Why are stars different colors?

Daniel and Kelly dive deep into the hearts of stars to explain why they all twinkle a little differently. See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Daniel Whiteson?

  This is an iHeart Podcast. Guaranteed Human.

• 0:05Speaker 01

  Hey, I'm Niall Rogers. This fall, I'll be driving across the country, meeting some of today's coolest and most fascinating artists. We'll explore the stories before the moments that changed everything. Because every musician remembers the moment where it all began. Experience Before the Moment. A Lincoln Original Series on the iHeart Radio app, YouTube, or wherever you get your podcasts.

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• 1:50Daniel Whiteson?

  The best view in the universe is not from the top of the Sears Tower or the Burj Dubai. It's not from some seaside hotel. It's not from the top of Mount Everest. It's from your backyard on a dark night. You can look up at the universe and see across billions and billions of miles. Every view on Earth pales in comparison to the vista you see across this ocean of space. Though they are impossibly distant, you can still see balls of gas furiously fusing, burning brilliantly enough that they can be seen with your naked eyeball. And if you look closely, you'll notice something incredible. There's a rainbow of stars out

• 2:31Daniel Whiteson?

  there. White, blue, yellow, red. It's a fabulously colorful universe. What does that mean? Why are stars different colors? We'll be digging into the physics and countering some usual pop science misinformation along the way. Welcome to Daniel and Kelly's extraordinarily fabulous colorful universe.

• 3:04Kelly Weinersmith?

  Hello, my name is Kelly Wiener Smith. I study parasites and space and I love looking at the night sky.

• 3:10Daniel Whiteson?

  Hi, I'm Daniel. I'm a particle physicist and I don't have a favorite color.

• 3:16Kelly Weinersmith?

  I do. Mine's purple. Why don't you have a favorite color?

• 3:19Daniel Whiteson?

  I don't get the whole principle of choosing a favorite. Like, I like the colors. They're all nice. Why do I have to pick one and say, this one's my favorite color? Like, I like purple. It's nice. I like orange. Yellow is pretty good, too. Blue is so soothing. I like the colors.

• 3:34Kelly Weinersmith?

  Groan.

• 3:35Daniel Whiteson?

  Do you have a favorite child?

• 3:36Kelly Weinersmith?

  You know, you and Zach. I'll ask Zach, what's your favorite blobble? And he's like, I don't see any reason to pick a favorite. And I'm like, because people use this as a conversation starter and you're a conversation killer.

• 3:50Daniel Whiteson?

  I was just gonna say, it's a way people start conversations, but it feels to me like dishonest. It's false. It's like, it's not what you really think or feel. It's just like, oh, you like red. I like green. You know, it's not sincere. So it doesn't start a good conversation.

• 4:04Kelly Weinersmith?

  I think it's implied that like, you're not married to that color for the rest of your life. I was doing a book signing. I was like, oh, so tell me something about you. And he's like, I hate small talk. I was like, oh, okay.

• 4:16Daniel Whiteson?

  And that's the end of this conversation.

• 4:18Kelly Weinersmith?

  I did stop talking. I was like, okay, all right. I don't know what else to say. Now you've ruined the question I was planning on asking you today because it involves the word favorite. But I was gonna ask you about your favorite or the best star viewing experience you've had. Like, when was the moment when the sky was most clear and you could see the farthest?

• 4:35Daniel Whiteson?

  I think one of my favorite observing moments is not actually looking at stars because, you know, stars are hard to make out any features for because they're so far away. They're basically just points of light, but stuff in our solar system that you can actually see. And back in the mid 90s, I was working on a project over the summer, right when Comet Shoemaker Levy was about to slam into Jupiter. And I had access to a super high speed camera. And so we pointed at Jupiter, hoping to catch really high speed photographs of the impact to see like, you know, planet sized plumes of fire emerging from it. It was super cool.

• 5:11Kelly Weinersmith?

  I mean, I do feel like I asked you something like, what's your favorite color? And you answered macaroni and cheese.

• 5:18Kelly Weinersmith?

  But that is a very cool story. Did you see what you wanted to see?

• 5:21Daniel Whiteson?

  Yeah, we got to see Shoemaker Levy impact on Jupiter. Unfortunately impacted around the back of Jupiter. So we didn't see the collision itself, but we did see the plume coming up over the limb of Jupiter. Really pretty amazing. I sort of remember it in black and white. I can't really answer your color question, but Jupiter does have a lot of beautiful colors. See, it doesn't just pick one.

• 5:41Kelly Weinersmith?

  That's amazing. And you keep sort of wiggling around my questions, but that's fine. That's fine. What you're saying is interesting, so it's fine for content.

• 5:48Daniel Whiteson?

  And I'd ask you what your favorite color is, but you're wearing a color right now. You literally have paint all over your arms. Does that mean that that's your favorite color?

• 5:56Kelly Weinersmith?

  Well, that's my favorite color for my laundry room renovation that I'm working on right now. But my favorite star viewing moment, I got really lucky once. So I was in Costa Rica and I was in a community where there was a big leatherback sea turtle conservation project going on and leatherback sea turtles can get distracted by lights. And then they move towards those lights and then they don't go out to the ocean after they hatch like they're supposed to. So the whole community had very dim red lights and they turned off all of their bright lights at night. And so we were sort of in the middle of nowhere and all the lights were off. So my job at 2am was to guard the hatchery so that raccoons wouldn't dig up the leatherback sea turtle eggs and eat them.

• 6:35Daniel Whiteson?

  Wow.

• 6:35Kelly Weinersmith?

  So it was like 2am in Costa Rica. It was the middle of nowhere. All the lights were out and it was just the most amazing view of the Milky Way and, you know, shooting stars. And at any time, there could be baby turtles. Like, that's peak Kelly life right there. I'd much rather spend time with my kids, but that was amazing.

• 6:52Daniel Whiteson?

  That sounds like a magical moment. Did the night sky look anything like your outfit does right now?

• 6:58Kelly Weinersmith?

  Yeah, except a fewer holes in it. This is also what I wear when I'm doing some renovation work. I don't know, it's comfy.

• 7:04Daniel Whiteson?

  For the audio only listeners, Kelly's wearing something which looks like it has star patterns on it.

• 7:08Kelly Weinersmith?

  It does, and my daughter has a matching outfit that we wear sometimes and she still wants to wear it, even though the dress has holes in it. Whenever she wants to wear a matching outfit, I am all in because I imagine it's like weeks or months before she's too old to want to do that. So every day it's an automatic yes.

• 7:24Daniel Whiteson?

  You've got to savor those moments when the kids still want to spend time with you because pretty soon they're going to grow up and move on.

• 7:31Kelly Weinersmith?

  I know, I know, your kid's going to college.

• 7:34Daniel Whiteson?

  And kids are not the only thing, growing up and changing and moving on. Everything in the universe is going through its own life cycle, including the stars in the sky, burning and glowing and changing colors as they do.

• 7:46Kelly Weinersmith?

  Yeah, so when I was listening to your introduction, I was realizing that I don't appreciate all the colors of the stars in the sky. I just think of them as white and sort of look at them as sort of a blurry something. I don't pay attention to the individual colors, the palette of the universe. So I'm excited that next time I look at the night sky, I'll pay a little bit more attention and I'm excited to learn more from you today.

• 8:10Daniel Whiteson?

  Yeah, the universe is very colorful, but it's not just to please you or to start conversation among 2 a.m. sky watchers who can't otherwise talk to each other. It's for physics reasons and the colors in the night sky are going to tell us a lot about what's going on inside those stars.

• 8:26Kelly Weinersmith?

  As long as it's not for chemistry reasons.

• 8:31Daniel Whiteson?

  So this is actually part one of a two-part series about why stars have colors and what it tells us about the universe, which ends up with a really interesting story about yet another overlooked female astronomer.

• 8:43Kelly Weinersmith?

  Oh, I love when I'm surprised while we're recording. I didn't realize this was a two-parter.

• 8:49Daniel Whiteson?

  I just realized this is a two parter because I'm preparing that other episode. And now I'm understanding, oh my gosh, this is the perfect setup for that episode. So good job, Daniel, realizing that in real time.

• 8:59Kelly Weinersmith?

  Welcome to Star Week.

• 9:02Daniel Whiteson?

  This is how carefully we plan the episodes on the podcast. All right. So before we dig deeper, I was curious if people knew why stars had different colors. And so as usual, I went out to our group of wonderful, clever, hilarious, well-informed, good-looking volunteers to ask them, if you would like to join their ranks for future episodes, please don't be shy, write to us to [email protected] and we'll hook you up. In the meantime, think about it for yourself. Do you know why stars are different colors? Here's what our volunteers had to say.

• 9:36Speaker 04

  We know that color here on earth is based on different wavelengths of light. So I'm assuming that the same principles apply to stars.

• 9:46Speaker 05

  Redshift, the including distance from and speed away from the observer. Color of the star is related to its size.

• 9:53Speaker 06

  So the older the star, the more it's been able to fuse, the heavier elements, you get different spectrum.

• 9:58Speaker 01

  Due to their temperature and the elements.

• 10:02Speaker 07

  I know that big hot stars are described as blue and small cooler stars described as red and middly ones like our sun are yellow. I think the color of stars depends pretty well solely on their age. I think stars are different colors based on the levels of hydrogen and helium in the star.

• 10:24Speaker 01

  Red ones are shy and as they get closer, they turn red. Blue ones are kind of cool. So they're going away from you. They don't care. Different elements of different colors and stars throughout their life fuse loads of different elements. I think stars are different colors because of the gases that are in them and also because of the temperature of the star.

• 10:46Speaker 04

  Probably due to their composition and elements they are made up of, and perhaps also their size and perhaps just how they're burning their energy output.

• 10:56Daniel Whiteson?

  Also, the universe believes in diversity, equity, inclusion, and that's why they're different colors.

• 11:01Speaker 05

  Their temperatures are different and that causes the black body radiation peaks to be at different wavelengths. Stars are different colors because they have different elements and they burn at different temperatures, which changes.

• 11:13Speaker 01

  The color we see. I'm really not sure why the stars are different colors, but if I had to guess, I would say it had to do something with where they were at in their life cycle and the gases that they are made up of. I think stars are different colors because the way they expand and like the gases inside of them, like as they expand and get hotter, I think they like change colors and stay those colors their lifetime. I would think that stars are different colors because they emit different light particles. But I think a non-sciencey answer would be that they have a cool aura.

• 11:50Kelly Weinersmith?

  Amazing answers. I chuckled and some really clever insights here. What did you think, Daniel?

• 11:55Daniel Whiteson?

  I think there's a lot of interesting stuff going on here. The picture I'm putting together of what people imagine causes the different colors of stars is that there's different fusion happening with like different ingredients, maybe you have more metal here or more metal there, and that's somehow changing what's going on and that's changing the burning of the star. The way you can like put copper in a Bunsen burner and it turns green, for example.

• 12:17Kelly Weinersmith?

  That is my understanding coming into this conversation is that I do think it depends on what elements are getting burned and how hot. But often I'm wrong.

• 12:27Daniel Whiteson?

  So I suspect a lot of you listening out there probably think the same thing, which is why we do this segment to orient us and understand where we need to take you from your current understanding to a deeper, more physical understanding. So we hope that by the end of the episode, you have a deeper view of why stars have different colors, because the answer is a little bit more subtle than anything we heard from our volunteers.

• 12:48Kelly Weinersmith?

  All right. I love it. Let's dig in and let's start with the basics. Tell me about light and color.

• 12:52Daniel Whiteson?

  Right. So some of the listeners commented that different colors mean different wavelengths. And that's true. Of course, we're interested in like, why is light emitted at different wavelengths? But fundamentally, it's important to understand the physical mechanism here, like why are we seeing different colors? And they're right, because light is electromagnetic radiation. The universe is filled with electromagnetic field and that field can ripple. And when electrons in Alpha Centauri wiggle, they cause ripples in that field because electrons are connected to that field, right? When electron moves, it wiggles that field and that wiggle is what we see as photons, for example. And those wiggles have frequencies and the frequencies correspond to different wavelengths and those wavelengths correspond to colors.

• 13:34Kelly Weinersmith?

  I feel like a bunch of pieces actually just clicked together in my head that should have clicked together much earlier, but there they are. Okay, click.

• 13:42Daniel Whiteson?

  And the same mechanism is like how radio works. Why is there an antenna? Because electrons are going up and down in the antenna. And as they do so, they make the photon field wiggle. Just the same way like if you're holding the end of a jump rope and you go up and down, you make wiggles down the jump rope. It's exactly the same mechanism.

• 13:58Kelly Weinersmith?

  Hmm, I think the piece is just unclicked.

• 14:00Daniel Whiteson?

  Oh, no.

• 14:02Kelly Weinersmith?

  I think I don't understand the electrons going up and down the antenna thing. Is it just radio waves are traveling through the antenna? And that's radio frequencies?

• 14:10Daniel Whiteson?

  Well, an electron has an electric field around it, right? If the electron just sits there, the electric field doesn't change. What happens if the electron moves up? Well, the electric field also has to move up. Well, if the electron field moves up and then down, the electric field moves up and then down. But it doesn't do so instantly. Like if you wiggle an electron in an antenna that's a mile away from me, I don't instantly see a change in the electric field. It has to propagate. And so if the electron is going constantly up and down and up and down, then it's making waves up and down in the electric field that propagate out away from it. And if I'm a mile away and I have electrons in my antenna, those wiggles in the electric field are going to push on my antenna's electrons, which are going to go up and down. And then I'm going to read that out as current.

• 14:53Daniel Whiteson?

  So yeah, the electrons in the sending antenna wiggle the electric field, which wiggles electrons in the receiving antenna, which move and can be picked up by my electronics.

• 15:02Kelly Weinersmith?

  Click.

• 15:02Daniel Whiteson?

  Can we re-click?

• 15:03Kelly Weinersmith?

  Yep.

• 15:03Daniel Whiteson?

  Yay.

• 15:05Kelly Weinersmith?

  Thanks.

• 15:07Daniel Whiteson?

  And so the different colors are different wavelengths of those wiggles. Photons are wiggles in the electromagnetic field, but they can have different wavelengths, right? They can be really, really narrow. So they're like very high frequency, like ultraviolet or purple. They can be really, really long. So they're like radio waves or red light or infrared light. And there's a potential source of confusion here. Remember, these things are quantum mechanical. So photons are discrete units. Like you can have one photon or two photons, but you can't have 2.781 photons. But you can have photons of any energy. That's a continuous spectrum. So there's an infinite number of frequencies. On the spectrum.

• 15:46Kelly Weinersmith?

  So does that mean there's an infinite number of colors?

• 15:49Daniel Whiteson?

  Hmm. Yes. Great question. Right. So there are an infinite number of frequency choices for a photon, but colors are things that we experience in our mind. Right? There are responses to signals on the optic nerve. So let's talk momentarily about the physics and the biology of what's going on there. You have photons that enter your eyeball and they hit the back of the eyeball. And the back of the eyeball, there are three different kinds of cells that respond to different colors. They have proteins on them that operate like little switches. And when they absorb a photon of the right color, they flip that switch and they send a signal up the optic nerve. This whole continuous spectrum of photons gets converted into three numbers. How much did you turn on cone one, cone two, and cone three?

• 16:31Daniel Whiteson?

  And then your brain interprets those and generates the experience of the color. So the color is actually in your mind. It's not on the photon. Right? We say red photon or green photon when we're being sloppy, but really the photon has an energy and the experience of color is only in your brain.

• 16:48Kelly Weinersmith?

  And we got a great question on our Discord channel, which you can join by going over to danielandkelly.org and clicking the invite and Quicksilver the Durg.

• 16:59Daniel Whiteson?

  Sorry, who was on the Discord?

• 17:01Kelly Weinersmith?

  Quicksilver the Durg? Anyway, this individual going by the moniker Quicksilver the Durg, wanted to know something about why we see the colors that we do see. And actually, our distant ancestors had two kinds of light cones and we have three. And so why do we see the colors that we see? The answer really at the end of the day is we're not 100% sure. But we think what happens is that there was a gene duplication event. And these happen every once in a while in our genome. And so now instead of having two different kinds of cones, you had two different kinds of cones, but three different genes for those cones. And over time, selection tinkered with that extra new gene. And we ended up with the ability to see the color red.

• 17:44Kelly Weinersmith?

  We could already see things like green and the thought was that seeing colors like red allowed our primate ancestors to differentiate between different kinds of fruits. And in particular, whether or not the Fruits were ripe. And there are some macaques that only have two types of cones and some macaques that have three types of cones in the same species. There's this variability. Some studies have found that if you have three types of cones, you get ripe fruits quicker and eat them quicker, which shows a benefit. But other studies and other kinds of macaques haven't found that because it's biology. So it depends. There you go.

• 18:17Daniel Whiteson?

  And I think you put your finger on it because the important thing here is the ability to distinguish different colors, right? But I think that depends not just on like the number of different kinds of cones you have, but also the processing power behind it. I know that like mantis shrimp are famous for having like more than 10 different kinds of cones, but they actually apparently are terrible at distinguishing different colors because they have almost no neural processing behind it. I think they might just like experience 10 different literal colors. Whereas we have a huge number of different colors that we can distinguish because we can do this interpolation, right? That's what your brain is doing is it's getting like, oh, a little bit from the bluish cone, a little bit from the reddish cone, a little bit from the greenish cone. And it's saying, okay, what color would give me this pattern? And then sort of inferring what

• 19:02Daniel Whiteson?

  color might be there. But again, the experience of color, the reason like red is reddish and blue is bluish, has nothing to do with the photon. That's something your brain has assigned to it, has invented, has fabricated, right? In principle, it could make up a new color. If you got a new cone implanted that was sensitive to the ultraviolet, your brain could like invent a new experience, not one that's a combination of the other ones, but like a real novel experience to represent the signal from the UV cone.

• 19:29Kelly Weinersmith?

  And there are insects that can see in the UV and it makes me so sad when I look at a beautiful flower and think there's layers that I'm missing. But anyway, I'm guessing based on our examples that you and I both read An Immense World by Ed Yong, because we both seem to have cherry picked examples from that fantastic book.

• 19:45Daniel Whiteson?

  It is definitely a great book, a lot of fun, and recommend everybody read that, especially if you're interested in physics and biology, because it covers both of those topics about our experience.

• 19:54Kelly Weinersmith?

  Okay, so the three kinds of cones that we have determine what colors we can see in the night sky. Are we missing a lot? Or are there ultraviolet stars out there that we're missing?

• 20:04Daniel Whiteson?

  We're definitely missing a lot because we can only see a certain range of photons, ultraviolet and infrared, everything below that and everything above that we can't see. And the sky definitely is bright in those colors. That's why, for example, we have radio telescopes and infrared telescopes and ultraviolet telescopes because stars and other different phenomena emit differently in those spectra. And if you can see those, you can see different kinds of things. Also, the universe is transparent differently in different frequencies. Some frequencies of life can go through dust clouds and others can't. And so if you want to see through dust clouds, you've got to change your frequency. So absolutely, the night sky looks very different outside the visible range. Our star, of course, peaks right in the center of the visible range, which again is no coincidence, right?

• 20:49Daniel Whiteson?

  If you're going to evolve vision, you might as well evolve it to be able to see the most common photons that are around you. So if we had evolved around a redder star, for example, probably our visible range would be lower. The sun is an unusual star. Most of the stars out there are redder than our star. Our star is yellower than most stars out there. So we may have a different visual range than most of the aliens.

• 21:12Kelly Weinersmith?

  So are you postulating that the color of the star determines the colors that we see? Because I feel like that's not an evolutionary argument that I understand. Does the color of our star impact the color of things on Earth that are important for food, mating, or running away from a predator?

• 21:28Daniel Whiteson?

  I think it makes sense for us to be most sensitive to the photons that are most present here on Earth for that range. Whether we can experience two, or seven, or three, or whatever, I think is a different question. But for us to be sensitive to the most common kind of photons makes sense to me.

• 21:43Kelly Weinersmith?

  All right, I'll give you that.

• 21:45Daniel Whiteson?

  You're skeptical. Why are you skeptical?

• 21:46Kelly Weinersmith?

  So I totally follow the argument that the most common kinds of photons in the environment would be the ones that we're good to sense. But if for some reason those didn't translate into an increased ability to find your food or an increased ability to recognize a mate, then I don't necessarily think that selection would hone in on those. And there's so much variability in what animals can see. You know, there's plenty of dichromates, so they don't have three kinds of cones and they get along just fine. So yeah, how do you explain the variability then?

• 22:17Daniel Whiteson?

  That's a great point, and I can give you an example to support your point, which is neutrinos. Neutrinos are everywhere in our environment, but pretty much useless because everything is transparent to them. And so it wouldn't be great to develop a neutrino eyeball, even though they're everywhere. So you're right, just being ubiquitous isn't enough to have to be useful. And so I think the combination of the fact that they are everywhere and stuff on earth tends to not be transparent to them, makes them useful for us to see things. So I think you're right, it's more complicated than just the physics of this region has to also be like useful in your environment, for sure. I have no idea why you would have two or seven cones.

• 22:55Kelly Weinersmith?

  I guess both of our fields are useful. You ponder the question Daniel and I were just debating during the break. And when we get back, we'll talk about why stars have different colors.

• 23:14Speaker 01

  Hey, it's Niall Rogers, host of Before the Moment, a Lincoln original series. I'm driving across the country, meeting some of today's most fascinating artists. We're exploring the stories before the moment that changed everything. This week, I'm joined by breakout singer-songwriter Alex Warren. I'm like a kid when I play music and I make music and I'm constantly asking questions. You know, whether it's an instrument I haven't played, or it's a melody that seems foreign. You know, this every night you play a show and it's the same set, but it's not the same show. At a certain point, your brain kind of goes on autopilot. When I'm in the studio or if I'm making music, that's where that curiosity kind of comes back, you know? I haven't lost it yet, but I'm also on my sophomore album.

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• 25:26Kelly Weinersmith?

  All right, and we're back. You look up at the night sky and there are lots of different colors of stars, though I'll be honest, I missed that in the past. I'm gonna be paying much more attention now. So Daniel, why do those stars have different colors?

• 25:38Daniel Whiteson?

  So there's sort of three big factors that control what color a star is in our sky. One is what it's made out of, another one is its temperature, and the last one is its velocity. So let's do those in order. What it's made out of. This is the one that's connected to your experience of like putting copper in a Bunsen burner and seeing it glow green. That's because atoms are quantum mechanical little objects and they have energy levels. You know, electrons around an atom can't just be at any energy willy-nilly. There's a ladder of energies there. And because there's a ladder of energies there, they can only eat and emit photons of certain energies, the ones that let them go up or down the ladder. So when an electron is around an atom and a photon comes by,

• 26:20Daniel Whiteson?

  if that photon has just the right energy to take it up one or two or seven rungs on that ladder, it can eat that photon. You might think, I don't care about what photons the electron eats. It actually does matter what it absorbs and what it reflects, because what it reflects is what you see, right? If you're looking at something that's blue, it's not blue because it's eating blue photons. It's blue because... Because it's eating everything but the blue photons. It's reflecting blue photons back at you. Remember, you see something blue because some light in the blue range has hit your eyeball and you responded to that. So it's important what energy levels an atom can absorb. And the same is true also in the reverse. Atoms emit light. That's what's happening to the copper in your Bunsen burner.

• 27:03Daniel Whiteson?

  It's getting hot and the inverse process is happening. An electron jumps down energy levels and gives off a photon, a green photon in the case of copper.

• 27:11Kelly Weinersmith?

  That always felt very counterintuitive to me that the color that I'm seeing is the color that's being given off by something. It almost feels like everything is disguising itself in some way. I don't, I don't know why my brain had trouble wrapping itself around that.

• 27:24Daniel Whiteson?

  I totally remember the first time I understood that. I was like nine and I was like, oh my gosh, things that are blue are not actually blue. They just look blue. And then I realized, well, maybe that's what it means to be blue, to look blue. But yeah, I had the sense that like, if I could see through these photons to what things actually looked like, I could perceive a deeper truth to the universe. But the universe is kind of a construct we've assembled in our head from our experience of it. And there's a lot of that that's imagined and that's, you know, put together from our experience. And the way we paint these colors on things is just one layer of that.

• 27:57Kelly Weinersmith?

  Yeah, man.

• 28:02Kelly Weinersmith?

  I totally agree, though.

• 28:03Daniel Whiteson?

  Yeah, but that's the joy of physics is it helps you separate what's really out there and what you know about it, what you don't know about it, which parts you're building in your mind. Super fascinating, even if it is philosophical and could never be understood. So the next piece to understand is why things have different colors, right? We said that atoms have energy levels and electrons can jump up energy levels, eating photons or down energy levels, emitting photons. And those photons have specific energies corresponding to those gaps. Well, different atoms have different energy levels. Copper and mercury and helium and hydrogen all have different energy levels because the solutions to the Schrodinger equation are different. You've got different numbers of protons and the whole configuration of the atom is different. Every atom has its own unique ladder, which means that they have like a fingerprint.

• 28:49Daniel Whiteson?

  If you take an element and you take a gas of it and heat It up and then you take the light from that, pass it through a prism to spread it out. You'll see that each element has its own unique fingerprint, its own spectrum. They'll have these little bands, like a blue one and a red one, or a different element will have like a green band and the blue band will be shifted over. So you can tell what something is made out of just by looking at a spectrum of a hot gas. So there's two things that are happening here. You have hot gas, it glows in certain colors, right? And if you have more oxygen, you glow in these colors. You have more neon, you glow in those colors. And our whole next episode is going to be about using spectroscopy to understand what stars are made out of. But there's also absorption, right?

• 29:29Daniel Whiteson?

  If a star has an atmosphere and that atmosphere is mostly neon, for example, then it's going to absorb that light. So if you have a hot blob of gas, it emits light to those frequencies. If you have light that passes through an atmosphere, then it absorbs those frequencies.

• 29:45Kelly Weinersmith?

  So it feels to me now like we have an infinite number of combinations of elements that could be emitting and like gases that could be absorbing. How do we make sense of the output, given that there's now so many different options? And it's probably not just one element that's emitting at a time. There's probably a couple of different things that are emitting and I'm overwhelmed.

• 30:04Daniel Whiteson?

  Yes, it is overwhelming. It's a hard problem and it was solved by a very clever young lady around the turn of the century. And we're going to talk about that in the next episode.

• 30:12Kelly Weinersmith?

  Okay.

• 30:12Daniel Whiteson?

  But even if you mix all these elements together, what you get are a bunch of different spikes. Like these are narrow emission lines. You can't put them together to get a broad spectrum. And that's not what most of the light from the stars is. This is like people's conception, but most of the light from the star comes from a completely different process. It's not from atomic emission and absorption. It comes from the plasma inside the star, just glowing at a certain temperature.

• 30:36Kelly Weinersmith?

  What? All right. We were all wrong, audience and friends of mine. Okay. So Daniel, let's move on to the next thing then. Tell us more about this thing that actually contributes most of the color.

• 30:46Daniel Whiteson?

  Yeah, so we said that the color of stars comes from what they're made out of, and that has to do with atomic absorption and emission, mostly in the atmosphere of stars, but also from its temperature. And so that's what we're going to dig into next. And there's a process here that has a terrible, absolutely terrible, very misleading name. It's called black body radiation that describes why stars glow at certain temperatures. And it's terribly named because a star doesn't seem like a black body, right?

• 31:11Kelly Weinersmith?

  Nope, you guys did it again.

• 31:12Daniel Whiteson?

  But in physics, we have a model of some hypothetical object with no reflection. Like any photon, you shoot at it, it will absorb it and just like heat up. And even in the hypothetical version, it's not black. It just means it doesn't reflect. It glows because of its temperature. Everything in the universe that has a temperature and is made of charged particles will glow. Like if you have fluorescent lights in your ceiling, you look up, that has a hot gas in it that's glowing because of its temperature. Or you're glowing right now, Kelly, because of your temperature. Not in the visible light, but if I put on night vision goggles or infrared goggles, I would be able to see you emitting light.

• 31:48Kelly Weinersmith?

  What is the physics definition of glow? Is it just releasing photons or something?

• 31:53Daniel Whiteson?

  It's just releasing photons because you have charged particles in you, and charged particles are always interacting and moving around and whizzing. And when they do so, they emit photons. Like, an electron can't change directions without emitting a photon. That's how it does it. An electron is flying through the universe and it wants to curve along a magnetic field or something, it's got to push out a photon in the other direction to conserve momentum. Now you have a big blob of gas with all sorts of charged particles whizzing around. They're going to be constantly emitting photons, and they emit a very broad spectrum. Not like the atoms we talked about, where have very specific energy levels. This black body radiation is a very broad spectrum, but it has a peak that depends on the temperature. So really cold things tend to emit in the infrared. Warmer things emit invisible.

• 32:37Daniel Whiteson?

  So for example, you take a piece of metal, it's glowing right now. Even if it's cold, just in the infrared, you can't see it. Put it in the oven, heat it up, put it in the forge, it starts to glow red. And then it glows white, right? White hot is very hot. Why? Because now it's hot enough to be glowing in the visible. Keep heating it up, it'll start to glow in the ultraviolet. You won't see it anymore, but it'd be super duper hot. The temperature of something determines the peak of its emission, right? So everything in the universe that's made of charged particles glows at a certain temperature, and the temperature controls the frequency of the emission.

• 33:11Kelly Weinersmith?

  Okay, all right, a few questions. So when an electron kicks out a photon, is it still an electron?

• 33:17Daniel Whiteson?

  It's still an electron because a photon is neutral. Yeah.

• 33:19Kelly Weinersmith?

  How many photons can an electron kick out? Can it just do that all the time?

• 33:25Daniel Whiteson?

  Oh, my gosh, what an amazing question. It makes me think of electrons like having a bag of photons that it could run out of, right? Like fuel, you think of like rockets, right? And they need some source. No, an electron has an infinite number of photons. Remember, these are just wiggles in the electromagnetic field, right? And so it's not like it's kicking off some substance. But in order to conserve momentum, you have to have some momentum move through the electromagnetic field and some momentum move through the electron field.

• 33:52Kelly Weinersmith?

  Wow. Okay, all right. So say you've got some iron and we talked about iron having emission line spectrums where the electrons get excited and they jump to different levels and they emit light. And then you heat that iron up and now it's glowing. Are you saying that the glow is more important than the jumping of the levels?

• 34:16Daniel Whiteson?

  The most general answer is that it depends on what it's made of and its temperature. In the case of stars, it's mostly black body radiation. Like when we look at the spectrum from the sun, it's almost all black body radiation. So that's definitely the most important effect. And there's a combination of the two things. So you have the star, which is glowing, very broad spectrum, which peaks right in the middle of our visual spectrum for our star. But then the sun has an atmosphere and this atmosphere is gas and it's made out of various stuff. And that atmosphere will eat that spectrum. So the light we get from the sun is a huge black body peak with a few lines removed. That's absorption spectra. So if you look at the spectrum from the sun, you see reds and yellows and greens and blues.

• 35:00Daniel Whiteson?

  But then you see these black spots, these things that have been removed. That tells you what's in the atmosphere of the sun. You can see the hydrogen lines have been removed, the sodium lines, the helium lines, the magnesium lines. And so that helps you figure out what's in the atmosphere of the sun. So there's two things happening there. There's the broad glow inside the sun, which we're not fully seeing because parts of it have been removed as the sun's atmosphere. Eat some of those frequencies.

• 35:23Kelly Weinersmith?

  That's amazing.

• 35:24Daniel Whiteson?

  Yeah. And so the last piece we talked about for what determines the light that comes from a star is the velocity. And a bunch of listeners pointed this out, that things that are moving away from you will be red shifted, just like a police siren moving away from you will have a lower sound than a police siren moving towards you. It's just the Doppler effect. And so a star emitting light as it moves away from you is going to get red shifted. And we use this red shift to measure that velocity. It's like a hugely important thing in astronomy. A star moving towards you would be blue shifted. And we can tell the red shift and the blue shift because we know the typical fingerprints. Like you get a star you've never seen before and you want to measure its red shift. Well, you might think, well, how would I know? It's impossible. If the star has hydrogen lines, but they're shifted by a few nanometers, and it has magnesium lines,

• 36:09Daniel Whiteson?

  and those are shifted by the same number of nanometers, then the most likely scenario is, oh, this isn't made of some new, super weird metal that looks like magnesium and hydrogen, but shifted. It's just magnesium and hydrogen, and it's been shifted due to the red shift. So you can fit these lines, these spectra, these absorption and emission lines to tell you the frequency shift of an individual star. And so that definitely affects the color of the star in the sky. And almost everything in the sky is moving away from us. So they're all red shifted.

• 36:38Kelly Weinersmith?

  I'm having one of those moments where I'm just so proud of our species for figuring all of this out. Like, that's so many different things to keep track of. It's so incredible that we got there and that we understand all of this stuff. And anyway, way to go, humans.

• 36:52Daniel Whiteson?

  This is the amazing thing about astronomy is that you cannot leave the Earth mostly. But you've got to figure out the puzzle of the universe from these clues. And we didn't assemble this experiment in this way. We stumbled over these things. People had moments of insight where they realized, hold on a second, this pattern actually reveals this thing about the universe. Those are incredible realizations, so powerful. And let me just qualify what I said a moment ago about everything moving away from us. Galaxies are moving away from us, and those are all red shifted. Most of the stars in the Milky Way are held together by the gravity of the Milky Way. And so those aren't red shifted and moving away from us. The stars in the night sky are the ones from the Milky Way. Those are the ones we can see. They're not red shifted. But other galaxies are red shifted.

• 37:35Kelly Weinersmith?

  All right. Well, that is amazing. Let's take a break. And when we get back, we're going to talk about the color of our very own sun.

• 37:50Speaker 01

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• 40:06Kelly Weinersmith?

  All right, we're back and Daniel wants to tell us about why Neil deGrasse Tyson is wrong.

• 40:11Daniel Whiteson?

  I don't want to say that. I do want to dismantle a lot of pops eye myths that you hear.

• 40:17Kelly Weinersmith?

  From Neil deGrasse Tyson.

• 40:19Daniel Whiteson?

  I didn't say that. I didn't say that. I just want to say that, you know, on this podcast, we like to dig deep, well past the usual pops eye explanations and tell you what's really going on. So, you know, we're interested in the colors of stars in general, but of course, our sun is near and dear to our hearts. Why does the sun have its particular color? And the sun's color is actually fascinating because if you were out in space and you were looking at the sun, it's white, right? And what does it mean for the sun to be white? White isn't like a color. It's a mixture of the colors. And it's sort of the way your brain responds when you see a very broad spectrum of light across the visible range. So out in space, the sun is white. Here on Earth, because of the effects of the atmosphere, the sun looks a little yellower, right? Because

• 41:02Daniel Whiteson?

  the atmosphere tends to scatter blue light, which is why the sky, which doesn't have a color of its own, looks blue, right? Because it's reflecting blue light. Air itself is not blue, but we see it as blue because it's reflecting blue light. And so the sun has some of the blue removed, which makes it look a little yellower to our eyes.

• 41:20Kelly Weinersmith?

  Oh, that's so cool. And again, yet another thing that we had to account for to understand all of this stuff. Amazing.

• 41:26Daniel Whiteson?

  And the reason the sun glows at this color is because of its temperature. The surface of the sun is about 5,000 Kelvin. And what we're seeing when we look at the sun is its surface, right? We can't see through it. It's opaque. Another bit of pops eye you hear a lot is that it takes like thousands of years for a photon to go from the center of the sun to the surface. And like, I don't even know what that means because, you know, what's happening is photons are made at the center of the sun, but the sun is opaque. So then they just get reabsorbed and they contribute to the overall heat of the sun. An individual photon doesn't go from the center of the sun to the edge of the sun. It just heats up the sun and the surface emits a new fresh photon.

• 42:03Kelly Weinersmith?

  Oh, interesting. I mean, I can't say that, you know, Ada came home from the playground and it was like, did you know that a photon at the center of the sun takes, you know, To get to the edge. I had never heard that before, but now I'm enlightened.

• 42:15Daniel Whiteson?

  I hear that a lot. And so you can mimic the same spectrum that the sun makes by heating up like a piece of tungsten to the same temperature. If a 5,000 degree filament of tungsten, it will emit in the same spectrum as the sun, right? This is not 100% true because the sun is not a perfect black body radiator, right? It does reflect some things. It's not a perfect absorber. So the peak of the distribution for the sun is actually in the green spectrum. And so this is why you might hear some pop side folks being like, did you know the sun is actually green?

• 42:45Kelly Weinersmith?

  Okay. Well, so now let's take down that hypothetical. Did you know the sun is actually green person?

• 42:51Daniel Whiteson?

  So saying that the sun peaks in the green is actually saying if you had a perfect black body radiator at 5,000 Kelvin, it would peak in the green. But we don't have a perfect black body radiator. Our sun actually peaks in the violet. And where it peaks depends a little bit on how you're doing the accounting, like, are you doing in wavelength? Are you doing in frequency? These things have a nonlinear relationship. And so there's some like calculus that goes on there, but which gets accounted into which bin? And so it's not really even true saying that the sun peaks in the green. And even if it peaks in the green, you would still see it as white in space. It's not like out in space, the sun is green or violet or any of these things. Really, it's just most accurately said that there's a broad spectrum

• 43:33Daniel Whiteson?

  that if the sun were a perfect black body radiator would peak in the green, but it isn't. So it peaks in the violet.

• 43:38Kelly Weinersmith?

  Which is my favorite color. So thank you, universe. There we go.

• 43:42Daniel Whiteson?

  Why don't I see it on your arm? Then why aren't you painting your laundry room violet?

• 43:46Kelly Weinersmith?

  If we had been doing this podcast when I was painting my bathroom, you would have seen my purple arm.

• 43:53Daniel Whiteson?

  Well, I have purple on the wall behind me as well. So I'm a big fan.

• 43:56Kelly Weinersmith?

  There you go. It's a great color. Best color. All right. So tell me about how we get colors for some other stars.

• 44:01Daniel Whiteson?

  Yeah. So we've learned that star color depends on its temperature and on what its atmosphere is made of and its velocity. So most of the stars in the universe are smaller than our star. The sun is a big star compared to the average star, which is more like a red dwarf. So smaller stars have less gravitational pressure. So they're not as hot, so they burn and they glow in the red more than in the yellow. So that's why they're called red dwarfs, right? So smaller stars burn cooler and slower. They're longer lived and they tend to be redder. So a lot of the stars out there in the universe are cooler on their surface than our star. But there are some that are hotter.

• 44:40Daniel Whiteson?

  Blue giants, for example, super enormous stars, very hot on their surface because of the crazy fusion happening at their core. They tend to glow in the blue. If you look at the spectra of stars, you see a really big range. There are fewer that are bluer because those are bigger and burn brighter and burn out faster. Red stars tend to burn a lot longer. Like our star is going to last billions of years. Really big, huge blue stars can sometimes only last millions. Whereas little tiny red giants, we think maybe can last hundreds of billions or even longer. Much longer than the age of the universe. So we don't really even have a measure for it.

• 45:18Kelly Weinersmith?

  Is there any relationship between the temperature at which a star burns? And what we think its ability to sustain life is?

• 45:26Daniel Whiteson?

  Yeah, great question. We have this one clue, right? That we have life around a yellow star and not a red star. And does that mean that life only happens around yellow stars? Or are we weird and unusual and most of the aliens out there look up at a red sun? Yeah, we don't know. Red stars tend to be a little bit more variable sometimes. And so there's arguments there, but it's all based on this N equals one. So until we meet the aliens, we won't know the answer to those questions.

• 45:52Kelly Weinersmith?

  I mean, I feel like if you could be at the right spot next to a red star, that would give you more time for life to pop up. But I guess if it's more variable, not necessarily, and it's colder and what does that do? And anyway, all right, yeah, we need more data.

• 46:02Daniel Whiteson?

  We definitely need more data. And what's in the star definitely does have an impact. Like if there's magnesium in the atmosphere, then those lines are removed from the star. But that doesn't really change the star that you see from the naked eye. Cause you can't tell that individual slice of the spectrum has been removed. It still looks red to your eye. But if you pass these things through a prism, you will notice that different stars, even if they have the same temperature, have different spectrum because they have different lines removed. And that tells you what they're made out of. Most stars. Still totally hydrogen. Like, the universe started out all hydrogen, mostly still hydrogen. But the elemental mixture does affect the lines in the atmosphere. So it will affect the color of the star technically.

• 46:43Kelly Weinersmith?

  So we've talked about a lot of different things that influence star color. When I look at the stars at night, am I understanding correctly that the thing that mostly determines the color I see is how hot they're burning?

• 46:54Daniel Whiteson?

  Yeah, which is mostly determined by how big they are.

• 46:57Kelly Weinersmith?

  Okay.

• 46:57Daniel Whiteson?

  So yeah, redder stars are smaller and bluer stars are bigger.

• 47:02Kelly Weinersmith?

  Got it. I'll appreciate that so much more now.

• 47:05Daniel Whiteson?

  And it's incredible what we can learn about these stars just from the pattern of the light, or that we even figured out that you can break up light into components and that it contains all of this useful information, right? Always makes me wonder what haven't we yet figured out? What information is landing on the planet screaming itself, screaming clues about the universe that we haven't yet figured out? That in a few hundred years, people will be like, oh, my gosh, you guys were such idiots.

• 47:30Kelly Weinersmith?

  You know, I wonder the same thing in biology. What are the things that we're missing that people will laugh at us for in the future? But, you know, all you can do is take incremental steps forward and then be willing to take those steps backwards when you learn that you're wrong.

• 47:42Daniel Whiteson?

  Yeah, and take things apart and try to learn about them, right? Exactly. Pass your parasites through a prism. See what bits they're made out of.

• 47:49Kelly Weinersmith?

  Tell me what you find out.

• 47:52Daniel Whiteson?

  And everybody out there, come up with your favorite color and your favorite parasite.

• 47:55Kelly Weinersmith?

  Oh, what's your favorite parasite, Daniel?

• 47:59Daniel Whiteson?

  All the ones that are not in me.

• 48:01Kelly Weinersmith?

  Oh, that's a lot. That's a lot. You must really like parasites.

• 48:04Daniel Whiteson?

  I approve of their choice to not be inside me right now, yes.

• 48:08Kelly Weinersmith?

  Well, you know, as we were telling a listener the other day, they don't have a lot of choice, but I'm glad that they're not in you.

• 48:14Daniel Whiteson?

  And I hope that this has helped you appreciate the beauty of the night sky. Next time you're out camping or on a beach in Costa Rica, protecting turtles from raccoons, you'll appreciate that the night sky is colorful and that those colors communicate information about the nature of the universe. What's going on in the hearts of these big furious balls of fusion?

• 48:34Kelly Weinersmith?

  Enjoy the night sky, friends.

• 48:42Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

• 48:48Daniel Whiteson?

  We really would. We want to know what questions you have about this extraordinary universe.

• 48:54Kelly Weinersmith?

  We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

• 49:00Daniel Whiteson?

  We really mean it. We answer every message. Email us at questions at danielandkelly.org.

• 49:07Kelly Weinersmith?

  Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D and K Universe.

• 49:16Daniel Whiteson?

  Don't be shy. Write to us.

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