Ep. 664: The First Stars

The Sun is a third-generation star, polluted with the metals from long-dead stars. Astronomers have also discovered second-generation stars, with very low metallicity. But theories suggest there must be a first generation, with stars made from only pure hydrogen and helium. Can we ever find them?

Show Notes | Transcript

Show Notes

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NASA Artemis (NASA)



Chinese space station epitome of aerospace workers’ wisdom: designer (CGTN)

InSight Mission – NASA’s InSight Mars Lander (NASA)

Overview | Sun (NASA)

Stars – Stellar Populations (Astronomy Online)

What is stellar magnitude? (EarthSky)

Population I (Swinburne University)

Population II (Swinburne University)

Population III (Swinburne University)

What is a globular cluster? (EarthSky)

Two planets around Kapteyn’s star: a cold and a temperate super-Earth orbiting the nearest halo red dwarf (MNRAS Letters)

Pair-Instability Supernovae: What might they look like? (Astrobites)

Gamma radiation (ARPANSA)

Hydrostatic Equilibrium (Swinburne University)

Positron (Swinburne University)

Potential First Traces of the Universe’s Earliest Stars (NOIRLab)

Quasar (Swinburne University)

Gravitational Lensing (Hubblesite)

PDF: First Stars and First Light: The Epoch of Reionization (NSF)

What is the Cosmic Microwave Background? (Universe Today)

New insight of AGC 198691 (Leoncino) galaxy with MEGARA at the GTC (MNRAS)

Elemental Abundances (Center for Astrophysics)

Hubble Space Telescope

American Astronomical Society

241st AAS Meeting (AAS)

Supermassive Black Hole (Swinburne University)

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Transcriptions provided by GMR Transcription Services

Fraser Cain:              Astronomy Cast, Episode 664: The First Stars.

Fraser Cain:                 Welcome to Astronomy Cast, your weekly facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain, I’m the publisher of Universe Today. With me is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the Director of CosmoQuest. Hey, Pamela, how are you doing?

Dr. Pamela Gay:         I am doing well. How are you doing?

Fraser Cain:                 Good, good. So, we did a wrap-up of the space news in 2022 and – while 2022 like many years sucked – for space it was the best, right? We had Artemis, we had the James Webb space telescope, we had DART, we had the Chinese space station, we had so many amazing things come together. It feels like it was probably one of the best years in space in my career. So, it’s amazing. It was an amazing year and I don’t see as many exciting things coming up for 2023, but who knows?

Dr. Pamela Gay:         That is true. See I’m still mourning InSight. The InSight mission was my favorite little Mars sitter/lander.

Fraser Cain:                 But it lasted twice its lifetime.

Dr. Pamela Gay:         It did. And so I guess I’m still mourning. I’m still mourning the little lander that tried so hard.

Fraser Cain:                 Well, what more could you have asked for it, right?

Dr. Pamela Gay:         A giant earthquake.

Fraser Cain:                 It should have lasted for four times as long? Ten times as long? They can’t all be opportunity.

Dr. Pamela Gay:         I know. I know.

Fraser Cain:                 Right. All right. The sun is a third-generation star polluted with the metals from long-dead suns. Astronomers have also discovered second-generation stars with very low metallicity. But theories suggest there must be a first generation with stars made from only pure hydrogen and helium. Can we ever find them? So, first, please explain the generation numbering scheme for stars because it’s ridiculous.

Dr. Pamela Gay:         Okay.

Fraser Cain:                 We have a pop I star?

Dr. Pamela Gay:         Yeah. Astronomers –

Fraser Cain:                 And there are pop II stars?

Dr. Pamela Gay:         Astronomers shouldn’t be allowed to name things or set the directions of calibrations. So, just like astronomers screwed up in terms of calling the brightest stars magnitude zero – meaning faint stars are magnitude 20 – we decided that our current population is number I. We’re number I.

Fraser Cain:                 Right because we’re the first star that we ever discovered.

Dr. Pamela Gay:         Well, yeah, we are the first star we ever discovered sort of I guess. You fell asleep the first night and then you found all the rest of them?

Fraser Cain:                 Yeah.

Dr. Pamela Gay:         But anyways, so the generation of stars older than us – that doesn’t have as many metals – can’t probably form planets the same way. Those are population II and as illogical as it can possibly be, the first generation of stars the universe ever formed is population III.

Fraser Cain:                 Right. So, it makes sense. The first star that we ever discovered, they’re population I. The second stars that we ever discovered; those are population II. And then we’re hoping someday we’ll find the third stars – the type of stars – which will be population III, still theorized. Okay. So, that is the – and then how do you define them? As an astronomer, when do you take a star and put it into the pop I bucket and put it into the pop II bucket, and the theoretical pop III bucket?

Dr. Pamela Gay:         So, pop III means these stars are primordial ingredients. They are made of the exact same stuff that was produced in the Big Bang – hydrogen, helium, maybe a trace amount of lithium and beryllium – definitely a trace amount of lithium and beryllium. But then pop I are all the stars like our sun that are being formed currently. These are things that are two to three percent of atoms heavier than hydrogen and helium and the stuff in between is the population II.

So, once you start getting to the point – and I’m sure this is gonna get redefined as the formation of planets plays a larger and larger role – but pretty much once you start getting to the point that you’re not seeing planets forming, that’s where you have population II stars.

Fraser Cain:                 Although I’m sure someone’s found planets are on pop II stars or will shortly?

Dr. Pamela Gay:         The issue is population II stars these are objects that are generally less than a percent of heavier atoms. And if you don’t have heavier atoms, you can’t form planets. So, one of the amazing things is globular clusters out there. These are population II stars orbiting our galaxy. They don’t have planets so far and we have looked as hard as we can possibly look at things that far away and they’re just not showing up.

Fraser Cain:                 So, there’s one known type II star that has a planet?

Dr. Pamela Gay:         Are we sure, sure?

Fraser Cain:                 Well, it’s called Kapteyn’s Star. It’s a M1 red sub dwarf and the metallicity is about 14% of the sun, which classifies it in the type II star.

Dr. Pamela Gay:         How do you always find these exceptions?

Fraser Cain:                 Because I have a hunch. Look, okay, look. Here’s my standard operating procedure, right, is that I think all I do is report on surprising new discoveries and new things moving forward and so it just it feels like somewhere there is a headline, “Astronomers find the first planet around a type II star.” Yeah, anyway, and I’m sure they’ll find many more because “Astronomers shocked to discover. Theories overturned.” So, yeah, all right, we’re gonna talk more about the first stars in a second – which we know we don’t know of any. Or do we?

Dr. Pamela Gay:         It’s true. We don’t yet.

Fraser Cain:                 We’ll be back in a second. Oh, wait a second. I may have news for you then. We’ll be back in a second.

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Fraser Cain:                 And we’re back. All right. So, we’ve classified the pop I star, the pop II star. Let’s talk about the pop III. So, these are theoretical. As you say, they are made of the pristine material left over from the Big Bang – pure hydrogen and helium – at the exact constituents left over when the universe had cooled down that these atoms could form. How would they be different from a star like we have today? I’m gonna guess no planets, but then even that like I’m sure at some point somebody’s gonna be like, “We found planets around – We discovered pop III stars and we also discovered a planet” a gas giant, who knows?

Dr. Pamela Gay:         All right.

Fraser Cain:                 Pop III gas giants. Yeah, please continue, but what would they be like?

Dr. Pamela Gay:         All right. So, one of the weirdest things in trying to understand stars is what are the effects that these heavier elements have on the ability of the star to form and radiate heat? And it turns out that if a star is made of pretty much just hydrogen and helium, the light that is forming in its core just radiates out. And because it’s radiating away and not interacting as much with the protons and electrons because the energy levels just aren’t right, the star is able to become much more massive before it starts pushing out its outer layers, and a star that’s more massive – in this case 100 to 200, 300 times the mass of our sun – it’s only going to live for a million-ish years.

So, we’re in this situation where the very first stars formed roughly 100 million years after our universe formed and then they only lived for a million years before they underwent this super-weird form of supernova where the star essentially eats itself from the inside out, and leaves nothing behind except for a spray of light and heavier elements.

Fraser Cain:                 So, you don’t get a black hole. You don’t get a neutron star. You just get the thing detonating completely?

Dr. Pamela Gay:         It’s called a pair-instability supernovae and –

Fraser Cain:                 Can you talk about this? You say it eats itself from the inside out, but what is actually happening with pair instability?

Dr. Pamela Gay:         So, you have – in the core of the star – gamma-ray photons are getting produced. And these extremely high energy gamma ray photos they’re going to be interacting with protons, with electrons – and not as efficiently as if they were metals – and they’re also going to be interacting with each other. And ideally, the rate of energy production is such that the star is in thermodynamical equilibrium. Gravity is pushing inwards, heat and light are pushing outwards.

Again, the light isn’t as effective here because it’s mostly just flying through the atoms. And when these gamma rays interact with each other, they will produce an electron and a positron. This is a particle and an antiparticle that are able to then later interact with each other if they hit each other later and produce more gamma rays but – because of the kinetic energies involved – some of the energy is lost in this process. The lower energy gamma rays are going to lead to a changing situation where the outer layer of the star isn’t being supported as well. The star begins to collapse down.

As the star collapses down, it heats up. More gamma rays are produced. Gamma rays interact with each other because there’s more of them. It’s easier for them to interact. They produce more positrons and electrons. These interact and annihilate and the star is literally eating itself out of having matter and energy through the loss of energy to kinetic energy. It’s wild and what gets me is we think we may have actually been able to see the chemical fingerprint of one of these kinds of supernovae in the light of distant quasars.

Fraser Cain:                 Okay. Well, that was the evidence I was gonna bring up.

Dr. Pamela Gay:         Yeah.

Fraser Cain:                 So, then, yeah, yeah. And so I mean we see those pair-instability supernovae. We’ve seen these detonations before but just from heavier stars.

Dr. Pamela Gay:         Right.

Fraser Cain:                 Just stars with other elements like pop II or pop III – pop I, sorry. I got to keep this straight. But the second and third-generation stars but they’re very massive.

Dr. Pamela Gay:         Yes.

Fraser Cain:                 They have exploded as pair-instability supernovae and just completely vaporized themselves – no black hole, no neutron star.

Dr. Pamela Gay:         Right.

Fraser Cain:                 Just kablooey.

Dr. Pamela Gay:         Yeah.

Fraser Cain:                 Yeah. Okay. All right. So, they [inaudible]. Now you were starting to make some estimates of their masses – 100 times the mass of the sun, 200 times the mass of the sun. Do we have a sense of how big these things can get?

Dr. Pamela Gay:         This is where you end up with a lot of different people arguing in the literature. There are some folks that are like, “Above 300 solar masses, the energy being produced is going to cause no additional material to be able to fall onto the star. It’s going to be quenched.” There are those that are like, “At 150 solar masses, that’s gonna happen.” There are those who are like, “At 1,000 solar masses, that’s going to happen.”

Fraser Cain:                 Right.

Dr. Pamela Gay:         The safe bet is it’s occurring in the hundreds of solar mass range because – as you pointed out – we do see some massive stars in our modern universe. These are objects that are 150 solar mass to 200 solar mass when they first form and they do have these pair-instability supernovae when they die. So, just how big the very first ones get? We’re looking at hundreds is a safe bet, but exactly how many hundreds we’re not sure.

Fraser Cain:                 And there could be all kinds of weird dynamics that are going on. There could be magnetic field lines, there could be accretion disks, there could be ways to get beyond pushing that material back away from the star and that’s why it’s – I saw a simulation where they thought that maybe you could get into the tens of thousands of times the mass of the sun.

Dr. Pamela Gay:         Yeah.

Fraser Cain:                 They’re crazy. And then what is a supernova? They simulate in a supercomputer what a supernova would look like when a 50,000 solar mass star goes through this pair instability process.

Dr. Pamela Gay:         And then that light doesn’t get very far because the early universe was not yet ionized again, re-ionized.

Fraser Cain:                 Right.

Dr. Pamela Gay:         So, all of this is going on behind opaque layers of gas hiding from us.

Fraser Cain:                 Huh. We think about these. Sorry. We think about the cause of microwave background as this like someone came and turned off a switch, but it wasn’t like that.

Dr. Pamela Gay:         No.

Fraser Cain:                 It was this long, slow, gradual process of the clouds lifting over the course of several millions of years. And so could you have had these first-generation stars forming within this fog of the early universe?

Dr. Pamela Gay:         Yes. That’s exactly where they would have been forming and we’re starting – through gravitationally lensed galaxies – to be able to see the age of reionization, to see early galaxies that still have these clouds of neutral gas around them. But the very first generation, the closest we can come to hoping to be able to find them is that there are pockets of gas out there that just somehow got isolated and left alone for the fullness of time and there’s one – AGC198691 – I have to look at its license plate number.

It is one-twentieth of a percent made of atoms heavier than helium and it has one-fortieth the metallicity of our sun, which means that it’s population II stars. But it is so metal-poor that it starts to give us hope that maybe we’ll get lucky and – while there’s not anything in the nearby universe that is pristine material just starting to form stars – maybe we’re going to be able to find some lensed system from the first billion years of the universe that we’re able to start catching the light of those first supernovae.

Fraser Cain:                 So, then I have a two-question. I got to remember to bring up the other thing before we end the show, but anyway – So, first let’s talk about trying to observe them. So, you mentioned that they – even though these stars are ludicrously bright, even though their supernova are brighter probably than any supernova we’ve ever seen – they are cloaked in the fog of the early universe and will be difficult to see both their existence, but also their death.

Dr. Pamela Gay:         Yes.

Fraser Cain:                 So, how could we theoretically see them?

Dr. Pamela Gay:         If they formed in a pristine galaxy that – or a pristine blob of gas that managed to escape the first round of star formation like a blob of cookie dough without chocolate chips that escaped onto the counter. That escaped gas is probably the best hope we have of seeing these things. Now, the other problem that we’re dealing with is – if you go back to some of the earliest episodes of this show – we talked about the missing G dwarf problem, this idea that low-mass stars have the capacity to live longer than our universe has been around. So, the very first lower-mass stars to form should still be out there shining.

And this is where we get ourselves into trouble of just how effective was the mixing and how much metal does it take to start forming lower mass stars? And these are things we’re still trying to figure out. So, on one side, you have people looking for the oldest low-mass stars that were the very first low-mass stars to have ever formed. And on the other side, you have people trying to find pristine pockets of gas in dwarf irregular galaxies, in other forms of dwarf galaxies, that just don’t have anything to increase the mass of stars as they form – increase the metallicity of stars as they form.

Fraser Cain:                 Right, right. Now, you mentioned maybe through lensing.

Dr. Pamela Gay:         Yes.

Fraser Cain:                 So, Hubble never could.

Dr. Pamela Gay:         No.

Fraser Cain:                 But I know that using gravitational lenses in the way that Hubble could see the farthest galaxies, Webb could be able to see the farthest stars.

Dr. Pamela Gay:         And this is where – at the time that we’re recording this – we are about a week and a half away from getting the first deluge of science to come from the JWST at the American Astronomical Society meeting.

Fraser Cain:                 Oh, it’s the JWST meeting. Oh, yeah. I never even thought about that.

Dr. Pamela Gay:         Yeah, yeah.

Fraser Cain:                 You’re right. This is gonna be chock-o-block, isn’t it?

Dr. Pamela Gay:         They’re gonna have a whole lot of early results from JWST being announced and I’m kind of afraid that anything I say – other than JWST was specifically built with finding first stars in mind – is gonna be out of date within weeks of this episode going live.

Fraser Cain:                 Yeah, that’s interesting.

Dr. Pamela Gay:         So, stay tuned.

Fraser Cain:                 We should maybe do in that episode.

Dr. Pamela Gay:         We have it scheduled.

Fraser Cain:                 Oh, it’s already scheduled? Okay.

Dr. Pamela Gay:         Yeah, yeah.

Fraser Cain:                 All right. Yeah, yeah, we’ll do a first science result – first proper science announcements – from JWST. That sounds great. So, what would it take then to build? What kind of instrument could see them directly? Because I know there was a space telescope – the Origins Space Telescope – that was shelved and that was going to be a nine to 12-meter class infrared observatory – essentially a super-duper version of JWST. That was hoped could maybe find evidence of the pop III stars.

Dr. Pamela Gay:         Right. So, what we need is something that is capable of seeing the faintest light from dwarf galaxies that are being gravitationally lensed or from galaxies that aren’t yet fully fledged, fully full of light. This goes back to how do galaxies form? We have this notion today that they form both through the massive collapse of giant pockets of gas, but also through smaller pockets that collapse into dwarf systems and then merge together.

Those dwarf systems that have less mass in them will form stars at a slower rate. Big things form faster, small things form slower. By being able to see dwarf systems that are being gravitationally lensed and are super faint and are in the earliest days of the universe so their light is redshifted into the infrared, we can get back about as far as we can get back.

Fraser Cain:                 Right.

Dr. Pamela Gay:         It’s hard. It’s hard.

Fraser Cain:                 Yeah, yeah. Now when you think about the size of these stars – and we talked about how they don’t seem to form – they probably don’t form black holes, but there’s got to be some kind of link. Could there be some kind of link between these first-generation stars and the supermassive black holes – which are also a mystery – because they don’t seem to be able to form quickly enough to have the mass that they have already in the universe that we see them? Is there some connection between these first-generation stars and these monster black holes?

Dr. Pamela Gay:         At a certain level, yes because you’re, again, at this problem of, “If you have an atom –” Technetium is my own personal enemy because in stellar spectrum, technetium it has electrons bouncing around at all of the interesting, visibly apparent in your standard optical telescope colors of light. So, you’re trying to study whatever – small magnesium hydride was what I was working on – whatever small molecular lines or atomic lines and technetium is there going, “Hi. There are a few atoms of me. We’re gonna dominate everything.”

And what’s happening is – as the energy tries to radiate out from the center of the star – it’s absorbed in by all of these different energy levels of the electrons in the technetium and then re-radiated as the electrons jump between energy levels going back down. So, you have this one-two whammy of a whole lot more electrons involved in the energy levels and a whole lot more energy levels increases the ability to re-radiate that energy and this causes a pressure essentially. So, the light goes out, it gets absorbed into these atoms, and that supports the outer layer of the star. If you don’t have all of these energy levels to absorb in the protons, the protons just fly through.

So, this allows giant stars to form. Well, if you’re in a early galaxy and you have collapsing mass that is able to heat up and much more effectively radiate out energy, you can first of all get bigger that way. And then models that look at adding in turbulence to again allow more mass to get down in there than you could through a nice, calm collapse, this one-two punch of lower metallicity and significant turbulence allows you to start getting at supermassive black holes maybe.

Fraser Cain:                 Right. There’s an interesting experiment you can do talking about turbulence. You can take a bottle of pop – what you may call soda – and you turn it upside down and it glugs out and it’s very slow. But if you give it a spin, then the water pours out because it gets an air hole coming up the middle of this vortex and the thing will just empty in a heartbeat – just boom, completely empty – and so turbulence can have a tremendous effect. So, one idea – if the turbulence models are right and if they do just keep gaining mass forever – is you don’t get the pair-instability supernova. Instead, you just get this direct collapse into a supermassive black hole.

Dr. Pamela Gay:         Yeah, yeah.

Fraser Cain:                 That would be crazy. That’s amazing. Yeah, to think that however many million times the mass of the sun could all collect together into one spot and then just directly turn into a black hole.

Dr. Pamela Gay:         And it all comes down to how big were these slight overdensities and under densities of mass in the early universe?

Fraser Cain:                 Yeah. Wow.

Dr. Pamela Gay:         If we had had a different distribution of overdensities, we might have become a universe of nothing but supermassive black holes.

Fraser Cain:                 Right.

Dr. Pamela Gay:         So, there’s your food for thought for the day.

Fraser Cain:                 I love it. All right. Well, that was fantastic, Pamela. Thank you so much.

Dr. Pamela Gay:         Thank you and thank you to all of our patrons out there. We wouldn’t be here without you and this week I want to thank Brian Cagle, David Everson, Bruno Leitz, Alex Raine, and I’m gonna pause and say we now have an add-free distribution of this show on an RS feed through Patreon.

Fraser Cain:                 Nice.

Dr. Pamela Gay:         That you can only get if you’re a Patreon. So, I’m gonna keep going. We’re thanking Michael Prochoda, Burry Gowen, Stephen Veit, Jordan Young, Jeanette Wink, Kevin Lyle, nanoFlipps, Barre Andre Lysvoli, J.F. Rajotte, Venkatesh Chary, Andrew Poelstra, Brian Cagle, David Troug, Aurora Lipper, David, Gerhard Schwarzer, Buzz Parsec, cacoseraph, Laura Kittleson, Robert Palsma, Jack Mudge, Les Howard, Joe Hollstein, Frank Tippin, Gordon Dewis, Alexis, Adam Annis-Brown, Richard Drumm, William Baker, WandererM101, Zero Chill, Felix Gutt, Androsetz or Astrosetz, William Andrews, Gold –

Roland Warmerdam, Jeff Collins, Simon Parton. Kellianne and David Parker, Jeremy Kerwin, Rob Cuffe, Harald Bardenhagen, Matthew Horstman, Alex Cohen, Phillip Walker, marco iarossi, David Gates, Scott Kohn, Scott Bieber, Justin Proctor, Matthias Heyden, Claudia Mastroianni, Kseniya Panfilenko, Daniel Loosli, Jim Schooler, Gregory Singleton, Disasterina, Cooper, Tim Gerrish, Tim McMackin, Jeff Willson, Paul D Disney, Eran Segev, NinjaNick, Kenneth Ryan. And don’t you love that there’s fewer episodes in January and there’s that many more names at the end of the episode?

Fraser Cain:                 Fantastic. Thank you everyone for supporting the work that we do and we will see all of you next week.

Dr. Pamela Gay:         Bye, bye everyone.

Announcer 2:              Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a Creative Commons Attribution license. So, love it, share it, and remix it, but please credit it to our hosts – Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, AstronomyCast.com.

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