Sun is shown 'too yellow' in media

[alexch (Alex)]

Originally Posted by Paul Haese

At the risk of starting something here I respectfully disagree regarding how us solar imagers portray the colour of Sol. It is a yellow star. Click here for NASA page.

If Sol is supposed to be very similar to Rigel Kentaurus and its smaller twin then when I view that star through a telescope it appears as an orangey yellow colour. I have heard many people say it appears as this colour. While the colour of Antares is slightly more red and more orange. Antares's twin appears green to me, but most likely this is due to the orange fresnel rings mixing with the white light of the pup.

Anyway feel free to respond.

Ditto. The colour of the Sun if we could see it without over-saturating our receptors would be very similar to Rigel Kentaurus and appears peach-yellow.

The black body trajectory curve does not quite get into the green wavelength and there should be no green stars:

http://casa.colorado.edu/%7Eajsh/colour/cietoppm.gif
( image from http://casa.colorado.edu/~ajsh/colour/Tspectrum.html )


Alex

[sheeny (Al)]

They say a picture is worth a thousand words...

Here's a picture (Miss Jane) to help explain what was saying.

The spectrum is a standard library spectrum of a G2V star, same as our sun. The smooth curve is the Plank curve for 5700K i.e. the black body radiation curve for our sun. Refer to this as you read my previous explanation. It may help.



Al.

[overlord (Charles)]

The colour changes because of the energy output changes. It used to be more yellow in the 20th century but has become white. The moon used to be slightly yellow as well and now it's the same colour as the fluro street lamp. I'm not the only one who thinks this.

[alexch (Alex)]

Originally Posted by sheeny

They say a picture is worth a thousand words...

The spectrum is a standard library spectrum of a G2V star, same as our sun. The smooth curve is the Plank curve for 5700K i.e. the black body radiation curve for our sun. Refer to this as you read my previous explanation. It may help.



Al.

If we convert the spectral irradiance of the Sun to a photon flux distribution (our eyes respond to photons rather than wavelengths) then the curve peaks in yellow (attached) and is rather flat because blue photons need more energy (E=hf). The high intensity light with such flat spectrum is perceived as mostly white.

Now if we look at the path of the blackbody colour in chromaticity space (Planckian locus, image attached) then the blackbody colour changes from deep red to orange, yellowish white, white to bluish white as the blackbody gets hotter ( http://en.wikipedia.org/wiki/Planckian_locus ). If you could measure the the Sun with the colorimeter from space then its colour would correspond to peach-pink.


Cheers,
Alex

[sheeny (Al)]

Originally Posted by alexch

( http://en.wikipedia.org/wiki/Planckian_locus ). If you could measure the the Sun with the colorimeter from space then its colour would correspond to peach-pink.

I think we are saying the same thing Alex - almost.

The second sentence in Wiki:

It goes from deep red at low temperatures through orange, yellowish white, white, and finally bluish white at very high temperatures.

The normal sequence of colours is of course, red, orange, yellow, green, blue, etc. The black body temperature substitutes white for green because the the particular mix of frequencies we have evolved to see as white has peak intensity in the green part of the spectrum. A black body cannot get to what we perceive as green (refer to your colour space chart).

You are talking perceived colours, and I am trying to explain the perceived colours in terms of absolutes (based on my limited background in spectroscopy).



What's your source for the "photon flux" chart? That has me entirely intrigued. As I understand it, and instrument corrected spectrum is directly proportional to photon count. A CCD has a frequency response curve (which often peaks around the green part of the spectrum for pleasing mono images BTW) but once the spectrum is corrected for instrument response, the spectrum intensity has to be proportional to photon count. That's the basis of spectroscopy.

The photon flux chart you have posted looks to me like a raw spectrum captured on a camera with a peak response in the red.

Al.

[alexch (Alex)]

Originally Posted by sheeny

I think we are saying the same thing Alex - almost.

The photon flux chart you have posted looks to me like a raw spectrum captured on a camera with a peak response in the red.

Al.

Yes - the same thing. This has been a good thread - the Sun is called a yellow star and it got me thinking why is it called yellow if its spectrum peaks in the green wavelength as you noted earlier.

I did a bit of searching and came across this paper:
http://www.spectralcalc.com/blackbod...RadianceV2.pdf

"Many devices and systems respond in proportion to the number of incident photons, and it is useful to express radiometric quantities in terms of photons per second rather than watts. Dividing the spectral radiance Lν by the energy of a photon, hν, gives the spectral photon radiance Lvp."

There are example curves that show Lv and Lvp for different temperatures. "Note that Lν and Lνp have different frequency
dependences. Although the peak frequency is proportional to T for
both quantities, Lν peaks at a higher frequency than Lνp"

As far as I can tell, the lower frequency peak of Lvp is independent of the detector response, however it must be factored into the response curve of a particular detector because it responds to incident photons.

The image itself was taken from a rather questionable source:
http://www.science20.com/solar_fun_o...he_sun_part_ii
but it was easier to comprehend visually than table data.

Cheers,
Alex

[multiweb (Marc)]

I think there are enough greenie's things going on with the Sun at the moment. I like it yellow.

[sheeny (Al)]

Originally Posted by alexch

The image itself was taken from a rather questionable source:
http://www.science20.com/solar_fun_o...he_sun_part_ii
but it was easier to comprehend visually than table data.

I have to agree. Questionable source. I think he meant to start with a spectrum like the ones that CSIRO produce for solar panel design which plot W/m2 v wavelength and work back from there to photon flux (which is what we in spectroscopy call intensity). Instead I think he's started with a normal intensity (photon flux) spectrum and divided it by the Plank energy, but I am surmising. There's not enough rigorous logic in the article to decipher it properly.

Al.

[sheeny (Al)]

Originally Posted by multiweb

I think there are enough greenie's things going on with the Sun at the moment. I like it yellow.

I like it yellow too. Its traditional!

However, I used to like to try to be a purist once and I started doing my Ha shots as red - because that's the colour that Ha light is. Then someone pointed out that Ha light was a darker red than I was using... and that was the end of that! Its harder to see the detail in red, so I personally have settled on yellow for my images. White is the correct colour (the right shade of green) to represent the sun in but that means a BW image and sometimes a bit of colour... well it just adds some colour to the shot.

Al.