The True Color of the Sun

[Helio]

This thread moves a few posts from another in order to focus on the Sun's color....

 

[Helio]

To my knowledge, there is no formal paper on the Sun’s true color — defined as the color we would see of the solar disk when viewed from above our atmosphere and properly attenuated. I doubt there needs to be a paper, to be fair.

My avatar is an image from the McMath-Pierce solar observatory that demonstrably falsifies the yellow hypothesis that became incredibly ubiquitous.

Also, how it became “yellow” is quite a story born out of early stellar spectroscopy by Rutherford, Secchi and others.

 

[Helio]

From rod...
Helio, FYI. I view the Sun regularly using my 90-mm refractor with glass, white light solar filter that fits on the dew cap in front of the telescope optics. Common observations range 30x to 70x or so in my stargazing log I keep. I viewed the Sun seven times in June (6th-16th), observing sunspot AR2765 reported at spaceweather.com. When I see the Sun, at 0800 the Sun appears somewhat yellow with tint of slight orange around the limb. Later viewing the Sun near 1400 EDT, much higher elevation angle and the Sun appears more white with slight tint of yellow around the limb. I do not observe using H-alpha filter though. However, this is not solar observing *above our atmosphere*

 

[Helio]

rod, have you ever projected the solar image on a white or "gray" card? If not, just do a 2mm diameter hole in a card and allow sunlight to project the image. For a higher altitude Sun, you won't see any yellow, though by diffraction at the edge, perhaps you will. Refractors and especially filters will change the color. The limb coloring you note is because of the CLV (Center to Limb Variation). The limb temperature is that of the upper photosphere and it is cooler (5000K) whereas the lower region of the photosphere is seen when viewing the central region (6,390K) -- that's a lot of temperature difference. Yet even a 5000K blackbody doesn't produce a yellow result unless an atmosphere or filter is used.

I think the Baader filters give a white Sun, except near the horizon. My filter also gives a yellow results.

 

[Helio]

For those who take a literal interpretation of the Sun's description as a "yellow dwarf G2V" star, and many do, then it may helpful to know that the "yellow" is more about how the Sun's spectral lines match other stars of similar color. Rutherford (1860s), apprently, classified stars by their color first and then their spectral lines. Fr. Secchi, who is likely held to be the father of stellar classification, used a different scheme based on stars of similar spectra regardless of their colors.

The solution to the true color of the Sun -- defined as its disk's color as it would appear to us if we could see it from space and properly attenuated for normal vision, is demonstrably not yellow and the solution is, fortunately, easy to see.

To cut to the final leg of the chase.... my avatar is from a picture taken by Dr. Potter and Roy Lorenz of the Sun on their projection table at the McMath-Pierce observatory at Kitt Peak. [Until recently it was the world's largest solar telescope.] They also measured its intensity with a light meter to confirm it wasn't too bright to see any color variations.

Also, there were no filters used in the telescope. This image is as it appears for those who take the tour into the projection room. [They were adjusting the tracking system and only a dime-size image was on the large projection table. I was curious what it was and was told it was a projection of Venus. Pretty cool!] The color plastics in this avatar image are there to demonstrate that this as a color image and not a b&w image, and for any necessary color calibration.

But, and this is key, the white color we see is a result that comes after sunlight has encountered color extinctions caused by our atmosphere's scattering effects. When the Sun is near the horizon, we know it goes through a color metamorphosis due to Rayleigh Scattering effects. The scattering that take place is well known to be blue-end of the spectrum, which is why the sky is blue. [Ozone also causes a blue effect after sunset, surprisingly -- Chappuis effect.]

And, those spectral extinctions are well-defined and they vary with altitude. Thus, to get the true color of the Sun, those atmospheric extinctions must be added to the color seen in the avatar image, but we don't need to get fancy, fortunately, since we know what colors are removed from the sunlight - mainly the blues.

So, if we add mainly blue light to a white image, is there any way yellow could be the result? Nope! The Sun will never appear to the eye in space as a yellow star. QED

Surprisingly, I've witnessed astronomers and at least one very active solar amateur argue that the Sun is white because it contains all the colors of the spectrum, so it must be white. But, all stars emit all the visual colors, so by that overly simplistic view, then all stars are white, including red stars, yellow stars and blue stars. Ug. They know better, so it reveals how this topic is a discussion of scraps that fell of the lab table; not that big a deal, apparently. To be fair, the Sun's true color won't change one iota of science, but it's still interesting. And it would be nice to see a non-yellow Sun on occasion especially if it is matched with properly colored planets.

Color is the product of the spectral emissions of the light source and the spectral reception of the eye/brain (retinex). [For reflective surfaces, those surface properties will also need to be taken into consideration, which may need to consider Rayleigh Scattering at the surface as some birds exhibit.]

But for color rendering, the product is two dimensional: wavelength and intensity. Red stars, for instance, emit all the colors but the blue part of their spectrum is very weak.

BTW, most of the red stars appear orange to most people, but not all, so there are subjective elements that can come into play for star coloring, but, IMO, not the Sun. Some have said, well, since color is subjective then we can't really argue true colors. But that is only partially true and in some cases; is a red apple not red?

Further, the human eye system has incredible color correction ability, not unlike most cameras. The eye very quickly captures the whitest and brightest objects to help the brain adjust all the colors accordingly. For instance, If you look at an older car's yellowish looking headlight in the daytime, then look at it at night, you'd think someone changed the bulb because it will appear noticeably whiter. This is called color constancy.

So, with that said, even if the Sun had a hint of yellow, the human eye system would color-shift it to a image, especially given the lack of brighter objects nearby of a different color.

This topic is interesting because we can get into Planck distributions, computer color modeling, and other issues but I am convinced the avatar image nicely demonstrates how the Sun can't possibly be a yellow, or yellow-white star.

[So, by adding blue light to this image, could it appear blue, say in the hotter central region? That's another story.]

Also, the history of star color and classification is remarkable because, for instance, photography was black and white when Canon did her remarkable work classifying perhaps 300,00 stars by their tiny b&w spectra she saw on all those glass plates.