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

Metallicity

Star formation and stellar populations

STELLAR POPULATIONS AND THE HISTORY OF THE UNIVERSE

Stellar Populations in the Galaxy

Stellar Populations Of Globular Clusters

Stars – Stellar Populations

Transcript

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 gravitation