Solar Disk in True Color
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:
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.
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.
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.
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.
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.]
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.
Last edited by George; 2007-May-02 at 03:48 AM.
We know time flies, we just can't see its wings.