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Fotofan11460
2015-Mar-10, 11:07 PM
I'm trying to clarify the sun's color for my 8th grade students. How do I explain why the sun is actually white, but it appears yellow on the Hertzsprung Russell diagram? Thanks for your help.

grapes
2015-Mar-10, 11:11 PM
I'll get our guy George right on this

George
2015-Mar-11, 12:14 AM
Great question, though it is one that should not exist since the answer is easily determined by astronomers, especially solar physicists.

You may wish to do a simple experiment to help your class see for themselves because it takes us directly to the answer.

Punch a small hole (~ 2 mm in dia. for a 6. ft focal length) in a thin board -- a board large enough to cast a broad shadow. Then project the Sun's image onto a white piece of paper. If the sun is relatively high in the sky, you will get a white image and only a white image, no hint of yellow. You are seeing its true color.

My avatar is an actual image taken from the projection room of the world"s largest solar telescope -- McMath-Pierce at Kitt Peak. It is unfiltered and the color plastic pieces are there to demonstrate its color accuracy.

The yellow Sun story is quite interesting and I am still hunting down its origins. The H-R diagram was initially not a color chart. Since color and temperature are related, it was added. During the early days of spectroscopy, it appears that the Sun was inadvertently considered to be yellow because the Sun's spectrum was comparable with other stars, such as Capella, which happens to be cooler and a bit yellow. The discovery of stellar spectra (mid 1800s) and its importance to understanding stars greatly outweighed what little a star's color contributes to science. So color assignments were not studied closely, and the subjective nature of vision also serves to discourage much of an effort.

To add to the confusion, early spectra were in black and white, so color was really of no importance as astronomers worked diligently to create an effective classification system. Annie Jump Canon accurately classified over 200,000 stars from the tiny little spectragrams on glass plates, and she gave us today's classification system (OBAFGKM). [At least one account states she devised the mnemonic - "Oh Be A Fine Girl Kiss Me", but I am doubtful on this.]

Some will say that the Sun is white because it contains all the colors. [Recall Newton was the one who developed the use of prisms to separate the colors giving us the Solar spectrum. He, by the way, called sunlight "perfectly white".] Unfortunately for this "it has all the colors" claim, every star produces some proportion of all the colors (including reddish and bluish-white stars), else it isn't hot enough to be a star by definition. So it is white because the Sun's temperature of about 5,850K produces an almost flat photon distribution of all the colors. Every color has essentially the same number of photons per sec entering our eyes or cameras.

Other evidence abounds. Snow reflects sunlight evenly (~ 98% the same) and we all know that yellow snow is no place to go. :)

For a more complete, but corny, answer to your question, you might carefully consider...
http://www.scientificblogging.com/solar_fun_of_the_heliochromologist/blog/the_color_of_the_sun
http://www.scientificblogging.com/solar_fun_of_the_heliochromologist/the_color_of_the_sun_part_ii
http://www.science20.com/solar_fun_of_the_heliochromologist/the_color_of_the_sun_revelation

LookingSkyward
2015-Mar-11, 12:17 AM
George, that, right there, is why I love this board! Salute!

dave

George
2015-Mar-11, 12:42 AM
For an unusual adventure, some may dare to take in this color discovery account....

Heliochromology at Stanford's Solar Center (http://solar-center.stanford.edu/suns-color-story/index.html)

George
2015-Mar-11, 12:47 AM
George, that, right there, is why I love this board! Salute!

dave

Wow, thanks Dave. I am not accustomed to receiving 25% of anyone's posts in the form of accolades. :)

This is a great topic to illustrate just what us amateurs can stumble onto as we ride along with other Cosmoequestrians, or whatever we're called.

In fact, it was Phil Plait's lack of a hard color answer that finally pushed me, and others here, to have fun with it and hunt the answer down. Professionals helped too, including ngc3314 who quickly used his skills to offset atmospheric losses by looking at a hotter star ( a proxy) that had the Sun's spectrum after atmospheric extinctions. It, of course, appeared white to him as a result.

LookingSkyward
2015-Mar-11, 12:56 AM
heh - I think you misread my post count, but honestly, the added detail above is even cooler!

d

George
2015-Mar-11, 01:17 AM
heh - I think you misread my post count, but honestly, the added detail above is even cooler!

dyes, I sure did, sorry. :).

[Added: I did what I think the yellow sinners [sunners] did, ironically. We all tend to see what we want to see.]

malaidas
2015-Mar-11, 01:20 AM
Nice blog.

LookingSkyward
2015-Mar-11, 03:00 AM
:)

Shaula
2015-Mar-11, 04:55 AM
To add to what George said - on the HR diagram the colour assignments are done from the BV colour index. This is a difference between the magnitude of a star in the Blue band and the Visible band (yellow to green). This value is corrected in a pre-defined way such that A0 stars have a BV colour of 0. Star with negative BV are then called Blue, stars with a BV between about 0.0 and 0.5 are called white, 0.5-1.0 is yellow, 1.0-1.5 is orange and >1.5 is red (all numbers very approximate!). So the Sun appears yellow on an HE diagram because in astronomical terms (BV index terms) it is yellow. A more caveat laden way to say it would be that the Sun appears yellow on the HR diagram because in terms of its BV colour index its is yellow when compared to the arbitrarily chosen white reference. We could, if we wanted, redefine what star we call white to the Sun. Then you would find that you would have many more blue category stars and fewer red ones, you'd just be shifting the (again, arbitrary) divisions. It doesn't really have much to do with what colour the Sun actually appears to the eye!

WayneFrancis
2015-Mar-11, 06:01 AM
I think the real question is answered by the evolution of our brain and visual system. We perceive it as white because that is how our brain has evolved. We evolved to see the main part of our sun's emissions. It is hardly surprising that our eyes can be easily tricked into perceiving light as "white" and our brains do a very good job at making us think we are seeing white when we are actually seeing another colour.

profloater
2015-Mar-11, 08:42 AM
I used to light paintings and by lighting using low temperature light (dimmed incandescent) in a closed space, no daylight, you can easily test perception of a painting. You see all the colours but the paining is judged as warmer by most people when compared to lighting the same picture by high temperature light , say natural north sky (blue) light. We assume the ambient light is "white" and then judge all objects against that. In daylight the main sky is either blue or white mostly, so then we judge the sun as yellow. You cannot do that lighting trick in a day lit gallery, the lighting just looks yellow then.

Hornblower
2015-Mar-11, 12:06 PM
Great question, though it is one that should not exist since the answer is easily determined by astronomers, especially solar physicists.

You may wish to do a simple experiment to help your class see for themselves because it takes us directly to the answer.

Punch a small hole (~ 2 mm in dia. for a 6. ft focal length) in a thin board -- a board large enough to cast a broad shadow. Then project the Sun's image onto a white piece of paper. If the sun is relatively high in the sky, you will get a white image and only a white image, no hint of yellow. You are seeing its true color.

My avatar is an actual image taken from the projection room of the world"s largest solar telescope -- McMath-Pierce at Kitt Peak. It is unfiltered and the color plastic pieces are there to demonstrate its color accuracy.

The yellow Sun story is quite interesting and I am still hunting down its origins. The H-R diagram was initially not a color chart. Since color and temperature are related, it was added. During the early days of spectroscopy, it appears that the Sun was inadvertently considered to be yellow because the Sun's spectrum was comparable with other stars, such as Capella, which happens to be cooler and a bit yellow. The discovery of stellar spectra (mid 1800s) and its importance to understanding stars greatly outweighed what little a star's color contributes to science. So color assignments were not studied closely, and the subjective nature of vision also serves to discourage much of an effort.

To add to the confusion, early spectra were in black and white, so color was really of no importance as astronomers worked diligently to create an effective classification system. Annie Jump Canon accurately classified over 200,000 stars from the tiny little spectragrams on glass plates, and she gave us today's classification system (OBAFGKM). [At least one account states she devised the mnemonic - "Oh Be A Fine Girl Kiss Me", but I am doubtful on this.]

Some will say that the Sun is white because it contains all the colors. [Recall Newton was the one who developed the use of prisms to separate the colors giving us the Solar spectrum. He, by the way, called sunlight "perfectly white".] Unfortunately for this "it has all the colors" claim, every star produces some proportion of all the colors (including reddish and bluish-white stars), else it isn't hot enough to be a star by definition. So it is white because the Sun's temperature of about 5,850K produces an almost flat photon distribution of all the colors. Every color has essentially the same number of photons per sec entering our eyes or cameras.

Other evidence abounds. Snow reflects sunlight evenly (~ 98% the same) and we all know that yellow snow is no place to go. :)

For a more complete, but corny, answer to your question, you might carefully consider...
http://www.scientificblogging.com/solar_fun_of_the_heliochromologist/blog/the_color_of_the_sun
http://www.scientificblogging.com/solar_fun_of_the_heliochromologist/the_color_of_the_sun_part_ii
http://www.science20.com/solar_fun_of_the_heliochromologist/the_color_of_the_sun_revelation

My bold. Let me repeat my opinion that this photon flux distribution is not necessary for a sensation of white, as is indicated by our ability to adjust to a different distribution in whatever ambient light we have over a rather wide range. For all we know, a being on another planet with 3000K ambient light could evolve receptors that are more sensitive in the blue range where the photon flux is greatly reduced. This hypothetical being might have a set point that would perceive an M star in a dark field as white, rather than the reddish orange we typically see.

profloater
2015-Mar-11, 12:20 PM
Another point is that we do not have narrow filtered RGB cones, they are all gaussian so for example in yellow monochrome light we can get perception of colour from the different reflections off surfaces because we are seeing that yellow as three signals from our RGB cones. It's not true colour it's an interpretation and we accept the ambient as "white"

George
2015-Mar-11, 02:59 PM
To add to what George said - on the HR diagram the colour assignments are done from the BV colour index. This is a difference between the magnitude of a star in the Blue band and the Visible band (yellow to green). This value is corrected in a pre-defined way such that A0 stars have a BV colour of 0. Star with negative BV are then called Blue, stars with a BV between about 0.0 and 0.5 are called white, 0.5-1.0 is yellow, 1.0-1.5 is orange and >1.5 is red (all numbers very approximate!). So the Sun appears yellow on an HE diagram because in astronomical terms (BV index terms) it is yellow. A more caveat laden way to say it would be that the Sun appears yellow on the HR diagram because in terms of its BV colour index its is yellow when compared to the arbitrarily chosen white reference. We could, if we wanted, redefine what star we call white to the Sun. Then you would find that you would have many more blue category stars and fewer red ones, you'd just be shifting the (again, arbitrary) divisions. It doesn't really have much to do with what colour the Sun actually appears to the eye! Yet what is "yellow" if not color, especially to the general public, and hence perpetuating the contradiction. I think it should be changed especially since astronomy is predominantly about the information from light, so why miss the mark on color so obviously?

On the other hand, change may prove a little odd for at least two reasons: the B-V filter values you show would be diluted in color assignments since three or four classes are white stars; we would need to drop the "dwarf" term since the Sun is not a white dwarf. [And for more headache, many white dwarfs are bluish-white since their temperatures are as hot or hotter than O class stars.] Pluto's reclassification was easy. :)

Although I am trying to research the history on this, I strongly suspect that it originally was an inadvertent error, though not arbitrary at the time unlike today for the reasons you state. It may have come from the clout of Father Secchi who, after studying about 4000 stellar spectra gave us the first significant set of five types (classes) of stars. He also gave color assignments to these classes including yellow for the Sun (Type II, IIRC). But he also gave red and deep red to two two of the classes. I suspect pollution in Rome may have contributed to this color conundrum since stars are rarely seen as red without significant extinctions. This might explain why a white star may have been seen as yellow or yellowish-white.

Regardless, there has been significant improvement with this color issue, especially in the few modern textbooks I've seen. If and when we see most of the H-R diagrams changed then we will have arrived, no doubt.

George
2015-Mar-11, 03:20 PM
My bold. Let me repeat my opinion that this photon flux distribution is not necessary for a sensation of white, as is indicated by our ability to adjust to a different distribution in whatever ambient light we have over a rather wide range.Agreed, but it makes for an effective argument for white, right? Pedantry would have corrected my statement since it is slightly misleading to state that each color has the same photon flux. That was a simplified statement but what I really wanted to say, but went simple instead, was that the photon flux density distribution of the Sun, as seen from space (AM0), across the visible spectrum is flat. Interestingly, since yellow is the smallest band of color within the visible spectrum, it is the least likely color that would be seen for the Sun. [Also interesting is the fact that the photon flux density distribution does have a peak....yellow (ug!). But it is a tiny pimple not really a peak. :)] If we were paid to establish a definition for white, why not a flat distribution, though it is arbitrary but only to a point (no pun intended)? [Although this seems contrary to your argument, it is empirical enough to be useful I think.]

What ambient lights do you see near the Sun that might alter a color determination? [For "true" color, I want to see the Sun from space (no atm. effects) and appropriately attenuated (> 10,000x) to the comfort zone of photopic vision.] Color balance and constancy are very important to understanding the colors we see. For stars, this is especially important with the variations people see in the color of double stars. But given a single disk of a star and only a black background, these color-altering effects hardly apply. Perhaps the better question is to see the color variation across a star's disk due to the CLV (Center to Limb Variation) since we only see the integrated distribution.


For all we know, a being on another planet with 3000K ambient light could evolve receptors that are more sensitive in the blue range where the photon flux is greatly reduced. This hypothetical being might have a set point that would perceive an M star in a dark field as white, rather than the reddish orange we typically see. Yes, color is a mental sensation that is complicated and will vary with most Earth species, as well as, prospective aliens.

George
2015-Mar-11, 04:03 PM
I used to light paintings and by lighting using low temperature light (dimmed incandescent) in a closed space, no daylight, you can easily test perception of a painting. You see all the colours but the paining is judged as warmer by most people when compared to lighting the same picture by high temperature light , say natural north sky (blue) light. We assume the ambient light is "white" and then judge all objects against that. In daylight the main sky is either blue or white mostly, so then we judge the sun as yellow. You cannot do that lighting trick in a day lit gallery, the lighting just looks yellow then. Right, I blame artists, too! *wink* And for that matter, a lot can be explained with the misapplication of Crayons and white paper.

Also, the only time we can enjoy looking at the Sun is while it hugs the horizon, which necessarily makes it at least yellowish-white, though more often yellow or yellow-orange. This ads to the misconception as to its true color.

George
2015-Mar-11, 04:15 PM
Another point is that we do not have narrow filtered RGB cones, they are all gaussian so for example in yellow monochrome light we can get perception of colour from the different reflections off surfaces because we are seeing that yellow as three signals from our RGB cones. It's not true colour it's an interpretation and we accept the ambient as "white" Yes, amazingly, the brain is able to compensate for different lighting as it attempts to reveal the colors we would see under normal daylight. This is color constancy and it is also important in camera color-correction processing. The brightest object or objects will be shifted toward a white rendering and the other colors shift with it. I like to use the yellow-white car headlamp example since they are yellow in bright daylight but almost completely white at night. [The newer car lights are not as yellow, however.] This shift to white for bright serves all the more for a white sun since it is the "brightest light in the room". It just ain't yeller' and if it was, it would be shifted toward white due to color constancy.

George
2015-Mar-11, 04:17 PM
Nice blog.
Which one? The Stanford frolic? :)

[The other three were done in my normal, overly glib spirit and with the normal number of grammatical errors, unfortunately. At the time I wrote it, I had guessed it would get 50 hits. The last I checked, it was well over 100,000 total for all three, though the number of complete reads may actually be close to my original estimate. :)]

Hornblower
2015-Mar-11, 06:10 PM
In all fairness to those who envision the Sun as yellow, I must say that if I glance at it with averted vision it looks yellowish white. I get the same sensation looking directly at a light bulb that is too bright to look at comfortably. It looks yellower than a sheet of perfectly white paper that is illuminated by it, until I squint enough to dim it significantly. Then my tint sensation shifts to neutral white. It appears that when my color vision is overloaded with white light it suffers a yellow shift.

Shaula
2015-Mar-11, 07:12 PM
Yet what is "yellow" if not color, especially to the general public, and hence perpetuating the contradiction. I think it should be changed especially since astronomy is predominantly about the information from light, so why miss the mark on color so obviously?
I don't see why it should be changed - the Sun has a yellow tint compared to reference stars. Would you argue that we need to change the term redshift? After all, for wavelengths greater than a micron it is not a shift towards the red part of the spectrum!

chornedsnorkack
2015-Mar-11, 07:44 PM
What ambient lights do you see near the Sun that might alter a color determination?

For daytime Sun, the diffuse light scattered from blue sky. Quite appreciable total compared to the direct rays.

George
2015-Mar-11, 10:15 PM
In all fairness to those who envision the Sun as yellow, I must say that if I glance at it with averted vision it looks yellowish white. I get the same sensation looking directly at a light bulb that is too bright to look at comfortably. It looks yellower than a sheet of perfectly white paper that is illuminated by it, until I squint enough to dim it significantly. Then my tint sensation shifts to neutral white. It appears that when my color vision is overloaded with white light it suffers a yellow shift. My vision is similar since I see a yellow tint just outside the limb during a quick glance, and it is true for light bulbs as well, though more of an orangish yellow for me.

Of course this will be true for most if not all white stars at 1 AU, just in case it gets swapped out for a newer model. :) So, for the cause of science, shouldn't its true color, as I defined earlier, be the preferred color? The setting (or rising) Sun is the one we mostly see and its color is yellow, yellow-orange, or orange, though on rare occasions red. None of these, however, represent its unfiltered view. Why pick any of these as its color? We wouldn't want to judge the color of others feet by the color of their socks. Above our color distorting atmosphere and behind her harmful blaze lies her true color. That, IMO, is how we should judge a star's color. With advanced astronomy coming from high elevations or from space, this makes some sense to me.

George
2015-Mar-11, 10:29 PM
I don't see why it should be changed - the Sun has a yellow tint compared to reference stars. This yellowing is not of the eye, but color is meaningless without it, so why not use the color the eye beholds when unadulterated by pollution and other filtering effects?


Would you argue that we need to change the term redshift? After all, for wavelengths greater than a micron it is not a shift towards the red part of the spectrum! Sure, if a better term came along that could apply to the entire spectrum. How many people will think a white sun looks yellow because of redshift, which is a misapplication, though not your intent, no doubt? Isn't white the better term over yellow? A more accurate evaluation comes from the B-V values regardless of what colors get painted along the H-R plot. So why can't these be adjusted?

I suspect the problem isn't the idea or even need for change, but its implementation; how could it get done? My hope has been that the knowledge of its whiteness would slowly slide into the textbooks and other media, though never, or rarely, Hollywood for artistic flare.

George
2015-Mar-11, 10:32 PM
For daytime Sun, the diffuse light scattered from blue sky. Quite appreciable total compared to the direct rays. True, but I was applying this point to the color definition I used for the Sun (seen from space and without its blaze), thus with only a black background.

chornedsnorkack
2015-Mar-12, 05:29 AM
But how to remove Sunīs blaze?

One obvious way is Mie scattering. Which has very much less blueshift than Rayleigh scattering. (Though not none. Like most transparent substances, water and ice have dispersion, such that blue light is refracted, and therefore Mie scattered, slightly more than red.)

If you compare Sunīs disc filtered to the point it is clearly seen as disc, but not dazzling, with its background, then the background is white/grey and does not have strong colour index contrast with the disc. Then compare the dazzling disc of the Sun, when it exits the clouds... and its background is blue, Rayleigh scattered clear sky. Yellower because of different contrast!
Does Sun turn white/gray when seen through/against background of white/gray water/ice clouds?

Shaula
2015-Mar-12, 05:33 AM
This yellowing is not of the eye, but color is meaningless without it, so why not use the color the eye beholds when unadulterated by pollution and other filtering effects?
Because then almost all stars would be white! A similar strand of logic could be applied to say that we should not call any star a dwarf or giant. Because they are all huge in human terms. So why not use the size we see them to be rather than this strange 'relative to other stars' term?

Hornblower
2015-Mar-12, 02:01 PM
I really don't think a definition, or lack thereof, of "white" for light emitted by a luminous object is of any consequence for spectral work. We can compare a star's color index with that of a convenient reference object and call that reference color anything we wish. What is important as an educational tool for the general public is a display that realistically shows just how pale the tints really are, rather than the exaggerated saturated colors I have seen so often all my life. Sky and Telescope did an excellent one a few years ago as an HR chart showing not only the tint but the relative diameters of main sequence stars over the whole range. I wish I could find it without a laborious search. I have the complete digitized set through 2010, but the built in search engine is almost useless.

George
2015-Mar-12, 03:34 PM
But how to remove Sunīs blaze?

One obvious way is Mie scattering. Which has very much less blueshift than Rayleigh scattering. (Though not none. Like most transparent substances, water and ice have dispersion, such that blue light is refracted, and therefore Mie scattered, slightly more than red.) Mie scattering explains cloud reflection of sunlight, and as you mention, the scattering is farily uniform across the spectrum. This is yet another strong argument for a white sun since clouds appear white even floating serenely in a upper sea of blue, at least for me. Of course, as the Sun gets lower, the incident light becomes more yellow and for longer sunbeam travel distance...pink.

George
2015-Mar-12, 04:24 PM
Because then almost all stars would be white! But we should use the color that the eye would behold under the same terms as we use for the Sun -- from space and appropriately attenuated. The red dwarfs will be the winners -- allowing telescopic determinations to show us what they are. Then there are the red and the occasional blue giants. Of course, we would be bleaching-out many of those yellow stars that have no hint of yellow to begin with, which I am reasonably convinced came from very early and inadvertent color assignments.


A similar strand of logic could be applied to say that we should not call any star a dwarf or giant. Because they are all huge in human terms. So why not use the size we see them to be rather than this strange 'relative to other stars' term? That's an interesting anthropomorphic approach. It is more obvious that the two sizes are used for comparing stars with one another. But you have a fair point for reconsidering those terms, especially since we need to change not only the "yellow" in the often stated "yellow dwarf G2V" description to its color of white, but also the "dwarf" term needs changing; the sun is not a white dwarf.

We now know that the Sun is a very large star by percentage, thanks to the flood of red dwarf discoveries. So this should be addressed, whether that was your intent or not. Cosmoequestrians should saddle-up on this and assist, but in another thread.

George
2015-Mar-12, 05:28 PM
I really don't think a definition, or lack thereof, of "white" for light emitted by a luminous object is of any consequence for spectral work. Agreed, it is somewhat superfluous, though color adjectives can be helpful, such as quickly distinguishing a red giant from a blue giant.


We can compare a star's color index with that of a convenient reference object and call that reference color anything we wish. Yes, the B-V value, and values from the other filters quantify the star much better than the use of an approximate color. But if one chooses to use a color, why knowingly be misleading? If a star has no hint of yellow, why call it yellow? Otherwise, why not have green stars for a B-V value of say 0.7 since a G2V is really white and the cooler K stars may be yellow. Or, if we keep the G2 stars yellow by convention, then green could be assigned to the B-V ~ 0.45 since green is just before yellow in the spectrum.

Yes, I am being facetious, but I think we should fix this minor astronomical discrepancy that lingers unnecessarily. Assuming I am correct, this all started accidentally so there is no need to be stuck on a convention that is demonstrably contradictory. The university peripatetics (Aristotelians; Earth-centered universe) were the top paid and highest teaching authorities. They were the ones that gave Galileo more fits, and for the most years, than his Church, at least prior to his Dialogue. If change would bring clarity and is far more accurate, why not? As you say, the B-V values say far more.



What is important as an educational tool for the general public is a display that realistically shows just how pale the tints really are, rather than the exaggerated saturated colors I have seen so often all my life. Sky and Telescope did an excellent one a few years ago as an HR chart showing not only the tint but the relative diameters of main sequence stars over the whole range. I wish I could find it without a laborious search. I have the complete digitized set through 2010, but the built in search engine is almost useless. It would be interesting to see how accurate their color choices are. The errors (as I see them) are still ubiquitous. Even just now, I went to Wiki for the color index values and, no surprise, they state, "For comparison, the yellowish Sun has a B–V index of 0.656 ą 0.005". Yellowish is an improvement. :)

chornedsnorkack
2015-Mar-12, 06:50 PM
But we should use the color that the eye would behold under the same terms as we use for the Sun -- from space and appropriately attenuated. The red dwarfs will be the winners -- allowing telescopic determinations to show us what they are. Then there are the red and the occasional blue giants. Of course, we would be bleaching-out many of those yellow stars that have no hint of yellow to begin with, which I am reasonably convinced came from very early and inadvertent color assignments.


There is one bright sunlike star - Toliman. (It is slightly yellower than Sun, thanks to Toliman B - but it adds just 30 % to its visual brightness, so we are talking of only slight yellowing).

Looking at Centaurus, how does Tolimanīs colour compare against Agena?

George
2015-Mar-12, 07:36 PM
There is one bright sunlike star - Toliman. (It is slightly yellower than Sun, thanks to Toliman B - but it adds just 30 % to its visual brightness, so we are talking of only slight yellowing).

Looking at Centaurus, how does Tolimanīs colour compare against Agena? I will defer you to Hornblower on color observations since I have yet to do much of an actual study. From Kitt Peak, however, on behalf of nascent heliochromology, I did some imaging of the solar twin 18 Sco:

20357

and another of 18 Sco, probably with better color settings because it better represents what we saw visually...

20358

I recently found an account by Father Secchi showing colors for many doubles -- Grundzüge der Astronomie der Fixsterne (Fundimentals of Astronomy of the Fixed Stars) , 1878. This may be a chapter on star color, but the book is in German. I am slowly translating this chapter. I would enjoy sharing his star color account (especially of doubles) with you both, and anyone else, to see if Secchi's account has evidence of reddening, likely due to his poor atmospheric conditions from Rome and possibly color aberration of his refractor.

ngc3314
2015-Mar-14, 01:16 AM
Looking at Centaurus, how does Tolimanīs colour compare against Agena?

(Traveling south of the equator for a meeting, so I just stepped outside and checked)

To my eyes, Alpha Cen is slightly yellow (distinctly not pure white), while Beta is just slightly bluish. This was at altitude about 45 degrees.

George
2015-Mar-14, 08:17 PM
To my eyes, Alpha Cen is slightly yellow (distinctly not pure white)...
20367

Could the K1V partner cause this jaundice view? I assume altitude and excessive particulates were not a problem, right? Surely the red dwarf isn't luminous to contribute that much color disruption? They both (binary) have > 150% metallicity, so is that a factor?

But wait, hold my horses! Is it possible that astronomers are at heart color affectionados? I'm beginning to think so and am wanting to say.... heck, why not? I am surprised I have never considered this possibility. Why can't we keep it yellow for affectionate reasons and use the spectral shift toward red argument to answer criticism. This would explain why I am the Lone Ranger in this thread in this color fight for the right of white.

In my early translation of Father Secchi, he seems to enjoy giving color descriptions, as do others here. His color descriptions of doubles seems almost jocund and with a touch of hyperbole (no pollution necessary). At least one of the "lesser stars" he describes as green, not meaning greenish because another one or two he does use the greenish term for a color description. Others are purplish (perhaps better translated as more of a violet color). He also never married, I think, but did he not have children -- all heavenly and by the millions? How far off am I with this astro-community supposition?

If we did a poll on whether or not to drop the "yellow dwarf" description of its G2V classification, what would graduate astronomers say? Would solar physicists agree? What about physicists? What trail would the Cosmoequestrians take?

This topic just keeps getting more and more interesting for me.

Hornblower
2015-Mar-15, 05:24 PM
I found one of my Sky and Telescope references, from September 1992:

The Truth About Star Colors
By Philip C. Steffey, Ph.D.

The author concluded that under normal viewing conditions stars of spectral types late A to early F are neutral white, and late F to early G are pale yellowish white. (More archaic terminology: Here "early" and "late" mean the hotter and cooler parts of the sequence respectively.) He provided color samples accordingly, with the caveat that the printed samples are only approximations and are at the mercy of the printing press. His opinion and color samples are in good agreement with my view of a defocused image of Polaris in my Celestron 8 telescope. Polaris, an F8 evolved supergiant, has nearly the same color index as the Sun. As I expected, a projected image of the Sun in a daylight environment looked more nearly neutral white than did Polaris in a dark field at night.

It would be interesting to see how accurate their color choices are. The errors (as I see them) are still ubiquitous. Even just now, I went to Wiki for the color index values and, no surprise, they state, "For comparison, the yellowish Sun has a B–V index of 0.656 ą 0.005". Yellowish is an improvement. In my opinion, for this purpose, accuracy is in the eyes of the beholder. If a preponderance of observers report that they get the same sensation from the color sample for late F or early G stars and from observations of the stars themselves, then I would be content to say that the samples are as accurate as the printing job will permit. I recognize the fact that two different observers might perceive the same yellowish white for the printed samples viewed in daylight, with their color vision adjusted to the ambient light, while seeing different colors when looking at the actual stars at night. My own sensation may change in the near future. If I get a chance tonight or in the next couple of days, I will check Polaris and Capella, along with the blue stars in Orion, and then see what happens when my right eye stabilizes after cataract surgery scheduled for Thursday.

George
2015-Mar-15, 07:29 PM
I found one of my Sky and Telescope references, from September 1992:

The Truth About Star Colors
By Philip C. Steffey, Ph.D.

The author concluded that under normal viewing conditions stars of spectral types late A to early F are neutral white, and late F to early G are pale yellowish white. (More archaic terminology: Here "early" and "late" mean the hotter and cooler parts of the sequence respectively.) He provided color samples accordingly, with the caveat that the printed samples are only approximations and are at the mercy of the printing press. His opinion and color samples are in good agreement with my view of a defocused image of Polaris in my Celestron 8 telescope. Polaris, an F8 evolved supergiant, has nearly the same color index as the Sun. As I expected, a projected image of the Sun in a daylight environment looked more nearly neutral white than did Polaris in a dark field at night.
In my opinion, for this purpose, accuracy is in the eyes of the beholder. If a preponderance of observers report that they get the same sensation from the color sample for late F or early G stars and from observations of the stars themselves, then I would be content to say that the samples are as accurate as the printing job will permit. I recognize the fact that two different observers might perceive the same yellowish white for the printed samples viewed in daylight, with their color vision adjusted to the ambient light, while seeing different colors when looking at the actual stars at night. My own sensation may change in the near future. If I get a chance tonight or in the next couple of days, I will check Polaris and Capella, along with the blue stars in Orion, and then see what happens when my right eye stabilizes after cataract surgery scheduled for Thursday.
Thanks, this is helpful and seems to match ngc3314's and others' color determinations, though I'm now inclined to dig into this further. Perhaps there is m much on this somewhere that I have forgotten or ignored. There seems to be a distinct color difference for stars when comparing them as a point-source and as a disk. The latter case being almost never seen due to lack of magnification and resolution, but it is the direction I encourage if a star's true color is to be stamped in stone. Perhaps we are a few decades too soon to address this special "true" definition.

I had always assumed our atmosphere, optical aberrations, etc. altered a star's "true" color, especially during the mid to late 1800's when color assignments and spectra first became associated. Yet I suspect there may be a simple explanation to the "contradiction"...the fovea.

There are no "blue" cones in the fovea -- the amazing central and high acuity region of the retina. Apparently, only about 2% of all the cones are blue cones, so the brain must augment their inputs. Only the red and green cones comprise the fovea and if we were to combine the receptivity of these two cones, we would find that a peak sensitivity of around 580 nm (yellow). Yellow should have an advantage for this reason. A disk, however, would put the blue color cones to work and whiten the result.

If it were this simple, then if we use equally bright O class stars they should not look bluish-white, yet they do. [Is this true for all of them?] The eye is very active in movement and the fovea is small, so blue cones will be active unless veteran observers are more patient and have less eye movement. I will hazard a guess and say that astronomers may have less movement, favoring yellow and disfavoring blue. However, blue is more enjoyable to look at, so perhaps there is subconscious, or conscious, eye movement to gain more blue color. I would be interested to learn if your trained eye would see color variation with eye-movements.

Also, and probably more importantly as we go deeper into all this, the green color cone is broader in sensitivity than the more narrow blue color cone. The green cone extends into the blue region enough for a blue signal to register with the brain's color processing, I suspect. If this is true, the eye-movement may not be needed for a steady fovea-view of a point-source star to yield a yellow tint for a white star and still look bluish-white for a bluish-white star.

I look forward to your color findings for Polaris and Capella.

George
2015-Mar-16, 12:13 AM
When defocusing, I assume the exit pupil diameter would still be best when equal to the eye's entrance pupil diameter to obtain maxium surface brightness. Is this right?

One other thought on a yellow result is that the photon distribution of greens and reds are essentially the same for a G2 star (or most F class stars, no doubt). Since the addition of these two colors produce yellow, perhaps this enhances a yellow result for the fovea region.

There must be a good answer for this contradiction, though I had previously blamed it primarily on atmospheric and optical circumstances. But now I suspect we might see a solar twin, like 18 Sco, appear yellowish white when viewed through a lunar-based scope as well. I did not when viewed from Kitt Peak, however. So I remain puzzled and dubious that yellow would be a normal result without some contribution from extinctions to stain any super white G2V star.

Hornblower
2015-Mar-16, 04:08 PM
When defocusing, I assume the exit pupil diameter would still be best when equal to the eye's entrance pupil diameter to obtain maxium surface brightness. Is this right?
No, it depends on the star. Unless the telescope's exit pupil is larger than the eye pupil, the brightness of a focused star and that of a defocused disk of any given apparent angular diameter is independent of the exit pupil diameter. I adjusted the size of the defocused image of each star to give an optimum surface brightness. That meant a large one for Sirius and a much smaller one for 2nd magnitude stars such as Polaris or Orion's belt stars.

One other thought on a yellow result is that the photon distribution of greens and reds are essentially the same for a G2 star (or most F class stars, no doubt). Since the addition of these two colors produce yellow, perhaps this enhances a yellow result for the fovea region.
For most stars I saw no appreciable difference in hue between a sharply focused point and a defocused disk, whether centered or somewhat averted. Only extremely bright Sirius seemed to wash out when sharply focused.

There must be a good answer for this contradiction, though I had previously blamed it primarily on atmospheric and optical circumstances. But now I suspect we might see a solar twin, like 18 Sco, appear yellowish white when viewed through a lunar-based scope as well. I did not when viewed from Kitt Peak, however. So I remain puzzled and dubious that yellow would be a normal result without some contribution from extinctions to stain any super white G2V star. I am not sure just what it is you consider to be a contradiction when referring to these pale hues. Here are my observations from last night:

Star B-V

Epsilon Ori -0.19 pale bluish white
Sirius 0.01 cold white
Procyon 0.42 warm white
Alpha Per 0.48 warm white
Polaris 0.60 pale yellowish white
Capella 0.80 pale yellow
Betelgeuse 1.85 pale orange

I would expect to see a star identical to the Sun as pale yellowish white under these conditions if the aforementioned color index of about 0.66 is accurate. Such a description does not contradict my observations, nor does it contradict Dr. Steffey's opinion. I can only conclude that it contradicts one or more of the following possibilities:

1. Your own observations.
2. Observations by other observers Dr. Steffey and I are not aware of.
3. Your own prediction based on personal reasoning from first principles.

Please clarify.

George
2015-Mar-17, 01:14 AM
No, it depends on the star. Unless the telescope's exit pupil is larger than the eye pupil, the brightness of a focused star and that of a defocused disk of any given apparent angular diameter is independent of the exit pupil diameter. I adjusted the size of the defocused image of each star to give an optimum surface brightness. That meant a large one for Sirius and a much smaller one for 2nd magnitude stars such as Polaris or Orion's belt stars. That makes sense and easy to tweak on the go.




Star B-V

Epsilon Ori -0.19 pale bluish white
Sirius 0.01 cold white
Procyon 0.42 warm white
Alpha Per 0.48 warm white
Polaris 0.60 pale yellowish white
Capella 0.80 pale yellow
Betelgeuse 1.85 pale orange
Those are colors I would expect you would find with the exception of Polaris. It's little companion is blue, but to suggest it causes a yellow tint for Polaris is highly unlikely due to its very weak magnitude. The other possible contributing factor is that Polaris is a yellow supergiant and hotter by spectral class. Could it be possible that it has a deeper photosphere allowing the absorption lines to give it an F-class rating, yet have a yellow color along its much cooler limb, unlike main sequence stars of similar class? [I have never read anything like this, but what if?] I suppose this could be easily tested by looking at other F-class main sequence stars for comparison. There has to be a reason for your, and others, yellowish color for Polaris because the Sun is white and I say this because....


I can only conclude that it contradicts one or more of the following possibilities:

1. Your own observations.
2. Observations by other observers Dr. Steffey and I are not aware of.
3. Your own prediction based on personal reasoning from first principles.
The whiteness comes from solar projections. As mentioned before, my avatar is one from Kitt Peak. Also, when amateurs project any of the thousands of solar images the result, as far as I know, are all white as long as the Sun has a respectable altitude (ignoring optical quality and aberration issues). During a solar eclipse, thousands of white eclipses are seen in the shade of trees as the leaves create a pin-hole projection effect, snow is white, the Moon is white, astronauts, like Ed White when on our first EVA, are white. It is only yellow after effects alter its natural "perfectly white" (Newton) color. Your the first to convince me there is more to the story.

One may ask about the role of effects, mostly atmospheric, has on these white projections, including snow. The atmospheric extinctions should be considered to determine what our host star's color might actually be (if attenuated and seen from above our atmosphere). But most here know what end of the spectrum gets the lion share of attenuation by scattering.... blue. So we add mostly the blue-end colors to any of our white projected images and we surely will not get a yellow result.

Curious is it not?

Hornblower
2015-Mar-17, 02:46 PM
The difference in color between Polaris and a main sequence star of type F8 is irrelevant to this discussion. I was concerned only with the published color index and its correlation with the star's appearance, and with my inference of how the Sun compares with these stars whose color indices straddle that of the Sun.

Here is how I would address the question of the true color of the Sun:

"The Sun's true color, as averaged over its apparent disk, is a mixture of all spectral colors that closely approximates that of a luminous blackbody at 5778K. To a typical observer the sensations of these mixtures are virtually identical. If viewed as a small spot in a dark field, as is done with stars, the typical sensation will be neutral white if viewed from a spacecraft above the atmosphere. If viewed from the ground the atmosphere will discolor it to a pale yellowish white."

Note that I stopped short of saying that it is white. For scientific purposes I would say that only if we had agreed, for whatever reason, to define that mixture as white. If a thorough query of a large number of observers chosen at random averages out to seeing that mixture as neutral white, that is good enough for me.

George
2015-Mar-19, 07:23 PM
[sorry for the delay; been out of state and running hard.]


The difference in color between Polaris and a main sequence star of type F8 is irrelevant to this discussion. If, somehow, we see color differences for the same type (and similar B-V index value), it should be of interest since the explanation may prove interesting.

I was concerned only with the published color index and its correlation with the star's appearance, and with my inference of how the Sun compares with these stars whose color indices straddle that of the Sun. Why is Polaris and alpha Centauri (B-V = 0.69) seen as pale yellowish white?

The better question is, why does a completely white projected disk image (as seen terrestrially) suddenly develop a yellowish tint when seen as a point source? [Assumes the proxies are valid.] Atmosphere can’t explain it, I don’t think. Eye response to point sources might. Remember that the solar limb temperature is only 5000K, so the projected disk should at least have a little yellowish tint along the limb, yet if there is one, it is very hard to see.


Here is how I would address the question of the true color of the Sun:
"The Sun's true color, as averaged over its apparent disk, is a mixture of all spectral colors that closely approximates that of a luminous blackbody at 5778K. To a typical observer the sensations of these mixtures are virtually identical. If viewed as a small spot in a dark field, as is done with stars, the typical sensation will be neutral white if viewed from a spacecraft above the atmosphere. If viewed from the ground the atmosphere will discolor it to a pale yellowish white." That is a reasonable approach, but I would prefer…


The “true color” of any luminous object is the product of its unaltered optical spectral energy distribution (SED) and the eye’s spectral sensitivity. A star’s SED is an integral result due to the center to limb variation (CLV) of its photosphere. Scattering and absorption by interstellar medium and our atmosphere will normally redden the color.

[nit… 5778K is the Stefan-Boltzmann temperature. 5850K is the Planck temp. that best fits the actual sp. Irr. of the Sun.]


Note that I stopped short of saying that it is white. For scientific purposes I would say that only if we had agreed, for whatever reason, to define that mixture as white. If a thorough query of a large number of observers chosen at random averages out to seeing that mixture as neutral white, that is good enough for me. Yet I am puzzled with the pale yellow results of likely solar proxies. Toliman is almost 800K hotter

Hornblower
2015-Mar-20, 12:09 AM
I am fully aware that the hue we see when looking at a typical unresolved star is a composite of the brighter, bluer hue from the center and the fainter, yellower hue at the limb, along with all shades in between. I see it clearly when projecting the Sun onto a white surface. My personal sensation the last time I did that was neutral white at the center and a yellowish or brownish tinge toward the limb. This was outdoors and of course was influenced by the ambient light, which was a composite of the integrated direct sunlight and the blue sky. I don't know what the colors would have looked like inside an otherwise blacked out camera obscura.

When asked why someone perceives a particular star as yellowish white when expecting neutral white for whatever reason, all I can reasonably say is that his or her visual system has characteristics which have evolved for eons, without regard to whether or not the thinking part of the brain considers it to be in compliance with some mathematical ideal.

I stand by my opinion that there is no such thing as a pure white emitter in any absolute first-principles sense. The best we can do is pick one that a preponderance of observers perceive as pure white and adopt it as a practical working definition for better or for worse.

Hornblower
2015-Mar-20, 02:58 AM
I was hoping to get a glimpse of the stars tonight and see how they look in my right eye after the cataract surgery this morning, but it is cloudy. The current blurry vision, which is normal and will take a while to clear up, would not have kept me from seeing the colors of the stars. I am curious to see how my brain adjusts. Right now pastels are distinctly bluer in my right eye, but if I stare at a sheet of paper under a table lamp with both eyes, it looks pretty much neutral white, just as it did last night. This is the visual cortex's phenomenal ambient light adaptation in action.

George
2015-Mar-20, 09:02 PM
I am fully aware that the hue we see when looking at a typical unresolved star is a composite of the brighter, bluer hue from the center and the fainter, yellower hue at the limb, along with all shades in between. I see it clearly when projecting the Sun onto a white surface. My personal sensation the last time I did that was neutral white at the center and a yellowish or brownish tinge toward the limb. This color variation is surprising to me but it may be identical to what I too will see. I look forward to doing some solar image projections; I should have already done so rather than rely on my old memory. Perhaps the Kitt-Peak altitude and great optics will explain such a pristine projected white disk.


This was outdoors and of course was influenced by the ambient light, which was a composite of the integrated direct sunlight and the blue sky. I don't know what the colors would have looked like inside an otherwise blacked out camera obscura. Yes, the blue sky may indeed cause some disk yellowing due to contrast with background color. When I look at a white sheet of paper in the shade of a large tree and with a bright blue sky, the white paper suddenly appears as faint bluish-white. A blue background should cause a dim white object to appear to acquire a yellowish tint.


When asked why someone perceives a particular star as yellowish white when expecting neutral white for whatever reason, all I can reasonably say is that his or her visual system has characteristics which have evolved for eons, without regard to whether or not the thinking part of the brain considers it to be in compliance with some mathematical ideal. I am hopeful that this subjectivity is less than what we may think. There are some tests for this.


I stand by my opinion that there is no such thing as a pure white emitter in any absolute first-principles sense. The best we can do is pick one that a preponderance of observers perceive as pure white and adopt it as a practical working definition for better or for worse.My photon flux distribution is, admittedly, a bit arbitrary, but its simplicity and purity may make it a reasonable convention. Because of the effect of color constancy and given the blackness of surrounding space, small variations to a flat distribution will also produce just as white a result, no doubt. Metamers demonstrate this dramatically.

Hornblower
2015-Mar-22, 02:42 AM
I would stick with a blackbody at 5800K or so as a primary standard for pure white emitted light, as it appears to be what our visual systems are adapted to perceive that way. To bring the photon flux at the blue end of the spectrum up to that of the red end, my analysis shows that we would need a temperature of some 7000K, in the spectral type F range. In the absence of atmospheric yellowing, I would expect this to be cold white, perhaps with a noticeable tinge of blue. To make the distribution flat, we would need to reduce the green part, perhaps enough to make the resulting mixture look pale lavender.

I am not at all surprised by the yellowing of the darkened area near the Sun's limb. I have seen effective temperature figures ranging from about 6300K at the center to 5000K near the limb, which match the integrated light from late F and late G stars respectively. From above our atmosphere I would expect to see cold white at the center and warm white at the limb. Once again these adjectives "cold" and "warm" are in the old psychological sense and are opposite the actual temperature of incandescent lights. It cracks me up to see photographers refer to an 85B filter as a "warming" filter that changes the color temperature from 6000K to 3200K in the same breath. "Reddening" would be a better description of the filter.

Hornblower
2015-Mar-22, 02:50 AM
I was hoping to get a glimpse of the stars tonight and see how they look in my right eye after the cataract surgery this morning, but it is cloudy. The current blurry vision, which is normal and will take a while to clear up, would not have kept me from seeing the colors of the stars. I am curious to see how my brain adjusts. Right now pastels are distinctly bluer in my right eye, but if I stare at a sheet of paper under a table lamp with both eyes, it looks pretty much neutral white, just as it did last night. This is the visual cortex's phenomenal ambient light adaptation in action.
So far, so good. My surgeon said it will take several weeks for my vision to fully stabilize, but just two days after the surgery I can already see much better with my right eye than I have for a long time. I just took a quick look at some bright stars with binoculars, and the ones that looked pale yellowish white before now look more nearly neutral white. Sirius looked a bit bluer. I will wait a few more days before using the telescope because the doctor told me to avoid heavy lifting for a week.

George
2015-Mar-25, 05:31 PM
I would stick with a blackbody at 5800K or so as a primary standard for pure white emitted light, as it appears to be what our visual systems are adapted to perceive that way. Is this an a priori approach? Evolution is passive and, long ago, this Wien peak was even less. Is there some study on this?


To bring the photon flux at the blue end of the spectrum up to that of the red end, my analysis shows that we would need a temperature of some 7000K, in the spectral type F range. Are you using a Planck distribution or actual sp. irr. data? 7000K seems high. [I'm out of state but I will be happy next week to use actual solar data to tweak your value if you are using only a Planck model.]


In the absence of atmospheric yellowing, I would expect this to be cold white, perhaps with a noticeable tinge of blue. Why would a flat distribution trigger a blue response?


To make the distribution flat, we would need to reduce the green part, perhaps enough to make the resulting mixture look pale lavender. The Sun and I would likely both cry. *wink* It is obvious that you are using a Planck model. If I have time, I will drag-up a sp. irr. plot comparison to show that the significant unfitness in the blue end. I attribute the variation from a bb dist. to the CLV, but it may be more of a hydrogen story at work.

Here (http://upload.wikimedia.org/wikipedia/commons/e/e7/Solar_spectrum_en.svg) is one, but their color choice is almost offensive. ;) [I also think they meant to state 5520C, not 5250C]

George
2015-Mar-25, 05:42 PM
I am not at all surprised by the yellowing of the darkened area near the Sun's limb. I have seen effective temperature figures ranging from about 6300K at the center to 5000K near the limb, which match the integrated light from late F and late G stars respectively. [Bhatnagar and Livingston in "Fundamentals of Solar Astronomy" state 6390K and 5000K.]


From above our atmosphere I would expect to see cold white at the center and warm white at the limb. I respects your logic, but look at my avatar and tell me if you see this distinction. Perhaps at very great attenuations some color will appear along the limb.

George
2015-Mar-25, 05:46 PM
So far, so good. My surgeon said it will take several weeks for my vision to fully stabilize, but just two days after the surgery I can already see much better with my right eye than I have for a long time. I just took a quick look at some bright stars with binoculars, and the ones that looked pale yellowish white before now look more nearly neutral white. Sirius looked a bit bluer. I will wait a few more days before using the telescope because the doctor told me to avoid heavy lifting for a week.I am delighted you are enjoying better vision knowing how important this must be to you and your love of astronomy. Thanks for the nice report, especially since your color vision is clearly improving! :) This brings us to an important conclusion.... ngc3314 needs cataract surgery! ;

George
2015-Mar-25, 06:19 PM
I should report that my pin-hole solar projection near sea level (~ 100') produced a very faint pale yellow result. But the background is a pale blue color due to the cloudless blue sky, so the color contrast effect should explain or contribute to this color result. I will be at McDonald Obs. for a night with the 36" in a few months, so perhaps this will address atmospheric effects for point-source G2 stars. I'll try to remain objective. :)

Hornblower
2015-Mar-26, 06:09 PM
Here is a link to an image from a Wiki article on Planck's law.
http://upload.wikimedia.org/wikipedia/commons/thumb/1/19/Black_body.svg/1280px-Black_body.svg.png
Here the Planck curves for blackbodies at various temperatures are plotted with power as a function of wavelength. As is mentioned in the article they all have the same shape, as can be shown by appropriate resizing. Some years ago before I lost the calculator I had written in a spreadsheet I confirmed that exactly. Today, rather than do the work of restoring the calculator, I just did some resizing in my PaintShopPro software and observed it myself. Thus I can derive a curve for any temperature from the ones in this image, with plenty of accuracy for the purposes of this discussion.


Are you using a Planck distribution or actual sp. irr. data? 7000K seems high. [I'm out of state but I will be happy next week to use actual solar data to tweak your value if you are using only a Planck model.]
I used the Planck curve over the visible range for 7000K, which made the blue end stronger relative to the red end than at the Sun's temperature. Then I converted the power level at each point to photons per second, starting with the red end unchanged and multiplying the power number at each successive point by the ratio of the wavelength at that point to the wavelength at the red end. The result was a rolling off of the curve from red to blue, giving a curve virtually the same shape as the power curve for the Sun's temperature. That is, nearly the same level at the red and blue ends and highest in the green range, a bit left of center.


Why would a flat distribution trigger a blue response?
If we are talking about the photon flux distribution, I would say it is because the visual cortex, at least in my case, is adapted to interpret one with a reduced count in the blue range as neutral white, regardless of whether the thinking part of the brain chooses to analyze it in terms of power or photon flux. I have never heard of a neurological argument against my line of thought.

Hornblower
2015-Mar-27, 01:45 PM
I respects your logic, but look at my avatar and tell me if you see this distinction. Perhaps at very great attenuations some color will appear along the limb. You mentioned Kitt Peak. Did you look at the image of the Sun in the McMath telescope, as some visitors are doing in this photo?
http://parallax.sci.csupomona.edu/wordpress/wp-content/uploads/2010/06/IMG_7610.jpg
If so, perhaps the great brightness of that large image washed out any sensation of yellowing toward the limb. It would be interesting to look at the same sized image in a dark room with a much smaller objective. The late Horace Dall did just that with a superlative 4-inch apochromat in his camera obscura in England. I could do that with a much smaller image using pinhole projection in a blacked out room. I have a room in which I can do that easily as it has only one window, on a south facing wall. That may be my next experiment.

Swift
2015-Mar-27, 02:43 PM
Since the OP dropped out a long time ago, and this has become a longer discussion, I've move the thread from Q&A to Astronomy.

But please, don't let this move color your thoughts.

Hornblower
2015-Mar-28, 05:57 PM
My pinhole experiment is a bust. The window is so dirty that the view is washed out, and cleaning it would be a beastly job. It is a double hung window with the lower sash all the way up to accommodate a large air conditioner, so it is impossible to clean all four surfaces without taking the machine out. Not worth the trouble at this point.

George
2015-Mar-29, 02:24 AM
You mentioned Kitt Peak. Did you look at the image of the Sun in the McMath telescope, as some visitors are doing in this photo?
http://parallax.sci.csupomona.edu/wordpress/wp-content/uploads/2010/06/IMG_7610.jpg .

The normal lighting in the room is from fluorescent lights. My avatar was likely taken under this lighting and it does accurately represent what one sees. I doubt strongly that this picture is an accurate color image of what those people saw with the lights off.

Hornblower
2015-Apr-04, 02:09 AM
So far, so good. My surgeon said it will take several weeks for my vision to fully stabilize, but just two days after the surgery I can already see much better with my right eye than I have for a long time. I just took a quick look at some bright stars with binoculars, and the ones that looked pale yellowish white before now look more nearly neutral white. Sirius looked a bit bluer. I will wait a few more days before using the telescope because the doctor told me to avoid heavy lifting for a week.
I finally got back to the telescope Wednesday night. I may not be a reliable observer of subtle pastel hues because my brain is caught between two conflicting signals from the two eyes. F stars such as Procyon and Polaris looked yellowish white in the left eye and bluish white in the right. Capella looked distinctly yellow in the left and nearly neutral in the right. Pollux was orange left and pale yellow right. Betelgeuse, with its more saturated orange, looked only slightly paler in the right eye. A and B stars were distinctly bluer in the right eye.

In a naked eye view, the Moon was yellowisn white in the left eye and bluish white in the right eye. Jupiter was the same through the telescope. What was really striking with the right eye was the much higher contrast in the cloud patterns on Jupiter, which were badly washed out in the cataract-afflicted left eye. So far I would say the surgery on my right eye was a resounding success.

George
2015-Apr-04, 02:38 AM
That is quite a color contrast between each eye. I'm unclear just what is going on. It seems your left eye has a red filter effect on it, but only slightly and under certain circumstances, apparently. Your Betelgeuse observation should have been a deeper orange than reported, I think. Perhaps the greater magnitude of Betelgeuse improves things, but nope, the bright Moon (pre-eclipse full tonight) shoots this idea down. Have you tried binoculars? Your average should be perfect. :)

Hornblower
2015-Apr-04, 12:21 PM
With both eyes using binoculars, the stars look much as they did before the surgery. It is as if I have an automatic white balance function splitting the difference.

Betelgeuse has a more saturated color than the other objects I observed. In my opinion that is why the difference between the two eyes is less striking.

Hornblower
2015-Apr-05, 01:06 AM
That is quite a color contrast between each eye. I'm unclear just what is going on. It seems your left eye has a red filter effect on it, but only slightly and under certain circumstances, apparently. Your Betelgeuse observation should have been a deeper orange than reported, I think. Perhaps the greater magnitude of Betelgeuse improves things, but nope, the bright Moon (pre-eclipse full tonight) shoots this idea down. Have you tried binoculars? Your average should be perfect. :)
Actually I was somewhat surprised that the difference between the two eyes was not greater, after looking at a photograph of a cataract-ridden lens lying on a white surface. It was a gruesome brownish orange blob. Of course the color in that view was from a double pass of the light, going through the lens and then reflected back to the camera. It would be paler with a single pass that corresponds to the function while still in the eye. The surgeon recommended a clear rather than a tinted implant when I told him that astronomical observing was important to me, and I concurred.