Carina Nebula. Image Credit: Hubble

Ep. 412: The Color of the Universe

What color is the Universe? Turns out this isn’t a simple question, and one that scientists have really been unable to answer, until now!

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This episode is sponsored by: Swinburne Astronomy Online, 8th Light,

Show Notes

  • The Cosmic Spectrum and the Color of the Universe
  • Cosmic Latte beige
  • Astronomy Picture of the Day
  • Black body curves
  • Emission lines
  • Tetrachromacy
  • Visible spectrum
  • Electromagnetic spectrum
  • Butcher-Oemler effect

  • Transcript

    Transcription services provided by: GMR Transcription

    Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit for more information.

    Fraser Cain: Astronomy Cast, Episode 412: The Color of the Universe

    Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain; I’m the publisher of Universe Today. And with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the director of Cosmoquest. Hi, Pamela; how you doing?

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

    Fraser Cain: Welcome back from the Czech Republic.

    Dr. Pamela Gay: Yes. It is a beautiful country. I highly recommend it; but if you go in the spring you may get hay fever and snow simultaneously, and that sucks.

    Fraser Cain: Yes. That – you bring all the crazy weather with you. So, just to let people know, we’re kind of nearing the end of the Preston-verse.

    Dr. Pamela Gay: Yeah; we started the show back in 2006; we had Rebecca Bemrose-Fetter do some editing for us for a while; I did some editing for a while – I should not be allowed to do audio editing – and then we hired an actual audio editor: Preston Gibson. He was an undergraduate here at SIUE at the time, and for almost ten years he has been the person that made our episodes sound awesome for all of you, and this is his last season.

    He’s worked with us while he was an undergraduate; he’s worked with us while he was a graduate student; he is now a full-fledged adult out in the normal workforce, and he is actually going to focus on his real career. And we’ve been so lucky and grateful to have him stay with us through so long, and it looks like we’re now going to have to figure out: do we find a new proto-Preston and start the cycle all over again?

    Fraser Cain: The cycle must continue.

    Dr. Pamela Gay: It does. But we’ve loved Preston, and we’re in the final countdown.

    Female Speaker: This episode of Astronomy Cast is brought to you by 8th Light, Inc. 8th Light is an Agile software development company. They craft beautiful applications that are durable and reliable. 8th Light provides discipline software leadership on demand, and shares its expertise to make your project better. For more information, visit them online at Just remember, that’s www dot the digit 8-T-H-L-I-G-H-T dot com. Drop them a note. 8th Light. Software is their craft.

    Astronomy Cast is proudly sponsored by, training videos with personality for software professionals.

    Fraser Cain: So what color is the universe? Turns out this isn’t a simple question, and one that scientists have really been unable to answer until now. Now this originally – astronomers have taken a couple of cracks at the color of the universe. Turns out it was actually a lot harder to figure out than they had thought. Now when we’re talking about color, what are we talking about?

    Dr. Pamela Gay: We are talking about – the color we’re going to use, because you can use this in a lot of different ways – is the fully dark-adapted eyeball’s response when faced with non-saturating light.

    Fraser Cain: And so when you just open your eyes, gaze out into the cosmos, the average color that’s falling upon your retinas, this is the color that we’re going to be talking about.

    Dr. Pamela Gay: Right. And in talking about this color, this is where it – see, you go outside at night and you’re like, okay, the sky is kind of midnight blue; we have stars, bunch of different colors; we have galaxies of a bunch of different colors, but no matter how good a picture you use, you can still find darkness between galaxies. So what we’re talking about is if you smear all the light out; if you took a inkjet printer of the universe and smudged it out before it dried, what color would you get?

    Fraser Cain: Okay, I’m ready. What color –

    Dr. Pamela Gay: Beige.

    Fraser Cain: You get beige! Is there a – is there like a pantone number or an RGB value we can get?
    Dr. Pamela Gay: So there’s an RGB value and I have to admit that when we relaunch Cosmoquest – we’re in the process of upgrading the designs – we’re going to make the background color on Cosmoquest the beige of the universe. It’s technically called Cosmic Latte beige.

    Fraser Cain: What’s the RGB number?

    Dr. Pamela Gay: I don’t know; Cory knows this. Sorry; my programmer, Cory Lehan, who makes Cosmoquest go; he has the RGB number. But it’s beige. It’s the color of – take coffee, have a shot of coffee, and the rest of it is a mug of cream, and that’s the color of the universe.

    Fraser Cain: Okay, here you go: The Hex triplet is FFF8E7, and the RGB values 255, 248, 231. So if you just do a Google search while you’re listening to this, for Cosmic Latte – You know, it is the kind of color – it’s a very light beige color. It’s the kind of color that you would, as you said – you’ve taken your coffee, your latte; you’ve thrown a lot of cream into it, on top of the latte already; and it’s this very sort of milky white color, but a little beige. Okay, great. Why is it that color?

    Dr. Pamela Gay: This is the reason we’re doing the show. It would be an awful sad show if all we did was discuss beige.

    Fraser Cain: No, no. Yeah, we’re done; this episode of Astronomy Cast is over, thank you very much everybody – No. The question is, you don’t see this color anywhere, so why is this color the average color?

    Dr. Pamela Gay: It’s the color you end up with when you add together all of the black body curves of all of the different stars; at all of their different red shifts and speeds, and evolutionary states, and masses. It’s the color you get when you add in the emission lines of all of the hot gas; it’s the color you get when you then subtract things out with all of the cold gas that is creating absorption lines. Add all that together, and it gives you this spectra, which tells you how much energy you expect to find at each different color.

    And once you have that spectra, we have the technology to turn that into – well, you could do it with a hue Philips light bulb if you wanted to. But more to the point, we also know our eyes’ response to these different colors. So then you start getting into the – given someone with a completely normal set of eyeballs; so someone who’s not color-blind, someone who’s not tetrachromal – if you look at what their chemical response to this color would be, what would they see, and the answer is beige.

    Fraser Cain: I love the idea that you’re gonna be making the background color of Cosmoquest that color, which is brilliant. I may steal that idea too and make that the background color of Universe Today. Because that is the best idea; and it’s like a super Easter egg that people won’t even realize. That it is truly the color of the universe.

    Dr. Pamela Gay: And it is eggshell colored, actually.

    Fraser Cain: Exactly. Okay, so those are the photons that are falling on your eyeballs, but that’s sort of like what we would perceive in the visible spectrum, right? Is that a true understanding of the average wavelength of all of the photons that are falling on us from around the universe?

    Dr. Pamela Gay: And this is the thing, is there’s a difference between the perceived color in the very biased human eyeballs we have, and the average color of the universe. So when you start getting into, “Well, what’s the average color of the universe” you have to deal with the fact that there’s this cosmic microwave background in the background of everything. It’s just that leftover light from when the cosmic microwave background formed.

    It’s microwave light; it’s light that the eyeball absolutely cannot see. Then we also have this background of gamma ray emission that’s coming from its own various sources; all of the hot gas that’s out there. So then you start getting into the, “Well, expletive, what am I taking the average or the mean of? Am I looking at what is – by numbers the average photon has this energy, it has this wavelength, how am I figuring it out? Do I want to use the mean instead?”

    And the truth is, there’s a whole lot more cold universe out there; giving off infrared, radio, all of these colors that we can’t see with our eyeballs. So, by numbers, the universe is going to be dominated by all of these cold red sources. By energy, it doesn’t take a whole lot of hot x-ray emissions to have a higher energy than the eyeball can see.

    Fraser Cain: Now one of the things that I find super interesting is that this is sort of a snapshot in time; that you could look at this color, and a future astronomer could go, “Oh, you were living ‘round the 13.8-billion-year time frame of the universe.” So if we were living at an earlier stage in the universe, what would that average color turn into?

    Dr. Pamela Gay: It all depends on exactly when in the past you go. And there’s a really cool paper that came out over the summer, where they’re talking about how we are essentially past peak energy in our universe, and everything is just going downhill from here. So, there’s a point in the far past where things in general were lower energy, if you grabbed any given cubic volume of space. So you go back to the cosmic dark ages, when everything was a boring, bland, neutral gas, and it’s a low energy state giving off basically 21-centimeter radio emission, in a lot of cases.

    Fast forward, you start having stars turn on, and suddenly the universe is filled with young, hot, blue stars. We have ultraviolet radiation coming out, destroying things left and right; ionizing radiation. And this is what lights up what ionizes our universe. So we went from radio –

    Fraser Cain: Yeah; it’s very radio colored. It’s very 21-centimeter colored.

    Dr. Pamela Gay: – to, “Wow, everything’s getting a sunburn” UV-colored, to settling down through the blues in the visual spectrum, and now we’ve hit the point where things are more on the reddish side of beige.

    Fraser Cain: Of beige. So in the past – and I actually have got a pretty cool picture here, I don’t know if you’ve seen this one; there’s one that shows you sort of what it would look like, the color in the past. And it’s blue. And then into the future it sort of turns back to even more of a salmon-y color as it gets back into the reds; as all these stars age, and all the blue star that’s pulling it back to the middle of the spectrum are gone and dead.

    Dr. Pamela Gay: And we’re just going to get redder and deader from here on out. There will be individual bursts of blue. Our own galaxy, when it decides that it’s going to become one with Andromeda and we form Milkdromeda, there will be this wild burst of star formation in this collision process. So locally, we’ll be super blue at that point. But that’s locally. As a whole, our universe is red and dead.

    Fraser Cain: This color, this is pretty silly and it has other purposes beyond just what to color your space website – when astronomers look at the color of objects out in the universes, that color, that average color tells them a ton of stuff, right? Like if you look at a – if I look at say, a galaxy cluster, and I figure out what the average color of that cluster is, what is that telling me?

    Dr. Pamela Gay: So, this is actually something I studied with my doctoral work. There’s this really neat trend – it’s called the Butcher-Oemler effect – where on maps when you look at fairly large, but not huge, clusters of galaxies in the past; they’re much bluer. They have a lot more star formation going on; blue means star formation. But as you get more and more into the modern universe, what you’re seeing is, well, those same fairly massive but not huge galaxy clusters; they’re getting redder and redder over time.

    And what’s happening is, all of the stuff that gets used in star formation is through interactions getting gradually stripped out of the galaxies and used up, and we’re running out of star formation in these systems. Now, if you look at groups like our own local group, low-mass groups of galaxies, they’re blue. They’re just always blue. Because we’re not interacting with one another very often; we’re kind of standoffish, and without these interactions, we’re not getting our gas stripped out of us; we get to stay active blue galaxies.

    It’s going to change slowly over time, but it takes time. And what’s even more awesome, is these massive, massive galaxy clusters. They pretty much instantly went red, and this is because so much interaction was going on; these massive systems of galaxies that are whipping around at much higher speeds.

    Higher speeds, more density, all of this means more collisions, more – we use horrible words; it’s called ram-pressure stripping and galaxy harassment. Basically more stuff that’s removing the gas and dust out of these galaxies. So less star formation, from the beginning of time. So we can tell star formation by looking at the color of galaxies.

    Fraser Cain: And then one of the other factors that come into play is that over time, these galaxies and galaxy clusters are moving away from us, right? So once again, you look at those colors to get sense about their velocity.

    Dr. Pamela Gay: Right. So if I look at an individual star, or an individual type of galaxy, that I know typically what kind of color it would have, when I look at it I may find that it’s a radically different color. And then when I start sampling different parts of the spectra, that’s where I get the details; I can go, “Oh, this Lyman-alpha line that should normally be out in the ultraviolet that’s the 2-to-1 transition in hydrogen.” It’s suddenly over here where I can see it in the visual.

    That blue light has all gotten shifted to the red, and this galaxy that normally should have been much more blue is now appearing a pretty red color. Or beige, as it turns out most galaxies appear when you take pictures of them. And the color is the – when we sample different parts of the black body spectrum, it’s telling us the speed, the recessional speed, the Doppler shifting, that this galaxy is participating in.

    Fraser Cain: And we can even get a sense of that as we watch them rotate. We can tell how fast they’re rotating, again, the way that color changes.

    Dr. Pamela Gay: And what’s really cool, especially if you’re studying supermassive black holes, you have this disc-like galaxy – and in this case I have a non-spherical disc-like whiteboard galaxy that those of you watching live get to see me rotating. So if you imagine a small disc, a plate, a tiny desk whiteboard, when you hold it up so that you’re looking at its edge, when you rotate it – and this is why I like using a rectangle – I can see one side of the rectangle is moving towards my eyes, and the other side is moving away.

    The side that’s moving towards my eyes, it gets blue shifted. The side that’s away from me gets red shifted. The rate at which things are moving in an actual galaxy which isn’t held together as a plate, the center’s rotating way faster because it’s down close to that supermassive black hole. So when you look at the shifting color across it, you have slightly-blue shifted, more and more blue-shifted, then massively blue-shifted as you get down into the center.

    And then right next to that, massively red-shifted, then a little less red-shifted, a little less red-shifted. So we can get at how big is that supermassive black hole, just by looking at the amount of shifting that occurs deep down in the center of that galaxy.

    Fraser Cain: Yeah, it’s amazing how precise that is; this concept of looking at the color and averaging of the color, and looking at what you would expect, to what you see. It’s a very precise tool that astronomers use for some of their most deep understandings about what things are made of; all kinds of things.
    It’s a really impressive technique, and kind of made frivolous by Drum and – I’m trying to remember the people who actually came up with this. I don’t know if you remember when that color was sort of first proposed, but they actually made a mistake – they thought it was green, right?

    Dr. Pamela Gay: Well, they didn’t just think it was green; they thought it was this kind of like, toothpastey, turquoise-on-a-bad-day, kind of left-your-gum-to-dry-in-the-car, kind of color.

    Fraser Cain: Yeah; 9CFFCE if you’ve got your color palette in front of you, if you need it in hexadecimal.

    Dr. Pamela Gay: And what the problem was, they were simulating this for how it would look for the human eyeball, and they screwed up their white point. And any of you who are a photographer know, you have to adjust your white point; otherwise things appear to be a false color. So you hold up that piece of whatever that has the white, the black, the couple of intermediate shades of gray, and this allows you to color-correct your images. Well, you also have to color-correct when you’re simulating colors. And they started with a white that was too pink, and it just cascaded horrifically into a false color interpretation.

    Fraser Cain: That’s pretty hilarious. And I remember we actually ran that story, the original one, and they go, “Oh, the universe is green.” And then they were like, “No, wait! It’s latte colored.” Some great names here: Cappuccino Cosmico; Big Bang Buff; Cosmic Cream; Astronomer Almond; Skyvory, which I like; Univeige, which I like.

    Dr. Pamela Gay: I like Univeige.

    Fraser Cain: Univeige is good. Primordial Clam Chowder. Cosmic Khaki. So they put the name out to vote, and a bunch of members at the Johns Hopkins University came up with – astronomers voted for the different names. And in the end the latte one was the one that was selected.

    Dr. Pamela Gay: As someone powered by coffee – I have my grumpy coffee today – as someone powered by coffee, I deeply appreciate that we live in a Cosmic-Latte-colored universe.

    Fraser Cain: In a coffee-colored universe. Cool. Well, you know what; I actually have nothing else to say on this subject. I literally – unless you’ve got something super interesting, we may need to just wrap Astronomy Cast up at this point.

    Dr. Pamela Gay: I think that is completely fine. And let’s encourage everyone to go out and enjoy a cosmic beige night out under the stars, galaxies, and everything else. And hey, we’re still continuing to think about what we’re going to do for the eclipse; we are doing something in St. Louis. It is going to be a primary hotel where we have events during the day; we’re still securing the rooms as well as recommended rollover, hotels, and I’m going to find a camping ground. Because I heard you when you guys said that you wanted, some of you, to be able to camp.

    So we’re working, getting those details; I just wanted to let you all know we are in the process of beginning the planning; and you know, hotels don’t want to let you reserve things more than a year out. So we’ll have info as soon as we can.

    Fraser Cain: Big thanks to Bill Gercog from the WCSH crew, I think, who suggested this idea for this week’s topic. Just to show you that we do love to listen to your ideas for show ideas. There’s been some wonderful suggestions in the last couple of weeks, many of which we have stolen, and many of which we are gonna be rolling out soon. So if you have ideas for the show, for Astronomy Cast, and you have some topics – now, we’ve done 412 shows, so –

    Dr. Pamela Gay: So double-check first.

    Fraser Cain: So double-check, yeah. And the way to double-check – I do this all the time. I go to Google, I type in Astronomy Cast, and then I type in the idea for the show, and then I’m like, “Oh, no, we definitely covered galaxy clusters,” “Oh, no, we covered Rayleigh scattering.” But if you find something that we haven’t done, we would love to cover some of your topics.

    Dr. Pamela Gay: This show is made for you guys; thank you for helping.

    Fraser Cain: Thank you. Awesome. Alright, thanks Pamela; thanks everyone for listening this week, and we’ll see you all next week.

    Dr. Pamela Gay: Yay.

    Male Speaker: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at You can email us at Tweet us @AstronomyCast. Like us on Facebook, or circle us on Google+. We record our show live on Google+, every Monday at 12 p.m. Pacific, 3 p.m. Eastern, or 2000 Greenwich Mean Time.

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    [End of Audio]

    Duration: 25 minutes

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    One Response to Ep. 412: The Color of the Universe

    1. Bob May 4, 2016 at 11:01 am #

      So … the universe is ‘cosmic latte’. Good to know. I’m still confused about the color of our sun. This podcast, and Neil DeGraase Tyson, say it’s white. Yet on the HR diagram, it’s classified in the yellows (G), not the whites (F). What’s the really going on here?

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