had to be another one in terms of electrons yet. They were still figuring this stuff out.
They just knew there's this pattern of spectral lines that looks like what we think is hydrogen, but we're not sure yet. It's similar. And so a lot of work had to be done to make that connection.
It's just, I love the fact that we identified these stars before we identified one of the most prominent elements in the universe. And these are stars that are at the end of their lives and weirdly behaving in some ways a lot like tiny M-dwarf stars. So they're out there convecting wildly.
And this also changes how they appear. Our sun, it has a core that's undergoing nuclear reactions. It has a radiation zone.
It is a convection zone. You don't generally see the enriched material from the center making it to the surface of the sun. It's just not a thing that's happening.
With Wolf-Ray stars, we see an atmosphere that is enhanced with carbon, nitrogen, oxygen, and that enhancement is due to the convection that is able to dredge up materials. And so you have massive convection going on. You have bloated out atmosphere going on.
You have massive amounts of wind and mass loss. And all of this is, in terms of the convection, very similar to what we see with tiny stars where you have that fully convecting atmosphere. And I just love that it wrapped around on itself.
There's just a lot of little cool details about these stars to kind of fall in love with before they explode.
Fraser Cain:
So there was a paper that just came out, or there was a news release that came out from Hubble. I think it was the Cat's Eye Nebula. Yeah.
And that has a Wolf-Ray star in it, in the center. And yet it's creating a planetary nebula. And traditionally, sort of my understanding was a planetary nebula came from main sequence stars like our sun, that at the end of their life, they become a red giant, they blow out their layers.
But actually, you can get them even from these extreme objects.
Dr. Pamela Gay:
And this is one of those things where we are always going to be rewriting our understanding. And we now need to do an episode updating our understanding of planetary nebula, because this is a new understanding. Planetary nebula are formed when stars of existing, it used to be we used to say stars of lower mass, are exhaling their atmospheres at the end of their lives.
We didn't think through the fact that not every planetary nebula is going to have a white dwarf in its core, because all of that material has come off of what used to be a 10 or less, nine or less solar mass star. What we're now realizing is there are temporary planetary nebula around some of these massive objects that are undergoing mass loss. And Wolf-Ray is one of these massive objects undergoing mass loss.
And so here, this is temporary. That star is eventually going to most likely undergo either parainstability or standard I ran out of fissionable, fusible material in my core, I shall explode now kinds of supernova. And that supernova is going to shred that nice, pretty planetary nebula we know today.
Fraser Cain:
And then you're right. So then you're left with a regular old supernova remnant.
Dr. Pamela Gay:
Exactly.
Fraser Cain:
Right. So you just sort of, you know, you might get a parainstability. I think our audience is perfectly comfortable with the idea of a core collapse supernova, right?
It runs out of fuel in the core, burns all the way up to iron, fusion energy collapses, the star implodes, you end up with some kind of either neutron star or black hole remnant and a supernova. But the parainstability, I don't think people are that familiar with this process. So what's going on in a parainstability supernova?
Dr. Pamela Gay:
So in the core of the star, it's running so hot that gamma rays are being produced. And as these gamma rays pass through the outer layers of the star, they're going along, interacting with things and causing pairs of, that's the name, pairs of electrons and positrons to be created.
Fraser Cain:
Right. So it's building antimatter in the star.
Dr. Pamela Gay:
And the combination of the electron and the positron existing side by side, they can then go, hi, we are a matter antimatter matched set. We shall annihilate now and release energy photons. And those photons from the annihilation are able to push outwards, creating the light pressure that supports the star.
Fraser Cain:
And they're also gamma rays. Exactly. Exactly.
Dr. Pamela Gay:
Now, the catch is that positron, electron, putting it into existence and taking it out of existence through annihilation, there's time that that takes. And if you generate positron, electron pairs too fast, you have too much of the gamma ray, too much of the light pressure going into particle, antiparticle creation, and it's not decaying fast enough to create the light pressure to support the star. So you end up with a pile up of these positron, electron pairs, not enough light going out and the star's like, you're not holding me up anymore, bud.
And the star just collapses.
Fraser Cain:
Right. But it also vaporizes.
Dr. Pamela Gay:
Right. I mean, it's a little bit of both. It's that Loki gif, both.
Fraser Cain:
Yeah, because I mean, you're not getting all of those additional elements up the periodic table. Exactly. You're getting this totally different process that blows the star apart.
And how these actually proceed at the highest levels, we still don't even fully know. Sometimes you get a black hole in the end, other times you don't.
Dr. Pamela Gay:
And we can't see the systems that go straight to black hole unless we know the star ahead of time. And so understanding, are we right? Is it correct that some of these most massive objects just calmly become black holes without going through the supernova process?
Fraser Cain:
Yeah. So there is like this fairly recent research that we reported on quite a bit. Actually, I'm going to be interviewing people from this work, but that they found one of these un-novas, where there was a star there and then the star is just gone.
It brightened up and then it's gone. And there's no supernova, even though the progenitor should have gone off as a supernova. And it seems like what defines whether you get the supernova or not is the amount of neutrinos that are being blown out of the center of the star while this process is happening.
So if you get enough, then the neutrinos, even though they never interact with anything, they're still interacting enough with the infalling material that you get the supernova. But if you don't have enough neutrino production, then the thing just collapses in on itself as a black hole. And it could be that it's the neutrinos that act like the brake that stops the process of it just imploding into a black hole.
But your mind just boggles at this idea that you just had this star that maybe had hundreds of times the mass of the sun, and then it just went boop, it's gone.
Dr. Pamela Gay:
And the understanding that we have of these objects is rapidly, rapidly evolving. So anything we say we understand today, as especially Ruben comes online, our understanding could change. One of the things that got me preparing for this episode is back in 2017, we only knew of 30 of these in our galaxy.
And so just the rate at which we're finding them, the rate at which we're coming to understand their mass range, the rate at which we're coming to understand the possible ways at which stars die, the fact that we've only realized that they can sit in the cores of planetary nebula recently, all of these things, we're only going to add to this changing knowledge. And that's kind of cool. Rare, short-lived phenomena are the wild west where we still have the most to learn.
Fraser Cain:
So play out this sort of time of the mass loss, where it is going through this point where it's shedding out all of these outer layers. I mean, I think people are familiar with different flavors of variable stars and sort of compare and contrast it to what happens with the sun. Like when the sun goes through its final days, turns into this red giant, and then it oscillates between a smaller star and a bigger star and will sort of throw this material out.
How does that compare to what happens with one of these Wolf-Ray stars?
Dr. Pamela Gay:
I think it's important to realize that Wolf-Ray stars are kind of the big star version of the gamma ray end of the electromagnetic spectrum. At a certain point, everything above a certain mass is called a Wolf-Ray star when it's evolved. So these things range in initial mass from about 30 solar masses to hundreds of solar masses.
That means they don't all behave exactly the same in terms of how long it takes them to do things and how wildly they do things. Looking at it more generally, you have a star that has evolved out. It's no longer burning hydrogen in its core.
It has pretty much run out of hydrogen. This is an evolved star. It's living millions of years, tens of millions of years, basically.
And as it has hit this evolved stage, it has winds that are going hundreds of kilometers per second. And so we have massive winds that are causing those emission lines that we see in the spectra to be broad. These are the kinds of winds that we expect in things that have massive inflows.
In this case, it has massive outflows. It's its own light pressure that is driving this, not gravity. And that is new and amazing for these kinds of objects.
They can be in binary systems.
Fraser Cain:
Right. I wanted to talk about that. Yeah, but just most massive stars are in binary systems.
And so extra mayhem.
Dr. Pamela Gay:
Right. There was a JWST image that recently came out that sees these layers of gas around a binary pair of Wolf-Ray stars. And what you get with these multi-star systems is you have high speed winds coming out from the star.
Those winds collide. Now, the way to visualize this is you have two sprinkler heads that are going around and around each other. And so they're creating both a spiral and a water slams and falls straight down, except it just compresses because there isn't gravity pulling straight down in space.
This creates an amazing shell structure where you can say, oh, yes, this is how we get planetary nebulae. It's rich in carbon. Carbon is something that will hold on to temperature in ways that hydrogen doesn't.
And so these are things that will glow in the infrared in ways that are perfect for JWST. Right.
Fraser Cain:
Yeah. So you're talking about how these produce a lot of carbon. Yeah.
You know, and another way to describe this as soot, that if you could go and collect this stuff, it would be very similar to the kinds of stuff that you would sort of find on the inside of a chimney, right? The smoke that's coming out of a campfire, same kind of stuff.
Dr. Pamela Gay:
The technical phrase is amorphous carbon. Yeah. Don't ask me to spell amorphous.
I'm just deeply amused that like carbon has so many specific names for its structures, but it's got a lot of bonds. It does. It really, really does.
And it likes to bond, unlike helium. And so these stars are producing carbon that bonds into amorphous carbon molecules. It bonds into other kinds of molecules.
We do see things like buckyballs in space. But the majority of this is the kinds of dust that then goes on over time to get spread throughout the galaxy that ends up seeding molecular clouds, that ends up seeding the creation of all of these weird and amazing molecules, amino acids that we find between the stars. And so we're in this situation where we have massive stars born fast, die fast.
You only find Wolf-Ray stars in the youngest of star forming regions and the most massive of star forming regions, because the number of stars you have of a high mass depends on the initial mass function, which depends on the mass of the system. So if you want to get more than one of these, you need to have a whole lot of mass. Yeah.
We do find them in collections. There are a few examples for this out there. And they're undergoing massive amounts of mass loss.
They're going to undergo supernovae, which means they're going to scatter that material everywhere. But in the interim, it's out there being a thermal blanket to the stars, holding on to that heat, radiating away energy. It's just really cool.
While being really warm.
Fraser Cain:
Yeah. I mean, one of the incredible discoveries that astronomers have been making recently is just how much of the precursor molecules for life are being found just forming in deep space. You're seeing organic molecules.
You're seeing amino acids. We're seeing alcohols, sugar, basic sugars, things that are like gum. Formaldehyde.
Yeah. It goes on and on and on. And and now it's like I think things that are like protein precursors as well, like it just every little bit and piece of what all the building blocks of what life needs.
And carbon, of course, is is key to all of them. And and it really feels now that these are the these are the source of a lot of the carbon that's out there, that you have these massive stars dying, throwing all this carbon into space, that you're not having to wait for these massive stars to die as supernovae. And it's different from what you get from colliding neutron stars, for example.
They give off the heavier elements, stuff that's beyond iron in the periodic table of elements.
Dr. Pamela Gay:
And we've always known that carbon, nitrogen, oxygen were coming from the atmospheres of stars. We misattributed how much comes from what mass star, thinking a lot of it came from smaller stars. We were wrong.
And and this is your reminder that all it takes is one big whale doing something to bias the system. All those little guys, they're trying their hardest. They're just not going to make the same dent.
And and these are the whales of stars.
Fraser Cain:
So what is a relatively nearby what are some examples of some of some famous Wolf Ray stars that that people can can be familiar with? So, I mean, the most massive star that we know of, which is our one thirty six A is a Wolf Ray star. Yeah, it's Wolf Ray star.
That's in the Large Magellanic Cloud. It is like more than 200 solar masses. It is ludicrous.
Dr. Pamela Gay:
And it's that star forming region in the Magellanic Cloud that is where we find a whole bunch of these. Yeah, there's a bunch of them in there. That I know of, there's only one that's naked eye visible and it has a really cool name.
It is Gamma Velorum. It is this really cool shock looking nebula inside of a greater region of nebulosity. That's the neat thing that happens when you're looking at things in star forming regions that still have unused material.
And and so you can see if you look at this through a variety of different filters, there's pictures all over the Internet that you have the the super hot ultraviolet light from the Wolf Ray star heating the surrounding nearby material associated with its mass loss. And then around it is the material not yet incorporated into a star that is glowing due to being a star forming region. In general, Wolf Ray stars have a huge influence on the nebulosity of their systems.
You can look at a system and see the scattered light from these Wolf Ray stars no matter where you look in the nebula, the scattered light carries that that spectral signature in new directions. But when you're looking for something that has that cool pair of reflection nebula and light from scattering, these stars really do the best.
Fraser Cain:
What is their connection to Gamma Ray Bursts?
Dr. Pamela Gay:
So, as we discussed many times before, Gamma Ray Bursts are one of those things we're still trying to figure out. So Gamma Ray Bursts, we think that hypernova are a special kind of supernova that are either caused by a fast rotating individual star undergoing core collapse supernova or a binary system where you have a massive star feeding material onto a companion star that has a strong magnetic field. And whatever the situation is, you have this resultant strong magnetic field that acts as a funnel for high energy particles in the early moments of the supernova.
And it is the particles heading down that funnel that are the Gamma Ray Burst. And so you will see with long, long period Gamma Ray Bursts, they can be up to tens of seconds, a couple I think have been over 100 seconds. And these longer period ones, you see the Gamma Rays, you look for the optical afterglow, it fades quickly.
And then at the location where you saw the Gamma Ray Burst, a supernova remnant will appear a few days later because the brightest part of the supernova actually comes from particle decays that take a few days to get their light together.
Fraser Cain:
So, you know, people are always worried about Gamma Ray Bursts and which is the which is the closest one to us? Not a concern. Well, the closest one that we know of is this one called WR104 and it is a Wolf Ray star.
And it is about 8000 light years away, which is still like definitely in the kill zone, like if it's pointed at us. It's not. But it's not, although when you look at the pictures of it, it's a little spiral that does appear face on.
But apparently the the angle is off of us enough.
Dr. Pamela Gay:
It's just enough.
Fraser Cain:
Yeah, that it's not going to not going to strike us. But this is an example which, as you mentioned, right, you've got the star, it's clearly got a binary companion because you're seeing this spiral shape and that that there's something about the interactions between the big star and its companion that sets up the conditions for a potential Gamma Ray Burst. But but as as you said, this is all new territory.
We're still trying to figure this out. And thanks to Vera Rubin, we're probably finding a couple of night now.
Dr. Pamela Gay:
So, yeah, it's absolutely wild. What is starting to occur? And I deeply appreciate the fact that it's AI all the way down where they're using machine learning algorithms to process the data in real time, split it up across multiple data brokers, and then people are using additional AI to triage the data.
The data brokers have its AI all the way down.
Fraser Cain:
A good a good use for AI. Yeah, I actually just interviewed somebody who runs one of the data brokers, the Noir Lab data broker. So I've got a lot of information in my head about that right now.
All right. There you go. Wolf Ray stars wrapped.
Thanks, Pamela.
Dr. Pamela Gay:
Thank you. And thank you so much to all of our patrons over on Patreon dot com slash astronomy cast. There are so many of you and I mangled your names so ruthlessly, but not on purpose.
Fraser Cain:
No, it's best half the fun.
Dr. Pamela Gay:
Yeah, it's true. If you too would like to hear me destroy your name, join us at patreon.com slash astronomycast. We're able to do astronomycast thanks to the amazing support of people like you.
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