Why’s the Sun not Green?
It’s Friday afternoon and time for a mildly frivolous post.
I’ve been recently been teaching first-year astrophysics students (and others) about the radiation emitted by stars, and how stellar spectra can be used to diagnose their physical properties.
Received wisdom is that the continuous spectrum of light emitted by stars like the Sun is roughly of black-body form, with a peak wavelength inversely proportional to the surface temperature of the star. Here are some examples of black-body curves to illustrate the point.
The Sun has a surface temperature of about 6000 K – actually, more like 5800 K but we won’t quibble. The peak wavelength for the Sun’s spectrum therefore corresponds to bluey-green light, which is why the Sun appears … er… yellow.
Anyone care to offer an explanation as to why the Sun isn’t green? Answers on a postcard or, preferably, through the comments box.
And while you’re at it, you might want to comment on why, if the Sun produces so much green light, chlorophyll is actually green?
In the Dark 
March 11, 2011 at 3:41 pm
Rayleigh scatterin’, innit? Removes a disproportionate amount of blue from ‘direct’ sunlight, making it more yellow.
Though I had heard that out in space, the sun appears white…
March 11, 2011 at 3:42 pm
The peak may be in the green part of the spectrum however there is a larger area covered by the ‘yellow’ light in the BB curve, allowing us to see the sun as Yellow.
So although the colour of a star is a good indicator of the wavelength it is peak at (lamdaT = 2.9×10^-3), it isn’t entirely accurate.
Or Compton Scattering in the atmosphere. Whichever you choose to believe.
I hope so otherwise i need to change my report anyway.
March 11, 2011 at 4:00 pm
The sun and the moon are fermions and cannot both be the same colour. The moon is green (cheese). Therefore the sun is not green. QED
March 11, 2011 at 9:54 pm
Why is the Moon green? Is it made of blue cheese that has gone mouldy? It’s been there a very long time: it must be past its best-before date by now.
March 12, 2011 at 10:17 pm
Wonderful what’s in Wikipedia nowadays. The moon-as-green-cheese saying has been traced back at least as far as 1546:
http://en.wikipedia.org/wiki/The_Moon_is_made_of_green_cheese
March 12, 2011 at 10:34 pm
PS Bryn: I always buy supermarket cheese when it’s within a day of its sell-by date. Not only is it half-price but it might even be nearly ready for eating. Supermarkets are paranoid about food poisoning and sell meat and cheese far too young.
March 12, 2011 at 10:57 pm
I’m really surprised that the idea that the Moon is made of green cheese goes back to 1546. I had always assumed that it must be from a late-19th century children’s story.
March 11, 2011 at 5:53 pm
Not sure about all that physics stuff but chlorophyll might be green because early plants had to compete with archaea, which were probably more numerous at the time and are red/purple. By reflecting green light and absorbing red, plants were able to exploit that environmental niche. Another interesting question is why archaea insist on being tiny and living in the least desirable neighbourhoods and why there aren’t huge purple trees. Maybe the intensity of green light causes more cellular damage and keeps things small…?
January 6, 2013 at 11:48 pm
If I am not mistaken they are called cyanobacteria, and I believe they are not as well adapted to muticellularity, do to the fact that they evolved in a reducing atmosphere which is now oxidizing. (O2, O3) Oxygen while damaging allowed for respiration as you and i know it and allowed for larger multicellular pant and animal bodies. Most people don’t realize plants need some oxygen as well as carbondioxide(CO2). and we need some co2 in our blood too for proper acid base equilibria, but we make plenty of co2 and don’t need to take any in.
March 11, 2011 at 7:41 pm
Dear oh dear oh dear! Scattering?
March 12, 2011 at 6:22 pm
Dear oh dear oh dear. No.
March 11, 2011 at 9:31 pm
peter – is the higher rate of recent posts displacement activity? what should you really be doing?
March 12, 2011 at 9:01 am
I wasn’t aware I was posting more than usual. Perhaps I’ve been blogging in my sleep.
March 11, 2011 at 9:49 pm
The Sun appears yellow? Really?
Surely when high in the sky the Sun appears white (essentially by definition: physiologically our visual senses have developed to interpret that spectrum as having a neutral colour).
(Of course, from British latitudes, atmospheric extinction will often make the Sun yellow, or orange or red, or with cloud invisible most of the time.)
March 11, 2011 at 10:46 pm
I agree – the sun is white, not yellow. If it was yellow, then when I wear my white shirt outside, it would look yellow. My theory is that we think it is yellow because when we draw it as children, we are make to colour it yellow.
March 12, 2011 at 10:32 am
I rather think kids draw it yellow because it looks yellow.
July 10, 2020 at 1:58 pm
😅😅
March 13, 2011 at 2:19 am
>> I rather think kids draw it yellow because it looks yellow.
I humbly disagree. Here again is an image of the sun on a white background. It is white.
March 13, 2011 at 2:22 am
http://casa.colorado.edu/~ajsh/colour/Tspectrum.html
But whatever you do, don’t mention the colour of the cosmos.
http://en.wikipedia.org/wiki/Cosmic_latte
March 11, 2011 at 10:49 pm
As for the chlorophyll question – I have thought a lot about this one also. Here’s the absorption spectrum for chlorophyll
which is primarily in the red and blue. Plant evolution just settled on a good workable approach, rather than a “designed” approach which you would think would have the maximum absorption at the peak of the solar spectrum.
March 12, 2011 at 10:48 pm
Cusp: Have a look at the remarkable book Life’s Solution by Simon Conway Morris. It’s about evolution at the biochemical level and Conway Morris argues that more things than you might imagine evolve convergently, ie would look pretty similar on other planets where life has evolved. He gives arguments for chlorophyll being like that. One of his points is that chlorophyll is some way from absorbing at the best wavelength for collecting light energy (which is why I am mentioning him).
As for Peter’s original question, the relation between what we perceive and the spectrum entering our eyes is a pretty complex business. We process the signals from our three types of cones, with their differing respponse curves, and our rods, not only additively but subtractively – ie we set them off against each other. Sometimes I think that physicists have it easy compared to biologists…
March 13, 2011 at 1:19 pm
Indeed.
I’d add that although most astronomers claim the Sun is white, just think how many varieties of “white” there are when it comes to buying paint!
March 16, 2011 at 7:22 pm
Or washing powder (“whiter than white”).
Isn’t the white paint diversity mainly to do with texture rather than the wavelength response curve?
March 16, 2011 at 9:46 pm
It’s partly texture, but I don’t think that’s the whole story. I know from 1st hand experience that white matt emulsion is quite difficult to match unless you use exactly the same stuff.
There are other oddities too. Anyone remember that “Daz blue-whiteness”?
March 16, 2011 at 11:01 pm
I take the point. ‘Black’ is physically well-defined (as a zero spectrum) whereas ‘white’ isn’t.
March 11, 2011 at 10:54 pm
And we often look at the sun through a solar filter, but if you project the sun with a pin-hole camera onto white paper – it looks white.
March 11, 2011 at 11:41 pm
If you make the same plot with frequency instead of wavelength on the x-axis, the peak is neither at green nor yellow.
March 12, 2011 at 3:42 am
Here might be the answer:
http://www.spitzer.caltech.edu/video-audio/150-ask2008-002-Why-Aren-t-There-Any-Green-Stars-
March 12, 2011 at 4:56 pm
That’s a good answer by Dr. Michelle Thaller. She agrees with Cusp and me.
Dr. Thaller has got a Ph.D. in the spectra of stars.
Admittedly, so do Cusp and I …
March 12, 2011 at 6:16 pm
OK, I think that’s the right answer. Nothing to do with scattering or as someone else with a PhD in stellar spectra tweeted:
It’s actually because our green receptors are less sensitive than the red and the blue. That’s also why grass is green – it reflects more green light, because it absorbs less efficiently there.
March 17, 2011 at 12:05 am
single color response of the cones peak at 555nm, so I don’t really buy this. The green cone may be less sensitive than the red, but they overlap strongly.
March 17, 2011 at 12:12 am
And just as I submitted that last comment, I realized that green signal on the red receptor will be conceived as red, not green. I concede.
March 12, 2011 at 9:10 pm
>> it reflects more green light, because it absorbs less efficiently there.
That’s what I said!
March 12, 2011 at 9:20 pm
So you did.
March 14, 2011 at 6:01 pm
http://en.wikipedia.org/wiki/Planckian_locus
March 17, 2011 at 9:59 am
Best answer today, 17th March: because God isn’t Irish!
February 14, 2012 at 1:25 pm
You should check: http://asterisk.apod.com/viewforum.php?f=24
April 24, 2012 at 12:28 pm
Because our puny mortal eyes are not adequate to the task of seeing the true colours of the universe. Our system of colour detection relies on 3 types of “cone” cells in the retina; red, blue and green. The various mixes of red light, blue light and green light that we can perceive give us our colours; red and green light together looks like yellow to us, blue and red like pinkish/purple etc. The sun might look green to us if our anatomy were different, but in our case the sun emits enough red and blue light to produce a yellowish white blend in our eyes instead of green.
May 24, 2012 at 1:29 am
The Sun has always looked green to me. That’s how I found this actually.
August 5, 2012 at 10:44 pm
Here is the real answer,
The sun may peak in the green but as stated above there is a greater area of yellow being emitted. However, because the sun emits all the colours of the visible waveband it appears white in space, and of course the moon is therefore white as it reflects the sun’s light.
Now, if you dim the sun’s light it becomes more and more yellow as it emits more yellow light than the rest, (hence yellow dwarf classification of our star).
Finally, the sun appears yellow to us during periods where the sun is low in the sky due to Rayleigh scattering. Blue light being more energetic than red light can interact with the atoms in the sky causing it to scatter toward the Earth and therefore to our eyes. Red light passes straight through the atmosphere back into space. This is also why the sky is blue. If you look at the sun when it is low in the sky, the blue light will scatter leaving only the “redder” parts of the spectrum to hit your eye. Therefore the sun appears yellow. (same process when you have a red moon, scatter sun light through the atmosphere leaves only the redder light to hit the moon and to our eyes.)
However, when high in the sky the sun will be white due to less scattering (less atmosphere to go through).
Hope that helps :P.
As for the chlorophyll answer. Chlorophyll is a poor absorber of green light but will absorb the other colours (blue, red etc) leaving only green light to be reflected back at the viewer.
June 14, 2013 at 4:45 am
it’s emerald green
April 4, 2014 at 1:47 pm
My thinking is that it is red and green. There is no colour yellow. When hydrogen (green) is used up we are left with a red giant (carbon).
December 2, 2014 at 11:20 am
Because over time the wave lengths change and spread out. So it appears yellow-orange
January 9, 2021 at 11:41 pm
I am wondering if we perceive the sun as yellow because of the specific cones our eyes have. The sun should look white, but we tend to perceive white and yellow as similar colors. I know as a child I often had trouble seeing yellow crayon drawn on a white paper.
Our cones peak in red, green, and blue for whatever reason (I don’t know, but it could be green and blue due to the sun’s peak wavelengths plus red for dangerous animal markings, or maybe it has to do with needing to see and identify plants, since it does somewhat match with the chlorophyl spectrum someone else posted).
Anyway, we have the 3 main cones, but not yellow. But yellow is in our brain’s color mapping system- we divide colors into red-green and blue-yellow contrasts. But in order to do that with only 3 cones, our brain does a work-around. It counts non-blue light that has equal parts red and green as yellow. This is just conjecture, but what if our brain considers yellow and white light to be similar, since they are both caused by activating multiple cones at once? Or because it is the color our brain goes to when it sees something that doesn’t quite match any one cone?
January 20, 2021 at 12:34 pm
[…] reminded me of a post I wrote in 2011 about why you never see any green stars. They say a picture paints a thousand words so here’s […]
May 27, 2022 at 7:15 am
whether sun is green or white. It depends on that who is seeing. Humans are trichromatic having three cone cells (red, blue, green) so for them these three cone cells mix colors in such a way that the sun appears white instead of green. But for other animals like dichromatic(most mammals) or tetrachromatic (birds, fishes, reptiles etc.) sun might appear different in colour. But in reality, the sun is green.
June 18, 2022 at 10:40 pm
Interesting thoughts and assertions. What do you think would be 3-4 characteristics of a well-designed experiment to establish the distinctions between the actual energy distribution emitted from the sun, versus the most commonly detected color of (radiated) sunlight by human earthbound eyes ??
July 18, 2022 at 1:01 am
Late to the party, here, but I’m working through a thermal physics book and I believe I have an important part of the answer. The commonly seen plot of spectral intensity vs. wavelength can be a bit misleading, due to the inverse relationship between wavelength and photon energy. If you look at a plot of the sun’s spectrum with photon energy on the horizontal axis, rather than wavelength (this plot gives intensity per unit photon energy), it peaks in the infrared! This surprised me since I have always been told that the peak wavelength is green. So, there is more energy leaving the sun at that particular IR photon energy than there is anywhere in the visible part of the spectrum. The rest of the answer would be a matter of perception and sensitivity of the RGB cones to various wavelengths. As for why plants are so wasteful of green light: it’s a less pressing question for me with this answer, now, but I did stumble across an excellent video from the SciShow YouTube channel about this very issue.