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?


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42 Responses to “Why’s the Sun not Green?”

  1. 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…

  2. 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.

  3. Anton Garrett Says:

    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

    • Bryn Jones Says:

      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.

    • Anton Garrett Says:

      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

    • Anton Garrett Says:

      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.

    • Bryn Jones Says:

      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.

  4. Mary Cav Says:

    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…?

    • 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.

  5. Dear oh dear oh dear! Scattering?

  6. peter – is the higher rate of recent posts displacement activity? what should you really be doing?

  7. Bryn Jones Says:

    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.)

  8. 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.

  9. As for the chlorophyll question – I have thought a lot about this one also. Here’s the absorption spectrum for chlorophyll

    http://generalhorticulture.tamu.edu/lectsupl/physiol/P24f1.gif

    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.

    • Anton Garrett Says:

      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…

    • telescoper Says:

      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!

    • Anton Garrett Says:

      Or washing powder (“whiter than white”).

      Isn’t the white paint diversity mainly to do with texture rather than the wavelength response curve?

    • telescoper Says:

      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”?

    • Anton Garrett Says:

      I take the point. ‘Black’ is physically well-defined (as a zero spectrum) whereas ‘white’ isn’t.

  10. 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.

    http://upload.wikimedia.org/wikipedia/commons/3/39/Ordu_eclips2.jpg

  11. Andrew Liddle Says:

    If you make the same plot with frequency instead of wavelength on the x-axis, the peak is neither at green nor yellow.

    • Bryn Jones Says:

      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 …

  12. telescoper Says:

    OK, I think that’s the right answer. Nothing to do with scattering or as someone else with a PhD in stellar spectra tweeted:

    Kurucz model of Sun peaks in blue (470nm, lambda vs F_Lambda), whereas energy peaks in yellow (575nm, lambda vs lambda*F_lambda).

    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.

    • 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.

    • 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.

  13. >> it reflects more green light, because it absorbs less efficiently there.

    That’s what I said!

  14. telescoper Says:

    Best answer today, 17th March: because God isn’t Irish!

  15. It is white but looks yellow due to the contrast with the blue sky. I’m sure that astronauts can confirm that when there is no blue sky the Sun looks white.

  16. Miss Cellany Says:

    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.

  17. Joey Hopalong Says:

    The Sun has always looked green to me. That’s how I found this actually.

  18. Physics Nerd Says:

    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.

  19. it’s emerald green

  20. charlie Says:

    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).

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