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Thread: Sun Color Challenge

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    Sun Color Challenge

    More fun with the color of the Sun…

    It seems like a long time since any salvo has been shot at this subject... perhaps.

    What is interesting and not too surprising is that over the last ten years or so a kind of osmosis seems to be proceeding as I’d hoped – the top Google search articles are all telling us that the Sun is a white star. This was definitely not the case in the past when yellow or yellowish-white was still the favored color for our host’s apparel. Of course, 90% or better of all the color graphics today still present it as a non-white object. But pretty is better than plain.

    So, if you are in the mood to test your color knowledge and are willing to put on your Hat of Pedantry to find the many nits, then I hope you will enjoy this. The list of sites comes from the Google-search articles listed in order. Grammar errors should be waived unless they cause "clear ambiguity". [Oxymorons and puns are fine, of course.]

    [ADDED] The standard board prize of an Ice Cream Sundae is hereby offered to each of the first two who can list a total of at least 30 color-related nits from those sites, though there may be as many as 40. [I could be wrong on a few, admittedly.]

    istockphoto-918241144-612x612 (1).jpg


    It should be noted that most sites are very close to "nailing it", but there is room for improvement on all these sites. I enjoyed several including Frasier's and Phil Plait's links below as well as Quora's. [It was The Bad Astronomer's indecision from year's ago on the Sun's true color that was the last straw necessary to generate the interest to resolve this surprising color conundrum, at least for me.]


    Link #9 has no technical errors. [Here brevity has been incorporated to make re-reading this rule easier. ]
    The search phrase used was “The color of the Sun”.

    1 Ask an Astronomer
    2 Standford's Solar Center [A great site for solar info.]
    3 Science ABC.
    4 World Atlas
    5 Astro Bob
    6 Can You Actually
    7 Wiki
    8 Quora
    9 Science 2.0 [The site director has requested a professional alternative, but the author has failed to provide such.]
    10 Fraser's Youtube vid [
    11 Phil Plait's star Crash Course #26 [go to 4:32 min]
    12 Scientific Scribbles
    13 Thought Co.
    ?? CASA (Boulder)
    Last edited by George; 2019-Jun-12 at 02:28 PM.
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    If I ever need to find you in a crowd, George, I’m just going to shout “Doesn’t the sun look nice and YELLOW today?”
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    Blue.
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    Quote Originally Posted by Trebuchet View Post
    Blue.
    I ain’t going there.

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    Anybody remember the guy who was spamming darn near every thread in the forum with "water is blue"?
    Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

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    Quote Originally Posted by Trebuchet View Post
    Anybody remember the guy who was spamming darn near every thread in the forum with "water is blue"?
    What? No, when was this?
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    The Sun is orange. I see it every morning on the way to work, hovering right above the horizon, and it is always orange.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    — Mark Twain, Life on the Mississippi (1883)

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    Quote Originally Posted by KaiYeves View Post
    If I ever need to find you in a crowd, George, I’m just going to shout “Doesn’t the sun look nice and YELLOW today?”
    I would likely shout, "Yes!" If you are looking at it then it would necessarily have to be at least a saturated yellow to allow direct observation.
    Last edited by George; 2019-Jun-12 at 01:22 PM.
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    Quote Originally Posted by Roger E. Moore View Post
    The Sun is orange. I see it every morning on the way to work, hovering right above the horizon, and it is always orange.
    Do you mean orangish-yellow? It usually requires afternoon air turbulence to bump the particle count high enough to produce the extra scattering needed to go beyond yellow. But then there is the sailor's warning, "red in the morning, sailor take warning; red at night, sailor delight." Though, with pedantry hat on, they likely mean orange or reddish-orange. Red is rare, though some people have vision that render more red than average.

    I recall reading about some self-tests on vision when wavelength could be assigned accurately to color. I think this was in the mid 1800s. One account was from an English scientist who claimed to see down to about 850nm, IIRC.
    Last edited by George; 2019-Jun-12 at 02:18 PM. Reason: grammar
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    When I see the Sun is "red", it actually looks orange-red on the horizon, meaning yellow with a ton of magenta dumped into it. When I was a magazine editor during the Precambrian Era, my art director worked with me to see colors, to actually look at them and evaluate them. This doesn't mean I get colors right, but at least I'm closer.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    — Mark Twain, Life on the Mississippi (1883)

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    I had thought about offering the standard prize of this board -- an ice cream sundae. Hmmm. Okay I think I will do so for the first two people who can find a total of 30 nits. [I'll edit the OP accordingly.]
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by Roger E. Moore View Post
    When I see the Sun is "red", it actually looks orange-red on the horizon, meaning yellow with a ton of magenta dumped into it. When I was a magazine editor during the Precambrian Era, my art director worked with me to see colors, to actually look at them and evaluate them. This doesn't mean I get colors right, but at least I'm closer.
    Under certain circumstances, I’ve seen the setting sun looking a color I would describe as “blood orange”, basically that same color.
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    Quote Originally Posted by KaiYeves View Post
    Under certain circumstances, I’ve seen the setting sun looking a color I would describe as “blood orange”, basically that same color.
    Yes, when pollen, aerosols, dust, etc. pile-up, so to speak, in the atmosphere then a great deal of additional scattering takes place, scattering away more of the blue end of the spectrum.

    Then there are a few unusual color events that can happen. I recall one member several years ago observed a lunar eclipse from Hawaii and witnessed an almost purple Moon. This may have coincided with volcanic activity producing particle sizes just right for selective scattering of the red end of the spectrum. This, btw, happens every day on Mars where the larger particles are just right to do this, which produces a blue sky surrounding the Sun, whereas we see a yellow or orange sky adjacent to the setting Sun.

    Your view when you go to Mars (actual NASA image likely close to true color, but enhanced):

    Mars blue horizon.jpg

    Bigger view of the sky and the credits at APOD here .
    Last edited by George; 2019-Jun-12 at 04:41 PM.
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    So to help someone or some two get an ice cream sundae....

    Take the first one....

    "The Sun emits a lot of energy in the visible range. In wavelength scale it is from 390 nm to 700 nm, and when you translate it to colors, you get all colors from violet to red, just as we see them in the rainbow. When you mix all those colors together you get white, and that is why white is the true color of the Sun. Check out photos of the Sun taken by astronauts (with no filters). The Sun appears white on them!"

    Any takers? Also, you can play off other's findings, which is why I am offering two prizes.
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    Quote Originally Posted by KaiYeves View Post
    What? No, when was this?
    Long, long ago. Probably in the BABB days.
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    Quote Originally Posted by Trebuchet View Post
    Long, long ago. Probably in the BABB days.
    Is this dusty thread it? Here
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    I have see the Sun look reddish orange at sunrise on murky summer days, when stagnant air is loaded with particulate crud.

    If I am not mistaken, that sailor's saying refers to vivid reddish illumination of high clouds at sunrise and sunset respectively. The clouds are lighted by grazing light from a clear sky beyond the horizon. That light is reddened by its long path through the atmosphere. Since the clouds usually move from west to east, the morning case often means worsening weather while the evening case usually means clearing weather. I once saw a spectacular sunrise followed by rapidly thickening clouds and 8 inches of snow before sunset.

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    Quote Originally Posted by Hornblower View Post
    I have see the Sun look reddish orange at sunrise on murky summer days, when stagnant air is loaded with particulate crud.
    Yes, and stars, of course, will do the same. I have wondered if a reasonable particulate count could be determined by noting the change in tint for given reference stars. Eta Cas is a good example as it is a white star, slightly hotter than the Sun, IIRC, and it has a "red dwarf" neighbor. When from McDonald obs. I saw Eta Cas as distinctly yellow, the red dwarf was surprisingly red. I began observing these from home (low elevation) and similar altitudes to confirm that it is actually a white star after all, else my confidence in this thread's topic would have been toppled.

    If I am not mistaken, that sailor's saying refers to vivid reddish illumination of high clouds at sunrise and sunset respectively. The clouds are lighted by grazing light from a clear sky beyond the horizon. That light is reddened by its long path through the atmosphere. Since the clouds usually move from west to east, the morning case often means worsening weather while the evening case usually means clearing weather. I once saw a spectacular sunrise followed by rapidly thickening clouds and 8 inches of snow before sunset.
    Yes, that's a good point.

    I thought the computer I'm on now had a few cloud shots I've taken now an then that demonstrates the color difference between nearby white clouds vs. stars much farther way with their pink color. If I find one I'll add it. The extinctions greatly increase both going the extra distance both ways, so it reddens all of them usually into a nice pink shade.
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    Quote Originally Posted by George View Post
    I would likely shout, "Yes!" If you are looking at it then it would necessarily have to be at least a saturated yellow to allow direct observation.
    Awww, but if you don’t march up to correct me, how am I supposed to find you in the crowd?
    Last edited by KaiYeves; 2019-Jun-13 at 02:57 AM.
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    Quote Originally Posted by KaiYeves View Post
    Awww, but if you don’t match up to correct me, how am I supposed to find you in the crowd?
    “George!” should work. But you can shout any solar color question to find me, unless it’s a solar physicists’ convention.

    Come to think of it, two of those folks are missing out on free ice cream.
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    Has color pedantry died? Have I underestimated the solar color question fatigue? Admit it, I have been self-restrained for quite a while, after all. Even ice cream has failed to get things going.

    Part of the reason I'm doing this is to note how even smart and knowledgeable people can get a little loose with logic.

    For instance, what's wrong with this syllogism:

    When all the colors of the visible spectrum are combined, white is the result.
    Every star by formal definition produces all the colors of the visible spectrum.
    Therefore, all stars are white.

    I think the rules of logic would call this a valid argument but unsound. The conclusion is true only if the premises are true, but unsound since one premise is false.
    Last edited by George; 2019-Jun-13 at 04:26 PM.
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    Quote Originally Posted by George View Post
    ... Have I underestimated the solar color question fatigue? ...
    Years ago on this topic I posted that sunlight is the very definition of white. Take a white piece of paper and see what color it looks like under the sunlight. It is white.

    Adding something new to my previous argument: Doing photo and video work you might start to find distinctions within what you'd call white. You can set white by color temperature. Do you like 5500K as white? Maybe you prefer the old incandescent look (in which case the sunlight looks blueish!)
    White is measured by our eyes in context, and there is no definition of absolutely perfect white.

    Not much else left to say on this dog-eared topic.
    Forming opinions as we speak

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    Quote Originally Posted by antoniseb View Post
    Years ago on this topic I posted that sunlight is the very definition of white. Take a white piece of paper and see what color it looks like under the sunlight. It is white.
    Yep, and the power of color constancy that pushes the brain toward white for the brightest light source is worth remembering. Yet this white push is limited. Take that same sheet of white paper in bright sunlight and walk into the shade of a tree; you might be surprised what you see, requiring you to resolve it in more than 500 ms.

    Adding something new to my previous argument: Doing photo and video work you might start to find distinctions within what you'd call white. You can set white by color temperature. Do you like 5500K as white?
    When it comes to stars, we have the capability of establishing the temperature range that would render white stars, at least for those along the main sequence. I think most would be surprised at how low the temperature would be before even the limbs become yellowish.

    Maybe you prefer the old incandescent look (in which case the sunlight looks blueish!)
    Yes, which is another example of color constancy at work. Incandescent lights look a lot whiter at night.

    White is measured by our eyes in context, and there is no definition of absolutely perfect white.
    Agreed, though "absolutely perfect" isn't a phrase used hardly ever within science and engineering. But it is absolutely perfectly wrong to say the Sun is a yellow star (meaning yellow to the eye and not a rule of thumb for spectrscopy); photopic vision isn't all that subjective.

    I'm pleased that most seem to get this these days, yet dumb logic deserves a little criticism. And there are other errors besides the example syllogism used above.

    I know you get it, but when others, who know better, make false claims to support the white conclusion, it's troubling enough to try and correct their logic.

    As for "perfectly white". The first time I saw "perfectly white" used was from Isaac Newton who used this to describe sunlight at the time of his prism experiments. Thus, "perfectly white" is being applied to at least an AM1 atmosphere and yet many seem puzzled if the Sun actually looks white when atmospheric effects have to be considered (and assuming proper attenuation to avoid blazing effects to the eye).

    If I had to produce a definition for perfectly white it would be a perfectly flat photon flux density distribution across the spectrum. The Sun comes very close to a very flat distribution (AM0).

    Not much else left to say on this dog-eared topic.
    I think "dog-eared" is an apt description of it for most folks, but there are cool nuances that can come from this topic because of the somewhat trivial nature of this topic. Even though astronomy and physics would change hardly one iota if the Sun were to be found to be yellowish-white, it's possible that both poor logic and false claims will be made due to the lack of necessary scrutiny while other topics bearing scientific efficacy receive adequate scrutiny. But, if we fail to get the easy stuff right, especially the very visible ones, then.... And the Sun is a very visible one!
    Last edited by George; 2019-Jun-13 at 07:42 PM. Reason: grammar
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    At the risk this will exacerbate the color problem, take a look at today's APOD. Isn't it possible some will look at the B-R results and falsely conclude the Sun is a yellow star? I think an A0 is the zero point for the B-R filters, or am I wrong?

    I think it would be grand if astronomers, especially, would tweak many of these color issues since it is such a wonderful aspect for all of astronomy. If for no other reason than to eschew obfuscation.
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    Quote Originally Posted by George View Post
    At the risk this will exacerbate the color problem, take a look at today's APOD. Isn't it possible some will look at the B-R results and falsely conclude the Sun is a yellow star? I think an A0 is the zero point for the B-R filters, or am I wrong?

    I think it would be grand if astronomers, especially, would tweak many of these color issues since it is such a wonderful aspect for all of astronomy. If for no other reason than to eschew obfuscation.
    It's hard to tell for sure since I have not been able to find an explicit listing of B-R as a function of effective temperature. The researchers using B-R appear to be more interested in clusters than individual stars. My best estimate is that B-R = 0 is typical of K stars, and that the Sun would be somewhere near -0.5. If I naively assume that in these studies zero is called white in the jargon, I would conclude that the Sun is bluish white, which is just how it looks when our ambient light is dominated by the equivalent of K or M stars.

    Yes indeed it would be wonderful if astronomers did not use common color names in ways that are at odds with our perception of star colors under ordinary observing conditions. I think the consensus is that the Sun would look neutral white in the absence of atmospheric discoloration.

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    Quote Originally Posted by George View Post
    Yep, and the power of color constancy that pushes the brain toward white for the brightest light source is worth remembering. Yet this white push is limited. Take that same sheet of white paper in bright sunlight and walk into the shade of a tree; you might be surprised what you see, requiring you to resolve it in more than 500 ms.

    When it comes to stars, we have the capability of establishing the temperature range that would render white stars, at least for those along the main sequence. I think most would be surprised at how low the temperature would be before even the limbs become yellowish.

    Yes, which is another example of color constancy at work. Incandescent lights look a lot whiter at night.

    Agreed, though "absolutely perfect" isn't a phrase used hardly ever within science and engineering. But it is absolutely perfectly wrong to say the Sun is a yellow star (meaning yellow to the eye and not a rule of thumb for spectrscopy); photopic vision isn't all that subjective.

    I'm pleased that most seem to get this these days, yet dumb logic deserves a little criticism. And there are other errors besides the example syllogism used above.

    I know you get it, but when others, who know better, make false claims to support the white conclusion, it's troubling enough to try and correct their logic.

    As for "perfectly white". The first time I saw "perfectly white" used was from Isaac Newton who used this to describe sunlight at the time of his prism experiments. Thus, "perfectly white" is being applied to at least an AM1 atmosphere and yet many seem puzzled if the Sun actually looks white when atmospheric effects have to be considered (and assuming proper attenuation to avoid blazing effects to the eye).

    If I had to produce a definition for perfectly white it would be a perfectly flat photon flux density distribution across the spectrum. The Sun comes very close to a very flat distribution (AM0).

    I think "dog-eared" is an apt description of it for most folks, but there are cool nuances that can come from this topic because of the somewhat trivial nature of this topic. Even though astronomy and physics would change hardly one iota if the Sun were to be found to be yellowish-white, it's possible that both poor logic and false claims will be made due to the lack of necessary scrutiny while other topics bearing scientific efficacy receive adequate scrutiny. But, if we fail to get the easy stuff right, especially the very visible ones, then.... And the Sun is a very visible one!
    My bold. My calculations show that we would need a blackbody temperature of roughly 8000K to get the photon flux equalized between the blue and red ends of the spectrum. This is in the middle of the A type range and would look bluish white from above our atmosphere. In addition we would need to subdue the normal blackbody crest in the middle, and this might give it a noticeably magenta tinge. No, I will stick with a blackbody at the Sun's effective temperature as a useful definition of a "perfectly" white emitter.

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    Quote Originally Posted by Hornblower View Post
    It's hard to tell for sure since I have not been able to find an explicit listing of B-R as a function of effective temperature. The researchers using B-R appear to be more interested in clusters than individual stars.
    I'm not familiar, being an amateur after all, with the filter use for clusters. I assume that the blue and red filters are the common choices used, as you suggest. After starting this thread then to see the APOD post showing the color distribution, I found that a B-R = 0 matches an A0 star from one of my simple searches. [I think I can find it easily if you wish me to, but time is short at the moment.]

    My best estimate is that B-R = 0 is typical of K stars, and that the Sun would be somewhere near -0.5. If I naively assume that in these studies zero is called white in the jargon, I would conclude that the Sun is bluish white, which is just how it looks when our ambient light is dominated by the equivalent of K or M stars.
    [my bold] I'm unclear what your're saying here. The hotter stars, the Sun in this case, are brighter and we (our vision system; retinex) will make that object the white source, so I don't see how it would have any tint of blue.

    Yes indeed it would be wonderful if astronomers did not use common color names in ways that are at odds with our perception of star colors under ordinary observing conditions. I think the consensus is that the Sun would look neutral white in the absence of atmospheric discoloration.
    Yes, and this board, beginning with Phil's initiation of it has played a role I suspect in convincing others who either didn't give it much thought and/or were okay with the "yellow dwarf G2 V" broad classification, crayola coloring, and other subtle influences. Sometimes a psuedo-pest comes along and gently nudges those who he, or she, greatly respects to get them to decide to take a more serious look at it.
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  28. #28
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    My ambient equivalent of K or M stars is the 3200K indoor lighting with an admixture of diffuse daylight leaking in through closed window blinds, making my bathroom sink look neutral white overall. A thin beam of sunlight, partially obstructed by trees and getting through a chink in the blinds, makes a spot on the sink that looks bluish white by comparison. If I open the blinds and let the Sun move clear of the trees, it overwhelms the light bulbs and looks neutral white, with the indoor light becoming a dull brownish color by comparison.

    As for color index, I checked the B-R plot of M13 in the linked APOD against published B-V plots of various globular clusters. The turnoff point which is typical of such clusters is to the left of B-R = 0, but is to the right of B-V = 0, in the more familiar B-V version of the H-R diagram. This is the first I have ever heard of B-R. I found another B-R plot, this time of M67. Go to http://astronomyonline.org/Astrophot...ate=CMDDiagram and scroll down to figures 6 and 7. Figure 7 is marked as showing red giants around B-R = 0.

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    Quote Originally Posted by Hornblower View Post
    My bold. My calculations show that we would need a blackbody temperature of roughly 8000K to get the photon flux equalized between the blue and red ends of the spectrum.
    Hmmm, I get 6,928K to match the photon flux at 390nm with 750 nm.

    I will double check my Planck equation work and conversions to see if I might be off. I have double checked them in the past.

    At 8,000K, using Planck's eq., I get 4.16E8 w/m2 at 390nm (9,010w/m2 at 1 AU and 1.58E8 w/m2 for 750nm (3,413 w/m2 at 1 AU.

    A significant problem in achieving a flat distribution is that stars ranging from about 4850K to about 9000K will have photon flux peaks within the visible spectrum. [3900K to 7500K produces peaks for energy distributions. Notice the irony since I am using the wavelength range of 390nm to 750nm for the visible range, thus finding temps. 10x the wavelength values (flipped) ] Therefore, using a bb distribution, there can never be a given temperature that comes very close to emitting a flat distribution.

    One typical error this thread was to point-out is that claims are erroneously being made about the Sun based on a blackbody model (Planck equation) and not the actual distribution (ie solar sp. irr.). The green peak claim, naively, appears correct since the solar peak for an energy distribution is about 501nm (5777K). But the actual peak is around 480nm (blue). Though the Sun in the IR band has a very tight fit with the Planck model, yet it doesn’t have a nice fit in the blue end of the visible spectrum. The Sun has almost a 10% energy bump for a large portion of the blue band when comparing actual sp. irr. (AM0) to the Planck model. This blue bump helps flatten its photon distribution but not near enough to give us a flat distribution. So, my suggestion of using such for a perfect white definition is close but probably not close enough even with the blue-end bumps for real stars, but how nice would it be if we could define white in such a way!

    It would be interesting to look at SEDs from F-class stars to see if their actual emittance would yield something close to a flat distribution since it's the blue end where the Sun gets droopy in the distribution. I think all F-stars are likely white in appearance (AM0 or AM1).
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    When I look at the published spectra, it does indeed appear that the Sun would look somewhat bluer than the perfect blackbody with the same effective temperature. It should be fairly straightforward to make satisfactory laboratory simulations. I don't think we need to duplicate the snaggletoothed shape of the solar spectrum, as the three types of cones respond to broad bands with lots of overlap.

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