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ngc3314
2004-Nov-11, 10:43 PM
At the risk of general brickbats - I just tried a simple numerical experiment on this pressing and vexing question, to see what sort of answers might not require detracting from the view on someone's suborbital flight. George Jacoby, Deidre Hunter, and Carol Christian published a digital atlas of well-calibrated stellar spectra some years ago, which I have kept a set of FITS files from. I just tried reddening some of these stars corresponding to zenith atmospheric extinction at Kitt Peak, to see what spectral type has a color as we see it which best matches the Sun above the atmosphere (alright, so I took some other G2 V star). To be picky, I asked where the ratio of (star below atmosphere/Sun above atmosphere) was flattest with wavelength across the range of best sensitivity for our eyes, 450-650 nm. The answer - ta-daah - is that above the atmosphere, the Sun has the color that we see down here from a star of spectral type F6 or F7. The brightest stars in about this range would be Procyon (F5), Polaris (F8), and both Delta CMa and Gamma Cygni (F8). So my answer, based mostly on how I see Procyon, would be "just about white". Not as blue as Canopus and not as red as Alpha Centauri (big duh!) among 1st-magnitude stars.

George
2004-Nov-11, 11:20 PM
Hmmmm. What a silly endeavor? What scientific contribution can come of this? Very strange from someone of your calibre. [-X
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Ok. Now that I am below the screen, YEAH!!!! =D> =D> Finally. Some cool work into the esoteric realm of solar chromatics as a function of rhodopsin activity.

Can't tell you how delighted I am to see this. 8) =D> Thank you, Thank you!

Now, can you explain it to me? #-o


At the risk of general brickbats - I just tried a simple numerical experiment on this pressing and vexing question, to see what sort of answers might not require detracting from the view on someone's suborbital flight. George Jacoby, Deidre Hunter, and Carol Christian published a digital atlas of well-calibrated stellar spectra some years ago, which I have kept a set of FITS files from.
Are these "solar twin" irradiance plots?


I just tried reddening some of these stars corresponding to zenith atmospheric extinction at Kitt Peak, to see what spectral type has a color as we see it which best matches the Sun above the atmosphere (alright, so I took some other G2 V star). To be picky, I asked where the ratio of (star below atmosphere/Sun above atmosphere) was flattest with wavelength across the range of best sensitivity for our eyes, 450-650 nm.
Isn't the ratio wavelength dependent? Above is almost a nice blackbody (peaking on the edge of blue). Below the atmosphere, the irradiance is pretty flat. The ratio is greatest at blue, less at green, much less from yelow to red.


The answer - ta-daah - is that above the atmosphere, the Sun has the color that we see down here from a star of spectral type F6 or F7. The brightest stars in about this range would be Procyon (F5), Polaris (F8), and both Delta CMa and Gamma Cygni (F8). So my answer, based mostly on how I see Procyon, would be "just about white". Not as blue as Canopus and not as red as Alpha Centauri (big duh!) among 1st-magnitude stars.
Is that a bluish white?

Are there any likely "true" (or "natural") color images of any of these twins from Hubble or other calibrated imager?

ngc3314
2004-Nov-12, 02:47 PM
At the risk of general brickbats - I just tried a simple numerical experiment on this pressing and vexing question, to see what sort of answers might not require detracting from the view on someone's suborbital flight. George Jacoby, Deidre Hunter, and Carol Christian published a digital atlas of well-calibrated stellar spectra some years ago, which I have kept a set of FITS files from.
Are these "solar twin" irradiance plots?

Sort of - except for other stars. For convenience, I used their G2 V star (HD 66171) instead of ours. In each case, it's a table of energy per unit wavelength versus wavelength, all sampled at the same points in the spectrum.


I just tried reddening some of these stars corresponding to zenith atmospheric extinction at Kitt Peak, to see what spectral type has a color as we see it which best matches the Sun above the atmosphere (alright, so I took some other G2 V star). To be picky, I asked where the ratio of (star below atmosphere/Sun above atmosphere) was flattest with wavelength across the range of best sensitivity for our eyes, 450-650 nm.


Isn't the ratio wavelength dependent? Above is almost a nice blackbody (peaking on the edge of blue). Below the atmosphere, the irradiance is pretty flat. The ratio is greatest at blue, less at green, much less from yelow to red.

Indeed. What I was checking, and sort of surprised to find, was how much atmospheric extinction can compensate for temperature changes within the visual band. Around F6-F7, the ratio is flat within a few percent, smoothing out the ripples due to spectral lines mismatching. The difference becomes dramatic as you go into the UV, but that doesn't particularly enter for visual color.




The answer - ta-daah - is that above the atmosphere, the Sun has the color that we see down here from a star of spectral type F6 or F7. The brightest stars in about this range would be Procyon (F5), Polaris (F8), and both Delta CMa and Gamma Cygni (F8). So my answer, based mostly on how I see Procyon, would be "just about white". Not as blue as Canopus and not as red as Alpha Centauri (big duh!) among 1st-magnitude stars.
Is that a bluish white?

Are there any likely "true" (or "natural") color images of any of these twins from Hubble or other calibrated imager?

There is plenty of data (such as calibrated spectra). But I know just enough about color representation and reproduction not to get slurped into that particular ideological battle.

Hmm. This is a cute enough factoid that I'm wondering whether we might want to put together a little letter to Sky and Telescope... unless, of course, Zwicky said it in 1938.

(By the way - the post above does not mark my first use of smilies. I had no idea that BBCode interprets Polaris' spectral type - eff eight - as the code to insert a smiley!)

George
2004-Nov-12, 04:08 PM
Are these "solar twin" irradiance plots?
Sort of - except for other stars. For convenience, I used their G2 V star (HD 66171) instead of ours. In each case, it's a table of energy per unit wavelength versus wavelength, all sampled at the same points in the spectrum.
Nice.


...What I was checking, and sort of surprised to find, was how much atmospheric extinction can compensate for temperature changes within the visual band.
Huh? Did you find a way around atmospheric "bleaching"?


Around F6-F7, the ratio is flat within a few percent, smoothing out the ripples due to spectral lines mismatching. The difference becomes dramatic as you go into the UV, but that doesn't particularly enter for visual color.
Are you saying you are simulating the shifting of F-type 8) (couldn't resist) stars toward red and getting a compensated above-atmosphere looking irradiance curve? #-o


So my answer, based mostly on how I see Procyon, would be "just about white". Not as blue as Canopus and not as red as Alpha Centauri (big duh!) among 1st-magnitude stars.
Are you seeing any hue at all? A simple strobe of the sun at zenith reveals no hue that I, and others, can perceive. This is logical since, below the atmosphere, the spectrum is as "flat as a pancake" (except for a few pecans near the edge :) (i.e. blue and shorter) )



Are there any likely "true" (or "natural") color images of any of these twins from Hubble or other calibrated imager?
There is plenty of data (such as calibrated spectra). But I know just enough about color representation and reproduction not to get slurped into that particular ideological battle.
Agreed. Trying to learn how to predict a final color is way "too much squeeze for the juice" (too many books, too little time, too little fruitful results). The only color rendering (computer) for our Sun (that I know of) produced a "pinkish peach" result. :roll:


Hmm. This is a cute enough factoid that I'm wondering whether we might want to put together a little letter to Sky and Telescope... unless, of course, Zwicky said it in 1938.
An astronomer (or was it physics prof.) published a cool article in Astronomy almost 2 yrs. ago. He determined the Sun was greenish. I emailed him, but (no surprise), he did not reply. IIRC, this was about the same time as the lime-green universe average that was found in error and was later shown as beige. IMO, the Sun's color deserved more attention than some average color concept.
Also, don't forget the engineering world. The lightining industry (at least some sites) will show you 5800K lighting to be...blue 8)

I suggest you find some naive soul who will follow your advice and conduct actual scientific experiments for you. Someone who might be interested in knowing the truth about the star known as "Sun". Possibly that person is here on the board, or even on this thread. Hmmm, that sorta narrows it down. :)

George
2004-Nov-12, 04:22 PM
Of course, "true", or "natural" (more popular NASA term), color images taken from space of Solar twin stars would, likely, do the trick. I believe there are web site pages that list these stars.