What color is the sun?

[Yeticus_Rex]

AKA…Rayleigh scattering and Mie scattering.

[Alan_Smithee]

@Alan Smithee

I think I can explain it.

If it is noon in San Francisco, it’s 8 pm in London. In London, the sun appears to be setting on the horizon. Because the light reaching London passes through the atmosphere at an oblique angle, the shorter rays scatter, making the sky in London appear red and orange. The blue rays spread out in the atmosphere, making the sky appear blue in Chicago. The light hitting Chicago directly doesn’t scatter; it only travels a short distance through the atmosphere, so we see the sun in nearly it’s true color. So the color of the sky at noon in Chicago doesn’t come from the light hitting the atmosphere above Chicago. It comes from the light hitting the atmosphere over London and being scattered.

Oops! I didn’t mean to switch from San Francisco to Chicago. Not that it matters, the sun setting in London makes the sky blue at noon in San Francisco and at 2 pm in Chicago.

[Blake]

@Keeve

Shouldn’t I expect the white objects to be just as colored as the sun itself?

They are, the trouble is that your brain overexaggerates the pinkness of the sun and underexaggerates the pinkness of a peice of paper. Once again try taking a picture of the sun and a peice of paper. Then zoom in using photo shop and actually analyse the colours. The differences will be minor.

I suspect that at least part of the problem is that you are looking at the sky/clouds around the sun and using that as your reference for the colour of the light sources, wheras the actual sun itslef is far less pink than that.

Suppose we go to a planet that has a red sun and look around. What colors would we see? I would expect to see nothing blue or green or yellow.

Nope. Red stars still produce light in all wavelengths, the only difference is that they produce less light in the higher wavelengths. As a result a green object will appear grey or a dull grey-green, while blue and yellow objects will appear blue and yellow, just slightly dull. You can see this without going to another planet, just strap a piece of red cellophane over your eyes and you will get exactly the same effect. Cellophane also filters out most of the non-red light, in fact it produces a more pure red spectrum than you woudl get under a red star.

Without artificial lighting, where would other wavelengths come from?

From the sun itself. The percieved colour is the result of averge temperatures. That doesn’t mean that none of the electrons in the star are ever excited enough to produce violet or even x-ray light, it just means that it’s less common than in a white star. Individual electrons on Earth are sufficently excited to produce x-ray and violet light and this isn’t a star at all. IOW even something as cold as a planet emits across the entire EM spectrum.

[mr.jp]

@Alan Smithee

I think I can explain it.

If it is noon in San Francisco, it’s 8 pm in London. In London, the sun appears to be setting on the horizon. Because the light reaching London passes through the atmosphere at an oblique angle, the shorter rays scatter, making the sky in London appear red and orange. The blue rays spread out in the atmosphere, making the sky appear blue in Chicago. The light hitting Chicago directly doesn’t scatter; it only travels a short distance through the atmosphere, so we see the sun in nearly it’s true color. So the color of the sky at noon in Chicago doesn’t come from the light hitting the atmosphere above Chicago. It comes from the light hitting the atmosphere over London and being scattered.

So the blue light wave comes in over London at an oblique angle. It then hits something, and is bounced all the way to chicago, where it proceeds to color the sky? And only short wavelengths do this?

[Keeve]

@Blake

Red stars still produce light in all wavelengths, the only difference is that they produce less light in the higher wavelengths. As a result a green object will appear grey or a dull grey-green, while blue and yellow objects will appear blue and yellow, just slightly dull. You can see this without going to another planet, just strap a piece of red cellophane over your eyes and you will get exactly the same effect. Cellophane also filters out most of the non-red light, in fact it produces a more pure red spectrum than you woudl get under a red star.

Yep, I’ve tried the cellophane bit, and that’s part of where my whole question is coming from.

I suspect that when you wrote “exactly the same effect”, what you really meant was “a similar effect”, because the red star produces all wavelengths, and the cellophane is a more pure red.

I had not realized that the stars produce such a wide variety of wavelengths, despite their apparent color. I must give a great big thanks to Strinka for posting that link to the illusions. I had seen it before, but had forgotten how powerful those illusions are. I am blown away by how easily my color perception was decieved.

[Alan_Smithee]

@mr. jp

So the blue light wave comes in over London at an oblique angle. It then hits something, and is bounced all the way to chicago, where it proceeds to color the sky? And only short wavelengths do this?

According to scotandrsn’s post, yes. The “something” it bounces off of is the atmosphere. Yeticus Rex’s links have the technical details.

[Chronos]

Except that the blue ray doesn’t get all the way to London before it bounces. It bounces somewhere way west, before it gets anywhere near London, and give the blue to the sky in that way west location.

[Alan_Smithee]

That’s true, but it also causes the blue in the sky in other parts of the world than the area west of London where it scatters, right? Otherwise, the sky would not be blue at noon, since the light coming from directly overhead isn’t being scattered.

[scotandrsn]

@mr. jp

So the blue light wave comes in over London at an oblique angle. It then hits something, and is bounced all the way to chicago, where it proceeds to color the sky? And only short wavelengths do this?

The blue is actually scattered at all angles, but, as mentioned before, at angles closer to perpendicularity with the earth’s surface, there’s less atmosphere to pass through than at angles closer to the tangential, so a goodly amount of the scattered light comes from the light initially headed in those directions.

At more extreme angles, according to some info in the links, longer wavelengths such as green are scattered out as well, leaving the setting sun redder.

The fact that the blue light is scattered out leaves light in the green-to-red end of the spectrum at high angles though, making the sun look perhaps a bit yellower than it is intrinsically, but it is in fact a yellow-white star.