Starship Asterisk*

Looking at the sun spectrum I could not help but wonder if the reason the Yellow-Green portion looks brighter is just because that is where the human eye is the most sensitive? What would this look like it it was corrected for this facet of human vision? It did get my interest that there were some areas where there were some rather obvious gaps with apparently no emission at some wavelengths. Are these missing elements or are there no elements which radiate at these wavelengths? This might be interesting to compare with the emission spectra of, say, a blue giant.
Bill Baka

Hi and welcome, Bill!

The yellow-green part of the solar spectrum is actually the brightest; radiation intensity peaks at about 500nm (varying between 483 and 520 nm), a wavelength which, on its own, would appear green-yellow.

Reproduced below is an image of the sun's radiation profile and its best-fit blackbody curve ("Solar Blackbody" link in the left navbar) from http://climate.gsfc.nasa.gov/~cahalan/Radiation/


A blackbody is an idealized object that absorbs all incident wavelengths - hence "black" - and is capable of emitting at all wavelengths. Ideal blackbody radiation peaks at a certain wavelength given by Wien's displacement law:

wavelength = (2.9 * 10^-3 m K) / T

Stars and the Sun are approximate blackbodies because a plot of intensity versus wavelength (or frequency; wavelength = c / frequency) produces a curve similar to the radiation curve of a blackbody of a particular temperature.

What's puzzling to me is that the Sun certainly does not appear green-yellow despite peaking at a green-yellow wavelength. The Sun looks more or less white, from what I've observed by accidentally glancing skyward on clear days. You bring up an interesting point regarding the sensitivity of the human eye to various wavelengths and how it affects our perception of solar radiation. Having evolved under the Sun's particular radiation profile, do humans perceive it as the "color" "white"?

Also, the Earth's atmosphere preferentially scatters blue sunlight, resulting in a blue sky and a less blue (thus yellower) image of the Sun. Is this effect significant enough that the sun appears bluish from above the Earth's atmosphere because humans are adapted to see yellowed sunlight as white? (I'm guessing "no")

The dark gaps in the solar spectrum ("Fraunhofer lines") are caused by neutral elements in the sun's atmosphere absorbing certain wavelengths of light and re-emitting them in accordance with kirchoff's laws of spectroscopy: a continuous electromagnetic spectrum source viewed through a "cool", low-density gas produces an absorption-line spectrum, as shown in the APOD. The sun's photosphere emits a continuous spectrum, but certain wavelengths are absorbed as the light passes through the photosphere's cooler upper layers. Each element absorbs characteristic wavelengths, which is how the chemical composition of the Sun and stars is determined.

The APOD does not seem to show CO2, ozone, and other absorption bands that the Earth's atmosphere would produce...How did the McMath-Pierce Solar Telescope scientists do that?

Blue giants as well as stars of other spectral types have characteristic spectra from which astronomers can determine attributes like chemical composition, radial velocity, atmospheric pressure (leading to luminosity class), and magnetic field strength (Zeeman effect): http://www.ph.surrey.ac.uk/astrophysics ... scopy.html

Is there a formal definition of "white"?

Perhaps there's a mathematical definition, and another which relates to human perception. Photographers will know this through the use of "AWB" on cameras (Automatic White Balance).