Main sequence stars spend most of their time being… normal. Fusing hydrogen into helium in their cores. Producing radiation. But as their stockpiles of hydrogen run out they switch to other fuels, starting to climb the ladder of the periodic table of elements. And this is when things get weird. As we get more and more observations of the cosmos, our understanding gets more detailed. In this episode we look at all the ways a star can die and the updates that we've learned in the past 20 years of Astronomy Cast.

Show Notes* What are evolved stars and how stars leave the main sequence * Main sequence basics: hydrogen fusion and stability * Role of mass in determining stellar evolution paths * Helium ignition & shell burning (helium flash) * Evolution stages: red giant → asymptotic giant branch (AGB) * Mass loss and its impact on a star’s final fate * End states: white dwarfs, neutron stars, black holes * Formation of planetary nebulae and stellar remnants * Case study: Cat’s Eye Nebula and evolving understanding * White dwarfs: composition, cooling, and crystallization * Cosmic timescales: stars vs nebulae vs galaxies * Observing stellar evolution through changing nebulae

Transcript:Fraser Cain:

AstronomyCast, Episode 787, Evolved 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 is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hello, Pamela.

Dr. Pamela Gay:

Hi, how are you doing, Fraser?

Fraser Cain:

I'm doing well. See, since someone said, oh yeah, Pamela always says, I'm doing well, like always has that stutter there. I've decided I'm just going to mix up the intros every time.

You need to get out of the rut. You need to be shaken up and just exist in this sort of place where you can no longer find any firm footing, that the future will be unbounded and unstable. This is my guarantee.

At least the greetings I provide to you will be anything but a certainty.

Dr. Pamela Gay:

As long as it's shaken and not stirred, we're good.

Fraser Cain:

There you go. So this is an interesting anniversary for me, which is that we are at about the one-year mark from when Universe Today removed all of the advertisements from the website. Or when I removed all the advertisements from the Universe Today website, anyway, we went ad-free across our entire existence and instead just relied on people to join our Patreon.

And we're doing great. And by great, I mean, after I made that pronouncement and said, okay, this is what I want to do. Otherwise, I'm going to start laying off people and I said, I'm just going to remove the ads because they're now, the value is down, whatever, 70, 80%.

Let's switch to Patreon. If we can cover that shortfall, then I'll just keep the business running as usual. And we got an amazing response.

People joined. The number of patrons jumped up to exactly where it needed to be, which was kind of crazy. And I was anticipating it to quickly drop off.

And instead, there's definitely a lot of people who sort of were there for that month just trying to help us out. But then it's been sort of refreshing at a level where our income across the entire year is now just absolutely predictable. And in a way that we can have no, like I was able to redesign the website.

It's completely, it's so fast. No ads in the newsletter, no ads in the podcasts, the absolute minimum amount of ads that YouTube will let us do on YouTube. It's great.

It's amazing. And I just, like, it's amazing how free I am to just think about how can we provide great content. I sit and look at, I built a tool that lets me slurp in all of the journals from ADS as well as Archive, as well as the NASA technical report server.

And then I just go through those 600 papers every day. And I don't think one second about managing advertisements and search engine optimization and AI slop and any of that kind of stuff. So yeah, I know there's some overlap here where people are both on, you know, are my patron as well as the Astronomy Cast patron and maybe the Cosmic Quest patron.

But just like, thank you so much. You did this, you know, you are participating at whatever level and it has made a dramatic change. I haven't had to, I've let the writers write as much as they want.

Our coverage has gotten better and it feels like it's this perfect balance. And teachers tell me that they can use my stuff in the class and they just, they don't feel embarrassed for the ads of sexy ladies showing up, you know, like who knows what you'll get when you have various Google ads showing up on your stuff. So it's just, it's the best possible world.

And I feel like now I'm unstoppable. So thank you. Yeah.

Dr. Pamela Gay:

Yeah. And if you folks want to do the same thing to Astronomy Cast, we still have about 20% of our revenue coming in through ads. And I'm answering emails from about 10 people a week who are not happy with the ads and not happy with the network role.

Yeah.

Fraser Cain:

Yeah. Yeah.

Dr. Pamela Gay:

And so it's like, it's beyond my control what podcast they're going to send you to at the end of ours.

Fraser Cain:

But it can be. It can be. In your control.

Dr. Pamela Gay:

Yeah.

Fraser Cain:

Yeah. All right. Main sequence stars spend most of their time being normal, fusing hydrogen into helium in their cores, producing radiation.

But as their stockpiles of hydrogen run out, they switch to other fuels, starting to climb the ladder of the periodic table of elements. And this is when things get weird. All right.

So let's first like set a baseline and just talk about the main sequence phase of stellar evolution.

Dr. Pamela Gay:

So stars for the most part, and this is an important caveat that almost always gets left out. For the most part, stars start out their lives burning just hydrogen in their core and they hang out there burning hydrogen in their core for millions to billions of years. But the most massive stars actually get to burning some heavier elements right off the bat because they are that big.

So what we see is if we do a plot of the brightness of the stars versus the color of the stars, the temperature of the stars, there is this really cool line that goes through it that is this is all the stars that have finished collapsing down from being protostars have begun to completely and stably balance themselves between light pressure outwards and gravity inwards. And that is where they're going to stay for the initial era of their life.

Fraser Cain:

Right. And this is the thing that you see when you look at the Hertzsprung-Russell diagram. There's a big line that is in the Hertzsprung-Russell diagram where all the stars, all the main sequence stars live.

Dr. Pamela Gay:

Yes. And this is, we call it the main sequence. And it's yeah, it's just where most stars are because that's where stars linger the longest.

So statistics.

Fraser Cain:

Yeah. And those other forms, I mean, there's like the CNO cycle, like there's other cycles that can happen in like a tiny fraction, even in the sun, but are happening more commonly in the bigger stars. But the one that we're looking at mostly is, is this, uh, this, the traditional hydrogen to helium.

So then sort of what leads to, you know, when would you call a star evolved? Is that, is that like a polite way of calling a person old? Um, oh, you're a very evolved person.

Very mature.

Dr. Pamela Gay:

It's definitely postmenopausal stars. We'll go with that. So, so the, these are the stars that, uh, their core has run out of its initial fuel.

And so that initial thing that it was doing to generate light and support itself against gravitational collapse has stopped. And the first thing that ends up happening is the star will collapse down a little bit because again, the thing it was generating light from has stopped. Now that process of collapsing will heat it up more in the core.

Um, that whole pressure volume relationship that we learned in high school works for stars. And you will then end up with a shell of hydrogen burning around that core and you will eventually end up with helium burning in the core going to neon. And then eventually a whole bunch of, of additional, depending on the mass of the object, a whole bunch of additional, uh, elements climbing up through all the various relationships.

So, right.

Fraser Cain:

So this, so this idea of it creating these shells. So is this sort of that because the, the temperature in the core has like, you need a minimum temperature in the core to even get fusion, like out of 4 million Kelvin or something like that. Temperature and pressure.

You need both. Yeah. Temperature and pressure.

Yes. Yes. But that is sort of like where you define the smallest possible, like the 0.08 solar mass red dwarf. That's when it comes online as a main sequence stars is when it is able to reach that temperature and pressure in the core. And that when you run out of the hydrogen in the core, you switch to the helium burning thing. That changes the temperature in the core, which then brings more hydrogen fuel online in addition to the fusion that's happening from the helium.

Right. Is it, am I understanding this right?

Dr. Pamela Gay:

So, so the shells and the core, you can end up with them going at a different points. So there's this, this thing called the helium flash when that core ignites. So you have the initial collapsing down a shell around the core of hydrogen will ignite as it gets the correct hydrogen and pressure density temperature thing.

And then that core ends up igniting as, as everything reaches a new set of temperatures.

Fraser Cain:

In this case, it's the helium in the core can now fuse. Okay. And then, but this must like something must happen at this moment when the, when now the helium has come online in addition to this, this hydrogen shell, what happens to the star?

Dr. Pamela Gay:

The star bloats out radically. And so now you have the main red giant branch, you have the asymptotic giant branch. You have all these different places that stars go to live and, and where they are on this plot depends on exactly what's going on.

And so our Larry's my favorite star, we're going to return to them often. They're chugging along on this flat line after they've undergone that helium flash. We have moving up, we have that hydrogen shell burning and exactly what's going on again is going to depend entirely on the mass of the star.

I'm just going to keep repeating that dependency on the mass of the star. And one of the things that foils us on the regular basis is we, we have a pretty good understanding of the initial mass function that stars will format. We know there's not that many big ones that end up forming out of the fragmenting molecular cloud.

We know there's a gazillion little ones that form and then they undergo mass loss and exactly how much mass loss is something we're still trying to come to terms with. So you end up seeing wild phrases like stars, less than eight masses should eventually become white dwarves, but then between eight and 20 masses, they all become neutron stars. Well, neutron stars like are less than two and a half solar masses, right?

Fraser Cain:

So where'd the rest of the mass go?

Dr. Pamela Gay:

And it's all the mass loss, right? And, and so we used to not understand exactly how much mass loss was going down. And as we realized it was like the majority of the stellar mass got lost, it, it caused wild changes to how we understood stellar evolution.

Fraser Cain:

So, so then like, yeah, so, so say a star like our sun, it goes through that mass loss process and ends up with like half its mass, right? Like essentially the core is all that remains and the rest of the, the outer layers have all been sloughed off into, into space. So, so you get this, I mean, you essentially just described the red giant phase.

Does this take a while or does this happen like when that helium ignites, does it happen very quickly?

Dr. Pamela Gay:

So quickly as a matter of, of perspective. So our sun will spend about 10 billion years as a main sequence star burning through its hydrogen. It will then spend tens of millions of years in each of the subsequent phases.

So we're going from billions of years just chewing through hydrogen to then in, in the grand scheme of stars dying is going to rapidly both give off its matter and go through the CNO cycle, becoming a, a little diamond of a core surrounded by mass. It's going to exhale into the surroundings before becoming a carbon nitrogen, oxygen rich white dwarf star.

Fraser Cain:

But it takes longer than 10 million, like you said, the various cycles, like it has to go through a whole bunch of phases, puffing out, shrinking back down, puffing out, shrinking back down. And so, I mean, like I had seen that it was on the order of hundreds of millions of years for that whole process to wrap up, to go through all the different phases. Yeah.

Yeah. Okay. Yeah.

Yeah. Yeah. Yeah.

Closing in on a billion. Okay. So then, so, and I think, you know, we've done a whole episode on, on red giants and I, I wouldn't be surprised if we've done an episode on aurora lari, but probably, probably, I don't know.

Who knows? We'll check. Someone will tell us.

But, um, but, okay, so you, you get this place. So essentially much more heat is coming out of the core of the star. The star is, is then bloats out because it, the, the, that balance between the gravity that's pulling inward and the radiation pressure, the pushing outward is now totally shifted.

Dr. Pamela Gay:

Yes.

Fraser Cain:

And now the star is much larger, but also kind of cooler and also cooler. But, um, so what brings it back down again?

Dr. Pamela Gay:

Um, it's not so much that it shrinks down again as it just gets rid of its atmosphere over time and it's just left with the core.

Fraser Cain:

Oh, that's amazing.

Dr. Pamela Gay:

Right.

Fraser Cain:

Right. So it's not like it is because there are those variables, like the cataclysmic variables, things like that, where they are, you know, they are pulsating, but in this case, no, you're, you're puffing out and then you're just letting this, this go into space. Goodbye.

Dr. Pamela Gay:

Yeah. Yeah. It goes, it goes away.

Fraser Cain:

And there are phenomenal pictures of stars where they're surrounded by just this, you know, diffuse glow of material that the star ejected in various previous generations. Although it takes, it takes so long that a lot of this stuff's gone. Like it's, it's only the last couple of sheds can you actually see?

Dr. Pamela Gay:

Well, and, and what we're starting to learn is, um, what we're able to see depends on when we're looking and how much things have had a chance to cool or self-destruct. All these stars are going to undergo massive amounts of mass loss in their end days. So one of the rides on that for the Hubble Space Telescope to get built was actually to figure out what the heck are planetary nebulae from the ground at that point when they were building the Hubble Space Telescope, we didn't really have adaptive optics.

We didn't have eight meter and bigger telescopes. So we didn't have ground-based resolving abilities to, to see all the fabulous details. We just knew there were these smudged out blobs of color that appeared to be a variety of different gases at different ionization levels.

And so Hubble starts looking at these things and is finding in the cores of many of them white dwarf stars, ultraviolet emitting hot young white dwarfs. And so you have two things going on in these situations. The material is still drifting away.

As it gets further and further away, it's not getting heated up as much. And then that white dwarf in the core is also cooling down. So planetary nebulae surrounding white dwarf stars are created in the final days of smaller mass stars that don't go supernova and end up with the core of the star left behind.

And we know it's the core of the star from looking at its composition. And what we're seeing is the outskirts of the star that just got exhaled. But the story gets more complicated because we also have discovered that some of the hottest stars, the most massive stars that also undergo massive amounts of mass loss have around them what look like planetary nebulae.

Yeah. Yes. And this is because they're giving off ultraviolet light.

Fraser Cain:

So what's it? It's the cat's eye nebula picture.

Dr. Pamela Gay:

The cat's eye nebula. Yeah.

Fraser Cain:

Yes. Is that what is that? Put this into your mind.

You want to talk about? Yeah. Okay.

All right.

Dr. Pamela Gay:

That's still annoying me.

Fraser Cain:

Like, yeah.

Dr. Pamela Gay:

You learn planetary nebula have white dwarfs in the core and then you learn you're totally wrong.

Fraser Cain:

And yeah. What's the neutron star doing at the middle of it? Yeah.

Right. So, so then, I mean, we talked about main sequence stars, but, and, and sort of their process of shedding out this material. And then of course, now I learned the inspiration is this just incredible picture of the cat's eye nebula released from James Webb combined with information from Hubble and other telescopes.

And it's just, it's an insane picture.

Dr. Pamela Gay:

Euclid. Yeah.

Fraser Cain:

Yeah. Yeah. It's just an incredible picture.

And so, and so now maybe the, the giant stars are making planetary nebulae as well.

Dr. Pamela Gay:

That are also short lived, but for a totally different reason. So in these cases you have massive stars, these, these can be 30 solar mass stars that are eventually going to become neutron stars. They can be more than 30 mass stars are eventually going to become black holes.

And some of them will just eat themselves entirely and nothing will be left, but whatever their ultimate fate, they have formed on their way to that fate. This glorious temporary nebula, and then they go supernova all over it. So when we're looking at things like the Crab Nebula, it is entirely possible that not only are we seeing the shock waves from the supernova moving out, but that material that is being disrupted is something that once looked like the cat's eye nebula.

And this idea that you can go through multiple forms of exquisite beauty in death with these stars from, from something that was shaped by jets, by the, the existence of companion stars, by the existence of a planetary disk, all shaping how material is given off, creating what looks like a 1980s spirograph of nebulosity.

Fraser Cain:

Yeah. It's interesting. You know, I had this sort of realization about how young these remnants are, planetary nebulae and supernova remnants that, um, you know, when we look at galaxies, you can be looking at galaxy and it's a little look roughly the same for billions of years.

You look at star clusters. Okay. Now these things are going to look kind of similar for a hundred million, a hundred million years, 10, you know, the really young ones like please, okay, now maybe 10 million years, the really, really young star forming regions, maybe they're in the millions of years.

You're looking at the Orion nebula and you're going to see that, that really heavy nebulosity before the stars, you know, maybe 10 million years. But when you look at things like say the veil nebula, various supernova remnants, planetary nebula that you might be looking at things that are only say tens of thousands of years, thousands of years old. And it's this moment, this very short moment of time when this thing is, is released and then it fades away into the, just the interstellar sort of gas and dust that's out there.

All of the, the forces, the winds, the interstellar wind that's blowing on these stars is just adding up and eventually fades this thing away into the background. And so everything we look out and see, we're seeing fairly recent events.

Dr. Pamela Gay:

We can see them changing with time. That's the thing I love, like the crab nebula there, there was an activity when I was a student where it would give you a, a pair of images of, of the crab nebula and you measure the angular separation between the edges of the nebula and the stars. And you could calculate the rate of expansion in arc seconds per year.

Well, we've, we've been now looking at these objects for going on a hundred to 140 years.

Fraser Cain:

Yeah.

Dr. Pamela Gay:

And, and this is allowing us to really see both how they're expanding away from their source of heat. And then we can look out and we can start to get a sense of how white dwarfs cool and how supernova cool. And it's such a singular moment in time that these things exist.

Fraser Cain:

So I just want to continue along the story of this stellar evolution. So these stars, they go through this point where they're using different, different fuel, they're using different elements, they're fusing different kinds of elements in their cores, walking up the periodic table of elements to whatever is their final set point and letting out these outer layers into space. How does this end for a, for a star more like the sun?

And then we'll talk about the, how it ends for the bigger ones.

Dr. Pamela Gay:

So for smaller stars, you end up with when we look out at white dwarfs, we often see them that are carbon, nitrogen, oxygen rich. These are your CO white dwarfs. They come from stars similar to the sun.

As you start getting to smaller and smaller stars, you eventually at the smallest little we'll actually have had a chance to die larger red dwarfs. That's a really dumb way to phrase all of that, but they will eventually run out of fuel in the fullness of time. We haven't seen this occur yet, but when they do, they'll just collapse down into being pretty much a solid helium blob.

And then we do see objects that are smaller than the sun and have had enough billions of years to run out of hydrogen in their core. And they've just collapsed down into basically helium white dwarfs. So we just see the moral equivalent of charcoal that's still glowing.

Nothing is keeping it lit and it's just going to cool down over time. They're coals. That's what we're looking at.

Fraser Cain:

I've been working on the story about this. So white dwarfs, they start out at 150 Kelvin.

Dr. Pamela Gay:

Ultraviolet. Yeah.

Fraser Cain:

And yeah. And so that temperature corresponds into the ultraviolet. And so they're very, very bright.

And then they cool down. Neutron stars start at 600,000 Kelvin, which is why they are in the X-rays when they first start out.

Dr. Pamela Gay:

And what's cool is these are crystals, folks.

Fraser Cain:

Yeah.

Dr. Pamela Gay:

And as they cool, their crystalline structures rearrange. And so we'll see jumps in temperature as they go from one crystalline structure to another crystalline structure. And the energy changes how it's being.

It's like when something goes to changing phase instead of just cooling off.

Fraser Cain:

It's cool. Yeah.

Dr. Pamela Gay:

It's actually very hot.

Fraser Cain:

Yeah. It's very cool. Now, that's hot.

When they talk about the cores of these stars, that they are literally diamonds, that they really are like one big crystalline diamond. So yeah, super cool. Awesome.

All right. Thanks, Pamela.

Dr. Pamela Gay:

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Fraser Cain:

Nice. All right. Thanks, everyone.

And we will see you next week.

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