Daniel and Jorge Explain the UniverseDaniel and Jorge break down the life cycle of stars and estimate how many more stars will be born in our Universe.
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Hi.
I'm David Ego from the podcast Inner Cosmos, which recently hit the number one science podcast in America. I mean neuroscientists at Stanford, and I've spent my career exploring the three pound universe in our heads.
Join me weekly to explore the relationship.
Between your brain and your life, because the more we know about what's running under the hood, bet or we can steer our lives. Listen to Inner Cosmos with David Eagleman on the iHeartRadio app, Apple Podcasts or wherever you get your podcasts.
Guess what, Well, what's that mango?
I've been trying to write a promo for our podcast, Part Time Genius, but even though we've done over two hundred and fifty episodes, we don't really talk about murders or cults.
I mean, we did just cover the Illuminati of cheese, so I feel like that makes us pretty edgy. We also solve mysteries like how Chinese is your Chinese food?
And how do.
Dollar stores make money? And then of course can you game a dog show?
So what you're saying is everyone should be listening.
Listen to Part Time Genius on the iHeartRadio app or wherever you get your podcasts.
Hey, it's Horehand Daniel here, and we want to tell you about our new book.
It's called Frequently Asked Questions about the Universe.
Because you have questions about the universe, and so we decided to write a book all about them.
We talk about your questions, we give some answers, we make a bunch of silly jokes.
As usual, and we tackle all kinds of questions, including what happens if I fall into a black hole? Or is there another version of you out.
There that's right? Like usual, we tackle the deepest, darkest, biggest, craziest questions about this incredible cosmos.
If you want to support the podcast, please get the book and get a copy, not just for yourself, but you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters, and.
For the aliens. So get your copy of Frequently Asked Questions about the Universe is available for pre order now, coming out November two. You can find more details at the book's website universe faq dot com. Thanks for your support, and.
If you have a hamster that can read, please let us know. We'd love to have them on the podcast. Hey Daniel, how far back do you know your family story?
Oh?
You mean like where my family comes from?
Yeah, you know, like how many generations back do you know?
Well, I actually met my great grandfather when I was a kid, and I know some family history back a couple more generations than that, So I guess that makes like four or five generations. How about you.
That's pretty good. I actually have only met my grandparents three to the four of them. But then it gets a little fuzzy. You know, they're all immigrants.
And we know at least one future generation, I mean, like our kids.
Yes, children are the future.
And hopefully there are enough future generations that one day we become fuzzy memories.
Well, I'm not sure if I will have great grandkids, but if I do, I'm sure they'll remember us, right, they can always listen to this podcast.
If nothing else, they'll remember us as a cautionary tale.
Assuming they care. I am hoorhammy cartoonist and the creator of PhD comics.
Hi, I'm Daniel. I'm a particle physicist, but I'm the first person in my family to get a PhD.
Hmmm, you're the first doctor or the first academic doctor.
I'm definitely the first doctor of any kind in my direct line of There is another Daniel Whitson out there, somewhere who I think has a degree.
Oh interesting, it's like your long gloss twin. Maybe another version of the multiverse version of you.
There's another Daniel Whiteson, which is a distant cousin of mine who's an excellent artist and lives in London.
Actually, oh, you should switch places. Sounds like he has a cruder life.
I'm pretty happy with my situation. Thanks very much.
Do you think you'll be the last PhD in your family?
I don't know. My brother got a PhD, but that might be it. I'm not sure which direction my kids are headed.
But anyways, Welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we sum up the knowledge and the thinking of many, many generations of deep thinkers and scientists and philosophers and explorers who are trying their hardest to understand this incredible, beautiful universe that we find ourselves in. We talk about everything that we know about science, things that we don't know about science, the way the universe works, and the way the universe definitely doesn't work. We talk about the latest results, the craziest thought experiments, and everything in between, and throwing a few banana jokes to keep you awake.
And hopefully not to slip you up. But it is a pretty big universe, pretty amazing, and science has discovered a lot about it. We know a lot about how the universe started, how it's evolved, and what explains everything we see today. And big job of science is also to think about the future and what's going to happen to this crazy cosmos.
That's right, because one thing we know about the universe for sure is that it's not static. It's not just hanging out like this forever. It's evolving. It's not quite alive, but it's definitely developing and changing and has different modes to it.
Yeah, the universe hasn't always been the way it is right now, and it's not going to be the same wave in the future. It's it's always changing, evolving and growing, hopefully getting more mature. Do you think the universe is getting more mature.
I'm not sure it's getting better behaved, but it's definitely getting older if that's what.
It's still getting into trouble. Is you thinking the universe that learns its lessons.
I'm not sure who's teaching the universe anything, but I think it's fun to realize that the universe seems sort of static on our time scale, like the universe is not going to look any different the day you die than the day you were born, but that these processes are happening on a much much longer timescale, and that if you look at the universe over like millions of years or billions of years, it's very much dynamic. It's changing, it's growing, it's evolving. It's going to look very different in five billion years than it does today.
Yeah, or not even billions of years, like millions of years. I mean, the Earth looked a lot different, and even our Solar system looked pretty different, right, even a few million or hundreds of millions of years ago.
Yeah, that's something where learning about. You know, the planets are doing a dance where they're moving in and they're moving out, and we think maybe Jupiter spent some time near the Sun and then Saturn swooped in and saved it and pulled it back out to the colder parts of the Solar System where it can survive. So there's definitely a lot of stuff going on if you look at the Solar system or our galaxy on like a super time lapse, you would think it was crazy.
Yeah, it's a big action movie, but only if you hit the fast forward button a couple of.
Times and then don't forget to rewind it before you return the tape. Just you know, be kind.
Hopefully some people out there get that joke. Probably not if you're under thirty or thirty five, you would get that at leasta, I guess maybe what's the equivalent now you had to take it off your Netflix queue. Maybe it's a courtesy to nobody. But yeah, it's a big, beautiful universe, and we often wonder what's going to happen in the future, and in particular to the things we see around.
Us, you can think about the sort of age and cycle of the stuff around us. Not just the planets are spinning around the Sun, but the Solar system itself is moving around the galaxy. It takes like two hundred and fifty million years to do one loop around the center of the galaxy for the Sun to orbit the center of the galaxy and came back to where it started. So from that point of view, you know, the galaxy itself is not that old, like it hasn't done that many spins it's like, you know, thirty to forty galactic years.
Old, still searching for itself, maybe still hasn't figured out what it wants to do with it the rest of its life.
It's like running through cosmic fields towards the Andromeda Galaxy. There's gonna be a very dramatic moment there where they both learn who they really are.
They're going to find themselves. They're going to become one with the cosmos and.
With each other exactly. And then within those galaxies there's stuff happening. Stars are being born, stars are dying and exploding. There's all sorts of crazy stuff.
Yeah, stars are probably the thing that people most notice or think about even when they think about space and the entire universe, and it's a big part of the universe. There are a lot of stars out there, and we kind of have to what's going to happen to all those stars eventually in the near future and forever perhaps.
Yeah, because stars do not live forever. Some of them last millions of years, some billions of years, some potentially trillions of years. But there are stars that formed and burned and died even before our Solar system came to be. The things that you and I are made out of, and that our solar system are built out of are remnants of ancient stars which shine through the universe and no longer exist. So we can look into the past and think about, like, how many generations of stars have there been so far that burned and died before we even came to being.
Yeah, it's kind of like people, you know, we humans have generations that come and go and each time they're a little bit different, hopefully.
And if we're made of star dust, then by analogy, are you made out of grandparent dust? Yeah?
Yeah, So there have been a lot of generations of stars since the beginning of the universe, and so today we'll be tackling the question how many generations of stars will there be? I guess, Daniel, that number could be finite. Is that the idea, like, maybe there aren't that many generations of stars that can happen, or is that number infinite?
Perhaps? Yeah, exactly, That's where we're going to dig into. There's a lot of different ideas about the future of the universe and how this might happen. A lot of things we still don't even really understand about the conditions for star formation, where it's happened in the universe where it's not happening in the universe, and all that's going to come together to give us a picture for how many times stars can burn and then explode and reform.
Yeah, I guess it's interesting what you said earlier is that maybe a lot of people don't realize that stars kind of have a lifespan, right, Like, you know, the Sun is not going to be there forever. It wasn't always there since the beginning of time. It came about at some point in the past, and it's not going to be there forever into the future, and in its place, maybe there'll be another star exactly.
The Sun came around sort of late in the universe. Universe fourteen billion years old. The Sun is only like four and a half billion years old, So there was nine or so billion years of stuff happening in the universe before our sun even existed, Right, that's a lot of time for stuff to happen. Stars formed and died, and then our star formed from the leftover bits of other stars and also you know untouched material from the Big Bang. And our sun will not last forever. It's only got a five more billion years or so and so it's a fun question to think about, like how many times can you gather stuff together to make stars, burn it, blow it up again, and then repeat. Can you do that forever? Or is there some fundamental limit to how many times the universe can get right?
Yeah? And if we run out of generations to stars, well, does that mean the universe will be dark forever? It's at the end of light of sunny days in the universe.
That's a dim weather forecast.
Man, I know it's a cloudy future.
Zero percent chance of radiation forever.
Well, need subblock. So that's that's a positive.
Don't invest in sunscreen companies people.
At least not in the billion year bond market. But anyways, we're wondering how many people out there had thought about this idea of solar or star generations and how many there will be in the future. So, as usual, Daniel went out there and ask people how many generations are stars? Do you think there will be?
And as always, I am deeply grateful to those of you who are willing to answer these crazy questions online. If you'd like to hear your voice speculating on difficult physics questions on the podcast, please write to us to questions at Daniel Adjorge dot com.
Here's what people had to say.
I think I heard somewhere there's been three generations of stars. So if we're supposed to the universe is supposed to last for billions of years more, at least four or five more.
I'd say, possibly, and unlimited amount of generations.
We don't really know what's going to happen in the future exactly.
You might you're smarter than me, but.
Yeah, I don't know. I don't know.
It depends on the time, depends if the time is infinite, I don't know. Probably they can be a lot, a lot of generations at eleven, Okay, how.
Many generations of stars will there be? I think? So, I know that we've got the original and then we've got the next generation. I'm gonna say, we're gonna have all the generations of stars.
I have no idea.
Well, we're in the third generations of star at the moment, even though this generation is called population one. I got Jorge has something fun to say about that naming logic. But I'm not sure how much of the matter in the universe has now been used to create stars. So let's say it's ten percent. Not sure it is, but let's say it's ten percent. There. We are three generations into the universe, so ten times as many, let's say thirty generations.
I've thought about this because we know how long the universe is gonna last and how long it's gonna be dense enough for new star formation. Our sun is like four billion years old. In the universe is like fourteen, So maybe there's been two or three generations there, and Sun's got another four billion years left and it's kind of an average star. Oh, so maybe that's we're just on the second generation right now. So maybe ten more generations, it's my best.
Guess, or five hundred billion seems like a good number of star generations. I mean, if we're taking the average life span of a star to be somewhere around five billion years, and you know, at some point, you know, all the stars exploding are decaying, so we're going to kind of reach a point where gas and dust is to spread out to create more stars. Yeah, we're gonna reach a plateau. So I'm just gonna guess five hundred billion years.
I guess infinite until the universe ends, because and you can't really count them because different star generations and at different times, because the life spans of stars is so different.
All right, So a lot of guessing. I mean, everyone seemed to have sort of an idea. Maybe they're like three infinite.
Five hundred billion. I like that as a guest. That's a good guess.
They said that sounds like a good number, Like, why not.
Sound like a good number? Yeah, next time they ask me if i'd like a raise, I'll say, yeah, you know what, five hundred billion seems like a good number.
Pennies or centy dollars.
Nano dollars or something.
All Right, So that's a pretty interesting question. How many generations of stars will there be? Are we gonna have stars until the end of time or are we gonna run out of fuel or ability to form these stars eventually in the future. Start us off, Daniel, I guess how do you define a generation of a star? And how did those kind of come about?
Yeah, it's a good question. How do you even define a generation of a star? It's actually not that easy to like crisply what generation a star is in, because every star is made out of a bunch of material that just happen to be around and some of it could be like leftover pristine material from the Big Bang, and other bits could have been in other stars. But I think it's important to understand this sort of process of how stars are formed and then burn and then die, so we can think about how often that can happen. And so you know, the way that stars form is that you have these big clumps of gas and dust, these things like these giant molecular clouds. And I think the thing that people might not realize is that for stars to form, you need cold stuff because what you need is gravity to pull this stuff together. Remember, the stars are a delicate balancing app between gravity that's trying to pull things together to make it more dense and pressure that's like resisting being compacted too much. But gravity is really weak. So to get a star to form, you need like a really pretty cold, pretty huge clump of stuff near each other and it gathers together to form a star and reach that critical density where fusion can start. So you need to start with something like ten to twenty degrees calvin.
And it has to be cold because if it's too hot, it won't clump together, like it'll just kind of keep missing itself. And it won't compress.
Yeah, because if it's too hot, that means that the particles are moving around too fast, and so gravity just can't slow them down exactly. So they need to be like basically motionless so that gravity can gently tug on them and pull them together into a star. If it's too hot, it just won't happen.
Right, And we've talked about in other episodes about how you need like a certain minimum amount of stuff, right, like more than a jupiter's worth of stuff. And also it's about kind of the kind of stuff it is, right, like, the only certain kinds of stuff will clump and form into a star.
Yeah, in order to get fusion going, you need like at least like eighty times the mass of Jupiter. You can get clumps forming that are smaller than that, they just won't necessarily start fusion. But typically these clouds where stars are formed are like a thousand light years long, and they have a huge amount of material like thousands to million times the mass of our sun. And inside that you have like denser regions, which astronomers refer to by the technical word clumps, and then inside that are like slightly denser regions they call cores, and that's usually where stars are actually formed, but it's not something we understand in great detail, you know, like what actually makes those cores, what triggers them to collapse into a star. Some people think like and nearby supernova has to come through and the shockwave there can trigger the collapse to make the star. But you're right. You can also be a variety of material. But you know, when the Big Bang happened, it made mostly hydrogen, and so most of the stuff in the universe is still hydrogen. It is a little bit of helium also, and so most of the stars in the universe are made of mostly hydrogen and helium. And then there's heavier stuff.
Right, And it's important that it's hydrogen or healium because those are sort of the lightest elements, right, And so to make a proper star you need fusion. And so it's easier to sort of merge together the lighter elements than it is to merge together like the heavy elements.
That's right. To fuse hydrogen, you need a lower temperature than you do to fuse helium, which is a lower temperature. Then you need to fuse lipium, et cetera. Et cetera. So if you want to start a sun, you better start with a lot of hydrogen. And something I think is funny is that astronomers refer to anything that's not hydrogen or helium as a metal, by which I think they just mean like, oh, this is a serious heavy element. But you know, like chemists are like oxygen is not a metal. You know, carbon is not a metal, but according to astronomers, it's helium, hydrogen or metals. It's like there's only three elements on the periodic table for.
Astronomers, hydrogen, helium, and metal. It's sort of like musical tastes, Like someone who's really into classical music thinks anything not classical is like heavy metal.
Intent that's right, it's either mozart or it's hip hop exactly.
Yeah, So you need a lot of hydrogen and helium, which is good because the universe started with a whole bunch of hydrogen and helium, right, But you can also make like if I have a whole bunch of oxygen, can I make a star out of just oxygen?
You could make a star out of just oxygen, and if you had enough of it and it was cold enough and near enough to itself. Then it could pull itself together with gravity, and you could even you know, get oxygen fusion happening to make heavier stuff that certainly as possible.
Could that happen naturally?
It wouldn't happen naturally because the amount of oxygen in the universe is tiny compared to the other stuff in the universe. So, you know, most of the stuff in the universe still hydrogen. You know, we've been burning stars for billions of years to make quote unquote metals like oxygen, but we haven't made that much progress. You know, it's still overwhelmingly hydrogen. So you make a random star, it's going to be mostly hydrogen helium. Now, the like fraction of metals in stars has been increasing as the universe goes up, just because there's more of that stuff around. But to get like a star sized blob of oxygen by itself, that's never gonna happen. Well, I mean, I guess in an infinite universe somewhere it's happening.
Right, or like we could maybe engineer it maybe somehow.
Does that sound like a fun project?
Let's make an oxygen Why do you do that?
What are you doing this weekend?
Oh?
I got some you know, house projects going and make an oxygen star.
Yeah, I cleanc bathroom and also make a star. All right, So then that's kind of how stars form. And I think we've talked in other episodes about kind of what happens then, right, Like, eventually all of that hydrogen and helium merges into heavier elements, and those merge into even heavier elements, and at some point the star burns out or kind of implodes. Right, there's a couple of possibilities there.
Yeah, as fusion happens, it creates heavier elements, and those heavier elements are then fuel for the next round of fusion, which are fuel for the next round of fusion. But at some point it stops. When you get up to iron and iron can't fuse and create energy. If you fuse iron and iron, you actually lose energy, and so that cools down the star and so it's sort of like it creates ash and eventually it's snuff the star out, and so the star is no longer able to resist the gravitational forces because it's not producing enough radiation pressure to resist that collapse. And then depending on the mass of the star, it either you know, just like cools down into a white dwarf, or maybe it goes supernova and blows its material out, or maybe it goes supernova and the core becomes a black hole. Depends on exactly how much stuff you started from. But yeah, eventually stars do fizzle out.
Right, So a star is born, it lives, and it dies, and that's one generation of star and then how it a next generation of star form? Then if it just snuffs out or if it turns into a black.
Hole, because it doesn't capture all of the materials into the star and sort of hold onto them forever. Even in the scenario where you create a black hole, a large part of the material of the original star is blown out into the universe. Like our sun for example, it won't go supernova and it won't go into a black hole, but still it's going to have this big red giant phase when it's burning helium and it puffs up to be really really huge, and a lot of the material gets really dispersed, and then it blows that stuff out into the universe. So when you end up with like a core that's left over from a star doesn't have all the material of the star. Some of the stuff from the star is blown out into the universe and then can get remixed into the basic ingredients of hydrogen helium to form the next stars. So that's how you can sort of recycle material that's made in a star into other future stars or planets or people.
Oh, I see, it's like when a star dies, most of its materials go out back into space, like you compost it kind of, you know, you don't throw it all in the trash, you know, it doesn't all get buried in a coffin, is put back into the soil.
Yees. Some of it is sort of lost to black holes or maybe to a white dwarf that eventually will cool into a black dwarf and can't really be used again. But a lot of it is tossed back out there as fodder or ingredients for the next generation of stars, which will have a higher metal percentage than the previous generation, just because this star has now made more metals out of the wrong ingredients.
Right, they'll be heavier metal, more extreme. But I guess you know, one question I have is, like, you know, stars are so far apart, like do you actually get mixing of like you know, left door star stuff from one star mixing with left door stuff from another star, or is it mostly like it all stays within the same you know, recycling area.
Stars are far apart, but you know they tend to be made in clumps. So you have like stellar nurseries, these big gas clouds where stars are made, and those gas clouds are collected both from like the interstellar medium and the intergalactic medium and also from old stars. So along a long timescale, this stuff really does move around and gather, and so stuff doesn't just like stay in one place. There are flows and ebbs and currents of all of this stuff.
All right, Well, so that's one generation of a star, and so you can have multiple generations as the stars and the stuff goes out and it comes back in. And so let's talk about how many generations there have been a star in the universe, and then let's talk about what the future holds. But first let's take a quick break.
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All right, how many star generations have there been and will there be in the universe? Daniel, we talked about one generation of a star, and we know that a star eventually dies, and all this stuff kind of goes out into space and then it maybe it gets combined, and then that stuff then by gravity, I guess it cools off and then comes together again, and then maybe it ignites and then creates a new generation of a star. But eventually that star will die. So and this happens over I guess millions of years or billions of years.
It depends a lot on the size of the star. So a larger star will burn hotter because its core has a lot more pressure on it, so its temperature is higher, so it burns through its material much much faster. So the bigger the star, the shorter its lifespan. Like a star is a few hundred times the mass of the Sun. It might only live like two to five million years. If a star is really really small, like much smaller than the mass of the Sun, like right on the threshold, it might burn for a trillion years, whereas our star, you know, the sun is going to burn for about ten billion years. So the serving size you get at the very beginning totally determines how long you will burn.
For interesting sort of like rock star some of them, you know, flame out after one album, and some of them have a long and a nice career.
I guess Bob Dylan is very low mass, right, because he's like on album fifty.
Yeah, Whereas you know Rick Ghastly was a super nova. So you're telling me that each generation of a star there's sort of no fixed time period like in human years, you can think of a generation as being roughly, I don't know, thirty forty years, but in a star it could be like one year or one hundred years.
Yeah, but it is sort of similar to humans, right. Some humans live a long time and have kids in their nineties, like Charlie Chaplin. Some people have kids when they're teenagers, and so over fifty years you might have like three generations. It's sort of similar with stars, and that's why I was saying earlier, it's not exactly clear, you know, it's to say like which generation a star is in, because if I have material from all sorts of sources, including two generation stars or three generation stars, or even pristine material left over from the Big Bang. So it's tricky, but you can sort of organize roughly into few generations when we look back into the cosmic history.
You mean, sort of like on the average, or maybe just looking at like our sun, we know how many generations it's at.
Yeah, well, we don't know actually how many generations the history of our sun features, because in theory, you could have a series of really really massive stars hundreds and hundreds of times the mass of the Sun that die and burn out and die and burn out, die and burn out. You could have, you know, material that's been in a hundred of those stars that's around there in the universe right now. So we don't actually know for any given piece of material how many stars it's been in. What we can do is look at the stars we see in our galaxy and in other galaxies, and we notice something interesting. We notice sort of like two rough populations of stars. Stars that seem to be sort of a lot older and have less metals in them, and then stars that tend to be younger and bluer and hotter and have more metals in them.
But then each of those would be in a sort of different generation, right, Like some of them might be you know, one hundred to ten generation and someone might be, you know, fifteenth or five.
That's right. But the interesting thing about these two populations, so we have like the older ones and the younger ones, the older ones we call population two stars is that they all seem to be really really old. They all seem to have formed like eleven, twelve, thirteen billion years ago and to still be around. So those stars probably are second generation stars at most because they formed very very early on in the universe and they're still around. So that limits like how many generations of stars there could be before them. The other population of stars we see, what we call population one. They're still forming today.
Oh, I see, you're sort of categorizing what you see out there in the universe. You're not categorizing by generation. You just sort of looking out there and categorizing by age, Like, Wow, those over there look pretty young, so they must be later generation. But these over here look really old, so they must be kind of like one of the OG generations.
Yeah, it's just like looking at humans and you're like, well, this bunch of people all are similar and come from a similar era and act the same way. So we'll call them boomers, right, and these folks will call gen zers. And you know, there's not a crisp delineation between generations between millennials and gen xers and whatever, but it's sort of a rough rule of thumb. Especially when we look out there, we notice, just like we noticed in human populations, these sort of clusters of stars that have similar properties.
All right, So then when you can categorize kind of by age out there, by what we see right now, and so step us through, what are the different populations.
So what we've seen are these two populations, population one in population two, And you know these are historical names given by a German astronomer back in the forties when he was looking at these stars and he saw that a bunch of them were bluer stars, and these were tend to be like in the galactic disc, right far from the center, and that closer to the center, you had more red stars, so we called them population one in population two. He didn't understand at the time what it meant that population two stars were probably much much older. In population one stars were more recent. But we know that now and it makes sort of sense because you know, as stars live in the galaxy, they tend to slow down in their orbit and sort of fall towards the center. So the center of the galaxies mostly these older stars that formed, like you know, eleven, twelve, thirteen billion years ago and have less metals in them than the other stars because they were formed in the time when the universe had less metal in it, it was more hydrogen. So those are the population two stars. They tend to be like making up the bulge of the galaxy. And then also they're in these weird globular clusters that sort of orbit the galaxy that we had a whole fun podcast episode about.
I guess maybe a question is how do we know how old they are? And how do we know how much metal is in them? Like is it from like the spectrum of the light that comes from them or the distance, Like do we know their age from how far they are, or like how can we tell their age, and is that polite to ask?
That's a little tricky their age. What you can do is you can look at the color of the star. Stars tend to be bluer when they're younger because bluer means hotter, and so hotter stars don't last as long, they don't live as long, and so if you see a star that's blue, that means it must be younger because they just don't have that long a life cycle. Like if you see a cat, you know that cat is not one hundred years old because there just aren't any hundred year old cats. So if you see blue stars, you know that they tend to be younger. Red stars are either old stars or they might be smaller stars. So it's not exactly clear always when you look at a star's color to tell its age. But what we do see is that population one stars tend to have a lot of blue stars in them, which means that they're young, whereas in the bulge there aren't very many blue stars, so that suggests that there aren't many young stars there, and most of those are pretty old.
Oh, I see where you're grouping them by location or by how old you think they.
Are, Well, both and there's a correlation there, right, the stars in the center tend to be redder and older, and the stars in the disk tend to be bluer and younger.
All right, So then I guess you can assume that the younger stars are maybe a later generation, right because they were born more recently, whereas the older stars they might be even one of the original generations, right, because you're saying they're fourteen billion years old that's when the universe started.
Yeah, we don't think that they're part of the original generation. They are like, you know, eleven to thirteen billion years old, so we think they formed, like you know, a billion years after the start of the universe. You also asked about how we can tell what they're made at it. That's another clue. When we look at the spectrum from the stars, how they're glowing, we can tell what's inside the star because different elements glow at different temperatures. They tend to like get excited at different energy levels, so they have like a characteristic fingerprint. So just as you said, we can look at the spectrum of light that tells us, like, you know, how much light is in there at this frequency, and how much of that frequency tells you exactly the composition of the star. And what we see is that these stars tend to have a lot less metal in them than the stars that are forming today, like even our sun. So these population two stars can have like one hundredth or one thousands as much iron in them, for example, as our sun. So that also suggests that they were formed earlier in the universe, when the universe had less metal in it.
Oh, I see, right, As the star progresses through its life, it makes metals, right, But then if it's been a lie for that long or burning for that long, wouldn't have made a lot of metals by now.
It makes metals, but you know, not that much. When a star burns, it's mostly burning helium and hydrogen. It doesn't convert all of that into iron, for example.
But the fast burning ones do create metals more quickly, I guess, or as each generation burns and is born, then you sort of increment in metallicity.
But not every star makes it all the way up to iron. Like, you need enough stuff in the star to create the conditions so you can get hot enough so you can fuse the stuff to make iron. Some stars stop after making helium or lithium or carbon or neon or whatever, depending on how much mass they have in the temperature that they achieve.
All right, so then what does that tell us about how many generations there have been in the universe? Since we know that, you know, the older stars are only a few generations in but do we know how many generations the younger stars are or what generation they're in.
Yeah, we're not one hundred percent sure, but the current thinking is that there are roughly three generations. So these stars we call population one. These younger stars like our sun, that are out there on the disc, including stars that are just forming and stars that are a few billion years old. These are the ones with a lot of metals in them, like they start with like one to four percent of them are made out of metal. So that's population one. And then the population two, the ones sort of in the bulge of the galaxy, the older ones. We call those population two. And we think that there was a population before that that very early on in the universe, before any of the stars that are burning today were formed, that there was an initial star forming phase, but that all those stars are gone now that those stars didn't last very long, just a few million years, maybe tens or hundreds of millions of years, but that those are all gone, and we call those population three stars. So we've never actually seen one because they're so old, they'd be very very distant and so very hard to make out. So that's sort of the rough timeline. Population three stars were the first in population two, and now today we're making population one stars.
M I feel like you picked the opposite order, like the higher the number of population the first in order of time.
I know, because the next generation of star is going to be what population.
Zero zero and then minus one. I think you named yourself into a corner there.
I think we certainly did. It's sort of like you know, deciding the charge of the electron, like whoops, that was not the best choice. But I don't think at the time we really understood the context is sort of the historical sweep of all of this stuff, or we definitely would have named it something else.
All right, So there was an original generation of stars, but those stars you say they're all gone because I guess in the early universe, thanks for sort of like hot and vol talent, so all those stars burned out pretty quickly.
Yeah, we think that those stars were really big. The very very first stars were like hundreds of millions of times the mass of the Sun. It was harder to make smaller stars early on because there wasn't as much metal, so it was difficult to get like a big blob of stuff to cool. Like, if you have some metal that can seed a core to gather stuff together, it's like a heavier spot. But if you just have a big cloud of gasses hydrogen, it's harder to get like a small clump together. So these first stars were probably like hundreds of times the mass of the Sun and then burning for only like a few million years and dying at supernova after making a little bit of you know what astronomers would call metals. But again, we haven't seen those, so they're hypothetical. Like we've seen really really distant galaxies, some of the first galaxies that ever formed, but we can't resolve the stars in those galaxies, so we can't say we've ever seen a population three star.
Interesting, they're only sort of in our imagination or in our I guess theories about where the second population of stars came from.
Yeah, it's like the oral history of your family, right, maybe you don't have any pictures of great great Greek Grandma Genine or whatever, but maybe your family knows something about what she liked for breakfast.
And that she was a rock star. Maybe it was very.
Bright back in Lithuania or whatever.
So then that first generation burned out and that give us the population two, which are redder but a little bit cooler. And then that population, some of that population burned out, and that's where the population one that we see now and like our Son comes from.
But our Son, right, it has a mix of stuff in it. It has some stuff, certainly from population two stars that burned out, probably some stuff from population three Stars, but also it has a lot of stuff that hasn't done anything since the Big Bang. Because our Son is seventy percent hydrogen and twenty eight percent helium, so it's only like one to two percent metals. So most of the stuff in our Son is basically on its first act since the Big Bang happened.
But we still think it's a third generation star.
Yeah, because it has stuff in it that was created from other suns like that one to two percent that really changes what a star is. And that stuff was made by other suns, and you know, like most of the Earth is that kind of stuff, Like most of the Earth is not hydrogen or helium. Right, So basically everything on our planet came from the death of a star, whereas the Sun is mostly pristine stuff from the Big Bang.
Still got a lot to go on and possibly more generations. And so I guess the question is how good is that stuff in the sun, Like it's it good for one more generation or a billion more generations? What will the future hold? So let's get into that, But first let's take another quick break.
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Hi, I'm David Eagleman from the podcast Inner Cosmos, which recently hit the number one science podcast in America. I mean neuroscientists at Stanford, and I've spent my career exploring the three pound universe in our heads. We're looking at a whole new series of episodes this season to understand why and how our lives look the way they do. Why does your memory drift so much? Why is it so hard to keep a secret, When should you not trust your intuition? Why do brains so easily fall for magic tricks? And why do they love conspiracy theories?
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The help of Stanford psychologist Jamiale Zaki.
It's really tragic.
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All that on the Happiness Lab, listen on the iHeartRadio app, Apple podcasts, or wherever you listen to podcasts.
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All right, how many generations of stars will there be in the future. We know that our son is sort of like the third generation star in the universe's history, and so the question is is this like the beginning of the history of stars or you know, are do we only have a few more generations left?
Yeah, it's a really fun to think about what the future might look like, and you have to like think deep, deep into the future of the universe. And I think there's sort of two big questions. You know, one is like, if you had perfect conditions for making stars so that you could always gather the hydrogen together and heal them together to burn it, how long would it take to burn all that fuel and convert it all to iron for example? You know, that's one question, But the reality is that the universe is not a perfect laboratory for making stars. You need very specific conditions. So I think we should also think about, like how long the universe will be conducive to star making. You might have the ingredients you need to make stars unburnt hydrogen, but that doesn't necessarily mean that you're going to be making stars.
Well, let's think about it, maybe just our star, Like what's going to happen to our star? We know we talked about how it's going to burn out and sort of blow up, get big and then cool down eventually, and then do you think another star will take its place? Eventually?
Well, our star is probably going to turn into a white dwarf, right, which is just like a hot lump of stuff. No longer fusion is happening, but it's still really hot and so it's glowing, which is why it's called the white dwarf. And white dwarfs are stable, like, we don't think they really do anything else. They just sit there and gradually cool off until they become a black dwarf, and that could take you know, trillions of years. We don't think there are any black dwarfs in the universe right now. There's a bunch of white dwarfs sitting there glowing hot, but none of them have had enough time to cool the black dwarfs. But that doesn't mean that our sun won't contribute to the next generation of stars. Because the white dwarf doesn't take all the material from the Sun. There's a lot of the material from the Sun that will get blown out into maybe a new planetary disc for that white dwarf, or just out into the interstellar medium and contribute to another generation of stars.
So our star it's going to become a white dwarf and then blow outed stuff out into space, which then is going to maybe combine with the hydrogen helium from other stars that burned out and then make other stars.
Yeah, this happens because the sun before it burns out, before it stops fusing, it gets really really big, and the outer shells tend to get blown out while the inner part collapses. So it's these outer layers of the star they're going to get blown out, and they can definitely contribute to future stars or a planet. There could be aliens ten billion years from now that are eating their cereal and thinking, wow, this could have been in another star billions of years ago, and they would be right.
I see, when you're eating your cereal, you are maybe. I mean everything is made in the core of stars, right, anything above hydrogen.
And yeah, unless you're eating hydrogen for breakfast, what you're eating was made in a star.
Interesting, not recommended, but not as filling or it is, I guess pretty filling.
Hydrogen the cereal. Oh, the humanity.
Oh I see. So it's not like a star blows up and then in its place in new star forms It's like it you need this kind of like mixing between the spaces of stars.
Right, Yeah, It's more like you know, when a tree dies and it gives its nutrients back into the ground. You don't automatically just get a new tree in exactly the same spot. It just provides nutrients for a future plant to gobble up and to build something else.
So you're saying one scenario is that this keeps happening for a long time. You know, you get more generations of star but eventually, like all of the hydrogen and helium in the universe is going to have been consumed, maybe could be consumed by stars and turn into heavier metals.
Yeah. I don't think that's likely to actually happen in our universe, and we can talk about why it'd be difficult to arrange that. But in theory, if you engineer the universe to be a perfect star making laboratory, then yeah, you could eventually burn all the fuel. Like it's one direction. Or we're not making more hydrogen, right, We're just decreasing the amount of hydrogen in the universe as years go by. It's not an unrenewable resource, right.
Right, Yeah, Because I guess you can fuse things together, but it's kind of hard. Once you made all this iron, eventually it's hard to like turn it back into hydrogen.
Yeah, these things are stable. Like if you go far enough and you make your rain, then that stuff is unstable. It'll break down, but it'll break down into something else stable. It's not going to go all the way back down to hydrogen. Helium is pretty stable, doesn't split spontaneously into hydrogen. So yeah, hydrogen is not a renewable resource. Like you know, we got a lot of it. It's going to take a long time to burn through all of it. But as the years go on, the hydrogen fraction of the universe is dropping.
So like, if you made a whole universe made out of iron, it wouldn't do anything, like you would just sit there.
It depends on the distribution, but it probably would form a lot of black holes. And that's pretty dense stuff. So you know, if you had like a solar sized blob of iron, it would collapse into a black hole.
All right. So then I guess technically, if you were to engineer the universe, like, how many generations of stars do you think we could get in?
It's hard to know. And I asked them astrophysicists this question, and they were all just like, whoa such a big number? I can't even imagine, you know, hydrogen of depletion. I got some estimates between one hundred trillion and ten to the one hundred years eat all the hydrogen in the universe years.
But how much is that in generations?
Well, it depends. Right. Stars can burn for maybe ten billion years or a trillion years, depending on their mass, or they could even burn short amounts of time if you have more massive stars. We don't really see a lot of really really massive stars. But so if you say, for example, the average length of a life span of a star is maybe ten billion years, then that's still a lot of generations, right then a trillion years has one hundred generations in it. One hundred trillion years, right, it's ten thousand generations in it. If it's even longer time spans, and we're talking thousands and thousands of generations. So from that point of view, like we're on generation three, we're really just getting started.
I would have thought that you could maybe just compute it from the fact that, for example, we're on our Sun is in this generation three, and it already has one to two percent of other stuff. So wouldn't each generation sort of take up another one to two percent of the hydrogen and helium.
Yeah, that's a good question. I don't think this stuff it gets thrown out in the universe is an equal sampling of the stuff that the star made, like the stuff in the cores of stuff that's most likely to get kept into the white Dwarf. Right, So the heavy stuff is less likely to get distributed out into the universe. So the next generation of star is not gonna have like all of the iron that was made in the previous generation. Most of that stuff is gonna end up either in the black hole or the white Dwarf or the neutron star that's left over.
But I guess if you're engineering it though, Like you know, I'm gonna burn my son. My son's gonna burn till it has one of two percent of other stuff. Then I'm gonna throw out all that one of two percent of stuff and take all the hygien and healien and make an ees star. Eventually. That's how I'm gonna run out, right, because I'm gonna be losing one or two percent each generation.
Yeah, if you were optimal about it, then I suppose you could spend one generation burning about a percent or so of the stuff in the universe, in.
Which case maybe we would run out at some point, right to throw one hundred generations maybe.
Yeah, So it depends a lot on how this stuff is distributed and how you organize it, and the mass of the stars that you are making, how long it takes for these things to happen. But yeah, somewhere between hundreds or thousands of generations of stars.
Wow, and then that's it, no more stars.
And then that's it, no more stars exactly. And in that scenario, the universe is dark. You know, it's just like populated by a bunch of white dwarfs that are slowly cooling to black dwarfs, and it's basically not generating any light.
And that's just like the ideal you know, horehem magical engineering scenario where you are able to extract all the hygien and hemum. You're saying that that in real life, the physics of like for me an U star are actually much harder.
Yeah, exactly. But before we leave that, I want to throw out one thing, which is that in that future really dark universe, there won't be any stars made, but occasionally you might get a bright flash of light because white dwarfs can actually create light, like if two of them come together, they can combine and give you like a type one a supernova. This is very special kind of supernova. And so for a few weeks it can like brighten up the neighborhood it's in before dimming again. So the deep deep our future would be mostly dark with these like occasional few week long bursts of light.
And then again this is still like the optimal scenario, right, so it could be one hundred to a thousand generations of star optimally, but really it could be a lot less.
Really, it could be a lot less. And when you look around in the universe, you notice something which is that the rate of star formation in the universe is dropping, Like we used to be making more stars sort of per year a billion years ago than we are now, Like the rate of new star creation is falling in our universe.
So meaning what would you put the estimate of the number of generations ever to add, Like would it be It might be less than one hundred, Then.
It might be less than one hundred. Absolutely, it might be less than twenty. You know, a lot of it depends on what's going to happen with dark energy in order to form stars. What you need is a bunch of stuff near each other, like these cold gas clumps near each other. But what dark energy is doing is that it's pulling galaxies away from each other, and so that makes it harder for like gas to clump together to fall into these galaxies and create new stars. And what we see around us is that there are lots of dead galaxies, galaxies with lots of material in them, but there's just no more stars being formed.
That's kind of sad. I guess they're like graveyard galaxies, nothing but embers.
Yeah, because they don't have any like more gas falling into them to stimulate star formation. And also sometimes in the center of these galaxies, the black hole that's formed, these super massive black hole can be emitting so much energy that basically heats up those gas clouds so that they don't form stars because remember these gas clouds have to be cold. So you can get in these configurations where you have this huge amount of mass stuck in a galaxy but no light being created, no star is being formed, and that's not something we understand very well. It's called star quenching. It's a really active area of research. But it might be that every galaxy is headed in that direction. It might just be a few more generations before all these galaxies sort of die and stop forming stars.
Oh wow, And then what's going to happen after that? Like won't the black hole eventually cooled down, and won't things cool down for that galaxy and maybe stars could kick back up again.
We don't know. We don't think those black holes will cool down. We think they just keep absorbing stuff. You know, stars will keep falling into them. They'll get it bigger and bigger. Some stars might get thrown out of those galaxies. Right the way that stars fall into the black holes, that they're basically like sort of bumping against each other, not physically bumping, but like exchanging kinetic energy. One of them slows down, the other one speeds up, So you'll throw some stars out of the galaxies into intergalactic space. And then these galaxies will just become bigger and bigger black holes. And if dark energy takes over, then you'll have these black holes separated by larger and larger distances. And so that's the future of the universe is these galaxies collapsing into ever distant black holes.
So it sounds like the answer then to the question is how many generations the stars will there be is maybe not that many, maybe only less than twenty to go before the whole universe is completely dark.
Yeah, and it all depends on what happens with dark energy. Like if dark energy decides, hey, we're done with this expansion, let's turn everything around and bring galaxies together. Remember, we just don't understand dark energy at all. We have no mechanism to explain what it's doing, which means we can't predict its future. And it might, for example, turn around and bring everything back together, which could stimulate whole new periods of star formation, you know, before the universe is squeezed back down into a new big crunch. So really, our ignorance of the number of future generations comes from our ignorance of the overall fate of the future of the universe.
So I guess the lesson is, don't invest in sunscreen, maybe invest in flashlight.
Batteries or anti dark energy devices.
I guess now you're saying that if dark energy reverses or goes away, then then you might get possibly more stars in the future, but right before the universe crunches down. Yeah, So it's not that useful.
Nobody'd be a bright ending.
We'll go out with a bang with a will be our goodbye tour. All right, well, I think that answers the question. It kind of makes you appreciate the sun right now, like it could be. Maybe how only in the middle of its like universal lifespan of stars.
That's right, we don't know how many times this magical thing will happen, that this blob of gas and dust and a little bit of heavier stuff will collapse into this incredible bright ball of heat that's capable of burning and burning and burning so stably that life has a chance to evolve on the surface of planets near it. We don't know how many more times that will happen, so we should definitely cherish this one experience.
Yeah, that means that, like you said, it's the only reason life exists at all, So you know, maybe the universe won't get them any chances in the future to make more life.
That's right, So we better figure out space travel before that.
Happens, and better flashlight technology as well. All right, well, we hope you enjoyed that. Thanks for joining us, See you next time.
Thanks for listening, and remember that Daniel and Jorge explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. How is us dairy tackling greenhouse gases? Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
Guess what Well?
What's that?
Ma?
I go?
I've been trying to write a promo for our podcast, Part Time Genius, but even though we've done over two hundred and fifty episodes, we don't really talk about murders or cults.
I mean, we did just cover the Illuminati of cheese, so I feel like that makes us pretty edgy. We also solve mysteries like how Chinese is your Chinese food? And how do dollar stores make money? And then of course can you game a dog show?
So what you're saying is everyone should be listening.
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