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Green stars?

  1. Sep 7, 2004 #1

    mee

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    in some photos, mixed among the red white and blue stars, are green stars. Any idea what causes this?
     
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  3. Sep 7, 2004 #2

    mathman

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    It sounds to me it is a false color phenomenom, i.e. whoever produced the picture decided to color some things green for some reason. Color is determined by surface temperature. Cooler stars are red, intermediate are white, and hotter are blue.
     
  4. Sep 9, 2004 #3

    Chronos

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    Most likely due to intervening dust and gas clouds.
     
    Last edited: Sep 9, 2004
  5. Sep 9, 2004 #4

    Math Is Hard

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    When you said green, it reminded me of something from my astronomy textbook, but I am not sure if this has any bearing on your question:

    "The average temperature of the photosphere (of our Sun) is about 5800 K, corresponding to a thermal radiation spectrum that peaks in the green portion of the visible spectrum with substantial energy coming out in all colors of light. The sun appears whitish when seen from space, but in our sky the Sun appears somewhat more yellow - and even red at sunset - because Earth's atmosphere scatters blue light." - The Cosmic Perspective, 3rd ed, Bennett, et al

    Anyway, just an interesting side note.
     
  6. Sep 9, 2004 #5
    Quasars' colours

    Very high redshifted galaxies appears as point-like objects (interpreted and defined as quasi-stellar objects and called quasars ) smaller than a galaxy depending on its great distances.

    Are those (point-like) distant galaxies what you mean by the red, white, blue and green “stars”?

    The explanation to the galaxies' changing color with the distance is, that their energy-wavelength curves (Planck curve) are moving towards the longer (red) wavelengths caused by the entropy effect that is the mechanism that forces the radiation's energy by dissipation--that is energy reduction and wave-elongation--towards equilibrium in the low temperature back-ground radiation.

    Depending on the distance, the peak of a galaxy’s energy-curve is moving (displaced) to red, and its gradient (proportion or distribution of energy over the optical spectrum) changes inclination (angle). So, compare with a near galaxy with a “normal” distribution gradient (ratio) over spectrum that appears white to us, the gradient changes its inclination with the distance and the radiation that reach us get more energy at its blue side (and later red side) of the spectrum.

    So, with further distance, the peak of the energy-wavelength curve is further displaced on its way to us, and the gradient’s inclination changes when the peak passes through the colour spectrum and the radiation’s colour change relative to the distance to us. The more distant galaxy, the more displaced is the peak over the colour spectrum, and the galaxies' colour changes with the distance, from white over blue to red.

    Consequently, with more distance to the galaxies, the more they are redshifted, and the more displaced is the energy-wavelength curve that causes the change of the light’s apparent colour that reach us. At the most distant galaxies (interpreted as quasars) the “Planck-curve” has moved by the distance to the redder end, so that the red colour that reach us has got most energy, and those galaxies appears as red. By this entropy-effect, the radiation's displacement and dissipation on its way changes the galaxies' colour with the distance.

    The "quasars" (=galaxies) just apparently huge radiation is caused by the entropy-dissipation effect (defined and predicted by Clausius who didn't agree with Boltzmann's provisional statistical entropy interpretation) that is demonstrated by the combination of the laws of physics by Wien and Stefan-Boltzmann that a galaxy's energy that reach us is displaced proportional to the distance according to this simplified algorithm E=z^4. So if we observe a galaxy that is redshifted 10 times (z=10), its emitted radiation is 10x10x10x10=10 000 times higher than the radiation that reach us. This drop of energy is the dissipating entropy-effect that drives the radiation forward.

    Maartin Smith at The Sloan Digital Sky Survey said about the energy mystery of the quasars and their unknown strange nature: they had discovered the 6.82 quasar that was redshifted from 1216 Angstrom to 8300 Angstrom:
    "Quasars are the most luminous known object; they emit between 10 and 1000 times the energy of our galaxy." (...) "The standard model for the energy source of quasars is accretion of matter onto a large (ca.100 million solar mass) black hole."

    My understandable explanation and simple calculation show that the radiation's emitted energy from this "6.82 quasar" is 6.82^4 (=2100) times higher than the radiation we receive.

    Look also at my reply at https://www.physicsforums.com/showthread.php?t=36805
     
    Last edited: Sep 9, 2004
  7. Sep 9, 2004 #6

    mee

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    No the "green stars" were in the foreground. Quite bright. Go here to see what I was talking about.

    http://antwrp.gsfc.nasa.gov/apod/ap030531.html
     
  8. Sep 9, 2004 #7

    mathman

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    There appears to be one bright white star with a green fringe. It seems to be some sort of optical aberation.
     
  9. Sep 10, 2004 #8

    mee

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    If it is an optical aberration why are not the other stars affected as well making them green?
     
  10. Sep 10, 2004 #9

    Nereid

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    The key to answering this question is here: "About the Image: Wavelength: Natural color reconstruction from three filters (I,B,U)". In other words, the HST took three separate piccies, in the I, B, and U wavebands. Then someone used an image processing tool - not unlike Photoshop - to create something more like a three-colour RGB image. Since two the of wavebands are, in fact, largely 'invisible' (I and U) to human eyes, the 'reconstruction' cannot be perfect; in the case of the bright 'green' star, it is clearly not even 'good'.

    It might be a fun thing to try and work out what sort of 'IBU' colours the 'green star' probably has, by reverse engineering the 'reconstruction' transformation.
     
  11. Oct 2, 2004 #10

    mee

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    "There is a big difference between ordinary novas (all are binary stars with a white dwarf companion) and supernovas. Antares does have a distant (green!) companion, but it is a red super giant itself, and will likely become a "Type II supernova" eventually. "

    Someone on a mailing list I belong to wrote the previous quote. He seems to think there are green stars and is quite a physicist from what I have seen in his other posts.
     
  12. Oct 2, 2004 #11

    Nereid

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    Ah the tricks our eyes play on us!

    Yes, Antares' companion does indeed look green through the eyepiece of an amateur's telescope! As do many other stars; one of the delights of double stars is the astonishing range of colour contrasts between the two stars. Sadly, it's primarily a 'brain trick' - you've probably seen those 'square in a square' cards? The inner square of any pair is, in fact, the same colour; the outer square has very different colours, and our eyes (brain, actually) think we see the inner square as having quite different colours. I'm told that artists learn early on to paint 'shadows' with coloured tinges - add purple, if it's a shadow in a yellow context, add green if red, and so on.

    The 'true colour' of a star is a plot of intensity vs wavelength (an example )
     
  13. Oct 7, 2004 #12
    Maybe the stars are just sick? Guess they ate a bad planet.
     
  14. Oct 12, 2004 #13

    Dear god...science humour....not as bad as the " h(bar) and grill"...that one's real bad.
     
  15. Oct 17, 2004 #14
    The green color is centered in the spectrum, so a star which emission has a peak in the green color, appears to be white. If we take a (chemical) photo, then the optics, atmosphere, etc may distort the colors and make stars appear in false color. Thats why a star appears to be green or else.
     
  16. Oct 17, 2004 #15

    mee

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    but I have heard of stars appearing green to the naked eye using backyard telescopes.
     
  17. Oct 17, 2004 #16

    achromatic refractor possibly?
     
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