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    The Color Of The Sun, Part II
    By George Cooper | July 3rd 2008 08:09 AM | 22 comments | Print | E-mail | Track Comments
    About George

    Amateur astronomer, B.S. M.E., self-acclaimed heliochromologist....

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    We learned from the prior blog that the Sun is much too bright for normal viewing with our sensitive eyes. A white “color” results when we observe objects that are both extremely bright and bright at all or most visible wavelengths. Please allow me to elaborate a little more on this issue.


    [Hang in there.]

    This white result can be quite deceptive since its true color could be any color once we attenuate the object’s intensity to a light flux conducive to our normal eyesight. The reason for this blinding effect is due to the behavior of our color receptors (color cones) when they are subjected to excessive light. We have about 6 or 7 million of these cones in each retina. Our cones come in three types – surprisingly, there are four types of cones for some birds – and they are often designated as our red, green, and blue color cones due to their spectral sensitivity to the respective wavelengths of light. Our trichromatic eyesight gets quite complicated since there is much overlap of their sensitivity with one another. Fortunately, the brain handles it amazingly well.

    [BTW, many animals are dichromatic. Whitetailed deer, for instance, have just two color cones: essentially, blue and yellow. It seems likely they are capable of seeing into the UV band. Regardless, these dears can probably see a somewhat bright sky shining upon hunters as hunters stumble through their darkness.]

    If all these color cones become saturated due to excessive amounts of light shining upon them, then the brain will produce a white sensation for the bright object being observed. This excessive flux eliminates any chance for the brain to determine the true color of the object; the objects true color is hidden behind the blaze.

    This blinding white result is what happens when we observe the Sun with the naked eye when the Sun is directly overhead, or, worse, when we look at it from space without the safety visor pulled-down from atop our space helmet. Once the Sun nears the horizon, where direct sunlight must pass through much more atmosphere to reach us, the Sun's intensity becomes more comfortable to observe. But when it is high in the sky, it is dangerously bright to observe.

    This same problem of excessive light flux also applies to electronic light sensors. The Hubble Space Telescope, for instance, is programmed to avoid looking in any region within about 50 degrees of the Sun, else damage can occur to its very sensitive instruments. The HST also avoids pointing near either the Earth or the Moon for the same reason, though it can point closer to these dimmer objects. [Actually, the HST is capable of imaging the Moon, but care must be taken.]

    With the exception of the cooler stars, black holes, and neutron stars, all stars emit light at all wavelengths of the visible spectrum. Their output is known as a Planck or blackbody distribution. They are not perfect blackbody radiators, but they are close. For this reason, our color cones will be activated in proportion to the relative spectral energy distribution (SED) of the light emitted from the star. The hotter stars will excite our blue color cones proportionately more so and will cause us to see these stars as blue-white. Red stars are much stronger in red than any other wavelength, thus we will see these as red or orange stars. This assumes, once again, that we are not blinded due to excess intensity from these colorful stars.

    Whew, that was a lot of dry stuff, and heliochromology is supposed to be fun. Ok, gander at this:

    Matt

    That is the Sun's spectrum upon my son's face. [Thanks Matt!!] All the colors are vividly present. Notice, however, how very little yellow light is apparent in this spectrum created by using a narrow slit and an F2 prism.

    So, what about the Sun’s true color? Well, let’s now look at the hard data: spectral irradiance:

    Sp Ir 

     

     

    This data comes from just one of hundreds of data sets taken of the Sun from space, AM0. The background color is added to show a reasonably accurate association of color for each wavelength. Notice that the peak energy (top curve) of the Sun is found at a wavelength of around 450 nm (Wiehrli ’85 data set is 450.5nm and Thuilier 2002 is 456.6nm). This peak is in the blue portion of the spectrum. Does this surprise you? Did you think it was yellow? But, this doesn’t mean the Sun is a blue star, however. Unfortunately, the blue peak and the narrow band of yellow are only pieces of the puzzle. There are other color phenomena that must be considered.

    Metamers also can contribute to a color determination and must be mentioned. We can produce one color (hue) by simply using two or three different colors. That’s how color is produced on tv’s and monitors. You know that, right? [Some green birds are not actually green but are blue, due to scattering effects, combined with yellow, which causes us to see green.] So, if we combine green light with orange and red, we might get yellow color result.

    Does this color combination give us a yellow Sun? Well, that’s a tough question, but rather than tackle it head-on you’ll be happy to learn we can dodge it instead, thankfully. Take a look at the second, lower line in the graph: the “Photon flux”.

    Our eyes, as well as, electronic sensors, are better represented as seeing photons rather than wavelengths of light. Since E = h f (photon energy = Planck’s constant times the frequency of the photon), then we can convert our spectral irradiance to a photon flux distribution. Notice how flat the distribution is. It is surprisingly flat. By sheer luck or madness, the peak photon flux just happens to be in the narrow yellow portion of the spectrum. I have seen several scientists claim the Sun is yellow because the peak wavelength of the Sun is in the yellow. However, there is no real peak; there is only a pimple, and a small one at that. One tiny pimple does not a complexion make, or destroy.

    What color do you think you would get if you have such a flat photon flux distribution? Isn’t white the color we are suppose to see? Well……you’d think so, but if it is, it isn’t a definition I’ve been able to find, at least not one that is backed-up with objective evidence. Yet, admittedly, it does seem that a white appearance would be a logical result.

    One problem with this idea comes from the sensitivity of our color cones. Will a flat photon flux distribution of light produce a "flat" signal to our brain? Nope, the combined spectral sensitivity of our color cones allows us to see green much more easily than any other color. The color we finally “see” in our mind is determined by a product of the spectral irradiance of the object as it enters our eye and the spectral sensitivity of our eyes (ignoring reflection properties if we are observing an illuminated object). [Many emergency vehicles are now bright green for this reason; it’s the last color visible as light dims. Red becomes black too quickly in dim light.]

    So, oh great heliochromologist, you ask, where does all this take us and why not just tell us the answer? Well, this is a quest, not a fast food approachwe could take advantage of computer models that produce color results given the spectral irradiance, or we could use a number of other models that are used in industry and in association with CIE standards. I did find one impressive computer model effort done to determine the Sun’s color and their result is... peachy pink! Yikes, it’s a girl star! [Ok, that’s a little rude on my part, and, in fairness, they actually stated it to be pinkish peach, but remember, I’m in this for fun; Fun in the Sun.]

    Fortunately, we also know the spectral irradiance of the Sun as observed terrestrially. The difference between an AMO and, say, an AM1 allows us to see just how much color is taken away (extinctions) by our atmosphere. Due to Rayleigh scattering, the primary color loss is in the blue portion of the spectrum. This is a very key point to the Sun’s color determination, because we must take into consideration this light loss caused by our atmosphere to all of our terrestrial observations.

    But the final result comes from what has been discussed and actual observations. The evidence that produces strong support for the color conjecture for the Sun comes from the following separate lines of evidence:

    1) Color images of several Solar twins taken from Kitt Peak. Especially, 18 Sco.

    2) The study of cloud colors and other objects, adjusted for atmospheric effects.

    3) Color images of the Sun from Kitt Peak.

    4) Spectral irradiance data

    5) Results from a new invention for heliochromology called a asterochromograph. [More than a colorimeter as it produces an actual true color spot by reproducing the spectral irradiance of any object.]

    These observational inquiries will be discussed and the final color result will be given in the next and last blog in this...Quest for the Color of the Sun.


    Comments

    This is stupid and makes no sense.

    Helio George
    What don't you understand? The last line in the article links you to the final part of the story. You might find it more informative as it reveals the Sun's color in this three part article. Since this is a scientific blog site, I wanted this story to be both fun and scientific. The physics is correct, but my sense of humor can be a bit too corny for some. I hope you give the third part a try and let me know more of what you think.
    Lighten Up! You're made of stardust!
    Helio George
    Hmmmm....

    The link to the final "episode" is missing.  Here it is...

    http://www.scientificblogging.com/solar_fun_of_the_heliochromologist/the_color_of_the_sun_revelation

    Color of the Sun: Revelation

    George
    Lighten Up! You're made of stardust!
    I'm not sure there are any real world perfect black body radiators.

    Helio George
    Well, some furnaces have holes in them. :)
    Lighten Up! You're made of stardust!
    Helio George
    Hi Kevin, If you want to depict the color of the Sun as would be seen from space and at a comfortable intensity level for the eye, then white is the only reasonable "color" to use. [see part III for more on this: http://www.science20.com/solar_fun_of_the_heliochromologist/the_color_of... ] If you are depicting a setting Sun near our horizon, then you can use pretty much whatever you like. Of course, an orange to yellow gradient is even better. At the horizon, the air mass (AM) is about 40x the air mass directly above us as seen at sea level, so scattering often produces a more orange effect for the portion of the disc nearest the horizon. Atmospheric particulates (eg dust) can greatly affect this. After a rain, for instance, the Sun will almost always appear yellow and not orange. Sunrises are also more yellow than sunsets, which can be yellowish-orange or orange or even a pinkish-orange. Red is very rare, however.
    Lighten Up! You're made of stardust!
    I found this blog when I was trying to see if the color of sunlight was different in different parts of the world. In your article you mentioned measuring it from space and from within our atmosphere. I know weather and atmospheric conditions can change the color of sunlight, but is the color different in a clear sky in the middle of summer in different parts of the world?
    I am an artist and take note of the color of things as I travel. I live in the southern US, have traveled all over the country and have spent some time in Massachusetts and Alaska. It really "feels" like the colors are different in different places. It could be because it's just a reflection of the surfaces being observed, but I wonder if it's more than that.
    BTW -- I don't paint the sun as yellow. I think artists may have started doing that because of the strong contrast between blue and yellow.

    Helio George
    Good question. There is a great history on sky color. Gotz Hoeppe;s book, "Why the Sky is Blue" gives many stories on the adventures of mankind on this topic. The cyanometer was a multi degree scale of blue. Mountains were climbed with this scale in hopes it would give helpful evidence that might reveal the reason behind the sky's color. Your keen eye would discern some color variations since no two skies are truly identical. Since all the colors within sunlight scatter but at greatly different proportions based on their wavelength (4th power advantage for the shorter, blue, wavelengths) then the amount of particles of air, and other particles such as dust, will determine the color you see. Sky color is affected by many variables: 1) The alititude of the Sun in the sky. It is the amount of atmosphere that sunlight must travel through that affects how much scattering any given observer will see. The more atmosphere sunlight must travel to reach you, the more blue light is scattered away from the direct view of the Sun, and the more blue light can be seen elsewhere in the sky. 2) Clouds contribute to the abundance of light bouncing around the atmosphere,which changes the proportions of scattering. 3) Air is essentially uniform in composition, I assume, across the globe, but there are more particles of air in a high pressure zone than when a low pressure zone comes along. More particles, more scattering, more color change. 4) Non-air molecule particulates. Fine dust, aerosols (natural and man-made), pollen, smoke, pollution, etc. if small enough can greatly increase the total number of particles sunlight encounters, thus more scattering. Orange or red sunsets are a given after strong winds in West Texas blow. The "blue" hills of certain areas are more blue than others because the trees emit particles small enough to allow proper scattering (Rayleigh Scattering). 5) Humidity is likely a factor. Rain clouds often look green shaded when the Sun is low on the horizon since sunlight scatters away the normal blues before reaching the storm clouds. This gives the next shorter wavelength (green) some advantage over blue. There could be more to this green story, however. 6) Altitude. The greater your altitude, the more you get away from the larger particle sizes such as dust and whatever, which only reflect white light that washes out the colors. Also, the less atmosphere overhead due to your greater altitude minimizes the amount of non-blue colors you observer, giving blue a stronger advantage. This also means you will see a diminished amount of blue light which gives it a more indigo, or darker, blue color. If the Sun was more prodigious in producing violet, then you probably would see a violet sky. I hope that helps. I'm no expert, however.
    Lighten Up! You're made of stardust!
    Sometimes the Sun appear really white (against a sky blue) to me with the corner of my eye . But with one of my eyes closed, Sun is really extremely bright and blindness :-) . This happens often when the elevation of the our Star above the orizon is around 45-50 degrees (rare more high but happens) and the Sun is not too high for this kind of really quick "natural observation" in a split second . I think i know how to do that, it's an instict but i'm really careful. I hope my english it's correct .

    Helio George
    Hi macro, Yes, many see the high altitude Sun as either white or white with yellow. The problem with such an extremely bright Sun, or any object of such brightness, is that our eyes (ie color cones) become washed-out. We have three basic color cones that some like to call them red, green, and blue since each repsective cone is more responsive in these colors. The problem is that once each color cone is overloaded with flux, then the brain will tell us that we are seeing white, but at a lower and comfortable intensity, the color of the Sun, or any excessively bright object when attenuated, will reveal its true color. Such an object could be almost any color as long as all three colors had excessive flux into our eyes. For the Sun, and stars, our atmosphere further complicates the color question since the shorter wavelength light will scatter away more than the others. I think these three articles combined present enough evidence to reveal that the Sun is definitely not a yellow star, but may be a white star, perhaps perfectly white.
    Lighten Up! You're made of stardust!
    Thank you very much for the reply :D ;) I think (but obviously i'm not an expert XD) the Sun is "cool white" (in a complete vision from million kilometers etc...) . You have written that the Astronauts in Space reported of a very bright white Sun . I think it's true , obviously the limb of the Sun is "less white" (or yellowish white) than the Center and something similar but it doesn't matter enough . The Center is roughly a very pale light blue/white :D so in a total vision maybe the Sun is cool white . (5777 kelvin ;)) The equal enery white point with Planckian Locus and Cie is around 5500 kelvin . Good "late afternoon" (here in Italy are 22.20 p.m :D)

    Helio George
    Bonjourno! [My mother's parents were Italian born.]

    The important point about our inability to see the Sun's true color is that its intensity prevents any visual determination of its true color. [This is why so many of the color attempts for the Sun are done using alternative methods besides simply looking at it. It is, ironically, too bright to see, which explains the astronaut's view.]

    If the Sun was actually distinctively yellow, red, green, or any color for that matter, with appropriate attenuation, it would still appear white without attenuation because all three of our color cones within our retina are completely saturated with the excessive photon flux from the Sun. I don't have a simple analogy for this, but maybe someone else does.

    One not so simple analogy can be found on our tv, or monitor. Send just enough electrons to the red, green and blue phosphors on a monitor to cause that pixel to appear bright white. Now send 10x this amount of electron flux to, say, just the green phospor. Will it then look green? It can't because all three phosphors are saturated, so it will still look white. This is what is happening when anyone looks at an object that emits all colors but at a -26.7 mag. level of brightness. It is even dangerous to the retina.

    Any system, or model, that uses a blackbody profile for an object's temperature could easily misrepresent the "true" color for a non-blackbody object. The Sun is not a true blackbody emitter, but it is close especially in the IR band, but less so in the blue end of the visible band. You will see peak temperatures stated for the Sun as being in the green, but this is based on the false notion that the Sun is a blackbody emitter. A quick glance at the spectral irradiance data, which is easily obtained on the internet (e.g. SORCE), reveals that the actual peak energy output is not green but blue. When we recognize that the photon flux distribution is the better model for our vision, then the peak output is, surprisingly, yellow. Yuk!! But the photon flux distribution is very flat; yellow is a pimple, not a peak. :)

    To obtain the Sun's "true color" -- as defined as the color we would see it if there were no color extinctions due to our atmosphere, and at a normal photopic level -- requires us to find ways we can reduce the intensity and adjust, if necessary for any color extinctions. This is a visual approach rather than a computer approach or other method. I think this is superior to any other method, since ultimately color must be a "seen thing" with the eye. Short cuts don't always work, but can be very effective at times, admittedly. For the Sun, I have seen odd results using some color models.

    Since it can be easily demonstrated that sunlight is white -- a simple pinhole in cardboard works fine for this -- then we simply have to consider what colors are extinguished by the atmosphere and re-add them. The result is that the Sun can never appear yellow to the normal eye.

    It seems unlikely to me that the Sun will appear bluish-white even though the hotter (6390K) center of the disk is almost 1400K hotter than the limb, but maybe it will. It would be great if he did have a little bluish center.
    Lighten Up! You're made of stardust!
    I love the white Sun vision :D Pure and powerful ! Thank you and cheers ! :)

    Anyway there is also the possibility that any person may view the light of the Sun more white according to the elevation of our Star above the orizon because the scattering of the blue wavelengths by the atmosphere is lower . So Sun disk at an elevation of 24 degree (viewed with the corner of the eye) above the orizon (in the midday too) is more yellowish of a Sun at 45 degrees maybe. Cheers

    Helio George

    There is an excellent book about the blue sky, and the history behind how we determined the reason why it is blue, that I recommend to all: "Why the Sky is Blue" Gotz Hoeppe (German physicist and editor). It is a fun read.

    Surprisingly, he spends almost no time on the changing of the Sun's color. I think he simply stated the obvious fact the sunlight is white. He does a nice job of explaining Rayleigh Scattering including both verisons of Willam Strutt's work before and after Maxwell's discovery that light is electormagnetic.

    Thanks for your comments, marco!
    Lighten Up! You're made of stardust!
    Thank you too George ! That's very interesting :D

    Nevertheless if by some romote chance Humanity could live on an habitable planet "around a K type star" at a distance from the Star that could makes for our eyes a -26.8 apparent magnitude, i doubt that a such Star would appear brigh "blinding" white to our eyes only for the great incident amount of light . ;) That star maybe would appear a bright light yellow less blinding !

    ...less blinding only for a psychological factor ;), in my opinion yellow is "less invasive". What i've said it' only an idea, maybe it has a basis maybe NOT...:D :D ; Cheers

    light-yellow it's yellowish white but never white as the Sun ;) ; anyway correct my opinion if it's necessary ;) . A good day :D

    Excuse me for this another message (my idea it's the perception of the color outside the atmosphere of the hypothetical habitable planet)

    Helio George
    A K-class star is still much hotter than the tungsten element glowing in a light bulb. It will likely appear white under normal circumstances assuming we are using the human eye. The brain somehow manages to whiten a bright light source. The example of noting the color of your car's headlights in the day vs. at night is just one way to notice what is known as color constancy. Your brain is an amazing color computer.

    There is a great deal more on star and sky colors found in forums such as www.bautforum.com .

    Here is just one thread: http://www.bautforum.com/showthread.php/132826-Does-sun-spectral-type-in...
    Lighten Up! You're made of stardust!
    Thank you for the reply ;) I don't know if an M style Star also would appear bright white in the same example ;) Bye and thank you for the time spent for my answers/discussions :D