So my husband and I were talking about Superman tonight and the various effects that the solar energy of a yellow sun vs. a red sun would have on him. It made me wonder, what color is the sun, really?
I’ve always thought of “light” as being white, including sunlight. We are taught in school about white light dispersing through a prism into the various colors of the rainbow. I did a little intertube research and it sounds like the color of our sun is indeed closer to white than yellow, but it sounded like the temperature of the star itself is what determines the color of the light emitted. I wonder, what would have to happen for the sun to actually turn red?
The sun’s light passing through the atmosphere tends to make it a wee bit “yellow”. White minus the blue that you see in clear blue sky is what makes it yellow. All that blue light in the sky is scattered sunlight. If you would like to know what the sun looks like, find a lonely white puffy cloud on clear blue sky day. THAT is the true color of the sun.
And yeah, the definition of white is somewhat arbitrary. And the “color” of a star is generally determined by its temperature. Hotter equals “whiter”.
The really hot ones tend to look blueish: their spectrum has peak energy at wavelengths shorter than are visible; within the visible spectrum they radiate more energy toward the blue and violet end.
Here’s a link to a site that discusses star color and temperature.
Yes, there is a peak wavelength in a star’s blackbody radiation curve. In fact, for our Sun its in the green. But that’s not exactly the same as saying that the sun is green.
For a star’s light to be visibly other than white, you have to look at the extreme conditions in which the peak of that curve is outside the visible: hot, high-mass stars whose peak is in the ultraviolet, which may have a blue or violet tinge, or cool-low-mass stars that may have an orange or reddish tinge, or stars in the late stages of stellar evolution, whose atmospheres are vastly swollen and cooled.
Of course, here we’re only talking about the radiation actually emitted by the star. Another thing affecting a star’s colour is the composition of its atmosphere and its immediate surroundings. Some atmospheric components absorb light at particular wavelengths, so that if these components are present in enough quantity, they affect the colour of the star. There is a set of what are called “carbon stars” whose atmospheric carbon absorbs so much blue light that they are very red. Surrounding gas and dust also tends to absorb the shortr wavelengths and redden the star’s visible light.
When you ask what the color of something is, it should also be specified whether or not you want to take the human eye into account. This is because the human eye is more sensitive to some wavelengths than others.
Nonsense. Without taking the human eye into account, things do not have colors at all, they just have electromagnetic radiation spectra that they emit or reflect. If someone asks about they color they are already implying that the human eye should be taken into account.
Close, but not quite right.
Even with the blue scattered out of it, sunlight is still overwhelmingly white. If it weren’t (as Phil Plaitt, The Bad Astronomer, pointed out) clouds, which derive their color from scattered sunlight, would be yellow. Sunlight is our very definition of “white” light, so it can’t be yellow. Plaitt acknowledges that he can’t say for certain why people think of the sun as yel;low.
I wrote a piece about this for Optics and Photonics News, and made the following suggestion – as the sun goes down in the evening, it passes through longer and longer portions of atmosphere, in which the light scattered out by the atmosphere undoubtedly does cause the sun to become colored – the familiar orange or red sun at sunset being the ultimate result. You can perform a calculation based on the scattering coefficients to determine what the apparent color of the sun is as it passes through thicker and thicker layers of atmosphere. I did so, and the trajectory of the locus of the sun’s color on the CIE Chromaticity diagram is a straight shot from “white” at the center of the locus toward “yellow” on the Spectral ,locus, until the atmosphere starts to get really thick, at which point it veers away toward the orange and then the red end of the scale.
In short, the sun is white at its zenith – and too bright to look at for long. As it descends, it gets slightly less bright and easier to look at, at least for brief periods. As it does so it starts to get tinged with color – the yellow that billfish notes is the result of scattering out the blue of the sky. This coloration gets more noticeable the lower the sun gets, but doesn’t really become significant untuil the sun is pretty low. It then quickly runs the gamut from yellow to orange to red as it sets. So the sun is, essentially white most of the time it’s in the sky. If you try to look at it, it’s too bright, but gets easier to look at near the bottom of its path, by which time its color is (as it has been tending all the time) to yellow. This, I think, is why the sun is universally seen as “yellow”, and depicted that way.
The best definition of white light would probably be something along the lines of radiation with equal intensity at all visible wavelengths. The sun does not exhibit that - independent of atmospheric absorption, its radiation is somewhat stronger at the wavelengths we call orange and yellow than at either the red or blue ends of our visible spectrum.
Nonetheless, I think your explanation of why people think of the sun as yellow is persuasive.
What part of “wee bit yellow” in my post did you not understand?
And you got the bolded part wrong. Clouds derive their color from the unscattered sunlight PLUS all that blue light from all over the sky that WAS scattered. They are generally MORE white (or more accurately more true to the original sun color) than the filtered sunlight you are seeing. They would have to be. They get back most?/some of the scattered light that turned it yellow in the first place.
Yeah yeah yeah, absorption, ground reflections, imperfect reflectors, lasing and a gazillion other things…
No, that’s not correct – “white” light is not made of equal portions of all wavelengths. see any text.
“Wee bit” is correct, but that’s clearly by itself not what makes the sun appear yellow – the amount scattered out at zenith is negligible. Some other effect is clearly responsible for the universal perception of a yellow sun.
No, I don’t – the bulk, by far, of the light from the clouds is direct multiple scatter from the sun. The extra scatter from light already scattered by the air molecules (that make the sky blue) is far, far weaker.
Probably because as someone mentioned there are stars that are hotter and therefore “whiter”/bluer than our old sun.
So by comparison, our sun is “yellower” than those stars and yet at the same time sunlight (in combination with our eye’s response) is one crude definition of white.
Unless you want to argue about the magnitudes of the effect this doesnt make sense (there is that wee bit thing again)
I have a taco with cilantro in it. Somebody takes out the cilantro. Somebody asks what a taco with cilantro taste like. So I put back in **some **cilantro. Does it taste exactly like the original taco? No. Does it taste more like the original taco than the cilantro free taco? Most very likey.
Clouds that are exposed to direct sunlight AND the blue light scattered are a better representation of the sun’s spectral look than the direct light thats had some fraction scattered away (ignoring the fact they might possibly be really crappy reflectors). If you wanna argue that the effect is so small as to be unobservable, fine.
What color is the sun? In order of betterness IMO.
Look directly at the sun (dont try this at home kids).
Look at a small, isolated, poofy cloud on a clear blue day.
Go into a good optics lab.
Catch a ride on the Space Shuttle.
And even IF there is no visually detectable difference in color between the sun as seen from the ground and that poofy cloud, the cloud still has the advantage that the intensity is low enough that you can actually stare at it and “study” its color in comfort.
Part of it is that pictorial representations of the sun, by ancient convention, use yellow. If you’ve seen a thousand pictures representing the sun as yellow, you will tend to associate sunlight with yellowness, no matter what visual evidence may be placed before you.
Now, that convention has its own origins. I can only speculate that it arose because yellow both connotes brightness and shows up nicely against both white and blue backgrounds. (And into this might play the fact that a blue background, for reasons of perceptual psychology, tends to give an impression of yellow cast to a white object placed against it.)
No text handy, so I did some Googling. There are lots of imprecise definitions such as “light composed of all visible wavelengths” without any mention of relative intensity.
But I did find such things as “contains all wavelengths in the visible spectrum at nearly equal intensities” and “light that stimulates all three types of cone cells in the human eye in equal amounts.”
Sunlight obviously qualifies under these definitions, but does not represent the archetype.
billfish, the amount of blue scattered out of the sun by passage through atmosphere anywhere except close to the horizon is pretty low – so low that you can’t call it “yellow”. Plaitt talks about this in his first book, which is why he refuses to say that he knows why the sun is yellow – it’s certainly not because of the small amount knocked off by rayleigh scatter.
Sunlight is pretty much what determines what we call “white”, and the paradox of the yellow sun is that it seems to be yellow, while light from it does not appear to be. Even if Rayleigh scatter from zenith light made it noticeably more yellow, we’d still have the same paradox – because we calibrate “white” pretty much by sunblight, so the sun can’t be yellow.
It you want to argue that the sun is yellow but clouds are white because Rayleigh scattering by the blue sky adds the blue back in, that woin’t wash., A white sheet of paper illuminated by sunlight let in through a window appears white, not yellow. So do white sheets, and anything else white. This is true also if you close off the windows with cardboard so that you’re mnot letting in light from the sky.
Similarly, clouds seen against farther away dark cloudds in the absence of blue sky also appear white.
In other words, Rayleigh scatter alone won’t explain why we imagine the sun to be yellow. The explanation I give reconciles the perceived yellow color of the sun with the fact of white sunlight.
as for the spectral composition of “white” light – see any text on colorimetry. The CIE Standard Illuminants B and C are based on Noon Sunlight and Average Sunlight, and have spectra determined by the output of the sun modified by scatter and absorption in the atmosphere. It’s not flat. See Warren Smith’s Modern Optical Engineering*.
Illuminant A is light from a tungsten filament – it’s not uniform either, but has a distimnctly yellow color. Light from a “white” LED is noticeably bluish. There are other, more recent standard illuminants, but none of them is flat, with equal contributions from all wavelengths, except for E – which is there purely as a theoretical concept. The reason is that such flat emission doesn’t exist in nature – or in the lab, unless you work at it by throwing away useful light. The Standard Illuminants correspond to real sources people use
I think this is the biggest factor. We usually see the Sun against a blue background, so it looks yellowish by comparison. And then, of course, this gets reinforced by art: The Sun looks a little yellowish, so a painter choosing from a small palette chooses yellow to represent it, and then it’s a lot easier to look at that painting than to look directly at the Sun, so folks assume the painting is right.
I think the art reinforcement isn’t a big part – kids seem to rediscover this all by themselves. As for the “contrast with the sky” thing, it’s hard to look at the sun directly when it’s up and bright – it’s not like those conceptual things where you compare two colors comfortably against the same background. The “yellow as it gets lower due to increased scatter from thickening atmosphere” makes more sense to me.
Actually, it probably does. Our eyes evolved as organs for processing the reflected light of the sun, so sunlight itself is the neutral archetype they’re operating in reference to.