Ep. 642: Is the Sun… Normal?


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We’ve always assumed that we lived in a perfectly normal system with a normal star and normal planets. It’s all… normal. But with our modern understanding of billions of stars, just how normal is our Sun, anyway?

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Show Notes

Patreon: Universe Today

Patreon: CosmoQuest

CosmoQuest-a-Con 2022

Sun (NASA)

The Fermi Paradox (SETI Institute)

What’s a neutrino? (Fermilab)

White Dwarf (Swinburne University)

Planetary Nebulae (Swinburne University)

Red Dwarf (Swinburne University)

Brown Dwarf (Swinburne University)

WISE (Caltech)


Rubin Observatory’s Legacy Survey of Space and Time (LSST) (SLAC)

Rubin Observatory

Roman Space Telescope (NASA)

Hubble Space Telescope

Main Sequence Stars (CSIRO)

Hertzsprung-Russell Diagram (Swinburne University)

What are red giants? (EarthSky)

Subramanyan Chandrasekhar (The Nobel Prize)

Spectroscopy (Swinburne University)

Metallicities and Stellar Populations (Case Western Reserve University)

Abundance Ratio (Swinburne University)

Gaia (ESA)

Uncovering the birth of the Milky Way through accurate stellar ages with Gaia (Nature Astronomy)

TESS – Transiting Exoplanet Survey Satellite (NASA)

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Transcriptions provided by GMR Transcription Services

Fraser: Astronomy Cast, episode 642: Is the sun normal? 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. I’m Professor Cain, publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute, and the director of CosmoQuest. Hey, Pamela, how are you doing?

Dr. Gay: I am doing well. It is exciting times around here. My hair and my camera have both decided it is time to celebrate the 80s and glitch in proper Max Headroom and frizzy, curly style –and I think this works. So, we’re gonna go with it.

Fraser: You have hot humidity working on both of them at the same time.

Dr. Gay: It’s true.

Fraser: Just recking cameras in here.

Dr. Gay: Yes.

Fraser: So, before we get into this week’s episode, I just wanna do a rare, shameless self-promotion for something that we’re doing on Universe Today. So, as you probably know – well, maybe people don’t know this, but –

Dr. Gay: No, tell them.

Fraser: – when you support the Patreon for Astronomy Cast, you’re not actually supporting me or Pamela. You are supporting the team that maintains Astronomy Cast week after week after week. Our editors, our produces, everybody, the server hosting fees, all of that. But actually, we don’t take a salary from this. It’s a nonprofit. But we both have Patreons.

So, for the Universe Today Patreon, which support the work I do with all of the –with all of the –giant team of writers that we have on Universe Today, all of that, the video editors, auto-editing, and so on, we’ve been hovering under the sort of 800 and 900-mark, and people have been asking me to do some kind of book club. And so, we decided we’ll do that if we can reach 1,000 patrons for Universe Today.

So, if you go to patreon.com/universetoday, join as a patron, help us reach that 1,000-mark. And it’s a bit of a race. We’ve mentioned this on our channel. We’re gonna try and hit 1,000 patrons before either Space Launch System or the SpaceX Starship launch. Can you help us? Just show that we’re the real rocket ship here. Pamela?

Dr. Gay: That is amazing. And I – yeah. We, over at CosmoQuest, we are in the process of doing our final push, just like Astronomy Cast, through to mid-July, and that hiatus, and we’re in the process, right now, of planning out this year’s CosmoQuest-a-Con. Normally, we do CosmoQuest-a-Con in July, but no lies, I have no air conditioning in my house, and I like my team. So, we are not going to do CosmoQuest-a-Con in July because I like my team.

So, instead, we’re doing in October, but our goal is to have everything planned out by July and to sell enough tickets that we don’t have to do a Hangout-a-Thon this year. So, we’re selling tickets. Links are over at CosmoQuest.O-R-G for our October event. It’s themed, Rockets and Robots, and we have an amazing slate of people. So, go get your tickets today.

Fraser: Awesome. All right. So, we’ve always assumed that we lived in a perfectly normal start system with a normal star, normal planets. It’s all normal. But with our modern understanding of billions of stars, just how normal is our sun anyway? All right. Are we normal?

Dr. Gay: No.

Fraser: What?

Dr. Gay: But would we want to be?

Fraser: Yeah –I guess it doesn’t –Yes. You know what? I’ve decided that, yes, we would want to be normal because then that would mean that all of the other yellow dwarf stars that we see out there probably have planets. All the planets probably have the same distribution as we have in our solar system. Probably a rocky world orbiting in the habitable zone, and that means that there’s gonna life everywhere. Yes, please. Yes. We wanna be normal. This should be the template for what the universe should be like. Everywhere you go, it’s just solar systems everywhere.

Dr. Gay: All right.

Fraser: But that’s not real.

Dr. Gay: No. No. And I suspect the Fermi Paradox wouldn’t be a thing if your version of normal was a thing.

Fraser: That’s true. True. True. All right. So, I guess, do you wanna start with –what’s some of the interesting characteristics about our sun and then start to compare that and how we know? So, I guess –let’s define our sun, for a second.

Dr. Gay: All right. So, our sun is –a 10 to the 33-gram star. I don’t know why we measure these things in grams. We do.

Fraser: Well, you can do 10 to the 33 kilograms, if you like.

Dr. Gay: It’s true. It’s true.

Fraser: Yeah.

Dr. Gay: It –glows a yellowy-white color. If we were seeing it without our atmosphere, our eyeballs would see it as much more white than it sees it as yellow, simply because our atmosphere is scattering out some of that blue light. So, our sky lies to us. Temperature-wise, it’s more of a yellowy hue, so those old incandescent bulbs that we should no longer have in our house – houses, those were more solar temperature than the blue LEDs that we’re dealing with. Our sun is about 1.3 percent metals which is anything other than hydrogen and helium, and it’s just out there combining atoms to end up with light shining our way and a variety of neutrinos coming out of those reactions.

It’s only in comparison that our sun get interesting. Really. It’s kinda boring to just look at it by itself.

Fraser: But boring says normal, but you’re gonna tell us that it’s not normal, therefore, it’s all interesting. Game, set, match.

Dr. Gay: So, in the Gaussian distribution, there is interesting on one side, there is completely boring on the other side, and there is average in the middle. So, I would say being boring is actually not normal.

Fraser: I don’t think that it’s boring. You know what? You’re gonna have – you’re – we’re gonna have to go through this episode and I will judge in the end whether or not it’s boring.

Dr. Gay: All right. All right.

Fraser: But right now, my instincts say, not boring.

Dr. Gay: Okay, fine.

Fraser: But let’s take apart some of these things. So, you described the temperature.

Dr. Gay: Yeah.

Fraser: You described the mass.

Dr. Gay: Yeah.

Fraser: You mention the metallicity.

Dr. Gay: Yeah.

Fraser: The color.

Dr. Gay: It’s also –it’s five-ish billion years old, depending on the model you use.

Fraser: The age –Yep. Sure. So, let’s pick one of those, and let’s set that in comparison to what we see across the universe. So, let’s talk about mass.

Dr. Gay: So, in terms of mass, if you simply assume there is one star in every possible mass bin, we’re in the middle of the distribution, but the issue is that the universe doesn’t have an even distribution of stars. The vast majority of the stars out there are much, much smaller. And so, this puts us up towards the top end of the by-count distribution on size with the majority of the stars being smaller. But then all those stars that are bigger than us, they live much more interesting lives.

So, in terms of the kind of evolution that we get, based on our mass, we’re just another star that’s going to end up as a white dwarf surrounded by a planetary nebula. So, we’re not going to have an interesting supernova, we don’t have a companion, so, we’re not gonna end up with a different kind of interesting supernova. We’re just gonna piddle out eventually.

Fraser: So, give us a sense because –I mean, when you think about it, when you stand outside and you look at the sky, you’re seeing bright stars.

Dr. Gay: Yes.

Fraser: You’re not actually seeing what is the vast majority of the Milky Way, which are the red dwarfs.

Dr. Gay: Exactly. It’s –estimated that no fewer than 80 percent of the stars out there are red dwarfs and smaller, and the reason that we say no fewer is because we’re still tracking down all those brown dwarfs out there. We’re still tracking down what is the difference in distribution of red dwarfs between different populations. So, in some areas, you’re going to get a higher fraction of red dwarfs formed, in some you’re gonna get a lower fraction of red dwarfs formed, but no fewer than 80 percent of the stars out there are smaller red dwarfs, and brown dwarfs.

Fraser: It’s interesting that we –because they’re so dim and because they’re relatively small and of low mass, we actually don’t know how –where the bottom is.

Dr. Gay: Right.

Fraser: Like, we can see the bright stars. We can see the stars as they explode –billions of light-years away, but it’s really hard to even figure out how many of the smaller brown dwarfs there are within a few dozen light-years of us. They’re just so hard to see.

Dr. Gay: And this is where we’re also suffering from the lack of infrared survey scopes. WISE did a really good job trying to look for them with its surveys, and, unfortunately, JWST –or fortunately, depending on which side of the observing proposal you’re on, it’s not gonna be used to just scan the skies looking to see what’s there, it’s going to be looking at specific targets of opportunity. And since these smaller stars give off the bulk of their light in the redder wavelengths –and they’re so faint, you really have to be looking in these colors of light that don’t really make it to the surface of our planet to be able to find them.

So, we wait, and we hope that eventually we get a good old survey scope up there to just crawl across the sky and infrared the way LSST is going to be crawling across the sky in visible wavelengths.

Fraser: We do need an infrared LSST in space.

Dr. Gay: We really, really do.

Fraser: Yeah. Yeah. Nancy Grace Roman is gonna split the difference, but it’s gonna – it has a much wider field of view than Hubble than James Webb is in infrared, not in the same way that WISE is –was.

Dr. Gay: Correct.

Fraser: So, having that and – ’cause that’s how you find the asteroid. That’s how you find the comet. That’s how you find the brown dwarfs, the rogue planets.

Dr. Gay: Yeah.

Fraser: There’s a lot of really interesting things that are moving –Let’s get that into the decadal survey for next time. Okay. So, we talked about size, mass. I guess size and mass are tied up with each other. Let’s talk about –

Dr. Gay: But not really.

Fraser: Oh. Uh-oh. Okay.

Dr. Gay: So, this is where stars get weird, as you have to look at, what is the energy generation mechanism in their core? And you balance out, what is the mass? How are they generating energy in their core? And that tells you the size. ‘Cause, you’re constantly balancing the light pressure outwards against gravity inwards. Right now, our star is a main sequence star and that actually makes it fairly average for where we are in the galaxy because stars linger on the main sequence longer than they linger anywhere else in the H-R Diagram, and the main part of our galaxy, while the majority of the stars are older than us, we still see plenty of stars hanging out on the main sequence.

Fraser: So, right now, our sun is a main sequence star. And so, that’s what the whole term, main sequence.

Dr. Gay: Yes.

Fraser: But as it reaches the end of its life, it’s gonna change into a red giant. It’ll have roughly the same mass, but it’ll be vastly bigger. So, I can see why the –I guess the point of us –of its life that a star is in will define that size-to-mass ratio because of how it’s burning its fuel.

Dr. Gay: Exactly. And this comes down to things like, when our star stops burning hydrogen in its core it’s going to initially collapse down, and as it does, it’s going to get hot enough to ignite a shell around that core. And that shell igniting will blow the star back out and it can go through these different phases of, just what is it burning this millennium? And the entirety of its old age is much shorter than its main sequence lifetime, but it’s also a lot more dramatic because this is the time when we start eating other planets. This is the time when massive amounts of mass loss leads to the formation of that planetary nebula and stars are beautiful in their old age.

Fraser: All right. So, we talked about the size, the mass. Let’s talk about the color. But that’s also kinda tied to the size and the mass and the fuel, right?

Dr. Gay: It’s all one thing. When you’re an undergraduate in astronomy, you end up solving suites of equations that were originally really explained well by Chandrasekhar that go through and balance out the temperature, the mass, the radius and show how all three of these things are directly related. So, at its current light, pressure, gravity, inward balancing point, it is giving off the maximum amount of its light in a golden yellow color. And this is where you get into a great deal of debate among people on how we refer to the sun and how we perceive the sun. From the surface of our planet, we actually get to see that golden yellow color, but it turns out human eyeballs are really bad at seeing the color green.

And so, if we went into outer space, even though the sun’s giving off the bulk of its photons in this golden yellow color, or at least that’s where the peak wavelength of the light is.

Fraser: I thought the peak was green – Okay.

Dr. Gay: It’s –so, it’s more to the yellow side of that. But our eyes don’t see all of that light coming out in the green. And so, it ends up perceiving the sun, if you’re above the atmosphere, as white.

Fraser: That’s really cool. And again, that’s –that is just how much –how the sun is generating its fuel. What stage of it is in its life. I wonder as the sun gets older, will it change color before it turns into, as it gets older, into its main sequence phase, will it change color?

Dr. Gay: It’s constantly, at a very slight level, changing color and this is part of what’s going to cause our planet to change over time. Our star started off much cooler. It then warmed up and exactly how it continues to evolve is actually gonna depend on its composition. And this is, strangely enough, one of the things that is still getting debated is, just what exactly is the composition of our sun?

Fraser: And I guess it’s difficult –well, how do astronomers figure out the composition of a star?

Dr. Gay: Normally what we do is we take its light, we shove it through a spectrograph, and we sit down and measure each and every individual line using software that allows us to say, okay, so, if this star has this many parts technetium, this many parts iron, this many parts silicon, and to a certain degree, there’s sliders that you can go up and down with that will adjust suites of chemicals together, and then you fine-tune it because we know supernovae give off certain ratios of elements. So, depending on what kinds of supernovae go into forming a star, you can know, okay, this slider goes up, this slider goes down, and adjust accordingly the kinds of elements that’ll come out at once.

Fraser: But isn’t that kind of misleading? Because you’re really just seeing the surface of the star.

Dr. Gay: And that is the problem.

Fraser: Right.

Dr. Gay: We are basically making assumptions about what kind of mixing has taken place, how things have come to the surface, mixing from the core up, how things have sunk from the atmosphere, down to the core.

Fraser: And so, if you’ve got those heavier elements, if you’ve got iron or gold or platinum –the things that we find here on earth, they’d have to be present. They’ve gotta be in the sun.

Dr. Gay: Yeah.

Fraser: They’ve all sunk down inside, and they’re not being carried back up to the surface through convection. We only see a tiny amount of uranium, plasma in the sun. And so, it’s really tricky to know that that stuff is there.

Dr. Gay: And this is where –we get into frustration because –we think the sun is 1.3 to 1.6 percent metallicity. The numbers that I find the most believable right now are the 1.3 percent metals –but we aren’t 100 percent certain. And the presence of these metals, the metals are capable of absorbing light as it comes out and having their electrons change energy levels as they absorb the photons, and this changes the opacity in the atmosphere of the star. It changes how readily the light is able to push out through the star. And this changes how the star is able to balance light versus gravity. This changes how the star is able to evolve.

And because we use our sun, which we see so much more detail on to ground our understanding off all the other stars out there, if we’re wrong on how we do the sun, we have to recalibrate everything else that we’re out there looking at, which is just one of those things that makes my heart beat a little bit harder than it probably should every time I think about it

Fraser: Right.

Dr. Gay: So, we’re in this situation where we can measure the smallest fractions in the atmosphere of the rarest of elements that we would never see in another star, and now we’re trying to take and use the information we learned from our sun to understand all those other stars.

Fraser: And so, I guess, in a perfect world, we could take a star, like the sun, dismantle it, separate it into its various elements, nice, neat little piles, and then use that –or maybe do a few stars and then use that as a way to then measure all of the other stars that we wanna look at.

Dr. Gay: Yeah.

Fraser: Because you could –that would give you a sense of how much mixing happened, how much separation happened, and how much was still present, etcetera.

Dr. Gay: Right.

Fraser: But we can’t do that. We can’t even do that with the sun.

Dr. Gay: No. No

Fraser: And so, I guess, we have to learn these constituent elements –through other means.

Dr. Gay: It’s one of these things where we, literally, start from the assumption of, if iron exists in this amount, then these other things probably exist in these other amounts.

Fraser: Interesting. Okay.

Dr. Gay: And we use this ratio of Fe over H, iron over hydrogen, as a surrogate to get at what to expect from the rest of the star and –even here, using our sun, gets awkward because our sun has an unusually high metallicity. It’s done in a logarithmic scale, and we decided our sun shall be the average star. You make these assumptions.

Fraser: Right. Normal.

Dr. Gay: Yeah. Yeah.

Fraser: Yeah.

Dr. Gay: So, we called the Fe over H of the sun, zero, and if it has 10 times less, it’s minus one –and it turns out that pretty much everything has less metallicity than our sun, even in the local area of our galaxy.

Fraser: Well, that’s interesting.

Dr. Gay: Yeah.

Fraser: I mean, that –I mean –again when we’re –well, isn’t it interesting that life formed here on Earth on a terrestrial planet that needs metals, and we happen to live on a –and I know there’s a bunch of these things like phosphorus.

Dr. Gay: Yeah.

Fraser: So, there’s these weird elements that are surprisingly abundant in the solar system and rare elsewhere out in the universe, and you go, okay, so, maybe our difference, the difference of the sun, had a meaningful impact on whether or not life could arise in the solar system.

Dr. Gay: And –this is where Gaia’s data starts to become so interesting, ’cause Gaia’s doing a survey of a large fraction of our region of the galaxy looking all the way out to the halo, and saying, okay, what is the distribution of metals? And we’re seeing just how few stars, comparatively, have the same amount as metal as we do. And now we’re also going through and, with tests, doing okay, so, how many of these things actually appear to have planets? And the original thinking was you had to be roughly as metal-rich as our sun or more metal-rich to have planets, and it’s turning out, you can’t go a whole lot more metal-poor – we use terrible terms. Metal-rich, metal-poor. It’s terrible.

Fraser: Yeah.

Dr. Gay: But you can’t look at stars that have two much less than one percent metals before you start not seeing planets, but they are out there. So, the places where we haven’t found planets are where your 10 percent less. So, globular clusters, for instance. Haven’t found planets in globular clusters yet. But there are systems out there that are more metal-poor where we have found some planets. So, that’s kinda cool.

Fraser: So, let’s put it all together then. We’ve talked about our mass, we’ve talked about the size, the color, temperature, the metallicity. I guess we haven’t really talked about the age yet, but with –I mean, you’ve got Gaia.

Dr. Gay: Yeah.

Fraser: And you mentioned TESS, which is getting a sense of the planets.

Dr. Gay: Yeah.

Fraser: Gaia telling us the locations. There are other surveys that are doing an amazing job of telling us the metallicity and the – and, essentially, the chemicals present in these other stars. Back to the original question, how does –.do you feel like the sun fits within the general distribution across all of the axes that you would want to examine and compare it to the rest of the Milky Way.

Dr. Gay: It’s –a bit bigger. It’s a bit more metal-rich. It’s a bit younger. And it’s hanging out doing its main sequence-thing in the middle of its main sequence life. So, I feel like our sun is the stellar equivalent of upper-middle class. It –

Fraser: Right.

Dr. Gay: It’s –

Fraser: It’s a yuppie.

Dr. Gay: It’s a yuppie.

Fraser: Yeah.

Dr. Gay: It’s not extraordinary in anything except for, maybe, metallicity, but there’s still stars out there more metal-rich.

Fraser: Yeah.

Dr. Gay: It’s just –it’s above average, but it’s not that above average.

Fraser: So, we’re not getting that weird, unique place that would tell us, of course, life formed here because this is the only place that life could’ve formed.

Dr. Gay: No.

Fraser: The Fermi Paradox –isn’t solved.

Dr. Gay: And we also don’t have something that’s gonna explode in an interesting way or live forever in an interesting way.

Fraser: See, you say interesting. I say terrifying. So, it’s possible that we have a different definition of our star exploding, but I know you wanna see a super-volcano. Which, again –not interesting. Terrifying. All right. Well, thank you, Pamela.

Dr. Gay: Thank you so much, Fraser. And thank you to all of you out there that do allow us to take care of all the people that make this show happen. This week, I would like to thank a group of our Astronomy Cast patrons from Patreon.com/AstronomyCast.

This week, Neuterdude, William Baker, William Andrews, Gold, Andy Cowley, Jeff Collins, Kellianne and David Parker, Jeremy Kerwin, Stuart Mills, Rob Cuffe, Harold Bardenhagen, Phillip Walker, Matthew Horstman, Marco Iarossi, David Gates, Nicky Lynch, Rando, Alex Cohen, Jim Schooler, Scott Bieber, Justin Proctor, Daniel Loosli, Mathias Heyden, Gregory Singleton, the Lonely Sand Person, Nial Bruce, Tim McMackin, Jeff Willson, Paul L. Hayden, Cooper, Nate Detwiler, Paul D. Disney, Alex Raine, and Steven Shewalter. Thank you all so much for allowing us to keep doing this show and keep bringing you science week after week.

Fraser: Thanks everyone, and we’ll see you next week.

Dr. Gay: Bye-bye –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.

This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep this showing going, please consider joining our community at Patreon.com/AstronomyCast. Not only do you help us pay our producers a fair wage, you also get special access to content right in your inbox and invites to online events. We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.

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