Date: October 13, 2011

Title: Dialogue Concerning the Two Chief Color Views of the Sun

Podcaster: George Cooper and Mike Edwards

Links: http://www.science20.com/solar_fun_of_the_heliochromologist/blog/the_color_of_the_sun

http://www.science20.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

http://solar-center.stanford.edu/suns-color-story/index.html

http://solar-center.stanford.edu/suns-color-story/page22.html

The first three are the links to a three-part blog, which is done a bit tongue-n-cheek. The third is the best of the three and presents the color conclusion, as well as, provides a natural color depiction of the Solar system.

The last two links take you to a multiple choice presentation on the paths taken along a quest for discovering the Sun’s true color at the Stanford Solar Center.

Some portions of the music background are courtesy NASA via the Cassini mission from their “Sounds of Saturn” audio file.

Description: The vast majority of media and text suggest a yellow color for the Sun, yet some textbooks say the Sun is white. The correct color determination for our host star seems to have gone without much scrutiny by the scientific community. So enter the amateurs to tackle this somewhat trivial and subjective topic, which is nevertheless quite interesting.

Bios:
Bio for George Cooper:
Amateur astronomer
Nascent heliochromologist J
Graduate mech. engr. (Texas A&M)
Business owner

Bio for Michael Edwards:
Musician
Home recording enthusiast
Sears employee
Great friend to George

Sponsor: This episode of “365 Days Of Astronomy” is dedicated to the people who promote and teach rational, non-magical thought in all walks of life.

Transcript:

Dialogue of the Two Chief Views of the Sun’s Color

George: Hey Mike, what’s the true color of the Sun?

Mike: It’s a yellow star, at least according to most of what is written and shown in illustrations and images, even in my textbooks, though a few have suggested other colors, including white. But I like yellow because that’s the Crayon color I used as a kid.

George: Ha, Ha. Yeah, but I’m pretty sure a yellow Crayon is not a required color to use for coloring the Sun.

Mike : George, what do you mean by “true color”?

George: The Sun’s true color would be the color most people would see if they were to go up into space, away from our color altering atmosphere, and see the Sun’s disk at a comfortable intensity level.

Mike: Well it sure is bright here and even brighter in space.

George: Yes, the Sun is thousands of times more intense than our photopic vision is capable of dealing with, which makes any color determination with the naked eye essentially impossible.

Mike: Ok. So you must be thinking you might know its color sine you raised this topic. So what color do you think it is?

George: Well, perhaps it’s white. Clouds are normally white and the Moon is white.

Mike: But I’ve seen yellow, pink, or even green-tinted cloud colors. Do clouds truly reflect evenly the same colors that shine upon them?

George: Well, yes, actually. This behavior is described in what is known as Mie Scattering.

Mike: But the atmosphere becomes the problem. It not only alters the colors that the clouds receive but also alters the colors that we observe coming from the clouds. This would also apply to our color perception of the Moon.

George: Yes, it’s our atmosphere that explains why the Sun appears yellow, orange or sometimes even red during a beautiful sunset. The shorter wavelengths of sunlight, primarily blue, will be scattered away so that we will see the longer wavelength colors within sunlight as the Sun sets, simply because the sunlight is passing through more and more atmosphere, so more and more scattering takes place.

Mike: I agree. I understand that this is called Rayleigh Scattering. Overhead, at sea level is one atmosphere, and on the horizon it is almost 40 atmospheres. This also explains why the sky is blue because blue light, which scatters more, is the light we observe. But this doesn’t explain why the Sun is yellow or yellow-white overhead.

George: But the true color fo the Sun must be seen without any atmospheric effect. We have to consider all colors emitted by the Sun in space before sunlight reaches our atmosphere. It is not surprising that the Sun emits all the colors of the spectrum. Some have said that this is a good argument for the Sun being a white star since a combination of all the colors makes a white object.

Mike: Well, that’s a bit incomplete. In fact, every star emits all of the colors of the visible spectrum including blue, orange, red, and, of course, yellow.

George: Right. That’s a good point. It’s really the combination of these colors and, more importantly, the intensity of each color that determines the star’s true color. The cooler stars like Red Giants, for instance, produce much more intensity in the red end of the spectrum, so when you combine, in this case, the weaker other colors emitted by a red star, you will still see its color as orange or red. Similarly, the hotter blue stars are stronger in the blue end of the spectrum. Thus, they appear bluish-white. So it’s the dominant color or colors that determine a star’s true color.

Mike: True, but the peak wavelength of the Sun, I have heard, is in the green portion of the spectrum. So, George, are you now saying the Sun is a green star?

George: No. That is a misnomer. The peak wavelength of the Sun is not green. If you calculate a peak wavelength based on the Sun’s known effective temperature of 5,777 kelvins, which is about 10,000 degrees Fahrenheit, you will derive the green peak you stated. But the calculation works only for ideal radiators. The Sun is not an ideal blackbody radiator. In fact, the actual peak wavelength measurements for the Sun, as observed from space telescopes, can vary in wavelength from 450 nanometers to 480 nanometers depending on whose dataset you use. This result is a range of colors that varies from blue-violet to a blue-green color. No actual peak is in the green portion of the spectrum.

Mike: Well I’m surprised that there is such a wide range between the datasets. I thought this was the age of precision astronomy, so why such a range?

George: This range is due to the fact that the Sun doesn’t really have any sharp peaks. The range mentioned is more of a plateau of a bunch of little peaks. The Sun’s intensity is quite strong within this entire range. However, once again, it’s not the peak that determines the Sun’s true color but the combined intensity of all the colors emitted by the Sun, or any star for that matter.

Mike: Well, what about color computer models that can calculate a color result based on the Sun’s known spectrum?

George: Yes, and one popular result claims the Sun’s color is…peachy pink. Yeesh! Is it a girl star? Computer modeling for color is very difficult because it has to take into consideration not only the object’s spectrum but also the color sensitivity of the eye and brain. I am doubtful of the few results that I have seen from computer models.

Mike: So tell me George, how are we ever going to know the Sun’s true color?

George: Surprisingly, the answer is fairly simple. I finally realized it after staying up all night taking images of stars known as solar twins, such as 18 Sco, a star in the constellation Scorpius. The next day, my wife and I toured the McMath-Pierce Solar telescope at Kitt Peak Observatories near Tucson. They had a large projected image of the Sun on a pristine white table top, and the image of the Sun was completely white.

So here’s the key! Since we know that the atmosphere scatters primarily blue light, as we discussed earlier, then to determine the true color of the Sun as seen in space, we need to add these predominantly blue colors that our atmosphere has taken away back into this white image. Therefore, if we add blue colors to a white object, could we ever get a yellow or yellow-white result?

Mike: Nope. You’re right! That’s sure logical. It’s impossible to get yellow from adding blue and white together. But wait, were they using any filters on this telescope?

George: Nope, I checked. They were not using any filters.

Also, when we looked at their telescope’s high quality projected image, there was no hint of any yellow color, even along the edge, or limb, of the Sun. This is important because the temperature along the Sun’s surface varies considerably. It is about 5,000 kelvins, or about 8,500 degrees Fahrenheit, at the limb and almost 6,400 kelvins, which is about 11,000 degrees Fahrenheit, at the central zone of the Sun’s disk.

Mike: That’s surprising. Why such a variation?

George: Astronomers call it the CLV, or center-to-limb variation. It’s because the Sun, like all stars, is a ball of gas, so, at the central disk region, we are seeing deeper into the Sun’s interior where the temperatures are hotter than when we look along the Sun’s limb. So if the cooler limb region, which is simply the upper most portion of the photosphere, has not hint of yellow or yellowish-white, then there is no hope for the Sun to ever remotely have a color of any tint of yellow as seen from space.

So the Sun is, indeed, a plain white star.

Interestingly, if you want a little more evidence, though a little complicated, since the eye’s response is best represented by photon flux, it turns out that if you convert the energy intensity for each wavelength of the Sun’s spectrum to a photon flux, you will discover that the photon plot is almost flat as a pancake, except at the blue end of the spectrum where the photon flux is weaker than the other colors.

This surprisingly weaker blue end of the photon flux distribution means that after we add the blues back into our white disk image, they will not be enough to overcome the other colors to make the Sun a bluish-white star. The Sun will still appear as white.

Mike: Agreed. So our Sun is, indeed, a plain white star!
Perhaps, though, as I recall, Newton once said that sunlight is “perfectly white”.

George: Oh, you stink sore?

Mike: I nose sore.

End of podcast:

365 Days of Astronomy
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