Astronomy Cast Ep. 787: Evolved Stars (They're not dead yet!)

0:00It's the 365 days of astronomy podgang coming in three two one It's the 365 days of astronomy podgang coming in three two

1:00Astronomycast 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

2:04As 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

3:06And 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

4:08And 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

5:08We 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

6:10For 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

7:12Right, 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

8:15It'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

9:16And 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?

10:18So 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

11:21Yeah, 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

12:29But 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

13:31So 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

14:34To 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

15:34Right, 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

16:39So 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

17:42So 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

18:42Yeah 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

19:43But 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

20:49Or 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

21:53And 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

22:56So 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

23:58We 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

25:00Yeah 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 Thank you, and thank you so much patrons and seriously if our contributions go up

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29:36Cool. The 365 Days of Astronomy Podcast is produced by the Planetary Science Institute. Audio post-production is by me, Richard Drum. Project management is by Aviva Yamani, and hosting is donated by Libsyn.com. This content is released under a Creative Commons Attribution Non-Commercial 4.0 International License. Please share what you love, but don't sell what's free. This show is made possible thanks to the generous donations of people like you. Please consider supporting our show on patreon.com, forward slash CosmoQuestX, and get access to bonus content. Without your passion and contribution, we won't be able to share the stories and inspire the worlds. We invite you to join our community of storytellers and share your voice with listeners worldwide.

30:36As we wrap up today's episode, we're looking forward to unraveling more stories from the universe. With every new discovery from ground-based and space-based observatories and each milestone and space exploration, we come closer to understanding the cosmos and our place within it. Until next time, let the stars guide your curiosity. Thanks for watching!