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Thread: The Colour of Stars

  1. #1 The Colour of Stars 
    Forum Sophomore Tharghana's Avatar
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    Astronomy has always been one of my side interests. I do ask a lot of questions, but isnt that what a scientific mind does?

    Anyway, my question is what determines the colour of a star, is it purely Temperature, or does it also have something to do with Chemical Composition. I do know that Red Stars, burn at very low temperatures (In a stellar sense) and Blue Stars burn at very high temperatures, but following the the spectrum, Green is Missing, I have never heard of a Green star, why is that, and is possible for a star to be green, possibility a star that has Boron content. (due to the fact Boron burns with a bright green flame.) Or Is there some of factor, disabling stars from being Green.

    Would it also be possible to be a star so High it temperature is out and of the Visual Spectra, and therefore invisible to the naked eye, like a star thats colour is purely Ultraviolet.


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    http://www.astronomycafe.net/qadir/q72.html


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    Forum Sophomore Tharghana's Avatar
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    So technically A-Class Stars are green, we just don't see them as green?
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    Quote Originally Posted by Tharghana
    So technically A-Class Stars are green, we just don't see them as green?
    That's one way of putting it, but I wouldn't say it that way.

    They have a mixture of colors with the peak being green. We see a mixture of colors, having a more or less equal distribution of colors in the visible spectrum, as being white. Sunlight is an example.
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    Forum Sophomore Tharghana's Avatar
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    I understand now.

    But would a star with high concentrations of Boron in its atmosphere, would that appear to be green? Similar to how Carbon stars appear very Dark Red almost Crimson in colour.
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    Quote Originally Posted by Tharghana
    I understand now.

    But would a star with high concentrations of Boron in its atmosphere, would that appear to be green? Similar to how Carbon stars appear very Dark Red almost Crimson in colour.
    No. The color of light from a star corresponds to the black body spectrum for its temperature. The elements in the star show up as dark absorption lines, called Fraunhofer lines.
    http://en.wikipedia.org/wiki/Black_body_radiation
    http://en.wikipedia.org/wiki/Fraunhofer_line
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  8. #7  
    Forum Sophomore Tharghana's Avatar
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    Quote Originally Posted by Wikipedia
    A carbon star is a late type giant star similar to the red giants (or occasionally red dwarf) star whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere, and a strikingly red appearance to human observers.
    This is what I am referring to.
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    Moderator Moderator Dishmaster's Avatar
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    Interesting discussion! Here is a link that might provide some additional information:
    http://arxiv.org/abs/astro-ph/0108263

    The chemical composition generally has little influence on the appearance of stars (unless the differences are drastic). In the example you mentioned, I guess that the main source of the stellar colour in Carbon stars is the reduced surface temperature due to the additional cooling provided by the over-abundance of Carbon. So it is not primarily caused by a different line spectrum. See the comparison of spectral classes and in particular the different temperatures in the two links:
    http://en.wikipedia.org/wiki/Stellar...classification
    http://en.wikipedia.org/wiki/Carbon_star#Harvard

    The same should then be valid for Boron rich stars in which the Boron abundance compared to other elements is generally low. Please note that Boron has spectral lines in the UV range. It seems that it is often used as a diagnostic for investigating the evolution of young stars. Green spectral lines originate from doubly ionised Oxygen ions. It is strongly excited in ionised gas nebulae like the Orion nebula and can be easily observed when using dedicated optical filters.
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  10. #9  
    Forum Sophomore Tharghana's Avatar
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    Thanks for the Info. So a Star with a atmosphere of highly ionized oxygen atmosphere would appear to be green?
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  11. #10  
    Moderator Moderator Dishmaster's Avatar
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    Quote Originally Posted by Tharghana
    Thanks for the Info. So a Star with a atmosphere of highly ionized oxygen atmosphere would appear to be green?
    Maybe. I am not sure. But you would also need the right concentration of ionised oxygen. To my knowledge, such a star has never been observed.
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