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George
2007-May-01, 10:51 PM
This will probably be this amateur's only chance of contributing something “with legs” for this thread. [Happy B-Day, Nereid (not that I’m even close on the date).] So, here it is…..

Viewed from space, the Solar disk will look similar to the subsequent drawing. This assumes the observer is able to diminish the visible flux of the Sun such that it falls in the normal photopic range.

I bring this to the ATM because a non-yellow Sun is contrary to the majority of statements made regarding its color. Mainstream still seems to refer to the Sun’s color as yellow. [Not like this is hard mainstream science, admittedly, otherwise this would have been done 50 years ago. Thanks for waiting for guys like me. :)] Many H-R diagrams reflect some shade of yellow for G-class stars. It is rare to find even solar physicists use a non-yellow color adjective for the Sun. Of course, there are some astronomers, and physicists, who use a white “color” description. [I noticed Jeff Hester's (et al) textbook states the Sun as white.] But is it white?

So let me start with a colorful conclusion:

http://img443.imageshack.us/img443/3159/solardiskcoloroa6.th.jpg (http://img443.imageshack.us/my.php?image=solardiskcoloroa6.jpg)

It is white, with a light blue center. I don’t know just how blue it will appear in actuality, but I claim it will be noticeable.

Allow me to first address the white coloration. [Allow me to call white a color, too.] Also, all references of terrestrial Solar views are only when the Sun is at a relatively high altitude (AM2 or less).

Terrestrial Solar telescopes project unfiltered images of only a white Sun. Not that I’ve seen many projections. [Ok, I’ve only been to one, but at least it was the McMath-Pierce at Kitt Peak (World’s Largest!). Other terrestrial Solar disc views include my own pin-hole efforts (same as y'all), and direct observations using my homemade SAD (Solar Attenuation Device: a simple strobe). There were also a few naked-eye, quick glances at the Sun from time to time. [At least I didn’t use binoculars!]

The big argument that eliminates, IMO, any chance for a yellow sun comes from the white solar projections. Since our atmosphere greatly diminishes more of the blues and greens than the longer wavelength colors due to Rayleigh-Tyndall Scattering, how can the addition of these shorter wavelength photons into a white image, as would be observed from space, produce a yellow result? It won’t. In these Solar projections, even the limb region is white.

The following is an image from Kitt Peak’s McMath-Pierce projection room taken by Roy Lorenz (KP technician) along with some color references as per my request. It isn’t yellow.

http://img183.imageshack.us/img183/6413/sunspots3tv6.th.jpg (http://img183.imageshack.us/my.php?image=sunspots3tv6.jpg)

From space-based Solar telescopes, such as SOHO, we have spectral irradiance data that shows a peak intensity of flux at about 451 nm, which is almost violet.

http://img185.imageshack.us/img185/1600/irrcomparisonnovla9.th.jpg (http://img185.imageshack.us/my.php?image=irrcomparisonnovla9.jpg)

However, like plants with photosynthesis, it is more likely our eyes produce signal rates based on photon flux (see subsequent graph). [The colors are pretty accurate to the respective wavelength.] Since “blue” photons have almost twice the energy of red ones, this adjusts the peak intensity to a level more accommodating to our eye’s color cones. This photon flux peak of the Sun is yellow, as some sites have claimed. However, notice how the graph demonstrates that the general flux across the optical spectrum is nearly flat; it’s a pimple, not a peak! Combining all colors evenly will produce a white image. The slight “yellow pimple” in the graph will not change the white result.

http://img133.imageshack.us/img133/8053/solarfluxwy4.th.jpg (http://img133.imageshack.us/my.php?image=solarfluxwy4.jpg)

Further evidence can be found by imaging the best known Solar Twin: 18 Sco. Defocusing the image at a gradual rate will reduce overexposure of stars. The following, with permission, is a wonderful example imaged by Stefan Seip (Germany) of the Southern Cross.


http://img261.imageshack.us/img261/8261/starscolorexpandedhm0.th.jpg (http://img261.imageshack.us/my.php?image=starscolorexpandedhm0.jpg)

Using this technique on 18 Sco has produced white only results. Since I am a novice astrophotographer, better images are needed, which I intend to get accomplished now that Scorpius is up.

The BA has mentioned some other examples that lend support for a white, or near white, Sun: white clouds and white moon. Water vapor, as seen in white clouds, scatters light evenly and independent of visual wavelength.

The definitive evidence will come if the color spot from a Colorstat - a special colorimeter designed to reproduce any spectral irradiance from a known light source - will produce a color such that its sp. irr. will match the Sun's AM0 sp. irr. It has been built and has produced some true color spots of the Sun that are somewhat questionable. Ok, they kinda suck, but I'm getting there (blue dispersion has been hard to herd into the "spot"). [The new design adds 6000 strands of scrabled fiber optics to homogenize the color of the image our naked eye will observe.] Adding a spectrometer will verify that the color spot observed from the Colorstat is the true color we would see of the Sun, or any object of choice, (at similar intensity). [Ask me more and I'll tell you about it, but I already don't have enough time to make this a short post.]

As for the blue center….

This is due to CLV (center to limb variation). Since we are looking into the atmosphere of the Sun, we can see deeper into the interior of the Sun when we are looking straight into the central region. As we look toward the limb, we can not see as deep into the interior. The temperature difference is significant as a result. The central region is around 6400K and the limb region is about 5000K. [The net result produces an effective temperature of about 5777K and a Planck temperature of about 5850K.]

http://img185.imageshack.us/img185/9447/solarcutawayhb2.th.jpg (http://img185.imageshack.us/my.php?image=solarcutawayhb2.jpg)

http://img107.imageshack.us/img107/1609/solarlayersek8.th.jpg (http://img107.imageshack.us/my.php?image=solarlayersek8.jpg)

I am willing to risk my status as the possible world’s top heliochromologist and go out on a limb (then moving centrally) and claiming the center region of the visual Solar disk is blue-white due to the 6400K central temperature seen at the bottom of the photosphere.

In further demonstration of BAUTer bravado, I claim the solar core will be a rich blue color if it too could be seen at an intensity within normal photopic vision range, and, of course, prior to vaporization of the observer. Similar to the above Solar cut-a-way. In the optical range, it so happens that the Planck temperature of 15 million degrees, or more, is almost identical to a T^4 curve which is the same behavior found in Rayleigh-Tyndall Scattering that produces a blue sky. This too can be tested using the Colorstat as the sp. irr. of our blue sky is known.

I have left out a lot of details regarding why yellow is suspected, why astronauts have not seen this, and general info on how our eye works. But, I felt this might be a good start. My goal would be to get a polished version published. Hopefully, some entity would professionalize the Colorstat, too.

korjik
2007-May-01, 11:26 PM
I am pretty sure that the astronaut I know said that the sun looks yellow in space. Its been a few years but I am pretty sure it has come up.

I would also point out that I think that at least one of the many people who have been in space would have pointed out that the sun is blue in space if it actually was

I would also also like to point out that talking about the color of the sun is kinda pointless. Whatever the peak of the BB curve is, and whatever the peak of the eyes sensitivity is, neither is important seperately. When taken together, the sun is yellow.

If you really want to argue it, could you please tell me what color the grass, the sky, a red rose, or any number of other things really are?

George
2007-May-02, 12:26 AM
I am pretty sure that the astronaut I know said that the sun looks yellow in space. Its been a few years but I am pretty sure it has come up. I believe I read two accounts from astronauts that simply stated it was blinding white.

Since sunlight contains all colors at an intensity that exceeds the upper threshold of our color cones, only white is the logical result. Pehaps, for some, a yellowish-white is possible under such extreme conditions. [We did a poll (http://www.bautforum.com/showthread.php?t=20208)on this a while back.]

[Added: This actually goes further as the retinex system is really what takes the signals from the color cones and renders a color result. One interesting thing about this is how it amplifies blue light. Only a small percent of your color cones are blue, none are in the fovea region that gives us best acuity. Yet, the eye and brain compensate and boost the blue factor almost equal to the green and red signals.]

Astronauts, as far as I know, have never looked at the sun at a normal photopic level without some kind of filter that might alter its true color. They could simply take up a SAD deluxe and get the job done. [Another reason I am reluctant to finish the Colorstat.]


I would also point out that I think that at least one of the many people who have been in space would have pointed out that the sun is blue in space if it actually was. My prior answer should handle this one, too.


Whatever the peak of the BB curve is, and whatever the peak of the eyes sensitivity is, neither is important seperately. When taken together, the sun is yellow. Yes, it is the product of the spectral irradiance of the light source and the spectral sensitivity of our eyes that determine color, if in the normal photopic intensity level. Reflective properties come into play when this source illuminates objects.


If you really want to argue it, could you please tell me what color the grass, the sky, a red rose, or any number of other things really are? No, my argument assumes a coloration based on what the majority of people and cameras would see if in space and able to see the Sun at a reduced intensity level.

Of course, if you go far enough out into space, I forget how far but I calculated it long ago, you could use a telescope and observe this same color view. You would not see a yellow star.

But, you are alluding to the issue that color is subjective, and I accept this, but will still claim that my results are still verifiable.

Hornblower
2007-May-02, 03:16 AM
Our color vision adapts to the color balance of the overall ambient light, which we then see as white if it is a reasonable approach to an incandescent blackbody at 3000K or higher. We can handle a wide range of actual color balance in this way.

I have seen a narrow beam of sunlight strike a piece of white paper alongside of 3200K light bulbs, and it looks bluish white by comparison. I have also projected the Sun onto a screen under a clear blue sky, and then the disk looks slightly yellow overall, and even more so at the limb for reasons stated earlier in this thread. If I do a pinhole projection onto the same screen in a dark room, the disk looks white, period.

These are my personal observations. Others may or may not perceive them the same way. In any case the colors at high elevations are very much pastel, unlike the vivid yellows, oranges and reds we see near the horizon.

Suppose a sharp projection of the Sun is accompanied by ambient light that makes the limb look neutral white. The center will then look bluish white, as mentioned earlier. Unlike the images displayed in this thread, the center would be the brightest part of the disk.

George
2007-May-02, 04:04 AM
Our color vision adapts to the color balance of the overall ambient light, which we then see as white if it is a reasonable approach to an incandescent blackbody at 3000K or higher. We can handle a wide range of actual color balance in this way. Color balance is a very big issue as one adjacent color or tone can really affect the color we perceive.


I have seen a narrow beam of sunlight strike a piece of white paper alongside of 3200K light bulbs, and it looks bluish white by comparison. I have also projected the Sun onto a screen under a clear blue sky, and then the disk looks slightly yellow overall, and even more so at the limb for reasons stated earlier in this thread. If I do a pinhole projection onto the same screen in a dark room, the disk looks white, period. Yes, the dark room approach eliminates the color distortion caused by an adjacent color or background light. Fortunately, if we look at the Sun from space, we will not have this problem. So, I do not believe color balance is a problem, though, perhaps, some people might.

Thanks for sharing that experience.

I once made a dark room in the back yard by using black plastic and duct tape. Unfortunately, it was 100F and I learned duct tape adhesion simply melts away. It was a mess. :)


These are my personal observations. Others may or may not perceive them the same way. In any case the colors at high elevations are very much pastel, unlike the vivid yellows, oranges and reds we see near the horizon. That's interesting. Are blue stars more blue; red stars less red?


Suppose a sharp projection of the Sun is accompanied by ambient light that makes the limb look neutral white. The center will then look bluish white, as mentioned earlier. Unlike the images displayed in this thread, the center would be the brightest part of the disk. Wouldn't the ambient light tend to yellow the limb since ambient light includes more blues from blue sky light, or is it cloudy? :) But, perhaps, this still might produce a bluish center if the projection is large enough. I really don't know.

I don't think these are issues once our observer is up in space looking at the solitary Solar disk.

[BTW, I did add the Solar projection in the first post, as promised.]

Hornblower
2007-May-02, 12:51 PM
Color balance is a very big issue as one adjacent color or tone can really affect the color we perceive.

Yes, the dark room approach eliminates the color distortion caused by an adjacent color or background light. Fortunately, if we look at the Sun from space, we will not have this problem. So, I do not believe color balance is a problem, though, perhaps, some people might.

Thanks for sharing that experience.

I once made a dark room in the back yard by using black plastic and duct tape. Unfortunately, it was 100F and I learned duct tape adhesion simply melts away. It was a mess. :)

That's interesting. Are blue stars more blue; red stars less red?
An A0 star such as Vega, commonly called white by many astronomers, looks slightly blue to me, and B stars even more so. The K and M giants, commonly called red giants, look pastel orange through a telescope. I find that they look somewhat redder to the unaided eye, especially at twilight in a fading blue sky. Class G stars such as Capella look more warm white than real yellow. Once again, my personal observations.


Wouldn't the ambient light tend to yellow the limb since ambient light includes more blues from blue sky light, or is it cloudy? :) But, perhaps, this still might produce a bluish center if the projection is large enough. I really don't know.
In my hypothetical case I am controlling the ambient light to make the limb look neutral by comparison. This could be done indoors with a projection lens instead of a pinhole to get better resolution of the limb, and with the source of the ambient light suitably filtered.


I don't think these are issues once our observer is up in space looking at the solitary Solar disk.

[BTW, I did add the Solar projection in the first post, as promised.]
Out in space, with the Sun in a black sky as the sole source of ambient light, my guess is that we would see "cold" white with a slight yellowing at the limb.

Does this really need to be in ATM? So far it looks like an objective descriptive discussion of our color vision and the characteristics of color film and other imaging as applied to the color of the Sun and other stars.

George
2007-May-03, 01:22 AM
An A0 star such as Vega, commonly called white by many astronomers, looks slightly blue to me, and B stars even more so. The K and M giants, commonly called red giants, look pastel orange through a telescope. I find that they look somewhat redder to the unaided eye, especially at twilight in a fading blue sky. Class G stars such as Capella look more warm white than real yellow. Once again, my personal observations.
That makes sense to me. I imaged Antares using progressive defocussing and obtained a yellowish-orange. I will add my white images from 18 Sco when I get back to my home system in a day or so.

Thanks for sharing your color impressions.


Out in space, with the Sun in a black sky as the sole source of ambient light, my guess is that we would see "cold" white with a slight yellowing at the limb. That is what I used to think because of the 5000K limb Planck temperature. But, since the AM1 to AM2 projections produce a white limb, how can adding more blues and greens give a yellow limb? You may still be correct if indeed the central region is blue in color for the reasons you already gave.


Does this really need to be in ATM? So far it looks like an objective descriptive discussion of our color vision and the characteristics of color film and other imaging as applied to the color of the Sun and other stars. Well, this is still against mainstream conclussion for color, though it doesn't seem to be against any mainstream theory. I guess we are used to anti-mainstream theory in this forum, and, maybe that is a requirement.

It is, in a sense, a colorful theory. :)

Hornblower
2007-May-03, 02:51 AM
Out in space, with the Sun in a black sky as the sole source of ambient light, my guess is that we would see "cold" white with a slight yellowing at the limb.
That is what I used to think because of the 5000K limb Planck temperature. But, since the AM1 to AM2 projections produce a white limb, how can adding more blues and greens give a yellow limb? You may still be correct if indeed the central region is blue in color for the reasons you already gave.
First, a question. What are AM1 and AM2?

Back to my answer. I would be adding blues and greens to the ambient light that is dominating my vision, not to the solar image itself. In my thought experiment in space I am taking the Sun as is, with no atmospheric yellowing, and letting it be the sole source of ambient light in my spacecraft. My visual brain cells adjust to that and see it as white. I then look at the Sun through a perfect neutral density filter. I should see a slightly bluish white center and a slightly yellowish white limb averaging out to the ambient white.

Suppose the spacecraft had no windows except for the filter, and I chose to light the interior with 5000K artificial light which matches the limb. Then I should see a white limb and a center that is bluer than before, since my brain cells would be adjusted to the yellower ambient light.

Note that I am leaving the Sun's actual image unchanged in these hypothetical cases.

George
2007-May-03, 12:34 PM
First, a question. What are AM1 and AM2? I did fail to explain this. AM is Air Mass. From sea level to zenith is one atmosphere of air mass, or AM1. Thus, AM0 would be zero air mass (in space). IIRC, the horizon is about AM38.


Back to my answer. I would be adding blues and greens to the ambient light that is dominating my vision, not to the solar image itself. In my thought experiment in space I am taking the Sun as is, with no atmospheric yellowing, and letting it be the sole source of ambient light in my spacecraft. My visual brain cells adjust to that and see it as white. I then look at the Sun through a perfect neutral density filter. I should see a slightly bluish white center and a slightly yellowish white limb averaging out to the ambient white.
Yes, the brain adjusts for a white color constancy. I am not sure, though, that the effect will be strong enough for the case of seeing the Solar disk at a diminished intensity with only a black background.

If I click on the OP white solar disk to enlarge it, and in a dark room, I do not think I will see a yellow limb. [I will try this when opportunity allows.]

Yet, what you are saying certainly is not incorrect. Changing color temperaure on a camera will do just what you say. The eye works the same way as it adjusts its white standard to the light source. The Solar limb might not be enough change to allow a yellowish view, however.

upriver
2007-May-03, 05:51 PM
Very interesting discussion.

What would be the true color of the photosphere in this image?

http://www.lmsal.com/Press/SPD2003.html

Hornblower
2007-May-03, 08:02 PM
So far my lame attempts at simulating my hypothetical spacecraft view on the computer screen are inconclusive. I looked at a white spot with a variety of pastel backgrounds and it still looked white.

My educated guess in prior posts is just that, based on some of my other observations. Don't take it as gospel. I invite anyone else out there to share pertinent observations with us.

George
2007-May-03, 10:07 PM
What would be the true color of the photosphere in this image?
http://www.lmsal.com/Press/SPD2003.html
Ironically, it is the b & w image that is likely the only correct one. :)

Actually, if the intensity is low enough, the sunspots might look yellow-orange. They are round 4000K which does seem to give enough photon flux advantage to the red end of the spectrum. The penumbra might be yellow, perhaps.