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Re: [Phys-l] sun's true color



Well, how do you define color? The problem is what we see is heavily
influenced by a variety of factors.
1. There are 2 different pigments for one of the cones (green as I recall),
and which you get is genetically determined.
2. The distribution of cones varies from one person to the next.
3. The perception of color varies a lot from one person to the next so that
one person sees grey and another beige.
4. The eye-brain system compensates for the color of the incident light so
that a red square remains red under a variety of illuminations. It does it
by comparing colors across edges. This is why the 3 color model does not
work well for realistic situations. This was demonstrated by Land in the
60s and accounted for by his retinex model. Incidentally you can get fairly
good color reproduction of photos with only 2 colors in a totally darkened
room. So the 3 color model is actually a good model for captured images,
not for the eye.
5. When drawing isolated lines, the number of distinguishable colors is
very small. I recall it is around 8 or so.

I suppose the only "scientific" way to define it is to characterize the
color by the peak in the spectrum. Of course by that measure some stars
would have ultraviolet color, and others infrared. And there should be
"green" stars if the peak falls there. However, the eye might not notice
that.

So it is no wonder we can't agree on the color. The colors of the double
stars might not actually change, but it may be an effect due to a change in
the background which is influenced by atmospheric conditions.

John M. Clement
Houston, TX


If the color of a star is the color we observe with our eyes, then there
are indeed green stars. I'm surprised many astronomers say green stars
don't exist, because many observational astronomy buffs know that they
do. Of course the green color we observe is not the blackbody radiation
color itself, but is the color we perceive after the light has passed
through other stuff, either in the space surrounding the star, or in the
corona itself.

If you want to observe colored stars, it's a lot of fun to observe
double stars of contrasting color. Unfortunately these are often hard
to find by star hopping, but they are easily found if you know their
names or their celestial coordinates, and you have an 8-inch or larger
"goto telescope" that is accurately aligned.

There are two double stars in Hercules that are each classified as as
consisting of one red star and one green star. These are alpha-Hercules
and 95-Hercules. I always try to find these for my astronomy class, and
the students and I all agree that the "green" star truly appears green,
although some will say bluish-green, and the color does seem to vary a
little from time to time.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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