# The Yellow Sun

#### Helio

Perhaps for posterity, we should document some of the more egregious errors made by those who should know better regarding the Sun's color.

Here is a NASA video (not recent, apparently) colorfully showing how star temperatures are related to the color of stars.

The first part of the video correctly states, "Though the Sun's visible light appears white, it is really the combined light of the individual rainbow colors..." Also, that Newton (1665) demonstrated that "white is made of the bands of colored light..."

Elsewhere, one can find where Newton called this light, "Perfectly white".

Here is snip from the video...

At about 2:25 min. later, the Sun suddenly becomes very yellow. "Our Sun produces more yellow light than any other color because of its surface temperature."

But this claim is incorrect if one is looking at a typical energy distribution (spectral irradiance). Here's an example, with color bands added:

But, ironically, if one converts the energy distribution to a photon flux density distribution, then there is a peak in the narrow band of yellow. But I argue this is not a peak, but a pimple!

The video expands on the Sun's color to the other color of stars, appropriately noting that the cooler stars will appear more red and the hotter ones blue. But their coloring index is primarily yellow and there is no white star in this index!%#\$^2

#### billslugg

Sunlight is yellow in comparison to an LED flash light. We have a tiny window near the roof in our machine room, 65 feet in the air. During the day it projects a small circle of sunlight on the floor. If I shine a circle of my LED flashlight next to it, the yellowness of the sunlight is obvious.

#### Helio

Sunlight is yellow in comparison to an LED flash light. We have a tiny window near the roof in our machine room, 65 feet in the air. During the day it projects a small circle of sunlight on the floor. If I shine a circle of my LED flashlight next to it, the yellowness of the sunlight is obvious.
That’s cool. Can you give us a picture of this?

If you shine a bright yellowish light next to it, you might see a blue shift.

“Color constancy” describes how the brightest light will establish it as the “white source”causing our brains to shift colors for all else. Camera processing is very similar, apparently.

But, to describe the actual color of sunlight, it should be seen independent of these effects. Make a clean hole in a thin plate and use it to project a solar image through it. If the Sun is high in the sky, you will get only a white disk.

#### billslugg

Can't take pictures inside the plant. P&G security you know. I will try and replicate the situation somewhere else though. The key is to get a spot of sunshine on a white sheet of paper and have no illumination next to it so the flashlight can be shone there.
And, yes, the eye adjusts to the current white it is looking at. "Dance with the one who brung you". It is only a side by side comparison makes one look yellow and the other look whiter.

P&G is expert at making white clothing look white. Our Tide detergent has phosphors in it that absorb ambient UV light and re-emit it in the blue portion of the spectrum. Clothing washed in Tide will glow brightly under black lighting.

I was the youngest manager ever hired by P&G, held the record for 16 years. I was in the first incoming class (June '74) after Nixon signed Title 9. Prior to that, all P&G managers (with a few token exceptions) were white, male, ex-military with one of the five basic engineering degrees. All incoming managers prior to that were age 27-28. Our class was age 21. I was an October baby, Dad went to the school board and got me into First Grade at age 5 yr 10 mo. I was 17 when I graduated high school, 21 years 7 months when I started making paper for the company. I was the 82,849th manager hired since Wm Procter in 1837.

Today I am 69 and work for a contractor in a P&G plant, providing engineering consulting, operating the machinery, sweeping floors, cleaning U-drains, whatever they need. At a plant of 1700 people, I am the senior person. Not the oldest, there are two others age 71. Not the most years with P&G, I only had 24 years (left at age 45). But no one in the plant has an earlier P&G start date than me. I am treated with great respect. They call me "Mr. Bill". COVID has decimated us personnel wise and skill wise. I am glad to be able to help out.

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#### Helio

The key is to get a spot of sunshine on a white sheet of paper and have no illumination next to it so the flashlight can be shone there.
There are some cool optical illusions sites that ”dance” with colors,

Nice personal story! We‘re the same age.

#### billslugg

I have a good picture of the "yellow sunlight" but I don't know how to post it here. The only option appears to be to post an internet link. I used to use photobucket but it is a pay site now. Any ideas?

#### COLGeek

Moderator
I have a good picture of the "yellow sunlight" but I don't know how to post it here. The only option appears to be to post an internet link. I used to use photobucket but it is a pay site now. Any ideas?
Use imgur.com

billslugg

#### billslugg

Thanks to COLGeek!
This is a picture of an LED light on the left and sunlight on the right. The sun was only about two hours shy of sunset so some yellowing is due to atmospheric absorption of the blue. I have done this with the Sun high in the sky and it looks about the same.

View: https://imgur.com/a/HPiqVsy

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Helio

#### Helio

Nice. !

Camera processors attempt to match our vision (retinex).

Just to clarify. Removing the LED light, does the sunlight not look white?

I like to note how yellow car headlights (tungsten) are in broad daylight, but are nearly white at night.

#### billslugg

Yes, looking a just the sunlight, it looks white as can be. The LED ruins the party.

People with super white teeth annoy me. It looks so fake. A certain amount of yellow is normal, looks good. Pure bluish white -- yuccck!

Helio

#### Helio

To test the limits of color constancy, during a sunny day, one can hold a white sheet of paper while under a shade tree and it will appear bluish-white, not white.

#### billslugg

Yes, the illumination of the blue sky adds a blue tint. This is why diamond graders use northern skylight to grade diamonds. Big windows on the north side of a building, no direct sunlight.

#### Unclear Engineer

This discussion seems to be dancing around the definition of "white" light without really saying that it is a function of the spectral sensitivity of the cells in our eyes and our brains much more than the spectrum of the light emitted from the source.

So long as there are wavelengths that stimulate the light sensing cells in our eyes, our brains will try to differentiate differences in the areas that have different spectral distributions as well as different intensities, and perceive them as differences from the average.

Some people lack one or more of the 3 different cell types that have different peaks in their spectral sensitivities. We call such people "color blind", and subdivide them into groups like "red-green" when they cannot distinguish between those 2 colors. But, without them being tested to determine that they cannot see the difference between 2 colors, they think that they see "white" light just like anybody else. Fundamentally, they have a different definition of white that is based on the range of what they can perceive.

Similarly, all of us are blind to infrared and ultraviolet light frequencies, not to mention microwaves and gamma rays, all of which are still electromagnetic radiation. But, if there is life around other stars with much different spectra for their light emissions, we should expect each life form to "see" some main portion of the spectrum of their stars emissions, as filtered by their environment, as "white", because it is the differences from that "color" that provides useful cues to the situations around them.

For scientific purposes, an objective description of the color of a star can be based on its spectrum measured in light wave lengths and compared to the spectrum of wavelengths predicted to come from a "black body" (perfect emitter) of a particular temperature.

But, then again, language tends to get confusing for the general population. They may well wonder why we call something shining like mad a "black body" while also calling something that cannot emit any light a "black hole".

Our language is not only unconcise, it isn't even really consistent.

#### Helio

This discussion seems to be dancing around the definition of "white" light without really saying that it is a function of the spectral sensitivity of the cells in our eyes and our brains much more than the spectrum of the light emitted from the source.
Indeed, color is the product of spectral emissions and spectral eye sensitivity. There is some variation in eye sensitivity. With illumination, of course, the spectral properties of the material and even their size will alter what is reflected. Some birds, for instance, will appear blue because they have tiny particles that force blue light to be scattered (Rayleigh scattering). So, three variables are included in any reflecting color determination.

So long as there are wavelengths that stimulate the light sensing cells in our eyes, our brains will try to differentiate differences in the areas that have different spectral distributions as well as different intensities, and perceive them as differences from the average.
Yes and the lighting industry takes advantage of this by making more economical light bulbs that don't attempt to emit the entire spectrum but only that which is necessary to give us a white result.

Stars, of course, by definition will always emit light close to that of a blackbody.

Some people lack one or more of the 3 different cell types that have different peaks in their spectral sensitivities. We call such people "color blind", and subdivide them into groups like "red-green" when they cannot distinguish between those 2 colors. But, without them being tested to determine that they cannot see the difference between 2 colors, they think that they see "white" light just like anybody else. Fundamentally, they have a different definition of white that is based on the range of what they can perceive.
Right, and this seems to be understood so that we can argue that color is far more objective than subjective, especially with the vast majority of people.

Similarly, all of us are blind to infrared and ultraviolet light frequencies, not to mention microwaves and gamma rays, all of which are still electromagnetic radiation. But, if there is life around other stars with much different spectra for their light emissions, we should expect each life form to "see" some main portion of the spectrum of their stars emissions, as filtered by their environment, as "white", because it is the differences from that "color" that provides useful cues to the situations around them.
Agreed, but it's not a requirement due to the passive nature of natural selection, IMO. Snakes and other animals take advantage of the daylight beholders, and white tail deer see quite well in the near UV. Perhaps white tails may even improve in their UV vision as those that have better UV vision will more often notice hunters climbing in their deer blind (as I often have) when it is "dark" near astronomical sunrise, but not dark to the deer due to their ability to see the strong UV scattering in the atmosphere.

For scientific purposes, an objective description of the color of a star can be based on its spectrum measured in light wave lengths and compared to the spectrum of wavelengths predicted to come from a "black body" (perfect emitter) of a particular temperature.
Well... when this has been done, using computer models, one result (Boulder, CO) is "peachy pink", though I think they called it pinkish peach.

One problem, perhaps, is that stars are only close to blackbody emitters. The Sun for instance would have a peak wavelength in the light green (495.34nm using 5850K, or 501.6nm using 5777K).

But the actual peak is in the 470's nm. One data set shows it at 480nm. But the peak is always in the blue, not green, and certainly not yellow.

But, of course, we don't see it as a somewhat saturated blue star because of the points you make about spectral sensitivity across the spectrum. Seeing all these colors must be accounted for in determining the net color observed.

I've heard people, who should know better, say the Sun is whiter because it emits all the colors of the rainbow. But how dumb is that when we know that all stars emit all the colors of the rainbow. They aren't dumb, of course, but lazy on this topic.

The star color we see is, once again per your statement, requires not only what colors we observe but their relative intensity to one another. The flavor of cookie is not just what goes into it but how much of each ingredient.

Stars, especially ones like the Sun, are better understood (for color) when using a photon flux distribution compared to an energy distribution. Here (below) is what that looks like for the Sun. This is the conversion of a sp. irr. distribution taken from space (no atmospheric extinctions).

Such even distribution argues strongly for a white result, certainly not a yellow color result.

But, then again, language tends to get confusing for the general population. They may well wonder why we call something shining like mad a "black body" while also calling something that cannot emit any light a "black hole".
Agreed, and since they aren't really blackbodies, then all the more reason not to use this term except when deeper science is desired.

Our language is not only unconcise, it isn't even really consistent.
Yep, and too often my language gets in the way of what I'm saying.

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#### Helio

I might as well throw in for fun an instrument design that allows us to know fully the true color of objects, including exoplanets, for that matter.

It is the asterochromograph.

With any known reference light source, its spectrum can be altered with the use of a simple mask that adjusts each color to match that of a target's (star or exoplanet) known spectrum. An actual bright spot is produced to allow our eyes to make the color determination, superior to any computer model, IMO.

To verify the "spot" result is correct, a spectrometer can be used on this spot to produce a SED that is compared to the SED of exoplanet for a tight fit.

Here is a crude prototype I tried in my hunt for the Sun's color some years ago...

Here is the use of the solar spectrum that is altered by the use of the mask to produce the solar spectrum as seen in space. Notice how more blue light is allowed to pass to offset the loss of blue light due to atmospheric scattering. [The alignment makes use of the narrow yellow band of the Sun's spectrum.]

A "scrambler", using fiber optics, was being built to homogenize the light passing the mask, but when it became abundantly clear that the question of the Sun's color has a very simple solution, then I abandoned it.

I'm convinced a professional quality instrument could be made without great expense, and it might be useful for exoplanet science.

Catastrophe

#### Catastrophe

##### "There never was a good war, or a bad peace."
Helio, I would like to see larger versions of those pics. Clicking does not help (I can usually get to almost any size). The first gives me a slightly larger image bottom left, the other just drops me into an error frame.
Usually, when in such difficulty, I copy image and drop it into left imgur box, copy, copy from http etc., and then enter that here.

Cat

#### Catastrophe

##### "There never was a good war, or a bad peace."
Imgur: The magic of the Internet

There you go: click on ". . . " and download, and then keep hitting + option.

Just checked that. All OK, but you must click on open after download.

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#### Helio

Helio, I would like to see larger versions of those pics. Clicking does not help (I can usually get to almost any size). The first gives me a slightly larger image bottom left, the other just drops me into an error frame.
Usually, when in such difficulty, I copy image and drop it into left imgur box, copy, copy from http etc., and then enter that here.
The first two work for me, but I wanted to reduce them from the large image in the prior post, so I tried something different, but it didn't work properly (3rd image).

Does this work?,,,

#### Helio

Imgur: The magic of the Internet

There you go: click on ". . . " and download, and then keep hitting + option.

Just checked that. All OK, but you must click on open after download.
I tried that but the thumbnail image resolution is too poor to make use of the download.

I also clicked on "Next" and got a dancing person in a frog outfit. I assume those that think the Sun is green were having a party?

#### Catastrophe

##### "There never was a good war, or a bad peace."
OK #17 you can enlarge until it goes off the edges of the screen and goes all out of focus. Works on every image I ever wanted to see enlarged.

Cat

#### Helio

OK #17 you can enlarge until it goes off the edges of the screen and goes all out of focus. Works on every image I ever wanted to see enlarged.
I am confident the problem is my use of a low resolution thumbnail version. It serves as mainly a link to the image on the site of the host. This reduces the storage space for Space.com, btw, which they might appreciate.

#### Catastrophe

##### "There never was a good war, or a bad peace."
I used that very image to enlarge 'out of frame', but I cannot show it here.

Cat

#### Catastrophe

##### "There never was a good war, or a bad peace."
Great. I see you noticed my post. Sometimes (like now) it does show on screen.

Cat

#### Helio

Great. I see you noticed my post. Sometimes (like now) it does show on screen.
Do you prefer the large format in a post vs. the thumbnail version (assuming they can be made to work properly)?

I prefer the thumbnail images.