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Thread: Question about the Color of Stars.

  1. #1 Question about the Color of Stars. 
    Moderator Moderator Cogito Ergo Sum's Avatar
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    In the eighth episode of the TV show Cosmos: A Spacetime Odyssey, they talked about stellar spectra and their relationship with surface temperatures. Afterwards, I recalled that different characteristics of stars are summarized in the so-called Hertzsprung-Russell diagram:


    (taken from ESO)

    However, why are some colors missing on the x-axis of this diagram? Why can stars apparently not have a greenish or purple-like color?


    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

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    KJW
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    Quote Originally Posted by Cogito Ergo Sum View Post
    However, why are some colors missing on the x-axis of this diagram? Why can stars apparently not have a greenish or purple-like color?
    Have you every seen anything green hot? I think the constraint is in the black-body spectrum, noting that the x-axis is temperature.


    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Life-Size Nanoputian Flick Montana's Avatar
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    Well, stars DO burn green. Our plants reflect that light as evidence. Perhaps it is down to our ability to perceive certain colors of light?
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
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    Moderator Moderator Cogito Ergo Sum's Avatar
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    Quote Originally Posted by KJW View Post
    Have you every seen anything green hot?

    I cannot name something that has a high temperature whilst simultaneously appearing greenish.

    Quote Originally Posted by KJW View Post
    I think the constraint is in the black-body spectrum, noting that the x-axis is temperature.

    Are you implying that the temperatures for purple and green would imply surface temperatures too cold for stars?
    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

    ~ Arthur Schopenhauer, The Art of Being Right: 38 Ways to Win an Argument (1831), Stratagem XXXVIII.
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    KJW
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    Quote Originally Posted by Cogito Ergo Sum View Post
    Quote Originally Posted by KJW View Post
    I think the constraint is in the black-body spectrum, noting that the x-axis is temperature.
    Are you implying that the temperatures for purple and green would imply surface temperatures too cold for stars?
    No. I'm suggesting that the shape of the black-body spectrum is such that at any temperature where one might expect it to be green, the amount of red or blue would be too large for it to actually be green.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    KJW
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    Quote Originally Posted by Flick Montana View Post
    Well, stars DO burn green. Our plants reflect that light as evidence. Perhaps it is down to our ability to perceive certain colors of light?
    I think Cogito Ergo Sum was referring to light that appears green rather than light that contains green.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by Flick Montana View Post
    Well, stars DO burn green. Our plants reflect that light as evidence. Perhaps it is down to our ability to perceive certain colors of light?
    I think Cogito Ergo Sum was referring to light that appears green rather than light that contains green.
    What I'm saying is that it is perhaps our ability to perceive light in that part of the spectrum which is affecting the color. Do our eyes better detect radiation in certain bands of the spectrum? Green appears white, purple appears blue, etc?
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
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    Forum Sophomore Karsus's Avatar
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    Curious About Astronomy: Why aren't there any green stars?
    You're on the money. This site suggests that it is because of the sensitivity of our eyes and the way we perceive colour.
    Our sun burns at the right temperature to emit yellow-green light as its peak output, but it's also emitting light across the rest of the spectrum with a fairly even distribution across the visible part. Our eyes happen to interpret this combination of wavelengths as "white".

    Black body emission isn't going to give a distribution of wavelengths narrow enough around the green part of the spectrum to allow our eyes to pick it out. This site illustrates it pretty well. Blackbody Radiation
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    KJW
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    Quote Originally Posted by Flick Montana View Post
    What I'm saying is that it is perhaps our ability to perceive light in that part of the spectrum which is affecting the color. Do our eyes better detect radiation in certain bands of the spectrum? Green appears white, purple appears blue, etc?
    While it's true that the human eye has different sensitivities to different wavelengths, I think it's more likely that the narrowness of the green region and the broadness of the black-body spectrum would prevent a temperature that appears distinctly green rather than progressing from red to yellow to white as the temperature is increased.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Moderator Moderator Cogito Ergo Sum's Avatar
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by Flick Montana View Post
    Well, stars DO burn green. Our plants reflect that light as evidence. Perhaps it is down to our ability to perceive certain colors of light?
    I think Cogito Ergo Sum was referring to light that appears green rather than light that contains green.

    Yes, that is what I meant.
    My question is now solved, thanks to member Karsus.
    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

    ~ Arthur Schopenhauer, The Art of Being Right: 38 Ways to Win an Argument (1831), Stratagem XXXVIII.
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