Sun is shown 'too yellow' in media

[skysurfer]

In books, websites and planetarium software, the sun is shown in yellow which is not the reality. If it is yellow (or even reddish) then it is the scattering of blue light from the Earth's atmosphere.
High in the sky it is white.
A light bulb is actually yellowish, but that is 'only' 2700 K, but the sun is 5700 K.

Even F stars (7000-8000K) are shown as 'yellowish' but in the real world they are white with even a bluish tinge. This can easily be seen in bright F stars (Canopus, Procyon), particularly by observing with a telescope. Even bright G stars (Alpha Centauri, Capella) 'lose' there 'yellow' tinge when viewed with a telescope.
Strange enough that Capella in *daylight* does have its yellowish tinge but that is probably contrast to the blue sky.
A light bulb (or CFL energy saving lamp) from 1km away looks like a red M star, but when you view for example Betelgeuse with a 10cm or larger telescope it looks more yellowish. So the brightness has also an influence.

What are your ideas on this?

[naskies]

I think it's probably because the only time that most people are able to see the solar disc is usually during a sunset. I personally like seeing a golden or red colour in the images posted on here.

[Astroman]

I also think the colour yellow or orange adds warmth to the Sun. We have all grown up seeing those books etc... with the sun pictured in colour, I think it continues on through life. If we show a white sun, it wouldn't be as eye catching... I must admit, I have coloured my pictures of the sun to reflect the typical look what we see in books etc.... No harm in it.

[Nico13]

I had added some colour to my images not only because I liked the look of it but it helps in image sharpness and overall contrast that allows better granulation to show in the image as I only have Baader film filters.

[Zhou]

True the Sun is white but yellow is the colour we generally percieve it to be. To be honest I prefer diagrams, images and representations of the Sun to be in yellow even though it is not exactly scientifically correct.

[sheeny]

Do you ever wonder why there are no green stars?

The colour of a star is determined by the peak in the continuum of the star's spectrum according to Wien's Law.

For our sun, the peak is in the green part of the spectrum. We have however evolved to perceive the particular blend of frequencies our sun puts out as white. So we perceive all stars with similar spectra as white, and we do not recognise green stars.

Therefore the correct colour of the sun is green, based on Wien's Law, however, stars of this temperature have historically been called yellow.

Al.

[Poita]

So if I took a picture through solar film with a colour camera, and checked the RGB values, then the image would lean towards the green?
I like the description on this site, especially how Japanese children will often colour the sun red when drawing it...
http://solar-center.stanford.edu/SID/activities/GreenSun.html

As for colouring of images, I love the golden looking ones for a couple of reasons.

1) With the colour added, I find it easier to see the details than with a monochrome image.

2) It feels pleasing, creating the sort of emotional feeling I get when viewing a sunrise or sunset.

So I am okay with the false colour images generally.

[erick]

Everyone expects to see a yellow Sun, based on childhood drawings, so I regularly put a yellow filter on the eyepiece for public viewings of the sun. :D

[brian nordstrom]

:) True , but Antares small companion looks very green to me every time I've managed to spot it , a contrast effect ?
Brian.

[sheeny]

:)

No.

White light from the sun, has a certain shape across the spectrum. It is not a flat line across all the frequencies. The peak intensity of white light is in the green part of the spectrum. For red stars, the peak intensity is near the red end, for blue it is near the blue end. So, following Wien's Law, as I say, all white stars, have their peak intensity in the green part of the spectrum and are therefore technically green.

Some "white light" solar filters have a colour cast about them. That is, they don't block the same proportion of photons at all frequencies across the spectrum, the closer they are to flat linear (same proportion of photons blocked at all frequencies) the closer the image of the sun will appear to white.

Cameras, you must keep in mind, are manufactured to replicate what we see. So a colour photo through the white light solar filter, shouldn't look any different to what you would see, but often of course it does to varying extents depending on how well the manufacturer has built the camera to replicate what you would expect to see, and how you have various settings, such as colour/white balance, set, etc.

Am I explaining myself any better?:shrug:

So a peak intensity at green does not mean we see green. If the shape of the spectrum matches the sun, it will appear white. If the peak is steeper than the sun it will appear green. If the peak is flatter than the sun's spectrum, it will appear pink (Pink = white - green;)).

Even mono cameras often have peak sensitivity around the green part of the spectrum. This provides a mono image which is appealing to the eye, and this means we have to correct for the "instrument response" to do valid scientific measurements in spectroscopy.

Clear as mud?

Al.

[sheeny]

Whoops!!!!

:ashamed:

My apologies Brian!

I've just read you post again and for the first time read "Antares small companion" not "Antares".:doh:

Who is it that needs to get their eyes checked???:whistle:

Sorry mate!

:)

I can't say I've noticed the colour of Antares small companion, but if its the same temperature as the sun, it will appear white. Not sure what effect being a little bit hotter (probably) or cooler than the sun would have, but you generally don't here about green stars (because they are white;)).

I think you might be right about the contrast effect, though. If so, you could test the hypothesis by moving Antares just out of the FOV (or occluding it - maybe not so easy to do?). Once Antares is out of view, it should look white.

Al.

[Zhou]

The companion to Antares has always looked white to me. The perception of colour is such a subjective thing.

[brian nordstrom]

;) No worries Al , we all have our dyslexisc moments , me ,well ? all the time ...:lol:.
And Zhou , you are right there , but how do you se the colours in the traffic lights in the middle of the jewl box ?
I see red , orange and green very clearly in my 127mm apo , its a great effect . :thanx:.
Brian.

[Poita]

Unless they are Japanese, in which case they expect to see red from childhood drawings. Culture also plays a part in perception.

[Zhou]

I can't say I have noticed the traffic light effect in the Jewel Box, the "green" star has always appeared white to me and I have viewed the cluster through many types of telescope.

It is said that some people percieve the star Beta Librae (Zubeneschamali) as green. Again to me it looks white.

Sadly it looks like I may never see any green stars :P

[sheeny]

I can't say that I've ever seen or heard of the traffic lights in the jewel box. The ruby stands out...

If I ever get clear skies again I'll have to have a look for it!

Al.

[brian nordstrom]

yep Zhou , Zub ..... in libra , I have looked at all the stars in Libra , me being a Librean , and have seen it as orange , nice double that one .
I supose , as Kermit sang , " Its not easy being green " ? :lol:.
My refractors show green stars sometimes under good seeing , aye ? Al .
Very subjective , but nice at the eyepiece ..
Brian.

[sheeny]

I have a set of binoculars like that;)... but, ssshhhh... say no more, or everyone will want one.:P

Al.

[Zhou]

I definately cannot see any green stars (DSO's are a different kettle of fish). However, I have no problem seeing red. I saw some red shadings in M42 many years ago when I was using my 350mm dob near Oberon. I also suspect I may have seen a red hue in Proxima Centauri despite its faintness.

http://www.iceinspace.com.au/forum/showthread.php?t=93988

[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 (http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970717b.html).

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.

[alexch]

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]

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

Here's a picture (Miss Jane:P) 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]

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]

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]

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

The second sentence in Wiki:


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.:shrug:

Al.

[alexch]

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/blackbody/CalculatingBlackbodyRadianceV2.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_of_the_heliochromologist/the_color_of_the_sun_part_ii
but it was easier to comprehend visually than table data.

Cheers,
Alex

[multiweb]

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

[sheeny]

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:shrug:. There's not enough rigorous logic in the article to decipher it properly.

Al.

[sheeny]

I like it yellow too. Its traditional!:P

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!:rolleyes::) 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:P) 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:P.

Al.