Dec 14 2007
What Color is the Sun?
Last summer my mother visited me, and I took her to Kitt Peak for their nightly observing program. I would highly recommend this to anyone visiting the Tucson area. She seemed to really enjoy looking at the stars and learning about them. For Christmas this year I bought her Bad Astronomy by Philip Plait. The good thing about buying someone a book is that you get to read it before sending it away. I’m only through the first few chapters, but it’s a good read so far.
Anyway, back to the topic at hand. What color is the sun? Phil tackles this question in his book, but I’m going to comment here as well. I’ve been asked this question a few times, but mostly I like asking it to others. The reason? Because it doesn’t have an answer. Pretty much any answer given will be correct - unless you’re Sean Connery on Celebrity Jeopardy. The actual answer to the question doesn’t really matter, it’s the follow up - explaining that the sun doesn’t produce just one color, it produces lots of colors.
Below is the solar spectrum (courtesy Global Warming Art, see linked page for copyright information). The yellow “curve” is the radiation from the sun that is intercepted by the Earth at the top of the atmosphere. On its way to the ground, some of the radiation is absorbed or scattered. The amount of radiation we see at the ground is the red “curve”.
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What does this mean in terms of the question posed above? Well, as long as the person answering gives a color that corresponds to a wavelength between 250-2500+ nanometers they are right. My answer to the question of the color of the sun is always the same: green.
People always look at me weird when I say the sun is green, and I understand why. If you answer that the color of the sun is yellow or white people will think you’re normal. That’s because the sun does look white or yellow. But look again at the solar spectrum above. The peak is about 500nm. Do you know what color that is?
“Colors” of Light |
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| Ultraviolet Violet Blue Blue-green Green Yellow-green Yellow Orange Red Infra-red |
380-450 450-500 500-520 520-550 550-570 570-600 600-630 630-680 >680 |
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According to this page from North Carolina State University (from whence the table is derived), 500 nanometer light is blue-green! Actual 500 nanometer light is actually more green than the color of the font in the table. To me, it appears almost like the color used in the clocks on microwaves. Since I describe that color as green, I therefore describe the color of the sun to be green.
Of course, this doesn’t actually mean the sun appears green. I just went outside and looked. The sun still appears white. Don’t look directly at the sun! Ever! The reason the sun appears white is that the sun produces light in all the visible spectrum. And when those differently colored wavelengths of light are combined, they produce white - the opposite of shining white light through a prism.
The moral of the story? If some wiseass asks you what color the sun is, beat him/her to the punch and say it’s green. 
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And not coincidentally, our (for family-tree-wide definitions of ‘our’) body clock gets reset by daylight — blue-green.
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2082413
“… A fifth retinal photopigment, melanopsin, was recently discovered [17] and shown to be expressed in retinal ganglion cells (RGC) that are intrinsically light sensitive [18], with a maximum sensitivity between 420 to 480 nm …”
[brain dump of related links and tidbits here:
http://psychcentral.com/blog/archives/2007/03/12/light-and-dark/ ]
The sun above the atmosphere goes well into the vacuum ultraviolet, but the important point is that there is a humongous line at 124 nm, Lyman alpha (the first allowed transition in hydrogen) and smaller ones on the other Lyman series lines. These drive a lot of the ionization and ion chemistry in upper atomospheres. It is also a very variable part of the solar irradiance.