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B Why does Sirius A blink in many colors?

  1. Feb 5, 2017 #1
    When you look at Sirius, it blinks in white/blue/red and even green colors. Why does this happen?
    Please answer it with details(as my researched gave me answers like these: it is closer to horizon or closer to earths atmosphere,it is brighter , but these didn't helped me, i need a complete detailed answer) thanks in advance.
     
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  3. Feb 5, 2017 #2

    Jonathan Scott

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    This is what is meant by "twinkling". Search for "Why do stars twinkle?" for more information. Sirius is bright enough for this to be very visible.

    The effect is primarily due to the fact that the image of a star is so small it cannot be resolved, so all light from it comes via the same path, and is easily affected by atmospheric distortions. In comparison, the light from planets is typically much steadier, as the image is not quite a point and light coming via different paths tends to average out more smoothly.
     
  4. Feb 5, 2017 #3
    Basically this happens due to atmospheric refraction and to every stars but Sirius is the brightest one therefore it is easily observable
     
  5. Feb 5, 2017 #4

    Vanadium 50

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    Are you answering your own question?
     
  6. Feb 5, 2017 #5
    No i was just confirming, sorry for not writing it in reply
     
  7. Feb 5, 2017 #6
    Yes the 'twinkling' effect is entirely down to the light passing through Earth's atmosphere
     
  8. Feb 5, 2017 #7

    Chronos

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    Sirius is very hot. As spectral type a1 it has twice the surface temperature of the sun and is 25 times more luminous. It naturally emits more light in the blue end of the spectrum than most other stars.
     
  9. Feb 5, 2017 #8

    Ken G

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    What is still missing is the explanation. Yes it is because of atmospheric motions and density variations, we know that. Yes it is because of refraction. Yes the stellar image has to be small enough and bright enough to see it. But what remains to be answered is: why does it happen?
     
  10. Feb 5, 2017 #9

    russ_watters

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    Those were all reasons why it happens. Why doesn't that satisfy you?
     
  11. Feb 5, 2017 #10

    Ken G

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    Because I know all those reasons, and I still can't explain why it blinks different colors. The explanation must go further, at the very least it must explain why the intensity changes are different for different colors. I can think of several possibilities just off hand, I'm wondering what the right answer is. For example, I can imagine it might be like a focusing effect of refraction, but lenses show relatively small astigmatism. Perhaps that's because they're built to minimize astigmatism, so is that the reason? Or, I can think it might be something biological happening in the eye, about how the eye interprets brightness variations, like how you see colors in darkness after a light goes out. So we can ask, would a spectrometer measure color variations when looking at Sirius, or just an eyeball?
     
  12. Feb 5, 2017 #11
    I am pretty sure that the perceived twinkling is a real phenomenon and not an artifact of human vision, though I don't know if anyone actually tested that.
     
  13. Feb 5, 2017 #12

    jim mcnamara

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    @Ken G - here is an answer. https://www.scientificamerican.com/article/why-do-stars-twinkle/

    Which is really what @Jonathan Scott said more succinctly.
     
  14. Feb 5, 2017 #13

    Ken G

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    Actually, that's not an answer, it doesn't say a single thing about color changes. Even the analogy they use is not associated with any color changes at all!
     
  15. Feb 5, 2017 #14
    I think Change in colour can also be explained by atmospheric refraction, when white light passes through prism it scatters into 7 different colours and this is due to Refraction just like this when light passes through rarer to denser medium(Atmosphere) or vice versa, it refracts as well as scatters into different colours
     
  16. Feb 5, 2017 #15

    Ken G

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    What's more, most stars don't show color changes, they just show intensity changes, perhaps even a sense of jittering around in location. But they stay white. It's only a few stars, perhaps the brightest ones, that really go through all the colors, and very clearly so. So is this because the colors are really varying in intensity, and our eyes only detect it for the brighter stars, or is it something about how our eyes are responding to that level of brightness that produces an illusion of color change? If refraction is all that is going on, one might expect the refraction at different wavelengths to show much smaller variance than the refraction average, as happens for a prism or any kind of lens. So how could the lensing be so much different for different wavelengths, it would be like putting something under a magnifying glass and only having one color get magnified.
     
  17. Feb 5, 2017 #16

    Ken G

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    Sure, but when you look at an unresolved picture of a rainbow, you're going to see something white, not something flickering different colors. So that's why it is hard for refraction to do it, though I don't know if it is the key or not. You also mention scattering, so that's the issue right there-- we don't even know if the effect is due to scattering or refraction! Given that, how can we claim the question has been answered? All I'm hearing is, "the air does it somehow, or maybe the eye does, but I think it's the air." That's not much of an answer.
     
  18. Feb 5, 2017 #17
    Yes i think this happens to every stars but sirius is the brightest one therefore it is easily observable
     
  19. Feb 5, 2017 #18

    Ken G

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    Maybe that's true, but do we know it's true? And is there an explanation here anywhere? I think your question was a good one, and has still gone unanswered. For one thing, I still don't know the answer. What bothers me about it is that the explanations don't seem to even realize they aren't explaining it, beyond what we've seen already-- which says nothing about why the colors seem to change. When I stand in front of an oddly shaped mirror, I expect my image to distort in shape and total intensity, but I do not expect to see one color. Lenses have more aberration than mirrors, but still, to see one color, that's a heck of a lot of aberration. I suspect there has to be a lot of how the eye works in the explanation there. If not, it might be due to what is known as a "caustic", where you get a kind of perfect storm of refraction that gives a big magnification that might only be achieved at one color, and you only see colored caustics with refraction, because reflection has almost no aberration. and the caustics I've seen were either from reflection or from gravitational lensing (neither of which exhibits chromatic aberration).
     
  20. Feb 5, 2017 #19

    Ken G

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    Googling caustics gives me images like this:
    https://en.wikipedia.org/wiki/Caustic_(optics)#/media/File:Kaustik.jpg
    but nothing about color changes. You can see the caustics in that image are white, yet they are from refraction. Maybe variations in surface shape give less aberration than variations in density, but I'm still not seeing how the color effects are being explained here. Maybe the answer is that different wavelengths tend to pick out density variations on the scale of that wavelength, so the different scales of variations might produce different (and time varying) caustic patterns. That might at least explain it, but I still can't rule out that intensity variations at a given level could by itself appear to the eye like color changes.
     
  21. Feb 6, 2017 #20

    Drakkith

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    I'd guess that the effect is being magnified by the small diameter of the light beam entering your eye and the long distance between your eye and the region of the atmosphere undergoing most of the turbulence. At a distance of a foot or so you won't see much of a rainbow with a glass of water because the different colors can't separate much in such a short distance.

    You won't see any chromatic aberration with mirrors, as they have no dispersion.
     
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