Yes, our ability to see color correctly under unusual lighting, and incorrectly in certain cases, such as when other compartive colors are present, is quite surprising. Edwin Lamb is famous for his "Mandarin" color experiment.

However, the Sun is not illuminated by other light sources, nor is it adjacent to other colorful objects. In the blackness of space, if it is a yellow star, it will look yellow. A yellow light bulb will not look white if it "burns brightly in the night" and it is alone in the dark.

Our eyes seem to have evolved to use existing sunlight as the standard for pure white. This is not the same sunlight, however, as seen from space, which is different because of the color loses due to atmospheric extinctions. Nevertheless, the difference does not appear to be enough to alter the net color result if we could see the Sun from space with a properly reduced intensity.

Don't regress to the complacent mean; we need more volunteer heliochromolgists.

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Now we are talking about two different visual situations. In my previous post the Sun was the source of the ambient light that filled my peripheral vision. My experience was that my visual system had plenty of adaptive range to adjust to the blue tint of the car windows and see it as white.

If we look at the Sun through a neutral filter out in space from blacked-out surroundings, it will be like looking at defocused stars at night. Under these conditions our visual system seems to go into a default mode in which a G star is slightly yellow, an F more or less white, an A such as Vega "cold white", and a B such as Orion's belt stars slightly blue. This is my experience with a telescope near the zenith in the clearest sky conditions. In smoggy conditions during the summer they shift toward more of a yellowish tint.

The wild card here is the amount of atmospheric yellowing. My attempt at finding some typical ballpark figures for this has been a beastly job. I can find technical writing on the topic, but it is written for fellow professionals under the assumption that they already know all of the nomenclature, and it deals more with fine tuning of the calculations than in finding the initial rough figures, which presumably are already known. My best estimate is that the yellowing at the zenith is comparable to the difference between F and G stars.

If this is reasonably close, the Sun should look about the same out in space as an F star looks from sea level. I stand by my belief that it would look white.

Yes, this is important because what we can see here is easily testable. My work is to determine the color of the Sun as seen in space and at a normal photopic intensity level. I have not found any "true color" imaging attempt at this. Of course, someone is bound to eventually go up in space and, with the help of a strobe or neutral filter, simply look at the darn thing and resolve this color conundrum.

Hmmmm, that is interesting and I wonder why? Had you said G stars, as point sources, appear yellow to you and others, I would argue that the eye's fovea might cause a slight yellowing effect since there are no blue color cones in the fovea. Defocusing, however, should eliminate this possibility. [Surprisingly, the eye's constant movement counteracts this potential blue reduction, or yellowing, but I don't know how fully it handles stellar observations.]

When I looked at 18 Sco, the best known Solar twin, it appears white to me. Do you recall if your yellowish G stars were closer to the G9 class, or G2?

What is odd about your yellowing for the Sun [class of stars, G2] is the fact the solar projections from solar telescopes do not reveal any yellowing, unless some filter is used or, perhaps, smog has rolled in. Even the much lower temperature limb region (5000K) is still white. [On another realted thread, you may have seen the image I obtained from Kitt Peak's McMath-Pierce unfiltered scope.]

In an early attempt to determine the Sun's color, I glued a pencil to a paper plate, painted the thing black, and cut a ~ 0.5mm slot in it. [Appropriately, I coined it SAD (Solar Attenuation Device) and the operator a SAP (SAD Application Personell). No sense getting too serious about this colorful topic. It's more fun than hard science.] After spinning it using an electric drill, I was able to reduce the Sun's intensity by 99.9% and the Sun looked white to me, though it was still very bright, if not too bright. [For those who don't know know, once all the color cones are saturated, white is the color that would be seen, even if it wasn't a white object.]

Yes, I am almost sure there are extinction correction factors for various AM (Air Mass) levels, but I have never sought them. Since I have the spectral irradiance data for a few different air masses, I essentially have all I need. Some Solar data exists that is in increments of only a few thousandth of a nm!

Do you find yellowing regardless of telescope and mountain top, assuming good atmospheric conditions? I'm a little puzzled with this view. Perhaps a survey poll might help. [We did one on the Sun's color for midday quick glance views with a result of yellowish-white winning, though white was close.]

I'll bet you're right!

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I can certainly recheck my numbers and attempt to use a wider range of wavelengths. (Will have to wait until I get off work, and hopefully by then my internet access at home will be functional again.)

Well we're talking about an almost white light anyway, so the small variations in the brightness by wavelength make a difference as far as what hue we're talking about at such low saturation. Besides, you've shown that changing the selection of wavelengths causes noticeable changes in the final result.

I also suspect Hornblower is correct about F and G stars.

Naah, the blue-yellow process requires all three kinds of cones. Since blue cones are lacking the only color opponent process the fovea picks out is red-green. I've seen myself how a green object can look blue, a yellow object pinkish gray, or a purple object brown, and it's kind of a strange phenomenon to experience.

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Small changes can make a difference in color, but this is not so much true for objects that appear white. For example, the spectral irradiance of the Sun in the central zone at 6390K is different than that of the limb at 5000K, yet both regions look white, this is after atmospheric extinctions.

Yes, that was what I was saying, though I know only a little of how the eye works. Of course, since we are observing a relatively large disk in apparent size, the fovea limitation, if there is one, should not be a problem.

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My G example was Capella, which is a composite of G5 III and G0 III. It should be slightly yellower than a G2 V, and I could barely see a trace of yellow.

If my hunch about the default color vision mode at night is correct, we could be affected by yellowing of our eye lenses with age. I remember seeing an article about this effect in Sky and Telescope some years back, in which older observers reported different perceived colors of stars, but I cannot remember the details. I will search my back issues for it.

If it stays clear tonight I will do some spot checks to follow up on this discussion.

I wonder if the early lens coatings had a yellowing effect? I had not heard that our eye's lens could do this, but I wouldn't be shocked.

18 Sco is still pretty high early tonight. It is at the northern most edge of the constellation, 5.7 deg WNW from Han. 16 Sco is nearby and is a bluish A3 star. Both are about 5.4 mag.

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My spot checks tonight, under sparkling clear sky, looking at defocused bright stars in my Celestron 8:

Polaris, type F8 Ib, published color index +0.6
Slightly warm white

Altair, A7 V, +0.2
Neutral white

Vega, A0 V, 0.0
Cold white, perhaps hint of blue

18 Sco lost in trees, unobservable

Back to the eye lens topic. My father had cataract surgery on one eye at age 65, and he reported incredible color contrast. White objects looked blue in that eye and yellow in the other one. My mother reported the same thing after cataract surgery in her late 70's.

I will look at these tomorrow night with my Celestron 8.

I would have said green.

You have probably seen this before, but I like using it since it makes an obvious statement. It is of the Southern Cross.




Courtesty of Stefan Seip .

They hot ones look white until out of focus. Even the red ones look redish white. [Acrux, B1, is the lower right, and Gacrux, M4, is the top star.]

I don't know if color enhancement was used, admittedly.

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Last night's results using the defocusing technique:

Polaris appeared white, but at certain out-of-focus apparent surface brightnesses, it looked either a little, very little, greenish-yellow (dirty yellow, perhaps) white or, at times, greenish-blue white.

Altair appeared as bluish white, but the blue was very faint and was not noticeable at most of the defocus settings. The air mass value was AM1.2.

I took an image of Altair using a Canon 30D set for 5800K. Then I super saturated it to see what colors might be there.



Perhaps the atmospheric extinctions of more of the blues might help explain this odd blue-green result.

Vega revealed a distinctive, though faint, bluish-white color at AM1.1.

The following image of Vega is unenhanced:


This is not the result I should have gotten, right? I don't even have blue eyes. I don't know what is wrong and I am reluctant to blame the Canon 30D.

You seemed to see a hint of blue, too. Perhaps we should get others to help in this color conundrum by starting with Vega. We should come up with a snappy term for a "color pest", or maybe asterochromologist will do the job more respectfully.

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Visually you and I seem to be pretty consistent, with your eyes seeing slightly more blue than mine.

The saturation-boosted image of Altair suggests that the camera's original signal had a slight excess of green, and for all we know it might have been by design. If I were the manufacturer, my inclination would be to tweak its spectral response to get good overall tints on landscapes, cityscapes, human faces, etc. in typical daylight conditions. Possible use of the camera as an astronomical photometric tool would be way down on my list of priorities. An image on the screen that looks like a direct view of an incandescent blackbody does not actually have a continuous spectrum like the original. It consists of three color bands with some blind spots between them, something I can see easily by looking at a white line on the screen through a prism. If a colored object had some critical components in the blind spots, it might be slightly discolored when a white object is perfect. Tweaking for the colored object, if it is considered more important, would throw the white off slightly, and it could become easily visible if exaggerated by artificially boosting the saturation.

Your image of Vega is no surprise to me. I would expect it to have enough more blue to overwhelm any systematic green bias there might be.

Yes, this was especially true for films; Fuji film favored green and Kodak favored red, IIRC.

Yes, it is hard to get objects to maintain a nice 5800K temperature. Lamps have a CRI rating where the best rating matches the Sun's illumination.

Yes. The boosting software could be a problem, too. I am not sure how accurate the camera's software is at color control, but they do a remarkable job today in simulating what we see.

Regardless, I too am doubtful of the green result for Altair.

I have started a poll thread in the astrophotography forum hoping some will actually go out and observe it and let us know the color result they see.

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What does this have to do with the topic of this thread?

That's just spam. I hope the mods delete it. I think there's a filter on this board so people with 1 post can't post urls. I guess this is someone's attempt around this.

In Astrobiology class we discussed why the leaves of plants might be green, the same as the sun's peak output. I think the conclusion was that by reflecting the sun's peak energy, it prevents the leaves from overheating. In hindsight, this makes me wonder why we don't get different color leaves in cold climates.

I forget if this was a serious theory, or just off the top of our heads babbling (a lot of that happened in this class).

There was also something called the "red edge" and how our plants look red in IR photos. I forget if it had anything to do with the sun's output. I'm trying to Google for it now, but with no luck. It's very interesting stuff.

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I recall an article from earlier this year addressing this issue, but I can't find it. I think their conclussion was different.

Regardless, if they say the Sun peaks in green, they are wrong. [However, our peak responsiveness of our eye is green.] There essentially is no peak in the solar flux as shown in an earlier post. The photon flux peak -- since one does exist though it should be called a pimple, IMO -- is actually in the yellow [*gag*] portion of the specturm as seen from space. Terrestrially, the light flux pimple will rise in the orange region, near yellow.

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