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https://www.youtube.com/watch?v=jY1_q9a0UFc
Hosted by: Fraser Cain ( @frasercain ) and Dr. Pamela L. Gay ( @CosmoQuest )
Streamed live on Mar 16, 2026.
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.
Image Credit: NASA Galex
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It's the 365 days of astronomy podgang coming in three two one
It's the 365 days of astronomy podgang coming in three two
Astronomycast Episode 787 Evolved Stars
Welcome to Astronomycast from the fax space during the cosmos
We hope we understand only what we know but how we know what we know
I'm Fraser Kane, I'm the publisher of the universe today
With me is Dr. Pamela Gait, a senior scientist for the planetary science institute and the director of CosmoQuest
Hello, Pamela
Hi, how are you doing Fraser?
I'm doing well
Since someone said, oh yeah, Pamela always says, I'm doing well, I always has that stutter there
I've decided I'm just going to mix up the intervals every time you need to get out of the rot
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
As long as it's shaken and not stirred, we're good
There you go
So this is an interesting anniversary for me which is that we are at about the one year mark
From when the 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
Like we're just, and by great I mean, you know, after I made that pronouncement and said, okay, this is what I want to do
Otherwise I'm going to start laying off people
I said, I'm just going to remove the ads because now the value is down, whatever, 70-80%
Let's switch to Patreon if we can cover that shortfall then I'll, that I'll just keep the business running as usual
And we've 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 were there for that month to just try and help us out
But then it's been sort of refreshing
Yeah
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 podcast
The absolute minimum amount of ads that you people 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, 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, NASA technical report server
And then I just go through those 600 papers every day
And then I think one second about managing advertisements and search engine optimization
And AI sloping, and that kind of stuff
So, you know, I noticed some overlap here where people are both on my Patreon as well as the astronomy cast Patreon
And maybe the cost of my Patreon
But just like thank you so much
You did this, 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
The game
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
We're not happy with the ads and not happy with the networks
Pod role
Yeah
Yeah
And so it's like it's beyond my control what podcast they're going to send you to at the end of ours
It can be
It can be
And your control
Yeah
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 switched to other fuels starting to climb 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
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
Yeah
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 that is
This is all the stars that have finished collapsing down from being proto stars
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
Right, and this is the thing that you see when you look at the Hertz from Russell diagram
There's a big line that is in the Hertz from Russell diagram where all the stars, all the main sequence stars live
Yes, and this is what we call it the main sequence
And yeah, it's just where most stars are because that's where stars linger the longest
So it's just this
Yeah, and those other forms, I mean there's like a CNO cycle like there's other cycles that can happen
And 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 this, the traditional hydrogen to helium
So then what leads to, you know, when would you call a star evolved?
Is that like a polite way of calling a person old?
Oh, you're a very evolved person, very mature
It's definitely postmenopausal stars, we'll go with that
So these are the stars that 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
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 additional depending on the mass of the object
A whole bunch of additional elements climbing up through all the various relationships
Right, so this idea of it creating these shells
Is this sort of that because the temperature in the core has, you need a minimum temperature in the core to even get fusion
Like out of four 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 .08 solar mass red dwarf
That's when it comes online as a mean-sequence star 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
That changes the temperature in the core which then brings more hydrogen fuel online
In addition to the fusion that's happening from helium, right? Is it, am I understanding this right?
So the shells and the core, you can end up with them going at different points
So there's 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 it ignite as it gets the correct hydrogen and pressure density temperature thing
And then that core ends up igniting as everything reaches a new set of temperatures
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 they when now the helium is come online
In addition to this this hydrogen shell what happens to the star?
The star floats 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
Yeah, and and where they are on this plot depends on exactly what's going on
So our Larry's my favorite star was 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 have a pretty good understanding of the initial mass function that stars will form at
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 twenty masses they all become neutron stars
Well, neutron stars like are less than two and a half solar masses
Right, so we're the rest of the mass go
And it's all the mass loss
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 caused wild changes to how we understood stellar evolution
So then like yeah, so let's say star like our son it goes to that mass loss process and ends up with like half its mass
Like essentially the core is all that remains and the rest of the outer layers have all been sloughed off into space
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?
So quickly is a matter of perspective
So our son will spend about ten 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 to and through hydrogen to then in the grand scheme of stars dying
is going to rapidly both give off its matter
And go through the C&O cycle becoming 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
But it takes longer than ten million, like you said the various cycles
It has to go through a whole bunch of phases puffing out, drinking back down, puffing out, drinking back down
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, yeah, yeah, closing in on a billion
So then, and I think we've done a whole episode on red giants and I wouldn't be surprised we've done an episode on RLR
Probably, probably, I don't know who knows, we'll check some old tell us
But, okay, so you get this place so essentially much more heat is coming out of the core of the star
The star is then blows out because that balance between the gravity that's pulling in it
And the radiation pressure that's pushing out word is now totally shifted
Yes
And now the star is much larger but also kind of cooler and also cooler
But, so what brings it back down again?
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
Oh, that's amazing
Right, all right
So it's not like it is because they're those variable sorts like the cataclysm variables things like that where they are
You know, they are pulsating but in this case, no, you're puffing out and then you're just letting this go into space
Goodbye
Yeah, yeah, it goes away
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
So it takes a long, but a lot of it's gone
It's only the last couple of jeds can you actually see
Well, and what we're starting to learn is 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 reasons 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 only have adaptive optics, we didn't have eight meter and bigger telescopes
So we didn't have ground-based resolving abilities 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 nebulaes 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
Yes
And this is because they're giving off ultraviolet light
So what's it, it's the cat's high nebula picture?
The cat's high nebula, yeah
Yes, is that what, is that what put this into your mind that you want to talk about?
Yeah, okay, all right
That's still annoying me, like
Yeah
You learn planetary nebula, have dwarfs in the core and then you learn you're totally wrong
Yeah, what's the neutron star during in the middle of it?
Yeah
Right
So then, I mean, we talked about main segment stars
And sort of their process of shading of this material and then of course, now I learn the inspiration is this just incredible picture of the cat's high nebula
Yeah
Really from James Webb, combined with information from Hubble and other telescopes and it's just, it's an insane picture
Euclid, yeah
Yeah, yeah, it's just an incredible picture
And so, and so now maybe the giant stars are making planetary nebulae as well
That are also short live but for a totally different reason
So in these cases you have massive stars, these can be 30 solar mass stars that are eventually going to become neutron stars
They can be more than 30 mass stars that 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 high nebula
And this idea that you can go through multiple forms of explicit beauty in death with these stars
Yeah
From something that was shaped by jets, by 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 spyrograph of nebulosity
Yeah, it's interesting, I had this sort of realization about how young these remnants are
Or planetary nebulae and supernova remnants that when we look at galaxies, you can be looking at the galaxy
And it doesn't 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...
100 million?
Yeah, 100 million years, really young ones like please, now maybe 10 million years
Like the really, really young star forming regions, maybe they're in the millions of years, you're looking at their Ryan Nebula
And you're going to see that really heavy nebulosity before the stars, 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 old
Yeah
Thousands of years old
Because this moment, this very short moment of time when this thing is released
And then it fades away into just the interstellar sort of gas and dust that's out there
All of 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
We can see them changing with time, that's the thing I love
Like the crab nebula, there was an activity when I was a student where it would give you a pair of images 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 been now looking at these objects for going on 100 to 140 years
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
So I'm trying to continue along this story of this stellar evolution
So these stars, they go through this point where they're using different fuel, using different elements
They're using 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 star more like the sun?
And then we'll talk about the how it ends for the bigger ones
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 too smaller and smaller stars, you eventually
At the smallest little will 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
Yeah
The whole thing is keeping it lit and it's just going to cool down over time
They're coal, that's what we're looking at
I've been working on story about this
So they start out at a hundred, so white dwarfs, they started at 150 Kelvin
Ultraviolet, yeah
And so that's that temperature cores bonds to into the ultraviolet
And they're very bright and then they cool down
Neutron stars started at 600,000 Kelvin
Yeah
Which is why they are in the x-rays
Right, when they first start out
And what's cool is these are crystal spokes
Yeah
And as they cool, they're crystal instructors rearrange
And so we'll see jumps in temperature as they go from one crystal instructor to another crystal instructor
And the energy changes how it's being
It's like when something goes to changing phase instead of just cooling off
Yeah
It's cool
Yeah, actually very hot
Yeah, it's very cool, now that's hot
When they talk about the, you know, the cores of these stars that they're literally diamonds
They really are like one big crystalline diamond
So, yeah, super cool
Awesome, all right
Thanks very much
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