Why is the sun yellow?

This has been bugging me all day, and Googling for the answer only turned up two pages by astronomers who wish they knew the answer, so I figured I need to ask someone.

The sun is yellow. But sunlight is white.* Why?

A yellow lightbulb (or a white lightbulb behind a yellow filter, or whatever) gives off yellow light, while a white lightbulb gives off white light. The white moon gives off white light. And so on. I can’t think of any counterexamples, except for the sun.

I’m guessing there’s some obvious reason, and I’ll either figure it out in my sleep tonight or whack myself in the head when I read it here tomorrow… but right now, the best I can do is two theories, neither of which seems that plausible.

Maybe it’s because we see the sun in contrast to the sky, which is blue, so it seems to be the opposite, which is yellow. But that seems wrong–that would imply that against a red sunset sky we’d see a cyan sun, when actually we see a red sun. (Unless there’s some other factor there that massively outweighs the contrast or something?)

Or maybe our eyes register yellow when they’re overloaded with too much light? But that doesn’t seem to be true in other cases. (It’s a bit hard to tell what color you’re seeing when you’re being blinded, but the negative afterimages definitely aren’t blue.)

After that, I’m out of theories, at least for tonight.

Thanks.

  • OK, sunlight isn’t pure white as in “an even mix of every wavelength in the spectrum”, but as in “a mix that our eyes record as white” (probably for evolutionary reasons?). But it’s human psycho-optics that matter here, not pure optics. And in terms of pure optics, the Sun is more blue than anything else, peaking at 4800Å or so.

While I’m here, a related question:

The sun’s surface is around 5700K. (Actually, 5 different search results gave 5 different values, from 5500 to 5820. 5700 is a nice round number close to the average of those 5.) By Wien’s Law, it should be peaking at 2.9e-3/5700 = 5088Å. To get as blue as 4800Å it ought to be 6042K.

I can think of all kinds of possible answers: the sun obviously isn’t a perfect blackbody; maybe redder light gets absorbed or deflected more by interstellar dust or the outer reaches of our atmosphere or something; etc. But this seems like the kind of thing that any astrophysicist would just know off the top of his head, so there’s no need for me to guess.

Refraction. Ties with the classic of “why is the sky blue”.
As the light goes through the atmosphere it scatters. The shorter wavelengths scatter more. Blue is among the shorter of the visible light range so scatters across the sky leaving the sun making the direct path more white minus blue which seems more yellowish. As it goes through more atmosphere less short wavelengths scatter some too leaving it more yellow to orange to res as it “sets”.

What you describe is the correct phenomenon, but not the correct name. See Rayleigh scattering.

The sun isn’t really yellow except in children’s doodles. It looks white to me.

Isn’t yellow sun just a convention?

When I look at the sun - something I really try to avoid, I have a lot of trouble telling what colour it is. Its frickin bright! It completely overloads my visual abilities - I strongly suspect it is the same for everyone.

Yes I get a hint of ‘yellow’ I also get a hint of ‘blue’, I also get after images that may be purple or magenta. I see a searingly bright object that sort of flickers between bluish and yellowish white and hurts to look at.

These colours are not crayon standard, they are not like a painted surface. They are not stable, I can not get a good look at the sun (and neither can you). They are what you see when you look at something that overloads your visual system.

When I see the sun through thin (and to me , colourless) cloud, it is unquestionably white.

The only time you can actually look at it is when it is filtered by a lot of atmosphere and red on the horizon. I have seen it go from red through the spectrum to green (the green flash) as it sets. That is an awesome sight.

FluffyBob has it: The Sun is white. I think the only reason we think of it as yellow is by contrast with the blue sky.

The simplest answer is just to look at photos of the sun. It is always white.

And perhaps also by contrast with artificial light, which tends to be more blueish than sunlight.

Okay, first off, Rayleigh Scattering is a real effect. You pass any sort of light through the Earth’s atmosphere, and it’s going to preferentially scatter away more of the bluer light and less of the redder light. That’s why everything (Sun, Moon, stars, etc.) appears redder on the horizon and bluer at zenith.

This time-lapse photo of the moonrise illustrates this nicely: http://scienceblogs.com/startswithabang/upload/2009/09/red_sky_at_night_but_why/seattlemoon_stephens_strip2.jpg

But there is, as you correctly assume, also a component that has to do with human eyesight and the temperature of the star. Looking at our Sun, it produces lots of light of many different wavelengths, but its intensity peaks in the yellow part of the spectrum. No surprise that we evolved to catch the light where its intensity is greatest.

But there are two things further that go on here: one is that your eyes are better at seeing discerning mid-range and long wavelengths (greens, yellows, oranges and reds) than they are at short wavelengths (blues and violets). Want proof? Look at a rainbow; you’re way better at seeing the different shades of red than you are at, say, finding “indigo.”

The other, of course, is temperature. The Chinese, used to use the star Sirius – a notoriously “blue” star – as the standard for what “white” is. The colour of Sirius as recorded in ancient Chinese texts - NASA/ADS

If the Sun were just a few hundred degrees hotter, it would appear much, much whiter to your eyes than it does right now. But the atmosphere, as others have attested, does play a role. Want to prove it? Take a look at this picture of the Earth and Sun from space.

The Sun’s reflection off of the Earth’s oceans looks quite yellow, having passed through the atmosphere twice, but the sunlight itself – with no atmospheric distortion – isn’t quite perfectly white, either.

Of course, other animals have eyesight biased towards other wavelengths: bees can see ultraviolet light that humans can’t, while owls can see the infrared light that we can’t. If they saw colors the same way we do, the Sun would likely appear quite red/orange to a bee and white/blue to an owl.

So – and sorry for how long this post has been – the Sun gets yellowed (slightly) due to the atmosphere, but intrinsically, also has (slightly) more yellow-to-red light than violet-to-green light.

I agree.

Am I being whooshed here? Photos of the sun show it as anything from white to deep red, depending on where in the sky it is and how clear the atmosphere is.

Sorry. What I read from the OP was that he was asking why the sun the sun always seems yellow - not white. Following from that, I should have added that sun is white in photographs when high in the sky.

Thanks for all the answers! But now I’ve got too many answers, instead of too few:

  1. The sun isn’t yellow; it’s just a convention. (ChessicSense, FluffyBob, NineToTheSky)

  2. The sun is no more yellow that sunlight, but contrast with the blue sky (and maybe with artificial light) makes it look yellow. (Me, Chronos, Quercus, rayman5321)

  3. The sun is yellow because that’s what’s left when the blue light is scattered out. (DSeid, Machine Elf)

Let me go through these one by one.

First, if it’s a convention, it seems to be universal. It’s not just modern American kids who draw the sun yellow. Professional artists in every culture throughout time have drawn it that way. Cultures ranging from the Mongols to the Babylonians to the Romans to the Aztecs used a yellow sun as a religious symbol. Many cultures associated the sun with gold and the moon with silver (which I’m guessing is because they’re shiny yellow and white things, respectively). Also, everyone who gave physical answers on this page clearly believes it’s yellow.

I still like the contrast idea.* It can’t be the whole answer, or even the most important effect (or, again, the sun would be cyan at sunset instead of red), but is it possible that it contributes?

As for scattering, I think that’s the obvious answer I was looking for.

I immediately dismissed scattering as the answer, because it should also make ambient sunlight yellow too, because some of the blue light is scattered completely out of the scene.

But I was being stupid. Color isn’t an all-or-nothing thing.

The atmosphere should mean that ambient light is a bit yellowed, but less so than the direct image. Our eyes and brains evolved to define white as ambient sunlight, but the more-yellow direct image still looks yellow. Duh.

So, if you look from, say, the moon, the direct image should be exactly the same color as the ambient light, which should be slightly bluish Is that what astronauts see?

Anyway, again, thanks for all the replies.

  • As for Quercus’s addition of contrast with artificial light, that wouldn’t explain why the Aztecs, Mongols, Babylonians, Romans, etc. considered the sun yellow, when the only artificial light they had was orange (wood fires, candles, etc.), which should have had the opposite effect. Also, shouldn’t that affect ambient sunlight more than the direct image of the sun, not less?

I think there is some sensory judgement involved in deciding if it’s more white or more yellow, and when we do see a more colorful sun, it is always in the direction of yellow to redder. That is, there are no circumstances when you see a brilliantly blue or purple or green sun.

Preferential scattering of blue light to leave yellow light is a valid point. I think this is the mechanism by which the more colorful suns are more yellow or red.

There isn’t a single clear tidy definition of what white is.

Or, perhaps, saying sunlight is by definition white has some justification. Our eyes and sensation of color evolved around sunlight. We mostly use color to make judgement about things that are reflecting various parts of sunlight, or at least in evolutionary history that’s what we used it for.

What about astronomical images like this, where the sun photographed from space always looks yellow or orange? Actually, I think that image is actually ultraviolet or x-ray, so I suppose the colors aren’t real. Are they made to look like that just because of convention?

ethansiegel, thanks for replying to my second question–but I think you were actually answering the first one instead.

You gave lots of good info on the first question (the point about the double-reflected sun is what suddenly made it click in my head), but nothing for the second one.

This is probably my fault for asking two questions in the same thread, and being so long-winded about it. (Also, maybe I should have used nm instead of Å–if someone isn’t seeing Unicode properly they’d think I was giving temperatures instead of wavelengths.)

And I’m guessing you actually know the answer to my second question, too. So, let me try again.

The sun’s surface is somewhere around 5700K. It should peak at 508.9nm, which is yellowish. But it actually peaks at 480nm, which is bluish-green. Why?

Every page I found that gives the sun’s peak says it’s 480nm, and every graph (e.g., http://zebu.uoregon.edu/~imamura/122/images/solar_spectrum.png) agrees. Every page that I found gives a different temperature for the surface of the sun, but none of them are nearly hot enough to give a 480nm peak.

Sunlight - Wikipedia says “The spectrum of the Sun’s solar radiation is close to that of a black body with a temperature of about 5,800 K”–but it then gives a copy of that same image from above, which shows it peaking like a body well over 6000K.

The closest thing I found to an explanation is at Phil Plait's Bad Astronomy: Misconceptions says “The Sun, for example, should peak at about 5000 Angstroms or so, having a surface temperature of 5500 K. However, due to complicated processes, it actually peaks bluer than that, around 4800 Angstroms.” So, what are those “complicated processes”?

I’m assuming that 480nm is measured from space (or compensates for scattering); otherwise, the problem is even worse.

I just thought of a quicker better way to explain the scattering answer:

Ambient sunlight = direct sunlight + scattered sunlight.

Therefore, since scattered sunlight is blue, direct sunlight must be yellower than ambient sunlight.

I cheated a bit by not considering reflected sunlight. But generally, the ground is darker than the sky or the sun (and the exceptions are mostly when there’s a lot of blue ocean, white snow, or shiny metal around), so I think I can get away with it.

Good question. The sun is a yellow dwarf (G class), but G-type main-sequence star - Wikipedia says, “The term yellow dwarf is a misnomer, as G stars actually range in color from white, for more luminous types like the Sun, to only slightly yellow for the less luminous GV stars.”*

So, photos of the sun from space should be white. The orange false coloring must be by convention.

If it makes you feel any better, http://www.thesurfaceofthesun.com/images/mossyohkoh.jpg is a photo that’s false-colored in blue. But I think this might be a crank site, and even if it’s not, that photo is showing hot yellow contrasting with cool blue, which seems backward to me.

  • By the way, that same page also says, “Our own Sun is in fact white, but appears yellow through the Earth’s atmosphere due to Rayleigh scattering.” I wish this had been on any of the pages I’d actually looked at for the answer…

I’ve seen many different colors used for false-color images of the Sun by genuine non-crank solar physicists, including red, green, blue, and grayscale. I think that there are conventions for which false colors correspond to which actual wavelength bands, but I’m not sure exactly what they all are. Plus, of course, some are false-colored in red and blue for purposes of making 3D images.