Ep. 642: Is the Sun… Normal? (2)
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.
