Why are there no green stars?

By Phil Plait | July 29, 2008 8:00 am

Go outside on a dark, moonless night. Look up. Is it December or January? Check out Betelgeuse, glowing dully red at Orion’s shoulder, and Rigel, a laser blue at his knee. A month later, yellow Capella rides high in Auriga.

Is it July? Find Vega, a sapphire in Lyra, or Antares, the orange-red heart of Scorpius.

There are no green stars!In fact, any time of the year you can find colors in the sky. Most stars look white, but the brightest ones show color. Red, orange, yellow, blue… almost all the colors of the rainbow. But hey, wait a sec. Where are the green stars? Shouldn’t we see them?

Nope. It’s a very common question, but in fact we don’t see any green stars at all. Here’s why.

Take a blowtorch (figuratively!) and heat up an iron bar. After a moment it will glow red, then orange, then bluish-white. Then it’ll melt. Better use a pot holder.

Why does it glow? Any matter above the temperature of absolute zero (about -273 Celsius) will emit light. The amount of light it gives off, and more importantly the wavelength of that light, depends on the temperature. The warmer the object, the shorter the wavelength.

Cold objects emit radio waves. Extremely hot objects emit ultraviolet light, or X-rays. At a very narrow of temperatures, hot objects will emit visible light (wavelengths from roughly 300 nanometers to about 700 nm).

Mind you — and this is critical in a minute — the objects don’t emit a single wavelength of light. Instead, they emit photons in a range of wavelengths. If you were to use some sort of detector that is sensitive to the wavelengths of light emitted by an object, and then plotted the number of them versus wavelength, you get a lopsided plot called a blackbody curve (the reason behind that name isn’t important here, but you can look it up if you care — just set your SafeSearch Filtering to "on". Trust me here). It’s a bit like a bell curve, but it cuts off sharply at shorter wavelengths, and tails off at longer ones.

Here’s an example of several curves, corresponding to various temperatures of objects (taken from online lecture notes at UW:

Blackbody curves

The x-axis is wavelength (color, if you like) color, and the spectrum of visible colors is superposed for reference. You can see the characteristic shape of the blackbody curve. As the object gets hotter, the peak shifts to the left, to shorter wavelengths.

An object that is at 4500 Kelvins (about 4200 Celsius or 7600 F) peaks in the orange part of the spectrum. Warm it up to 6000 Kelvin (about the temperature of the Sun, 5700 C or 10,000 F) and it peaks in the blue-green. Heat it up more, and the peaks moves into the blue, or even toward shorter wavelengths. In fact, the hottest stars put out most of their light in the ultraviolet, at shorter wavelengths than we can see with our eyes.

Now wait a sec (again)… if the Sun peaks in the blue-green, why doesn’t it look blue-green?

Ah, this is the key question! It’s because it might peak in the blue-green, but it still emits light at other colors.

Look at the graph for an object as hot as the Sun. That curve peaks at blue-green, so it emits most of its photons there. But it still emits some that are bluer, and some that are redder. When we look at the Sun, we see all these colors blended together. Our eyes mix them up to produce one color: white. Yes, white. Some people say the Sun is yellow, but if it were really yellow to our eyes, then clouds would look yellow, and snow would too (all of it, not just some of it in your back yard where your dog hangs out).

OK, so the Sun doesn’t look green. But can we fiddle with the temperature to get a green star? Maybe one that’s slightly warmer or cooler than the Sun?

It turns out that no, you can’t. A warmer star will put out more blue, and a cooler one more red, but no matter what, our eyes just won’t see that as green.

The fault lies not in the stars (well, not entirely), but within ourselves.

Our eyes have light-sensitive cells in them called rods and cones. Rods are basically the brightness detectors, and are blind to color. Cones see color, and there are three kinds: ones sensitive to red, others to blue, and the third to green. When light hits them, each gets triggered by a different amount; red light (say, from a strawberry) really gets the red cones juiced, but the blue and green cones are rather blasé about it.

Most objects don’t emit (or reflect) one color, so the cones are triggered by varying amounts. An orange, for example, gets the red cones going about twice as much as the green ones, but leaves the blue ones alone. When the brain receives the signal from the three cones, it says "This must be an object that is orange." If the green cones are seeing just as much light as the red, with the blue ones not seeing anything, we interpret that as yellow. And so on.

So the only way to see a star as being green is for it to be only emitting green light. But as you can see from the graph above, that’s pretty much impossible. Any star emitting mostly green will be putting out lots of red and blue as well, making the star look white. Changing the star’s temperature will make it look orange, or yellow, or red, or blue, but you just can’t get green. Our eyes simply won’t see it that way.

That’s why there are no green stars. The colors emitted by stars together with how our eyes see those colors pretty much guarantees it.

But that doesn’t bug me. If you’ve ever put your eye to a telescope and seen gleaming Vega or ruddy Antares or the deeply orange Arcturus, you won’t mind much either. Stars don’t come in all colors, but they come in enough colors, and they’re fantastically beautiful because of it.

Note: this is not the end of the story. There are green objects in space, and some stars do appear green… but that’s for another post, coming soon. Promise.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

Comments (177)

  1. Celtic_Evolution

    Phil – please forgive this… but did you mean “Betelgeuse”? ;)

  2. Ronnie

    I bet the green objects are just ones which the intergalactic lighting engineers have placed enormous color gels in front of. Or maybe just small ones much closer to us. I’d bet that’s not too far off since I would imagine various dust clouds could tinge a light emitting object pretty well.

  3. Naked Bunny with a Whip

    Better use a pot holder.

    So many comments…none of which are acceptable here. *sigh!*

  4. So that’s what you meant when you said ‘posting will be light’. But not light green, I guess.

  5. Andre Vienne

    The lack of green stars makes me sad.

    I suppose I will have to destroy the universe and make a new one in its place, with proper green stars. <.<

  6. Thanny

    Sorry, Phil, but the sun *does* look yellow. You can’t deduce how the sun itself “must” look by examining how its reflected light appears. The difference in light intensity makes that an invalid comparison.

    Colors exist in our brains, not the outside world. It doesn’t just boil down to different wavelengths of EM. Different combinations at different intensities produce a range of results.

  7. Something that has bothered me ever since I learned that the blackbody curve of the sun peaks in the blue-green… Why are most photosynthetic plants green? Aren’t they reflecting the most abundant wavelengths of sunlight? Wouldn’t there be an evolutionary advantage (however slight) to purple basil over green basil?

    Cheers,
    Jeremy

  8. Spaceman Spiff

    “Look at the graph for an object as hot as the Sun. That curve peaks at blue-green, so it emits most of its photons there.”

    Phil – I think you mean that the Sun’s emitted *energy flux spectrum* peaks near 500 nm (green). The *photon flux spectrum* peaks at longer wavelengths (photon flux = energy flux/E(photon)), near 630 nm for a T=5777K blackbody. I don’t know the photo-biochemistry of the cones in the human eye, but I suspect that the photon number flux is as or more important than the energy flux.

    And of course, what we “see” of the Sun’s light down here has been moderately filtered by the atmosphere (molecular scattering and O2,O3 absorption bands).

  9. madge

    Found this the other day and thought wanted to share it with all of you good people. This seems like a good time…

    The Night Sky

    We
    are adrift
    in an ocean of stars,
    a shoreless ocean of
    unknowable depths,
    a vastness so great,
    it swallows all mind
    and all knowledge.

    The glow overhead
    is the light streaming forth
    from billions of suns
    which form our galaxy’s arms.

    Billions and trillions of galaxies
    float
    in the silent, velvet Black.

    If we turn from our stress
    and look up,
    it is there,
    still there,
    always there,
    and ever so quiet!

    So pause for a moment,
    an hour,
    a night,
    breathe in and feel
    Where we are.
    Perceive the Universe directly,
    in all of its exquisite beauty,
    its absolute mystery,
    its wordless wonder.
    and drink
    of the fullness
    of Infinite Possibility. by Peter Roth.

  10. Todd W.

    Well, in theory, someone with some physical abnormality in which the red and blue cones either aren’t there or don’t function would see the sun as green…not to mention pretty much everything else.

    As far as what color the sun appears to be, I can’t look at it quite long enough to discern that. Using a filter to dim it will affect the apparent color. Using the reflected light affects the apparent color, as well, as Thanny said. At least, that’s what the situation seems to me, without training in the physics of light.

  11. Kurt

    you write:

    “Our eyes mix them up to produce one color: white. Yes, white. Some people say the Sun is yellow, but if it were really yellow to our eyes, then clouds would look yellow, and snow would too (all of it, not just some of it in your back yard where your dog hangs out)”

    I am not sure I agree with this. The sun DOES appear yellow. Our eyes do mix them up but not in equal amounts as you say later “Most objects don’t emit (or reflect) one color, so the cones are triggered by VARYING amounts.”

    Where am i wrong?

    Also, I think the reason the sun peaks in the yellow-green and we just happen to see yellow must be due to evolutionary reasons. i have always wondered about this.

  12. Don Snow

    @ Phil

    “…, and some stars do appear green…” That’s a relief, because I remembered seeing a green star. Forget where I had pointed my telescope.

    Madge: thanks

  13. amphiox

    Jeremy Lusk:
    It isn’t just a question of absorbing the most light, I think, but also of absorbing the most light of the right amount of energy (wavelength) because the reactions of photosynthesis require the energization of electrons to specific quantum levels, and these require specific amounts of energy from the incoming photons.

    I believe, in simplified terms, that photosynthesis requires two photons. One somewhere in the blue range, and one somewhere in the red range. These get absorbed. The green range photons in the middle are not used, and are reflected. Hence we see green.

    (An interesting corollary of this is that sunlight is not the ideal combination of wavelengths for plants on earth! So much for intelligent design.)

  14. Celtic_Evolution

    @ Jeremy Lusk

    IIRC, Plants convert light from the blue and red spectrums most efficiently into energy… thus chlorophyll absorbs these colors more readily, and what’s not absorbed is reflected as green.

    Please, someone correct me if I’m wrong about that…

  15. Celtic_Evolution

    ahh… much more elegant answer than mine, amphiox… and ya beat me to it, to boot! :)

  16. blackbody curve (the reason behind that name isn’t important here, but you can look it up if you care — just set your SafeSearch Filtering to “on”. Trust me here

    I have my safesearch turned off on principle, and I tried… expecting goatse-like images. As a matter of fact, Google Image just gave me graphs like the one shown in the article and I assure you under the search box it said ” SafeSearch is off”. I did a Google Image search on the word “blackbody curve” without the quotation marks.

    What did I miss?

  17. amphiox

    Sunlight is white (white snow, white paper, Newton’s prism experiments, etc). But the sun’s disc appears yellow, both to naked eye and photography, on earth through the atmosphere and in space (I think). Don’t astronomers classify the sun as a yellow dwarf?

    At some point in the past I thought I knew an explanation for this, but I can’t remember what it is.

  18. Don Snow

    I think it looks yellow, too. Let me trot this explanation by, to say why both Phil as well as those of us who see yellow are right.

    We view the sun through natural filters. The gasses in our atmosphere, and the gassess in space, between us and the sun; and the EM heliosphere and magnetosphere. These all filter the photons before they reach our eyes. I accept Phil’s theory and physiology.

    That’s my layman’s speculation, with a few scattered facts.

  19. Celtic_Evolution

    I think maybe there’s a bit of mis-interpretation here. And maybe poor wording on the post.

    Sunlight is white. However, the sun does appear yellow. Phil even covers this in his book… pretty early on if IIRC…

    I also think this topic about sunlight being white but the sun appearing yellow has been discussed on BAUT a few times… in fact, here’s one discussion:

    http://www.bautforum.com/archive/index.php/t-7445.html

    I think the general conclusion was that there are several theories as to why this is but we don’t really know fully what causes us to perceive the sun as yellow.

  20. elgarak

    Re. google search “blackbody curve”:

    As Jorg above, I have safesearch turned off (I’m a big boy), so I did some google image searches.

    “blackbody curve”: Lots of blackbody curves. Nothing “unsafe”. All normal, sciencey stuff. Some images of scientists on page 3 or so.

    “blackbody”: About the same as above, but I only go to page 2 of search results.

    “black body”: Ahhaah! The space does all the difference :) .

  21. Dee

    Has anyone ever seen a green flash from the sun immediately prior to its setting. Why is it that our eyes are able to see that green wavelenght on its own?

  22. Doc

    If you look at the graph of the normalized response spectra of human cones on Wikipedia’s article on color vision (see URL below) you’ll see that at 500nm there is a dip in coverage (and is related to why green lasers are “eye-safe”), and that the red and green cones are stimulated in about equal amounts. This means the human eye will perceive light at this wavelength as yellow.

    I believe that the rods respond well to this wavelength though, which I suspect makes the perceived color “whiter”.

    http://en.wikipedia.org/wiki/Color_vision

  23. gopher65

    Kurt: The sun looks yellow when it is near the horizon, and so do clouds (or sometimes pink, or red, depending on the atmospheric containments). At noon the sun looks perfectly white. It’s just that we almost never look at the sun when it is high in the sky, because it is really bright, and we’d blind ourselves. We always look at the sun when it is near the horizon, and so we always see it as yellow, orange, or sometimes red. Since we always see a yellow sun, we think that the sun is always yellow. But that’s faulty logic. A —> B =! B —> A

  24. Overhead the sun appears white as indicated by the snow, clouds, etc. When you view it directly it is near the horizon (or else you do retina damage) and you are looking through a thick layer of atmosphere that filters the light to yellow or red. Some “white light” filters for observing the sun actually give a yellowish cast, some a blueish cast, and some filter evenly enough to maintain the white color. Photos of the sun in hydrogen-alpha wavelength are actually red, but are usually artificially colored in shades of yellow to red to give better contrast to the features.
    Antares has a dim secondary star that can, in good telescopes of 10″ or more, give a strong green color. This is an optical effect of complementary colors that results from its proximity to the brilliant red primary star. It may not be real, but the effect can be quite beautiful.

    Wayne Reed

  25. Thomas Siefert

    I’m with gopher65 on this, our perception of the suns colour is based on how it looked when we last looked at it.

    That said, bright sunny days always seem to have a yellow glow about them to my eyes.

  26. Brett

    The stuff about visual receptors is accurate, but I think it misses the point. The key is that the sun does not emit most of its photons in the greenish band. The spectrum is peaked there, so but that’s not the same. The fact is that the green visual band is very narrow, compared to red, yellow, blue, etc. We see green as a color like any other, but it actually represents a much narrower range of wavelengths than most other colors. (To see this clearly, look at a rainbow. A rainbow has to be pretty bright before you can even begin to see the narrow green band, while other colors are clearly visible.) So the sun emits far more photons in the bands adjacent to green than in the actual green.

    As to the question of why green is such a narrow band to begin with, the answer is purely evolutionary. Seeing green is much more important than seeing any other color, since green is the color of chlorophyll. So we have evolved extreme sensitivity for distinguishing different shades of green and thus distinguishing different plants.

  27. Celtic_Evolution

    Dee -

    When looking at the sun on the horizon, just as it sets, the light is traveling through the denser air near the surface, and moves more slowly and follows a path that curves slightly in the same direction as the curveture of the earth. Green light curves more than red and orange light, so the green light remains momentarliy visible after the red and orange light is no longer visible below the earth’s curvature.

    When conditions are right, refraction caused by atmospheric inversion can cause the appearance of a “flash” of green just as the sun dips below the horizon. Different atmospheric conditions will produce different types of “green flash”.

  28. Robert

    Don Snow: Could be a planetary nebula – they get their name from their green appearance. The light from these is at a very narrow wavelength, rather than the broad blackbody spectrum of stars, so we can see them as green.

  29. Andy Beaton

    Don Snow-
    I’ve also seen green stars – or at least whet appear to be green stars. They were white stars paired with very red stars in a double star grouping, and the contrast with the red makes them appear green.
    I hope this wasn’t a spoiler for Phil’s future post.

    I believe the reason the sun looks yellow to us is that – and I’m serious here – there are no white crayons in a box of 8. So we spend our childhoods drawing the sun with the closest crayon we can manage – yellow – and have convinced ourselves that that is the colour we see. I’ve looked at the sun on foggy days when you can get a quick glimpse, and it looks pretty white to me.

  30. Jeremy,
    That was a question bothering scientists for quite a long time! The answer has a couple of parts. The first part is that the ideal spectrum given is what we would see above the atmosphere; down here on the ground much of it is blocked. Sunlight at the bottom of the atmosphere ends up peaking in red. The second part is that plant’s don’t use the most *abundant* light, but rather the most powerful or productive light. Light that is slightly bluer packs slightly more of a punch than green. What happens when you add those two up is that at the earth’s surface, there is more energy per wavelength window in red (because of more photons) and blue (because the photons are more powerful) than in green.

    See http://naturalacts.blogspot.com/2008/04/color-of-plants-on-other-worlds.html for more details.

  31. DrFlimmer

    Another possible explanation why the sun “appears” to be yellow could be:
    Due to scattering of the blue light through the atmosphere, much blue light is taken of the “direct beam” of the sun (this is the reason why the heaven is blue – I think you can look this up in Phli’s book “Bad Astronomy” ;) ) and thus the sun appears more yellowish.
    Or it’s the “contrast” with the bright, blue sky that makes the sun look yellowish…

  32. Hey, Phil

    This is completely OT, but I have a widget on my google page called “Today’s Reason to Drink.”
    For TODAY, it’s this:
    “Another synthehol, barkeep! Today is the birthday of Wil Wheaton, the actor who played Wesley Crusher on Star Trek: The Next Generation.”

    I never really got into the whole Trek thing, but I get what a mancrush is all about, show the guy some love.

  33. For all the folks seeing a yellow sun, the yellow is coming from all the pollution you are seeing the sun through. It is also dimmer than it used to be. If you go to the southern hemisphere and then above, say 10,000 feet, you will see that the sun light is a brilliant white. Even as the sun get’s low on the horizon it stays white, until almost right at sunset. Same thing for the day time and night time sky. What most urban folks would call a “clear blue sky” is really a smoggy gray/white sky. And there are very few places on Earth left to see a truly dark night time sky.

  34. Tom Marking

    “I’ve looked at the sun on foggy days when you can get a quick glimpse, and it looks pretty white to me.”

    A more important question is what does “appears white” mean physiologically? There are no cone cells for white light so what does the color “white” really mean physiologically? Does it mean that the red, green, and blue cones are all firing at the same rate? If so then that doesn’t match the energy output of the sun which has a peak at green. The green cones should be firing more than the red cones and blue cones when we look at the sun. So why don’t we see at least a slight greenish tint to the sun?

    I think there’s more at play here physiologically than the BA has explained.

  35. Greg in Austin

    I’m not a biologist (or exobiologist), but this discussion brings up something interesting. If a planet around a blue star had plant-like lifeforms, could the grass there then be blue instead of green?

    8)

  36. “bright sunny days always seem to have a yellow glow about them to my eyes”
    Damn that glaucoma!

    Seriously, I think the perception of a yellow sun is also helped by comparing it to a blue sky (on sunny days, that is). Still, the position on the horizon is probably the leading effect.

    I know how you can make green stars: Put two gigantic mirrors on either side of a regular star, say at about a couple of AU, one of which has a small transmission coefficient. The mirrors form an optical cavity that can be tuned* to a green optical transition. There is bound to be some stuff floating around with a transition in that region. The star then excites the molecules, and the cavity mode will enhance the stimulated emission. I call it the galactic laser pointer, and it may come in handy when we need to zap hostile alien planets…

    *yeah, right.

  37. Alex

    Awesome post, fascinating.

  38. Todd W.

    @Tom Marking

    There may be a threshold for the cones’ firing rates. Because the sun is emitting so much, the threshold may be going at their max, hence the blend to white.

    Interesting tidbit that people may or may not have picked up on in Phil’s post. In light, the primary colors are red, blue and green, while with paints, they’re red, blue and yellow. Combine all the primaries together with light, and you get white. Combine all the primaries together with paints and you get black (well, really a nasty brownish-greenish crud, but theoretically you should get black).

  39. Nice use of the GOES 12 Solar X-ray Imager’s image of the Nov 2003 solar storm!

    http://www.agu.org/pubs/crossref/2005/2004JA010960.shtml

  40. Todd W.

    @Pieter Kok

    “I call it the galactic laser pointer, and it may come in handy when we need to zap hostile alien planets…”

    Or when we need to distract giant space kittens.

  41. johnny

    I liked the first half of your explanation, but the second half was lacking in clear cause-and-effect. I think a better explanation of the spectrum/eyesight interaction would be: mostly blue + plus some green + plus invisible UV = blue star. mostly red + some yellow + invisible infrared = red star. mostly green + some blue + some yellow = white star.

  42. Celtic_Evolution

    @ Greg in Austin

    Theoretically, one would assume that plant life on a planet at the same point in its evolution as, say, the earth, would follow a similar evolutionary path… that is, the plant life would develop mechanisms that would most effectively convert light into energy, and reflect light which does not convert as effectively.

    This, however, is where I would need the help of the more astronomy-learned on this site. I assume there would be factors beyond just what wavelengths the parent star emits the most light energy at… including atmospheric conditions for that planet, etc…

    But hypothetically, I think it would be possible for a form of plant life to evolve to reflect blue rather than green, if that were the most efficient way for that life to convert light into energy.

  43. Mr Lusk, the green photons don’t carry as much energy as the blue or red variety. Scientific American recently had an article about what colour plants would be, and why. Check this out for more info: http://www.sciam.com/article.cfm?id=the-color-of-plants-on-other-worlds

  44. madge

    My son has “colour vision deficiency” he can’t tell red from green and he says the sun looks green to him! :)

  45. Todd, it’s the difference between additive and subtractive color mixing.

  46. Thanny

    Preferential absorption and scattering of blue light could definitely skew the sun towards yellow. Equal parts red, blue, and green looks white to us, but if you reduce the amount of blue, it takes on a yellow tinge. You can verify this easily with any graphics program.

    It’s probably not that simple, of course, but there’s no question that the sun appears yellow, even at noon. And it has appeared so since long before crayons were invented.

    As for using “white” objects on the ground, any object which absorbed red and green more than blue would appear white under a yellow sun. Does snow actually reflect and absorb all visible wavelengths equally? Surely I’m not the only one who has seen high-quality printer paper that’s so white it’s actually blue?

  47. Thomas Siefert

    @Pieter Kok

    Would you believe that I read up on Glaucoma just this morning? I got an email from a friend this morning and he told me about a common acquaintance that’s being treated for an advanced state of Glaucoma.

  48. TheBlackCat

    Good post, Phil, but although the star side of it is correct the biology side is not entirely. It is a common misconception that the three types of rods are sensitive to red, green, and blue light, but this oversimplifies things in the same way that saying the sun is green oversimplifies things. All three types of receptors are sensitive to wide ranges of wavelengths, a couple hundred nanometers. Like a black body gives off different amounts of light at different frequencies, a cone has different levels of response to different frequencies. So a receptor will have a different level of response to the same number of photons at two different frequencies. What varies between the three types of cones is the peak response, that is the frequency to which they respond more heavily. One has a peak at 437 nm (in the blue part of the spectrum), one at 533 nm (in the green part) and the thrid at 564 nm (the red part). But the blue receptor is sensitive to light above 500 nm, for instance. As the frequency of light gets away from the peak response, the cell responds less and less to the same amount of light.

    This means that no single receptor can identify the frequency of light. The blue receptor, for instance, cannot tell the different between a dim blue light and a bright green one. This is because a dim blue light can cause the same level of response as a bright green one. The way our eyes tell what the color is is not by saying “alright, this receptor is active, therefore it is this color”. Instead, it compares across the receptors. So it says, “this receptor is firing this amount, this other receptor is firing this other amount, and the third receptor isn’t doing anything. The frequency of light that causes this pattern of response is this”. Then it reports that frequency of light as being the one that you are seeing. This is a bit of an oversimplification as well, since this comparison is actually done partially in the retina and partially in the brain.

    The problem, of course, is how do you tell the difference between a single frequency and a combination of frequencies? Light of one frequency will cause a different but characteristic level of response in each of the three receptor types. But it is possible to combine a bunch of frequencies in such a way that together they cause that same, characteristic level of response in the three receptors. So how do we tell them apart? The answer is that we don’t. There is no way for the retina to differentiate between different collections of frequencies that all cause the same pattern of response.

    That is what actually happens with stars. What would need to happen for there to be a green star is that a star needs to produce a distribution of frequencies that, when they stimulate the three cones, produce a level of response in each of them that is the same as the response produce by a single green light source. Since there is no way for a star to produce a distribution of frequencies with that characteristic, there is no way for the retina to see a star as green.

  49. Folks, don’t get the idea that blue cones are sensitive to blue light only, green ones to green light only, etc. The sensitivity-curves of the three kinds of cone have a surprising amount of overlap, and in particular there is no source of light that activates the green cones only. The situation is rather complicated down here on earth, within our eyes.

  50. I gave this question, why are there no green stars, as extra credit on an exam for my non-major astronomy survey course. The couple of times I gave it, I only had 2-3 students out of a hundred get it right. I cover the physics, not the biology, which is indeed the problem.

    As a science fiction writer I worry a lot though about how things are perceived, like starlight on an alien world. I had a lot of issues over what it would actually look like under the skies of a planet around an M dwarf: http://www.mikebrotherton.com/?p=632

    And I came up with some “green stars” too: http://www.mikebrotherton.com/?p=633

    I keep meaning to try to get a photograph of one of those.

  51. @Freelancer: Flying Spaghetti Monster bless you for that Google Gadget!

    @elgarak: That explains a lot! Thanks for pointing out the, ehm, “correct” search. Still pretty tame… ;-)

  52. Brent Marykuca

    Phil, this is the best post I have ever read on your site. Really interesting and clearly explained. Thanks!

  53. Jeremy Lusk: Others have already addressed you question, but I wanted to add that the plot that Phil showed gives *intensity* as a function of wavelength, not number of photons. To get the latter, you have take the intensity information and divide by the energy per photon at that wavelength (which goes as 1/wavelength). So, if I remember, right, you actually get more photons in the red than in the green-yellow part of the spectrum. Thus, since chlorophyll can only work with one photon at a time, using red light rather than green makes sense as long as red photons have enough energy to accomplish its needs. (Which, according to others, it doesn’t for all parts of the reaction. But for at least one stage, red suffices and is thus a good choice.)

  54. Todd W.

    @TheBlackCat

    Thanks for the bio lesson!

    On a side note, when it comes to people who claim to have seen ghosts, and who are not consciously lying about it, have there ever been any vision tests to see if their eyes are different than the average person’s? Now, I’m not saying ghosts exist, but if they are something that does not interact with matter in a way that we can currently measure, but which emit light in a spectrum not normally detected by human eyes…

    But then again, special equipment that can “see” in those spectra should be able to pick them up.

    Just an argument a friend made to posit some rational, physical explanation for why some people “see” ghosts.

  55. PG

    @Kurt- The “yellow Sun” is not an atmospheric effect, but an optical illusion. Your eye has “color fatigue” from the blue of the rest of the sky. Try this-after staring at the blue sky for a while, look quickly at something white (try using a piece of white paper) what color does it appear to be? The same phenomenon happens with the Sun’s light when viewed against all that blue.

  56. PG

    Similarly, shadows on the snow on a sunny winter’s day can appear to have colors.

  57. Loaf Of Bread

    Todd W. Says: “Interesting tidbit that people may or may not have picked up on in Phil’s post. In light, the primary colors are red, blue and green, while with paints, they’re red, blue and yellow. Combine all the primaries together with light, and you get white. Combine all the primaries together with paints and you get black (well, really a nasty brownish-greenish crud, but theoretically you should get black).”

    Good point.

    It gets even more complicated than that. Printers’ inks use a different set of primaries again: magenta, cyan and green IIRC. Mix them all together and you get black.

    And the colour theory I’ve read makes a distinction between additive primaries and subtractive primaries. Additive ones, which apply to sources that emit light, add up to white. That’s because you are adding the colours together. Subtractive primaries, which apply to objects that reflect and/or absorb light, take colours away producing black.

    We can do the same kinds of mixing and get similar effects working with complementary colours, that is, the ones directly across the colour wheel from the primaries.

  58. Celtic_Evolution

    great info, BlackCat…

    really enjoying this thread, by the way…

  59. Neal

    Nice Julius Caesar quote, I loled.

  60. Todd W.

    Another note on mixing colors. In stage lighting, you typically have “cool” lights on one side (blues, some purples, greens) and “warm” lights coming from the other side (reds, yellows, oranges, some purples). When a blue light hits something on stage, that object casts a yellowish shadow. When an amber light hits the same object, it casts a bluish shadow. And, if you have a light with a blue gel, one with a red gel, and one with a green gel, all hitting the same spot, you’ll get white (in theory). But, if you take all three gels and put them in front of the same light, you won’t get any light hitting the stage (in theory).

  61. Aramael

    You say “about -273 degrees Celsius” but what you don’t mention is that absolute zero is exactly 0 Kelvin! Coincidence — or conclusive proof of intelligent design?

    I kid, I kid — great article, I find the way the brain takes all these rather low-level inputs, and continuously synthesises them into what we perceive as the world — it’s incredible. It’s like each of us has an unbelievable special effects studio in our heads, and it does it in real time.

    Of course, time itself is an illusion, but hey :-)

  62. TheBlackCat

    Now, I’m not saying ghosts exist, but if they are something that does not interact with matter in a way that we can currently measure, but which emit light in a spectrum not normally detected by human eyes…

    It would have to emit light in either the infrared or ultraviolet frequencies. Some people can see a short distance into those frequencies under certain conditions, but it is easy enough to build cameras that work in those frequencies so photographing ghosts should not be a problem. Actually digital cameras often have some infrared sensitivity normally.

  63. Charles

    “Has anyone ever seen a green flash from the sun immediately prior to its setting.”

    Yes, several times in the Caribbean and Pacific. For years people who had never seen one said it was an illusion, an afterimage or perhaps too much rum in my head, but the green flash is real.

    Read Andrew Young’s fine explanation page with gallery links, etc. here:
    http://mintaka.sdsu.edu/GF/index.html

  64. Todd W.

    On the seeing ghosts thing, alternatively, rather than these people actually seeing something really there that the rest of us cannot, perhaps mutations or damage to the receptors cause them to see something that isn’t there, under the right conditions. At any rate, it may be interesting to see what would come of research into the visual systems of people who claim to see ghosts.

  65. SteveHB

    Phil, just wanted to say that i really enjoyed this post. it was the kind of post that got me reading BA and will keep me reading it long into the future. Well Done!

  66. Adeel Syed

    “Has anyone ever seen a green flash from the sun immediately prior to its setting. Why is it that our eyes are able to see that green wavelenght on its own?”

    That’s just because of the diffraction of the white light coming from the Sun because of our atmosphere, the Sun’s not doing anything at all.

    And about actually seeing green stars. I’m assuming it’s because of the Red/Blue shift because these stars are either moving away or towards the Earth. So the stars are originally not green, but they appear to be. Just a hypothesis.

  67. Todd W. sais: “I’m not saying ghosts exist, but if they are something that does not interact with matter in a way that we can currently measure, but which emit light in a spectrum not normally detected by human eyes…”

    In order to emit light, the substance these ghosts are made of must contain charges or electrical currents. That’s what Maxwell’s equations (or rather: QED) dictate. It is the only way photons of any wavelength can be produced or detected. However, substances that are made up of charges and currents feel other charges and currents (Coulombs law, Biot-Savart, etc.), for example in the atoms that make up, say, walls. This means that these ghosts cannot fly through walls. Moreover, it means that they are easily detected. Your assumption that they emit light means necessarily that they interact strongly with their environment.

  68. Cassius

    “The fault lies not in the stars (well, not entirely), but within ourselves.”

    I guess that’s why we’re the underlings. I was wondering about that. I’ll go tell all my friends.

  69. Tom Marking

    “All three types of receptors are sensitive to wide ranges of wavelengths, a couple hundred nanometers. Like a black body gives off different amounts of light at different frequencies, a cone has different levels of response to different frequencies. So a receptor will have a different level of response to the same number of photons at two different frequencies. What varies between the three types of cones is the peak response, that is the frequency to which they respond more heavily. One has a peak at 437 nm (in the blue part of the spectrum), one at 533 nm (in the green part) and the third at 564 nm (the red part).”

    Yes, this was the ingredient that Phil left out – response curves of the cones are not delta functions, they are wide. In general, given a set of N response functions that overlap figuring out the true spectrum of the emitted light is called the Unfolding Problem. It is a pretty tough problem to solve in mathematics if you have limited response data. Apparently the human brain must be doing something like this with only 3 response functions to use.

    Back when I was a nuclear effects guy back in the 1980′s we did the same stuff only for the gamma ray part of the spectrum. We were processing data that came out of experiments conducted on real exploding nuclear weapons at the Nevada Test Site. It used to take hours of Cray 1 supercomputer time to do these calculations, and the human brain is doing the same thing in a fraction of a second. Never let anyone tell you that computers are more advanced than the human brain.

  70. Madge (and anyone else) – if you like starry poetry, try this one of mine.

    Excellent post, Phil. Good question and equally good answer. I’ve still got a few niggling questions though, but I haven’t read all the posts yet so I might find my answers above. If not, I’ll be back!

  71. Richard

    It’s a decent post but has serious limitations. Unfortunately, while others have pointed out the limitations they haven’t resolved them. Well, not to my satisfaction.

    Take a look at the colour combination chart from wikipedia which you so conveniently included. On the second row, the third element from the right, there’s a high green with light red and blue. That adds up to green.

    Obviously that’s not what happens with the Sun’s light or that of any other star’s. But while people have pointed this out, they haven’t explained exactly what does happen. I would dearly love to know.

  72. Radwaste

    Guys, how can you forget or neglect all the processing done by the visual cortex? We see white because our physiology defines it as “white”.

    If we existed on a different planet, we would adapt to that environment and observe a completely different color gamut as “white”. Geez, that’s the whole reason Pantone® palettes and color compensation profiles exist: you get used to what you’re seeing, and so you have to produce accurate colors for display on a variety of media even though you’re viewing a red/green/blue composite.

    (Print ink is CMYK, by the way – cyan, magenta, yellow, black – because ink is a subtractive color process, and the inks are not perfect, so black must be added; you can find printers with seven color cartridges, built to overcome the inadequacies of ink.)

    You can even see this in temporary effects with a variety of optical filters. Carefully-built yellow lenses will improve your ability to observe contrast, and you’ll still have the ability to distinguish colors – albeit with some changes in accuracy – through cheap rose-colored glasses.

    Thanks for the explanation, Dr. Phil – but the rest of you, go read the Photoshop manual or get a subscription to MacWorld or PC Magazine. This gets explained about once a year on those pages.

    —–

    To expand on what Tom said: “normal” light doesn’t come from the nuclei of atoms. Even though the wavelength of a gamma ray – by definition from near a nucleus – could qualify as “visible”, this doesn’t happen enough to be detected unless an explosion is going on. Quite correctly, the atom bomb guys figured out they could find out more of what was going on in an explosion if they captured more of the emission spectra. So they set up ways to collect nuclear light. See the arc of mirrors at the Castle Bravo test site for one example.

  73. We may be doing some sort of white balance all the time. Try that with your dispplay: most of them have several color setup presets, often labeled “xxxx K” – try changing it and seeing how long it takes to adapt.

    Whatever you might want to call that color, the sun’s light is not white (as can be seen from the graphic) – it is not evenly distributed over the visible part of the spectrum.

    If it is possible to create such a perfect white light source, it would be a nice experiment to let people adapt to that and then have them see the world as it really looks like.

  74. CLM

    And here I thought it was because I was colorblind.

  75. Joe Meils

    So… does this also apply to my question as to why there aren’t any PLAID stars?

  76. Steve P.

    Good post, but quotes like the following always bug me when I’m reading about things I’m not an expert on:

    “…in fact we don’t see any green stars at all.”

    To be followed by:

    “some stars do appear green”

    My logic gland hurts.

  77. David

    Maybe I missed something here, so let me distill what I got from Phil’s post. Essentially, we don’t see green stars because pure green color is diluted by the remaining spectrum of emission, which averages out to white.

    So, my question is, why do we see red and blue stars? By the same logic, red stars would only be visible if there were no green or blue light being emitted. Are these stars only ‘kinda red/blue’, is it a question of the wide range of frequencies our cones associate with a certain color, or what?

  78. Ed

    Eeeer, now I wanna here more about these green objects and why I can see them and no a green star.
    You got me more confused than I was when I saw the title with the end of it. So get on with it.

  79. Ed

    and, black catz just making it worse now.

  80. Chanda

    Well said.

    From the mother of Rayleigh.

  81. hey, good post. As much as I defend your right to general geekiness and political commentary, it’s nice to see some astronomy in here from time to time.

    No, the sun isn’t a perfect white light source, but in space, it LOOKS awfully white. The reason it (and, well, everything else in space) looks yellow to us down here (and yes, Phil, it certainly does. Take note of the difference between natural light and artificial white-light. I didn’t even notice that scenes in The Matrix were blue- or green-tinted until it was pointed out to me.) has to do with the blue-scattering effect of the upper atmosphere. (which, yes, is also why the sky is blue.

    “Never let anyone tell you that computers are more advanced than the human brain.”

    Not more advanced, but certainly easier to apply to some kinds of problems. It’s frankly astounding how operations that would be trivial for our brains can’t even be done on modern computers (Some of them might not even be turing complete, but I can’t prove that,) but that our brains have such difficulty dealing with problems that a computer could solve in a fraction of a second.

    Of course, this difference is what makes computers a useful tool, when we have something so much more powerful — and interesting — packed into our heads.

  82. Tanalia

    The (subtractive) primary colors for printing (and paints or crayons) are cyan, magenta, and yellow — this is often simplified to the closest common colors of red, blue, and yellow for grade schoolers.

    Black is normally provided as an extra color because the limitations of pigments means they never mix to provide a really good black, and (for printing) it would be wasteful to mix colors to get black for normal text when applying black ink directly is simpler.

  83. Andres Villarreal

    A very big part of the answer of this question has to do with our eyes (and brain), not with physics.
    If our sun produced twice as much blue, what color would my white wall seem? _White!_ If you put red sunglasses on, what color would my white wall seem? _White!_
    Our eyes work a lot more around differences in light than around absolute amounts of light of any given color. That is why so many photographs taken by amateurs are filled with nasty surprises.

    Why is the sun (apparently) yellow? Because our eyes tell us that it is yellow, not because it is really yellow.

    And our eyes rarely see just the sun; we see a wide, brilliant blue sky around a small sun, so the relative lack of blue in the sun makes us “see” the sun as the opposite of blue, which, for our eyes, is yellow.

    If we look at the sun’s light with a spectrometer, the light is slightly bluish, not exactly white. And if we look at the sun with the spectrometer, the light is also almost white but slightly bluish.

    The conclusion is: you have to know a lot about your eyes before you can understand what you see!

  84. Andres Villarreal

    I like Matthias post a lot. But there is just one improvement: Our eyes do white balancing locally in every part of the image, not just globally for the whole width of our vision. Our eyes also do brightness compensation in every part of our vision separately, and that is why you can see the inside of your room and the strongly lit outside, simultaneously. If you use a traditional camera, with a roll of film, the white balancing is non-existent, and a film for outdoor use will produce ugly yellow pictures indoors, and a shot that includes indoors and outdoors at the same time will fail miserably either inside or out.

    A digital camera will do white balancing, and will show different colors for the sun’s light, depending on how you set it.

  85. llewelly

    OT:
    BA, what do you know about Nick Pope, and his latest Op-Ed in the NYT ? (link via PZ )
    He seems to be arguing that UFOs should be investigated by the US military – on the grounds that they might be terrorists. But the incidents he describes seem awfully familiar – very much like recycled ‘aliens are buzzing about us and scientists are in denial’ stories.

  86. John Baxter

    Phil, not only are your posts fascinating, but you have remarkably high quality commenters, whose comments are also fascinating.

  87. Crux Australis

    Thank you Tanalia! I always wondered why the ink colours were c,y,m while the pigment primaries were b,r,y. Thanks for clearing that up!

  88. Grahamf

    No, the sun is not white. It is pale yellow. Clouds, snow and other “white” objects appear white because they are illuminated by the yellow sun AND the blue sky. The sum of the light from the sun and the light reflected back from the sky is white. Yellow + blue = white in this instance. Of course your eyes can adjust to other whitepoints, but daytime sun plus sky is the de facto standard white granted to us by years of evolution. We didn’t evolve in outer space; we evolved under a blue sky + yellow sun. The article is otherwise quite good.

  89. Thom Boyer

    “Jeremy Lusk” and “Greg in Austin”:

    Scientific American ran a fascinating article in the April 2008 issue that addresses both these questions (The Color of Plants on Other Worlds, by Nancy Y. Kiang; available at http://www.sciam.com/article.cfm?id=the-color-of-plants-on-other-worlds).

    On the issue of chlorophyll reflecting the strongest part of the sun’s emission spectrum:

    “The energy spectrum of sunlight at Earth’s surface peaks in the blue-green, so scientists have long scratched their heads about why plants reflect green, thereby wasting what appears to be the best available light. The answer is that photosynthesis does not depend on the total amount of light energy but on the energy per photon and the number of photons that make up the light.

    “Whereas blue photons carry more energy than red ones, the sun emits more of the red kind. Plants use blue photons for their quality and red photons for their quantity. The green photons that lie in between have neither the energy nor the numbers, so plants have adapted to absorb fewer of them.”

    ‘Course, that explanation boils down to “plants are mostly green because photosynthesis doesn’t use the green.” Which implies that evolution just hasn’t yet found a way to harvest the most abundant portion of the solar radiation.

    On the issue of plant colors on plants growing in the light of other stars, the article summary has this to say: “Light of any color from deep violet through the near-infrared could power photosynthesis. Around stars hotter and bluer than our sun, plants would tend to absorb blue light and could look green to yellow to red. Around cooler stars such as red dwarfs, planets receive less visible light, so plants might try to absorb as much of it as possible, making them look black.”

    Interesting. But unfortunately it doesn’t definitively answer the question from “Greg in Austin,” since blue is not mentioned as a potential color for alien plant life (the other unmentioned colors [on the "Roy G. Biv" scale] are indigo and purple).

  90. jerry

    This is an amazingly great thread, and Phil, you should both be congratulated for it, and appreciative of your commenters for what it represents.

    a) I haven’t seen anyone called a f*tard
    b) I’ve seen lots of competing theories actually discussed and yet, no one has said,
    1) My theory is better, your theory sucks
    2) Anyone who believes in your theory abuses women and has a micro-weener

    I have been depressed lately that there are no science/sciency threads left on the net where semi-educated layman/lapsed physics majors and engineers could discuss science without all the invective brought over from the rest of the net.

    On the other hand Phil, I’m going to break those rules and say one day I hope you deck Richard C. Hoaglund.

  91. Oscar Ferro

    Just a little teaser, for those who wonder about those stars with a green appearance: here in the southern hemisphere, we can see a lovely open cluster in the Southern Cross, named “The Jewel Box”. If you observe it through a telescope, you’ll see an amazing array of multicolored stars, including some very delicate green ones. Not emerald-green, but a bluish green like a pane of window glass seen on its edge.

    Though I know there are no starts with such hues, they look very real, and I can tell you it’s not chromatic aberration on the part of the telescope.

    I know those colors originate in the observer’s eye, but I’m waiting anxiously for the promised explanation!

  92. Jeremy Lusk: “Why are most photosynthetic plants green? Aren’t they reflecting the most abundant wavelengths of sunlight?”

    The photon energy explanation mentioned above must have superseded the old hypothesis I learned about in high school. That is, green plants appear to be a relic from an era when blue/red/violet photosynthesizers were dominant. Green light was their waste product, in a manner of speaking, and the proto-plants had to survive on those scraps. Then something happened to the blue/red organisms and green plants have been dominant ever since.

    Seemed plausible at the time. Does the photo energy explanation rule it out?

  93. Helio George

    The BA has it right, the Sun is white. Anything else is bad astronomy. Yeah Phil!!! :)

  94. llewelly

    Some more about Nick Pope, who wrote the weird NYT op-ed.
    Nick Pope on a British MoD remote viewing study :

    Sceptics and cynics will doubtless say the whole project was a waste of time and money – details of the cost of this study have been withheld. But to me, such criticism shows a lack of imagination. When I ran the MoD’s UFO project I had to think the unthinkable. For three years I was in charge of the real X-Files. I discussed the possibility of carrying out a remote viewing project, but nothing came of it during my tour of duty. However, it now transpires that the study was eventually carried out. Even though we do not know whether the project continues to this day, I like to think it does. Drugs, bombs, criminals, terrorists – we live in dangerous and uncertain times. Sometimes the odds seem stacked against us. But maybe the strangest X-File in the MoD’s history has a final message for the bad guys: beyond your understanding and against all odds, we’re coming to get you. The psychic spies are on your trail.

    His ufo history contains some rather credulous bits:

    A British Crash Retrieval?
    Before we return to R. V. Jones, we will make brief mention of how US journalist Dorothy Kilgallen alleged that the British Government had recovered a crashed UFO. Writing in the Los Angeles Examiner on 23 May 1955 she said:
    “British scientists and airmen, after examining the wreckage of one mysterious flying ship, are convinced these strange aerial objects are not optical illusions or Soviet inventions, but are flying saucers which originate on another planet. The source of my information is a British official of cabinet rank who prefers to remain unidentified.”
    Writing in Flying Saucer Review (Volume 25, Number 4 and Volume 31, Number 1) Gordon Creighton, who had researched this story in detail, made it clear that he believed Kilgallen’s source was Earl Mountbatten of Burma. Indeed, it has been suggested that Kilgallen picked the story up at a cocktail party hosted by Mountbatten in May 1955. Kilgallen’s story has widely been dismissed as a hoax, but as we shall see, other events may put her claims in a new light.

    If that doesn’t set your kook meter going, I don’t know what does. But at least he’s dismissive of government conspiracy theories, claiming the British government is ‘indifferent’ or ‘incompetent’.

    (cross-posted to pharyngula)

  95. amphiox

    With regards to the question about the colors of plants adapted to lights of different stars: the key step in photosynthesis that requires the input of energy from photons, in simplified form, boils down to splitting the water molecule so that the energized hydrogen can be used as a reducing agent. The oxygen is left over and dumped as waste.

    The laws of physics and chemistry dictate what the level of energy required for this is. No single photon of visible light has enough energy to do it, but you can add up two of them to get the job done. And the two photons types of photons that happen to add up to the right energy are one blue and one red.

    This is the most energetically efficient way to perform the reaction, so my guess is that for alien plants (so long as they are water and carbon based), the color will in the vast majority of cases always be whatever the star puts out minus the pertinent reds and blues, which in most cases will be some shade of green. Only in very extreme cases, I think, would it be likely that evolution would be forced to adopt alternative reaction mechanisms using other wavelength photons. Around a very dim red dwarf star, maybe, that has very little in the way of blue light emission, maybe the plants would have to use an alternate pathway using three or four photons of various shades of red. Such plants would probably appear close to black in the light of their parent star. Alternately, around a very hot, energetic star with lots of UV emission, it just might be possible and preferred to perform the reaction with one single UV photon, in which case the plant might reflect all the visible photons and look white.

    But I think that the vast majority of possible stars out there are not so extreme, so most things that do photosynthesis will be green.

    (Caveat, the above only applies to plants living on the surface of their planets, receiving direct light from their stars. If we’re talking about plants living underwater, for example, everything I just posted goes out the window. We have photosynthetic red algae living underwater here on earth, and they’re -wait for it- red.)

  96. BMcP

    With safesearch on or off all I got for the first page was the graphs like the one’s above for “blackbody curve”, I was hoping for some illicit astronomy! ;)

  97. Cusp

    Stars are not monochromatic, so no green stars

    The sun is white.

    Plants have developed an efficient method to absorb in the red and blue – if they could absorb at all wavelengths, they would – but the current method is good enough to get on with things.

    End of post.

  98. Buzz Parsec

    Wow, BA, this is the longest sciency thread I’ve seen in a long time! Everything discussed here so far barely scratches the surface. There are loads of related topics people might find interesting, and lots of unexplored issues that will someday earn many people Nobel prizes. IOW, mega-cool!

    I was trying to explain black body radiation in a totally non-technical way to some relatives a couple of weeks ago, and it’s *hard* to do, I wish I could have pointed them at this post for an intro.

    Just one little side bit: someone asked about whether the lack of green stars could be due to the fact that some stars are red-shifted and some stars are blue-shifted, but there’s no such thing as a green-shift. Nope, the stars really are different colors. We can measure the positions of spectral lines, or the overall shape of light curve (such as the black body curves Phil posted above) with incredible precision (hundreths of nanometers) with a spectrograph, but these tiny differences are invisible to our eyes. To get a red or blue shift significant enough to obviously change the color of a star, it would have to be receding or approaching at a significant fraction of the speed of light, much faster than nearby stars move with respect to us. (Distant galaxies can move that fast, due to the expansion of the universe, but stars at those distances are much too faint to be seen individually, at least without a very large telescope.)

    This topic opens up all the related topics of spectroscopy, relativity, physiology, quantum mechanics, biochemistry, and a zillion other topics. Great can of worms to keep us all busy for a long long time! :-)

  99. Robbak

    When I had read half way through, this is what i thought (and it might still have some validity):
    We don’t see stars as green because the sun _is_ green. If there were such a thing as a green star, the sun would be it.
    But because all the light we see around us (except for artificial lights, which we design to mimic the sun’s light anyway) is that colour, our brains take that colour as white (“Take a white balance” in photography sense).
    We define white as “the colour of light from our sun”. Any star which differs from our sun, we see as a different colour: red, if it is cooler, blue, if it is hotter. But we don’t see “green stars” because we have defined “star green” as white.

  100. Jason

    Thank you Phil! This has kept me up many a night!

  101. Great write up. I never thought about TV projectors much, but while I was reading about stars, and how our eyes react to light, that lovely epiphany hit me. Projection TV’s have lamps that produce red blue and green, not because thats whats necessary to create a realistic picture, but because those are the colors that our eyes process! I love it when understanding in one type of science creates understanding in others.

    Thanks Phil, you rock.

  102. yy2bggggs

    Ah, TheBlackCat has the errors I see addressed. I would like to expound a bit though:

    Phil said: “An orange, for example, gets the red cones going about twice as much as the green ones, but leaves the blue ones alone.”

    Well, maybe. But “red cone” is a misnomer–the three cone types are L, M, and S, and the closer you associate them with R, G, and B, the more likely you are to be wrong. In this case, it probably is true that L is stimulated about twice as much as M somewhere in the range you would call “orange”, but the difference between the two has more to do with why you see orange than the amount of stimulation. (Furthermore, S plays an active critical role by not being stimulated, as opposed to the passive role one may assume with an RGB analogy).

    Color signals get filtered by opponent processes (see http://en.wikipedia.org/wiki/Opponent_process). The L and M curve sensitivities are compared to produce a red-green opponent process response (roughly L-M), and compare again to get a total brightness response (L+M; occasionally I see sources that report this as L+M+S, but most seem to agree that S plays little role in the brightness response). The blue cones are meanwhile subtracted out of the total brightness response for a yellow-blue opponent process (L+M-S). These are the signals that carry color to your brain–your brain never sees the cones.

    Furthermore, L is sensitive throughout the spectrum–it’s most sensitive at 560nm, which is a yellowish green. M’s sensitivity is somewhat like L’s shifted a bit towards the high frequency end, except that as you actually approach the high frequency end it drops off (it crosses the normalized L, leading us to perceive the highest frequencies as having a red component–which is why they look violet). S is mainly only sensitive at the high frequency end.

    What really causes the orange to look orange is more related to the fact that M drops off faster, leading to L-M being a bit larger, causing a red response (but in terms of raw stimulation, both L and M are getting their fair share at that frequency). Meanwhile, the overall brightness response (L+M) is also large, but the S cones don’t get stimulated much at all, leading to a high yellow signal in the yellow-blue opponent process. The red and the yellow are blended by the brain to produce orange.

    As for primary colors, everyone here is wrong. There are no three primary colors of light, nor are there three subtractive primary colors. The standard RGB light colors are sufficient to cover a lot of our potential visible spectrum, for sure, but there are colors we can see that cannot be produced using these candidate primaries. The “true” primary colors are all abstract; they exist in theory per the axioms of colorimetry but, due to the overlap of sensitivity of our cones in practice, cannot be realized.

    See Andrew Hamilton’s site for more details on primaries:
    http://casa.colorado.edu/~ajsh/colour/primary.html

    Andrew also has some very relevant information, with the coolest charts you’ve ever seen, on blackbody temperature versus color:
    http://casa.colorado.edu/~ajsh/colour/Tspectrum.html

    Phil: You should SO steal his charts the next time you bring this topic up!

  103. Brian

    Geez, why are all the comments in this thread leaning to the left?

  104. Our eyes are actually most sensitive to green light. This is most likely evolutionary for various reasons. The way you test this is to put up a spectrum in a darkened place and reduce the intensity until it almost disappears. The last color you see is the color you are most sensitive to, which for most everyone is green. That’s why green lasers appear brighter than red ones of the same power output. That’s also why green light will destroy your night vision quicker than any other wavelength.

    This seems in opposition to the idea of not seeing the green sun. But light from the sun must travel through our atmosphere to get to us. green – blue = yellow. Extending this idea into late afternoon produces an even redder-looking sun.

    The other explanations involving the wavelength distribution in our cones and color mixing fill out the rest of the story and explain why the sun appears white from space.

    But in the end, we are so sensitive to green light that we *should* see a green sun, except for the removal of the colors at the blue end of the spectrum by our atmosphere. I believe that if we could travel, say, 10 light years away and look back we would indeed see a green sun. And I believe the same would be true for any other G-class star viewed from space.

    –Terry

  105. StevoR

    Hope this doesn’t spoil the BA’s next post on this too much (Looking forward to it eagerly!)but the B8 type star Beta Librae or as its proper name goes (& I kid you not) Zubeneschmali appears green – to many if not all observers.

    There’s also Antares B, the blue dwarf companion to the red supergiant Antares A which was mentioned earlier and similarly the companion star to Rasalgethi (Alpha Herculis) which agains appears green or greenish tomany observers which wasn’t mentioned before.

    Contrast explains the two companions but Zubeneschmali (Beta Librae) is a single star – probably although one theory suggests otherwise – and is a different case. Incidentally, Gliese 581 the star with the much hyped “earth-like” (well Supersized venus-like more like!) planet orbiting it is very close in the sky to Beta Librae -same field or 1 degree away I think.

    We can sometimes see the Sun as green in the case of the green flash as noted by others here (THX for the photos!)and sometimes when stars scintillate (twinkle) low on the horizon they can flash green – both atmospheric effects.

    Of tangential noteworthiness – I’ve heard Japanese people have trouble telling grene and blue apart. Blue in Nihongo (Japanese) is ‘aoi’ and green is ‘midori’ (yes, like the drink!) but the usage is strange with ‘midori’ less used and ‘aoi’ sometimes or often used in place for green as well as blue .. if memory of Japanese class serves!

    Great post BA – can’t wait for part II! 8)

  106. StevoR

    tenacious said on July 29th, 2008 at 11:16 pm :

    “But in the end, we are so sensitive to green light that we *should* see a green sun, except for the removal of the colors at the blue end of the spectrum by our atmosphere. I believe that if we could travel, say, 10 light years away and look back we would indeed see a green sun. And I believe the same would be true for any other G-class star viewed from space.”

    (NB. Quote above should be italicised.)

    Alpha Centauri – especially Alpha Centuari A which is spectral class G2 V the same as our Sun – is 4.3 light years distant. If you’re in the southern hemisphere take a look at it tonight. It appears yellow-white NOT green. Tau Ceti is another G type or “yewllow dwarf” star (G8 V) just 12 light years away – & that’s visible in both hemispheres. Find yourself a star chart, locate it & take a look for yourself. Its not green either.

    Then there’s Eta Cassiopeiae (F9 V) which is nearly G-type -justafraction hotter and is 18 ly and a northern hemisphere circumpolar star. Ditto for Sigma Draconis which is a G9 type star (one class off type K) at 18.2 ly and for us Southerners there’s Delta Pavonis, a G7 type dwarf star lying 19.2 ly away. Sorry, tenacious but none of them appear green.

  107. Eddie Janssen

    It might please you all to know that somewhere in the 80s Erik Rohmer made a nice movie called “Le rayon vert” in which, IIRC, the boy and the girl watch a green flash at the beach in the last couple of moments of the movie!

  108. StevoR

    Zubeneschmali which is the only apparently single star that does sometimes appear green to some observers is a B8 type dwarf.

    This is the same spectral class as Rigel except Rigel is a far larger and brighter supergiant (B8 Ia not B8 V) which incidentally puts out more light in a minute than the Sun does in a month! (Source : Ken Croswell,“The Blue Witch” page 22 in ‘Sky & Telescope’ magazine May-June 2007.)

    Alpheratz or Sirrah (Alpha Andromedae) the star marking the start of the back leg corner in the Great square of Pegasus is another example of a B8 star – & this time a blue dwarf like Beta Librae. Alpheratz (which has been ‘mapped’ revealing major starspots or something if I recall right) appears white or blue-white certainly not green either making Zubeneschmali a very exceptional star.

    One theory (noted in ‘Astronomy Now’ magazine news August 1996) is that Beta Librae or Zubeneschmali is in fact a binary star but seen from an odd angle which means we can’t tell that its binary. A ‘… blue dwarf & yellow giant seen at right angles’ which combine their colouring for green~ness. I don’t know if that works or if further study has proven or disproven that though.

    I’ll add that I’ve checked and Gliese 581 – & its famous “earth-like” planetary system are located _very_ close to Zubeneschamali – 2 degrees North and shines at 10th magnitude – see page XIII in “2008 astro-schedule” lift-out in the December 07 Astronomy magzine.

    BTW. Zubeneschmali (the name means “northern claw’a reference to the nearby constellation of Scorpius to which all the stars in Libra once belonged.) is also a secular or historic variable – a star that has apparently changed
    in brightess over recorded history :

    ” …. although its {Zubeneschamali’s} present magnitude is larger
    than that of {Zubenelgenubi -Alpha Librae}, Erastosthenes in
    the third century B.C. , called Zubeneschmali the brightest of
    all stars in Scorpius and Libra, which implies that it exceeded
    even Antares while Ptolemy in the 2nd century A.D.,
    said that Beta Librae {Zubeneschmali} was as bright as Antares.”

    (Source : Page 246, ‘The Constellations’, Motz & Nathanson, Aurum Press, 1991.)

    I’ll also note that at least one histroic astronomer n English observer T.W. Webb for instance, was quoted as remarking on its “beautiful pale green colour.” (The Atlas of the Universe, Patrick Moore.) But then Patrick Moore writes : “It [Zubeneschmali] is said to be the only single naked-eye star with a greenish hue, but most people will call it white, and I have never been able to see any colour in it.” (Stars of the Southern Skies, Patrick Moore.)

    So I guess the best thing to do is for folks to check it for themsleves ..
    Personally, I sometimes see it as faintly greenish other times not ..

    Its also worth checking out the stars mentioned here on stellar-expert James Kaler’s website : Kaler’s stars website :

    http://www.astro.uiuc.edu/~kaler/sow/sowlist.html

    Pretty sure Kaler has something to say on the green stars issue too ..

  109. madge

    @ StevoR
    But my favourite star name has to be Zubinelganubi! ;)

  110. StevoR

    D’oh! Thought I’d got the italicising right – but clearly haven’t.

    The only bit in italics there was meant to have been the quote from Patrick More (& earlier the Lloyd Motz & Carol Nathanson quote which actually worked as intended.) Sorry folks.

    If only we could edit here … *sigh*

    (& if only the BA blog was still on the old host without that squashed far left lay-out, horrid colour scheme, ad nauseam .. *sigh* again.

    I really miss the pre-Discover blog server days … & yes that was a hint BA! :-(

  111. StevoR

    madge said on July 30th, 2008 at 1:09 am

    @ StevoR
    “But my favourite star name has to be Zubinelganubi!”

    Yep, that’s one of my faves too! ;-) Although variationsofnspelling exist I’ve al;ways used Zubenelgenubi for Alpha Librae, Zubeneschmali for Beta Librae and Zubenelakrab for Gamma Librae!

    However, my all time favourite the longest single star name is the Babylonian proper name for Delta Cancris (a.k.a. Asellus Australis) : Arkushanangarushashutu. (Correct spelling! Pron. “Arku-sha-nan-garu-sha-shutu!”)

    Then there are the Babylonian names for Zeta Tauri : Shurnarkabtishashutu
    (“Shur-nar-kab-ti-sha-shutu”)
    and Beta Tauri (El Nath comon Arabic name) : Shurnarkabtishashashiltanu
    (“Shur-nar-kab-tish-ash-iltanu”)

    Believe it or not with a little practice they do come rolling off the tongue! I love ‘em! ;-)

  112. StevoR

    Correction – Kaler has recently added Sigma Draconis to the 560 stars on his website :

    http://www.astro.uiuc.edu/~kaler/sow/alsafi.html

    Sigma Draconis carries the proper name of Alsafi (a new one for me) and is listed as type K0 NOT type G9 as I had it. My source was Dole & Asimov’s book based on Dole’s study ‘Planets for Man’, (Rand, 1964 – page 182.) so a bit outdated. I don’t think that makes too much difference – just one spectral class off. Besides given that its only 4th magnitude it wouldn’t be likely to display colour anyhow but still thought I’d set the record straight.

    PS. Have tried to post this a copule of times now without success – apologies if it ends up as a double post.

  113. George Kopeliadis

    Never thought of that! On the contrary I was convinced for the opposite with not much thought. Science Fiction writers have a lot to do with it :)

  114. Grand Lunar

    “When we look at the Sun, we see all these colors blended together. Our eyes mix them up to produce one color: white. Yes, white. Some people say the Sun is yellow, but if it were really yellow to our eyes, then clouds would look yellow, and snow would too (all of it, not just some of it in your back yard where your dog hangs out). ”

    I seem to recall you mentioned in your book about the sun being yellow. I assume you changed your mind?
    Not that there’s anything wrong with that. In fact, I agree with this logic.

    Incidently, I recall Neil Tyson’s book “Death by Black Hole” addressing this issue of the sun’s color and the lack of green stars.

  115. Bill Bones

    Sincerely, haven’t read all 113 comments, but I can’t resist being pedantic…

    There ARE green stars. The sun is green, just we don’t SEE it green because of our eyes…

    But now imagine an alien whose eye was sensitive to cyan, yellow and magenta. Guess how would it perceive the Sun? ;)

  116. Celtic_Evolution

    Thanks for the post, Bill… I encourage you to read through the posts for more information and discussion at about the very point you make.

    I learned quite a bit…

  117. @StevoR

    Where are you standing when you look at these G-type stars? Inside our atmosphere? Then you haven’t addressed my post at all. You even quoted me saying “from space” and then immediately countered with “southern hemisphere”. Do you understand *why* the sky is blue?

  118. Wayne Jepson

    From the moon, the sky appears black and the sun appears white. From earth, the sky appears blue and the sun yellow. This is due to atmospheric molecules preferentially scattering short wavelength (blue) radiation. The remaining light traveling directly from the sun to your eye (don’t stare!) appears yellow rather than white because its depleted in short wavelength light and thus dominated by longer (red-yellow) light. As someone else noted, snow still appears white because its receiving and reflecting the combined light from direct sunlight and light scattered from the atmosphere.

    http://en.wikipedia.org/wiki/Diffuse_sky_radiation

  119. yy2bggggs

    tenacious:

    “Our eyes are actually most sensitive to green light.”

    M and L cones peak at a much different frequency than S cones, but peak at very similar frequencies to each other. If these cones were scattered across our eyes in equal numbers, we would have problems focusing. As it turns out, M and L are the only cones that come into play for sharp resolution in moderate light or higher conditions, and S cones are scattered sparsely, with increasing frequencies the further from our center of vision you go (and indeed, they don’t even exist in the foveola). There are many more M and L cones (but not nearly as many cones as there are rods, though that’s for a different reason).

    This is my guess as to why we’re most sensitive to green. The opsins in L and M are chemically close to each other, so they probably have roughly the same sensitivity levels. They both trigger near green (L is more of a yellowish green). However, to actually get green stimulation, all you need is an M signal (which bumps red-green opponent process into green). To get reds, you need a significant difference between L and M signals, which requires much more light.

    The particular tuning of M to green simply allows the whole process to cover the spectrum (since M drops off quicker on both sides of the spectrum, the red-green opponent process (L-M) serves as our “middle-end” spectrum metric; M itself is the trigger for the central point; the yellow-blue opponent process, in contrast, is our “low frequency-high frequency” metric). So our particular layout gives us the richest set of colors covering the spectrum, takes advantage of opponent processes (which are more general than color vision), while reducing chromatic aberration given our particular eye design (single convex focusing lens).

    The only thing missing from this explanation is the evolutionary advantage of going trichromatic.

  120. @yy2bggggs

    Thanks so much for that explanation. I teach my physics students each year (incorrectly, I suppose) that since the sun produces more green-ish photons than any other wavelength, objects are illuminated mostly in green light. I have a lab where we illuminate different colored objects with light passed through different colored gels, and then I discuss color mixing and color subtraction. After we see the effects, we focus on the sun’s spectrum and consider what is truly ‘white’ light.

    I have never studied how the cones operate together in any detail. I will certainly educate myself further before I teach that material again. You’re posts were quite… illuminating. (Sheesh, that even sounded bad to me!)

  121. Brent

    Maybe I missed something. If we can’t see green stars because they would have to be emitting only green light, as green is one of our receptors, why can we still see red and blue stars, which are the colors associated with the other two cones.

  122. Wait, so tell me this: Are wavelengths near the green portion of the spectrum more in focus than the light at the ends? I know we can’t focus blue light because of the chromatic aberration, but is this also true for red? Even slightly?

    I guess I’m looking for a reason our eyes developed in a way that they became most sensitive to green light.

  123. Celtic_Evolution

    Brent -

    There have been a few posts that discuss the error with associating the cones of the eye with actual R, G, and B color… have a look back up 30 or so posts… this is covered… and if you put that information together with BA’s post, I think you’ll start to get a bit clearer picture as to the answer for your question…

  124. heafnerj

    Why was my post not allowed???

  125. TheBlackCat

    while reducing chromatic aberration given our particular eye design (single convex focusing lens).

    Our eye isn’t a single convex focusing lens, it is two. The cornea is responsible for about 75% of the focusing power of the eye while the lens is only about 25%, if my memory is correct (it may have been 80/20).

    The only thing missing from this explanation is the evolutionary advantage of going trichromatic.

    The advantage of trichromatic, as opposed to dichromatic I assume, is that you can differentiate more colors. You know color blindness? This is generally because of a genetic mutation resulting in someone being a dichromat. Because there are only two values being reported (the activity levels of two different types of cones) as opposed to three it is easier to get different collections of frequencies that produce the same activity level. Having additional receptor types would allow for either a wider range of frequencies or better color discrimination in the same range, but increases the complexity of processing. Tetrachromats are not uncommon amongst fish. Some animals, like turtles, have color filters to further improve color vision. Mantis shrimp have eight different color receptors plus four different color filters and can see from well into infra-red all the way into the ultraviolet (not to mention polarization sensitivity).

    I know we can’t focus blue light because of the chromatic aberration, but is this also true for red?

    Are you sure about that? I was under the impression our eyes did a very good job of correcting for chromatic aberration at least for small pupil diameters.

  126. heafnerj

    I tried posting this last night but it apparently didn’t get through the moderators so I’m trying again.

    One problem here is that there is no unique, physically meaningful “peak” to a blackbody spectrum. Planck’s law gives, as a function of temperature, flux *per unit wavelength* or *per unit frequency* and this is often overlooked and causes great confusion. If you plot flux per unit wavelength vs. wavelength, you’ll indeed get a “peak.” However, if you plot (for the same temperature) flux per unit frequency vs. frequency, you’ll get a DIFFERENT peak! The two peaks will not correspond to the same perceived color! In fact, the “peak” for the *per unit frequency* plot is in the infrared while the “peak” for the *per unit wavelength* plot is in the visible. The “peak” placement is entirely a consequence of which bookkeeping method (i.e. per unit wavelength or per unit frequency or per something else) we want to use.

    IT IS A MISTAKE TO ASSOCIATE ANY EVOLUTIONARILY CONNECTION BETWEEN OUR EYES’ PEAK SENSITIVITY AND SUN’S SPECTRAL OUTPUT.

    Sun is NOT yellow. If it were, white notebook paper outside wouldn’t look white. Sun *appears* yellowish because of atmospheric scattering. However, that scattered light combines with direct sunlight to make white notebook paper appear white.

    Introductory textbooks need to eliminate discussion of ***A*** physically meaningful spectral peak or at least describe that the traditionally discussed peak is merely a consequence of which “bookkeeping method” is used to plot a spectral distribution.

  127. I’m sure about the blue. I demonstrate this with my students every year. For a simple demonstration find a blue LED (on your computer or something) and attempt to make it appear as sharp to you as a red or green one. Get farther away to increase the effect. In my classroom I use a video projector and have the students walk toward the screen until they can resolve the pixels for each red, green, and blue. Red and green seem evenly matched, but I don’t *know* that they are, which was why I was asking.

  128. TheBlackCat

    @ tenacious: as yy2bggggs said the distribution of receptors is different. S receptors (S standing for “short”, as in wavelength) are more sparsely distributed throughout the retina, and are particularly sparse in the center where our visual acuity is highest. That means they cannot form as sharp an image as the M and L (medium and long) receptors no matter how low the chromatic aberration is. It doesn’t appear to me that your experiment can differentiate between differences in aberration and differences in spatial resolution.

  129. TheBlackCat

    According to this site, there is some chromatic aberration but the nervous system automatically corrects for it:
    http://www.telescope-optics.net/eye_aberrations.htm

  130. Coal Banks

    Throughout the universe there are inter-galactic nerdy drivers sitting in front of red stars, waiting….for the light to change to GREEN! LOL!

  131. amphiox

    Would not tenacious’ contention that G type stars would look green when viewed through the vacuum of space be easy enough to check? Just find a G-type star imaged by Hubble using the settings that produce “true” (human eye vision) color image?

    Or get one of the astronauts on the ISS to look out port (or during a spacewalk) at a G-type star? This wouldn’t cost anything extra as long as you’re not sending up an astronaut specifically to do just this!

    For that matter, aren’t there computer algorithms that can subtract the effect of the atmosphere to render an image into what it would appear to the human eye if the atmosphere was out of the way?

  132. StevoR

    tenacious Said on July 30th, 2008 at 10:49 am :

    “@StevoR : Where are you standing when you look at these G-type stars? Inside our atmosphere? Then you haven’t addressed my post at all. You even quoted me saying “from space” and then immediately countered with “southern hemisphere”. Do you understand *why* the sky is blue?”

    Yes indeed I do. Two words rayleigh scattering.

    & yes of course I’m observing from our atmosphere. Its a bit hard to stand at all in the weightlessness of space!

    But .. I would think if G-type stars looked green to the 200 plus (?) astronauts that have been beyond Earth’s atmosphere – and as Amphiox notes the HST and other space telescopes – then we’d have heard about it by now …

    Personally, I’d love it if you were right – green is my favourite colour & having G-type stars which are best for life shining verdant emerald in the black sky beyond the blue sky would be very apt and awesome! ;-)

    But sorry, I’m afraid it just ain’t so. :-(

  133. Spaceman Spiff

    There have been a lot of useful contributions to the question originally posed, and some misconceptions as well. I hope the BA writes a “Why are there no green stars – Reloaded!”, taking into account the many useful contributions here.

  134. Torbjörn Larsson, OM

    Having additional receptor types would allow for either a wider range of frequencies or better color discrimination in the same range, but increases the complexity of processing. Tetrachromats are not uncommon amongst fish.

    Increased complexity when taking advantage of it perhaps. I’m too lazy to check this late on the day, but I seem to remember recent blogging on a paper where they made rats go from IIRC dichromats to trichromats by adding a gene for a new pigment. Apparently they could immediately discriminate colors better, showing the plasticity of the system and the evolutionary path between differently chromatic eyes.

    And I also seem to remember discussions on possible (or possibly verified) human tetrachromats. Some pigments are X chromosome located (explaining the larger prevalence of men for color problems, I believe).
    If mutated that will result in females, that IIRC are X chimeras due to having random X’s downregulated, as tetrachromats.

    (But those mutated pigments will presumably be mostly very slightly different in spectral sensitivity.)

  135. I am probably confused, but I think it was settled long ago that Antares has the color olivacia subrucunda. Next thing you are going to tell me is that there is no such thing as luminiferous aether.

  136. Scott

    Nonsense! There are no green stars because they’re not ripe yet and so God hasn’t picked them.

  137. Oh, yea!

    If you come to my frame and travel with me, you will be able to see the greenary out there! :)
    (nice article)

  138. Elizabeth Madrigal

    My son and his co-writer gave your article an ‘UP’ (highest rating) in his column at wired.com’s weekly rating of Digg.com posted science articles and I can see why. This article is fascinating and I’m going to forward it to a few who love knowing ‘why’. Double kudos, however, as now I even have something interesting to tell my granddaughter who is in that ‘why, why, why?’ stage.

  139. Anne

    There really aren’t that many colours of blackbody radiation:
    blackbody_colours
    (feel free to use that Creative Commons image by the way). The very hottest stars are a sort of baby blue, as you can see here if your computer correctly reproduces the colours.

    Whether the sun is white or yellow is actually not really a well-defined question. So much of colour perception is in our heads that it doesn’t make sense to ask what colour an object “really” is. For example, a piece of “black” asphalt in dazzling sunlight might be sending exactly the same spectrum (and amount) of light into your eyes as a piece of white paper in dimmer light, but your eyes will correctly perceive the one as black and the other as white using context. Colour perception is even more complicated, as anyone who’s had to fool with the white balance controls on a digital camera should know. Optical illusions serve to make the brain’s interpolation obvious, but it goes on all the time.

    In particular, the claim that snow would look yellow if the sun were yellow is just plain wrong: white paper looks just as white under the Sun as it does under yellow incandescent bulbs or under ugly industrial fluorescents. The Sun usually looks yellow when you look at it, and there isn’t really a more absolute sense in which you can say it’s “really” white.

    Colour is a much more complicated business than the usual mumbo-jumbo about additive and subtractive primary colours that most of us got in school.

  140. Mike

    Why are plants green ?

    According to Fred Hoyle, green plants on Earth originated from unicellular
    photosynthetic organisms that came aboard comets.

    These unicellular photosynthetic organisms were born in giant molecular clods where the
    light from neighboring stars was reddened by the intervening dust.

    They needed a green photosynthetic pigment – chlorophyll , to absorb the red light optimally .

    Present day plants descended from these unicellular plants
    that came from space.

    Some plants developed photosynthetic pigments of other colors, but
    chlorophyll had a head start.

  141. nickenino

    Have you ever ear something about the Green Light? It’s a kind of (green) ray that one see when the last sun ray lights at a sea sunset.

  142. --Andrea--

    @@nickenino:

    I believe that someone answered this question earlier in the comments!! You should search and read it, I found it very interesting!

  143. Brian L.

    To amphiox, that statement about something like “the sunlight not being the best wavelength for plants”, that was poor observating, reasoning, and evaluating skills displayed there. There are other lifeforms that need the sunlight as well, different creatures and natural things at different wavelengths, and so the sun’s wavelength would not be very useful it was well-suited for only the plants. The sun is well-suited for good-enough-for-everybody-it-affects, and that’s something random chance couldn’t never precisely do with our lives just so happening to rely on it. That’s a design fulfilling MULTIPLE purposes…and btw, MOST “Christian” Creationist don’t even believe ALL of their own source of information, so one shouldn’t even base their understanding of Creation theory on theirs… The sun is PERFECT for what needs to be done for life here! It’s the EARTH that’s horribly flawed! LOL

  144. tomas kindahl

    What’s this nonsens! Of course there are green stars. But they’re incredibly rare. First of all: not all stars emit light according to black-body radiation, some stars have emission lines that overwhelms the rest of the spectrum. These emission lines tend to be in blue or red, because of the narrowness of the part of the spectrum which our eyes interpret as green. So we may observe purple or impossibly blue Wolf-Rayet stars — those stars that has the clearest emission lines. What we need is a star that has a corona dominated by green emission lines or bands. Most probably they’re out there, just awaiting discovery.

  145. Ben Nagorsen

    WZ Cas (near Beta Cas) is a nice example of a coloured double star (Spectral types N and A, distance 58″, around mag 8). Due to the contrast to the red N star the A star really looks green to me (and to most of my fellow observers). It’s like traffic lights without the yellow light.

  146. Pisces

    @madge…..”The Night Sky”

    Beautiful

  147. lila

    el horto dia mire por mi telescopio i mire acia una estrella i era de color berde es posible que fuera una nave de extraterestres?????????????????

  148. Mike

    “The x-axis is wavelength (color, if you like) color, and the spectrum of visible colors is superposed for reference.”

    Wavelength and color have almost nothing to do with one another. For example, while some pure colors happen to correspond to particular wavelengths, but other pure colors cannot be produced by any light of a single wavelength. This was already known to Newton and Helmholtz. It’s really a shame that many physicists still don’t know the first thing about color.

  149. Dann

    Couldn’t this all be much more simply described without starting up the notorious “colour is all in our minds” debate? It seem to be all about the way heat produces light in a curve. Stars that have curves that cross through the visible spectrum at an angle look either redder or bluer based on the angle, stars with curves that cross through more evenly or peak there look white because no curve can be steep enough to pinpoint that one juicy green frequency. We can obviously see green, its not in our eyes, its completely based on the way heat creates light. Am I wrong?

  150. Bridgit

    I saw a green star tonight! I thought it was strange.

  151. Khalid

    in 147 Mike Says on August 30th, 2008 at 2:52 pm

    “Why are plants green? According to Fred Hoyle, green plants on Earth originated from unicellular photosynthetic organisms that came aboard comets. …”

    Q1. Was there any scientific proof that unicellular organisms can ride a comet and fall on earth and evolved to become plants?

    Q2. are you (in 147) stating those as it is or as a science fiction joke? Fred Hoyle was also a science fiction writer.

  152. amphiox

    Brian L, #150;

    You really shouldn’t take a throwaway joke that seriously. It’s not good for the blood pressure.

  153. amphiox

    Another thing to consider regarding plants and green light is that of course plants have more pigments that just chlorophyll, and these pigments can absorb photons of wavelengths that chlorophyll can’t use, dissipate some of the energy, and re-emit a photon that chlorophyll can use. (Which is immediately absorbed by the nearby chlorophyll so we won’t see any of it).

    So it is not hard to envision the evolution of pigments or systems of pigments that could absorb green light photons and then pass on red and blue light photons to chlorophyll. A plant possessing such a pigment system could look black, and would undeniably be a more efficient photosynthesizer than any green plant.

    But evolution has not at least as far as I know stumbled on to this trick. And the reason could simply be because it has never been necessary. There are plenty of red and blue photons around, and there is no need to absorb those middle wavelengths, and manufacturing those other pigments is costly and perhaps isn’t worth it.

  154. No green stars since they are roughly black bodies, however: it would be possible to have a green glowing piece of material that was not a black body. It is not widely known, that the radiation spectrum applies per the absorptivity at each wavelength. So, if a body could remain purple at high temperatures (absorb green and reflect red and blue) then it would glow green at high temperatures like around 5000-7000 K. In principle the color could be rather saturated, as a sort of “complementary color.” Most materials can’t, but that’s a pragmatic issue. Maybe some weird metamaterials might pull it off. That would be neat, put some chunks of the stuff into a glass-blowing or refining furnace and see people’s reactions.

  155. 99% of Playstation blogs and sites today seem to be copy and past but this site is, keep up the excelent work – chaps

  156. There aren’t any green jelly beans, either. I doubt if it’s a coincidence. http://xkcd.com/882/

  157. Karl Zimmerman

    No purple stars, either; blue & red cones stimulated, but green cones quiescent…

  158. Now everyone should just go and read The Fault in Our Stars by John Green.

  159. Don

    Read all the posts….more confused than ever now!!! I have my own theory: I’m beginning to believe there are no green stars only because no element in abundance burns in green color.

  160. grob

    Much of my time is spent in the clean air of mountaintops above the clouds, and from there the sun always appears pure white to me and my colleagues, even when it is low on the horizon.

    It’s only when I’m in an urban environment, or some cloudy low-elevation site, that the sun seems anything other than pure white.

    And I’ve yet to see a green star, or even a greenish one, although I’ve seen many planetary nebulae and other kinds that display obvious shades of green.

  161. zhe tan

    can you help me ?i don’t konw why i saw the stars are all green?and I’m sure I’m not color-blind patients.

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