Ep. 496: Update on Stellar Populations (I, II, & III)

Another update show, this time on the various generations of stars, let’s get into it.
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Show Notes

Stellar populations
Population I stars
Population II stars
Population III stars
Star formation and stellar populations
Stellar Populations in the Galaxy
Stellar Populations Of Globular Clusters
Stars – Stellar Populations


Podcast Transcription provided by GMR Transcription
Fraser: Astronomy Cast, Episode 496: Stellar Update. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today, and with me is Dr. Pamela Gay, the Director of Technology and Citizen Science at the Astronomical Society of the Pacific and the Director of CosmoQuest. Hey, Pamela, how you doing?
Pamela: I’m doing well. How are you doing, Fraser?
Fraser: I am doing great, although I mentioned this – just the worst June ever. We call it June-uary because it’s so cold and miserable, not freezing, but the temperatures have been pretty low, and it’s been a rough June. And this is one of those few months that we get where it’s supposed to be super nice, and so, no, no luck.
Pamela: I’ll trade. We’re at 38°C.
Fraser: Wow.
Pamela: Yeah.
Fraser: That’s too hot. No way. No one should live like that.
Pamela: Yeah, I’ve been wearing shorts a woman my age should not be wearing out in public because it’s hot.
Fraser: Will you take our 10°C? Would you prefer that?
Pamela: Gladly.
Fraser: Really? I probably would too, actually. Alright. So, just another reminder, 500th show in St. Louis sometime in September. Where do people go to find out about the show?
Pamela: Folks, go to astronomycast.com. I don’t know where else you would go, and we have all the details under the Trips tab. Go sign up. It is either pay in full for $200.00 or put in a non-refundable hold-your-place for $50.00. This allows us to rent buildings that we can sit in and make a science.
Fraser: So, another update show, this time on the various generations of stars. Let’s get into it. That’s it. Quick intro. I was running out of time to prepare, and I didn’t have a longer intro. But we’re gonna be talking about the early generations of stars, the later generations of stars, what we’ve learned about how stars exist, both at the beginning of time and now in their various metallicity. There. I wrote an intro on the fly. Alright, Pamela, where do you wanna start? What have we learned that’s new about the populations of stars?
Pamela: Well, we have confirmed that the first generation of stars did indeed exist and were big and bright. Now, we theoretically knew they existed. We kind of knew they needed to exist or the universe wouldn’t be here, but the issue that we were having is people used to think the first generation of stars, the small ones should still be around because small stars live for forever, and it hasn’t been forever yet. Where are the small, first generation stars? So, when I started astronomy, we talked about this search for the Population III stars, and we couldn’t find them, and people got sad and upset.
And since then, we have indeed made observations that indicate that in the most distant, brightest galaxies observed, we have these giant, massive Population III stars that probably couldn’t have ever formed small and couldn’t have lived very long, and so this has led to much confusion in how we discuss the populations of stars.
Fraser: Well, let’s just sort of set the stage here just to give people an idea of what that early universe must’ve looked like and what those first stars would have been like. Back in the beginning of the universe, this is the first generation of stars. Of course, they call it Population III, but they are only made of the primordial elements left over after the Big Bang: hydrogen, helium, touch of lithium…
Pamela: Beryllium.
Fraser: Yeah. And so, what kind of a star do you get when you’ve got that much material relatively close together, and none of those pesky metals?
Pamela: Well, it turns out those pesky metals are really needed to help radiate energy in an effective manner in the outer envelope of the star, and without them, the star is like, “I shall grow bigger, and I shall grow bigger, and I will be really big,” by which I mean the small ones were 60 solar masses. And so, you end up with stars that are like, “And I shall die as a Type II Supernova fairly young.”
And so, where we used to talk about Pop. III stars being the first generation stars that we were desperately trying to seek, we now have a state of confusion because while there was a first generation of smaller stars, they just weren’t the first generation of stars, and we can look at them, and we can see the signatures of sometimes just one supernova’s materials getting mixed into the stellar stuff and things. And so, in talking to people about what Population III stars are, you have the camp of people who are like, “Population III stars are stars that are extraordinarily metal-poor.” Okay, that’s a nice, happily vague definition.
You have the people who are like, “It is the first generation of stars that were formed,” but aren’t specific on first generation by size, first generation total because the first smallest stars were forming at the same time that the biggest stars that formed were dying, so it’s kind of a mixed up generation because tiny things take time to form.
Then you also have the theorists out there who are like, “Well, maybe we do have Population III stars floating around out there, but their chemical enrichment from their own internal nuclear burning has gotten circulated up to the surface, or maybe as these stars orbit around the galaxy, they’re scooping up heavier metals, and so they’re in disguise.” So, Population III stars are whatever the author of a paper decides they are.
Fraser: Right. And I mean, the size. I mean, the biggest stars that we can get right now are about 60, 70 times the mass of the sun, maybe bigger than that.
Pamela: They have pesky metals in them.
Fraser: Right. So, what was the most massive stars that were possible, or do we just still kinda not know yet?
Pamela: So, it’s theorized that they could’ve been maybe even hundreds of times bigger. It’s not like we can go out and just casually look at them.
Fraser: Today –
Pamela: Today.
Fraser: Yeah, there are actually a couple of telescopes in the works, right? There’s the Origins Space Telescope, which obviously, I just did a video on all these different telescopes, so it’s all sort of in my head.
Pamela: And it’s not yet built, and we don’t know if it will be completed.
Fraser: Of course not, but its job is gonna be to directly observe Population III stars.
Pamela: And James Webb will be able to do a fair amount of work in this category as well. It will be doing it in the infrared where there are plenty of things to look at because the ultraviolet light from these extraordinarily distant stars gets shifted not just into the visible, but out the other side into the infrared, so we may even have our friendly lime and alpha hydrogen lines. This is the one-to-two transition in hydrogen. It may just be that those lines are right within the realm of what we can see with JWST.
Fraser: And even if they can’t see them directly, it should be able to do sort of what Hubble is doing and do these bank shots using gravitational lensing to see stuff that’s a little farther behind, and then when the Origins Space Telescope shows up, it should be able to see them directly.
Pamela: And what gets me is the very large telescope, which also works somewhat in the infrared, was able to see these stars from the surface of our planet, or at least infer them. It wasn’t able to resolve them or anything. But by looking at the brightest galaxy ever found in the early universe, they were able to just about two years, three years ago say these stars that we’re looking at in this distant galaxy, these are Population three stars. So, we’re seeing them; we’re just not resolving them well, and we’re pretty much seeing them in one system, but it’s a start, and it’s a start that doesn’t require a space telescope.
Fraser: Right. So, those are the mysterious Population III stars, the ones that are really just theorized, but other kinds of stars are seen here in the Milky Way, Population II and Population I, and there have been some interesting discoveries about all of these. The oldest stars in the Milky Way have been found, stars that are similar to our own sun and maybe formed with it have been found, so where do you wanna start with some of these other discoveries?
Pamela: I wanted to finish Population III star discussion with one more intermediate case. In a great press release last week by my undergraduate advisor Tim Beers, who is now at the University of Notre Dame, they introduce us to stars that they are not putting into Population III or Population II and are instead calling carbon-enhanced, metal-poor stars. So, these are stars that formed out of that first set of massive supernova, and they’re still out there waiting to be found. And what’s interesting is by studying these stars, we get insights into how our own galaxy formed that changes the picture we have to work with.
It used to be that we said Population II stars exist [inaudible] [00:10:29], and the outer halo, and in globular clusters, and in these old places. But these carbon-enhanced metal-poor stars that they’re finding, which are the first generation of small stars, they’re finding them in places like the disc of our galaxy which implies that the systems that they formed in later merged and built up our Milky Way galaxy.
So, it’s fascinating to think that the gas and dust that went on to form our sun came from systems that as we had predicted in observations are now starting to give us evidence for, those systems built up, and had some metal-poor stars in them, and now have the richest metallicity stars out there in them.
Fraser: It’s pretty hilarious. There’s this joke that astronomers have, right? There are only three kinds of elements in the universe?
Pamela: X, Y, and Z. Hydrogen, helium, and everything else.
Fraser: And metal, which I think is awesome.
Pamela: Carbon’s a metal as far as I’m concerned.
Fraser: Oh, you’re one of them as well. Okay, alright.
Pamela: Yes, I managed to convince folks to let me not take chemistry and instead take nucleosynthesis, graduate level as an undergraduate, so yeah.
Fraser: It’s all just metal to me.
Pamela: Yes.
Fraser: That’s awesome. Alright, so let’s talk about some of the other kinds of stars, then, and some of the interesting new discoveries that have been made in them.
Pamela: So, when it comes to Population I versus Population II stars, we don’t have a hard and fast this is what one is, this is what the other is. The way that we generally talk about it is Population II stars are going to be less luminous, they’re less likely to have planets, and this difference in what kind of systems they form and how bright they are is essentially the defining factor. The other way we define them is where they are. Globular clusters are pretty much pre-determined. These are going to be Population II stars, the metal-poor stars, whereas instead, we’re going to have the disc of the Milky Way where stars are continuing to form, these are Population I stars. Stars in open clusters, Population I stars.
And there are young Population II stars, which is deeply confusing because in general, Population II stars, the metal-poor stars, are thought to be old globular clusters. They’re old. But we also still have fairly pristine gas and dust out there, or mostly just gas because it’s pristine, and this fairly pristine gas is still capable of forming stars. And so, we can still get Population II stars forming, it’s just forming out of the stuff that is untouched by supernovae.
Fraser: Although, just in the last couple of weeks, there was a really interesting paper where they’ve got a new way to date globular clusters, and it may very well be that they are billions of years younger than originally thought, and so not 13 billion years old, but maybe 9 billion years old, which is still super old, but not old enough where you wonder if they’re older than the universe itself, so at least that settles that a bit.
Pamela: Well, we did used to think they were 15 to 18 billion years old, but the problem is you can’t walk up to a globular cluster and say, “Hey, when did you form stars?” So, the way we get at the age of globular clusters is by running simulations and by using the chemicals in the outer atmosphere to essentially radiocarbon date them. In this case, it’s cosmochronology dating of them. And some clever new models that look at binary stars in particular have reconstrained the age of these systems. Now, we all need to wait and see if these constraints stand up, or if globular clusters rebound to be the 12 billion that we thought because if they don’t, then our galaxy formed different than how we thought.
Fraser: Right. So, what are some new discoveries made about some of these other Population stars? I’ll sort of throw one in just to go, which is last year’s kilonova event, this idea that these two neutron stars collided together. One of the things that they generated was an enormous amount of gold, and strontium, and lead, and all of these heavy elements blasted out into space. Where did the heavier elements come from? It was always thought that they came from either stars at the end of their lives, blowing out material into space –
Pamela: Type II supernovae.
Fraser: – and supernovae when they go off, making stuff that’s higher than iron. And now, it looks like you’ve got potentially some and maybe even all of these heavier elements are actually coming from colliding neutron stars, and then they are going into these various solar nebulae and seating them with the heavier elements, making stars more like our own sun.
Pamela: What’s really cool about that particular result is it didn’t just rely on the results from looking at the gravitational waves. What actually happened is they looked to see how did these heavy elements build up over time in various systems, and you would’ve expected if it was Type II supernovae that you would’ve ended up with an early buildup of heavy elements because these are such short lived stars. And what they found is it was over eons that these heavy metals built up in star systems, and the only way that you get that is if you have to wait for the neutron stars to get around to merging, which can be a variable duration process.
Fraser: Still, kind of amazing. I’ve got a couple more, but have you got some more new information about these different stars?
Pamela: The one that continues to get me is we always had various predictions on, well, we expect the most metal-rich stars to be the ones that have planets, and we expect the most metal-poor stars to not have planets, and this is actually a piece of science that continues to be holding up. It turns out that it doesn’t have to be the most metal-rich stars. We do find stars that aren’t as metal-rich as our sun that have planets, but we still aren’t finding planets in globular clusters. We still aren’t finding planets in the halo of our galaxy. And so, this is becoming further and further evidence that the one thing that stops a planet from forming is lack of stuff to form out of, which seems obvious, but still has to be confirmed.
Fraser: Right. So, if you don’t have the heavier elements to form planets, you can’t form planets.
Pamela: It should be obvious, but because the universe likes to do unexpected things, we do have to confirm.
Fraser: Right. So, I would say one of the really interesting things that I’ve been tracking is – this sort of came out in the last couple of years, and this was something that I think if you had asked us 10 years ago, I guess when we started Astronomy Cast, we would’ve said, “Oh, this is impossible,” which is that astronomers think they found at least one of the sibling stars that formed in the same solar nebula that the sun formed out of four and a half billion years ago.
Pamela: And this is one of those things of science that you look at, and the frustrating thing is we can’t confirm it, so what has been found are stars that have essentially the same composition as the sun, the same ratios of this atom to that atom to this other atom over here. And we know that in the clusters where stars form, all the stars in the cluster essentially have the exact same composition because just like all the cookies that came out of one bowl of cookie dough are going to have the same stuff from the recipe, assuming you mixed thoroughly, all the stars in the open cluster are going to have the same stuff, assuming the universe mixed thoroughly, which it seems to do.
Now, as far as we’ve seen so far, every open cluster is just a little bit different. Every globular cluster, which also formed out of one blob of material, every globular cluster appears to be a little bit different. And so, when we find these stars that have basically twin spectra to our sun, it is a hint that this star has a history similar to our own. And when you run backwards on the kinematics, and there is nothing inconsistent with them having been in the solar neighborhood when the sun was forming, well, it comes down to these stars could be our siblings – there is nothing inconsistent with them being our siblings.
It is extraordinarily low probability that they would happen to have the exact same composition and kinematics, but not so low a probability that it isn’t possible that they’re completely different origins. So, it’s tantalizing, it’s fascinating. Actually, one of my favorite essays I’ve written talks about how we can never know our siblings, and I wrote it before this discovery. And dang it.
Fraser: Right, but as you said, we can never know for sure, right? We can only know that these stars are formed out of almost the exact same ratios of elements to within the error bars, and as we’ve had this conversation many times before, it’s all in the error bars is where the truth stands.
Pamela: And there’s two kinds of error bars, and we don’t always discuss both of them. The one set of error bars is just what is your observational error, how much error was there in your telescopic measurements. And then there’s the error that is what is the probability of this occurring kinds of errors that build up in our understanding, and those quite often don’t get discussed.
Fraser: What else have you got?
Pamela: This whole idea now that pretty much any kind of Population I star that feels like having a planet probably does have a planet. We originally thought that the smallest couldn’t have planets; now they totally have planets. We originally thought the largest couldn’t have planets because they’d push away all of the materials; now they have planets.
And one of my favorite results is there’s actually a planet that is being heated up on the outside by its hot star so much that the surface temperature of this planet is that of a K dwarf star. Its inside temperatures will be different, and a K dwarf will be significantly hotter on the inside, but the fact that a star can heat up the outside of a planet to be the temperature of a star is just a sentence I never thought I would have reason to utter. So, Population I stars are like, “We are here, and we have planets. All of us.”
Fraser: All of them.
Pamela: All of us.
Fraser: Which of course makes the Fermi paradox all the more puzzling, the Drake equation. Numbers are starting to slot into the Drake equation that, yes indeed, the number of stars that have planets is way beyond what anyone ever thought. Universe is weird.
Pamela: Universe is very weird, and at the same time, some of Lynette Cooks’ most amazing paintings of what it would be like to stand on a planet in a globular cluster, we’re not gonna be doing that, and that makes me sad.
Fraser: Anything else?
Pamela: I think the fact that this is still such a rich field is something that’s amazing. We are getting to the point that we’re starting to identify well VLTCs, the very first stars, off at great red chefs, work done by folks like Tim Beers and his team at Notre Dame are saying, “Here are these stars that formed out of those massive stars, and we’re not going to use, at least in our press release, Pop I, II, or III. We are simply going to call them carbon-enhanced, metal-poor stars. But, hey, look, these are the stars formed out of those first massive stars.”
We have identified things that we can say how many supernova went into it, so we’re soon going to be able to identify the path to get to the metal-rich universe we have in terms of earlier in the universe, we saw these massive stars, next we saw these carbon-enhanced, metal-poor stars, and some of them are still out there, but the bigger ones that formed, they went into this next generation of stars that we’re looking at and we’re seeing the signatures of one normal supernova. We are figuring out the chemical evolution of our universe, which basically boils down to at what point were different elements added to the mix.
Once we know that, it will start to give us limits on when could’ve the first planets begun to form. At what point did those first Population I stars capable of supporting planets begin to come into existence, which allows us to answer questions like could the Fermi paradox be solved by simply saying, “Hey, we’re the oldest planet out there,” which I don’t think we are. But by knowing that the majority of the planets out there either formed when we did or earlier or formed after we did and continued to form, this starts to give us a function of time solution to the Drake equation of at this point in history, there was this many planets, at this point in history, there were this many planets.
Adding that time dependence to the Drake equation will add a time dependence to how many civilizations we can expect and how many might have come to visit that we never saw or just stayed home as the case may be.
Fraser: So, one of the most amazing news releases that came out in the last couple of months was from the Gaia mission. This is, of course, the European Space Agency’s mission. It’s designed to track the positions, and directions, and chemical constituents of the various kinds of stars within the Milky Way, and it’s the latest release – I forget the number – well over a billion stars. Eventually, it’s gonna find 1 percent of the stars in the Milky Way. And one of the things that they created out of it was they made a – was it the Russell Hertzsprung diagram?
Pamela: Color magnitude diagram.
Fraser: Yeah. So, there’s this way of charting out stars based on their color and their brightness, and it’s always this simulation with these artistic versions, but they just took a bunch of the stars and just put them on this diagram and made a version of this with far more nuance than what had ever been done before. It’s just like a big sort of line, and people have made more fine-tuned versions of it, but by actually charting all of these stars into this diagram, you get these little jetties and little branches that come off, which is sort of much more detailed than anyone had ever seen before.
It’s quite beautiful just to see what all stars look like in one glimpse, and so I highly recommend – and I know this is a podcast, but if you can remember, check out the color magnitude diagram from the Gaia mission, and you’ll see all of these stars in one quick view.
Pamela: And what’s awesome about Gaia is because it has some of the most accurate astrometry measurements ever made, as it continues to take measurements over and over time, refining movement of these objects through the sky and parallax distances where those are possible to measure, we’ll be able to build region-of-the-galaxy specific Hertzsprung Russell color magnitude diagrams that allow us to say this region has been around this long, and this ability to plot out the evolution of stars in different places will – I mean, imagine creating a movie of the evolution of the stars in our galaxy by running all of these different regional color magnitude diagrams backwards over time.
Fraser: That would be really fun.
Pamela: And here is to Gaia having a long life and getting all the possible kinematic information it can out of as many stars as it can, so we can get these detailed models of how our galaxy is changing as stars’ orbit evolve.
Fraser: There’s been some just – I mean, now we’re gonna rabbit hole, so we should probably wrap this up – but there’s been some amazing animations that have come out of Gaia. One of the ones that I love is someone took the Kepler field of view, so all of the stars that were in the Kepler view, and then put them forward in time hundreds of thousands of years and did an animation, and so you see all these stars that are all sort of lined up in this grid that Kepler’s looking at, and then they all just spray out in all directions because all those charts are actually moving, and they’re all just these pinpoints in time.
So, not only are these [inaudible] [00:29:20] be seen in the sky, they’re not gonna be there forever. The constellations will change thousands, tens of thousands of years from now. Everything’s gonna look totally, totally different.
Pamela: We’ve already had a few stars that went from one constellation to another and didn’t change names and now continue to have great confusion to young astronomers.
Fraser: That is awesome.
Pamela: So, our universe is evolving, and we’re in it, and the most amazing part of all is we can understand it.
Fraser: Alright, well, thanks, Pamela.
Pamela: Thank you, Fraser.
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[End of Audio]
Duration: 31 minutes

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