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Why do planets and stars flicker?

  1. Jun 30, 2015 #1
    If you look up in the sky you can see both planets and stars. Sometimes you see them flicker, their luminosity oscillates, why does this happen?

    If we can perceive changes in their brightness from so far away wouldn't the object's brightness be changing in unrealistic amounts? shouldn't the object stay mostly the same brightness?

    Also, I think its only planets that flicker because stars are too far away, is that true?
     
  2. jcsd
  3. Jun 30, 2015 #2

    A.T.

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  4. Jun 30, 2015 #3
    Thanks,

    So does that mean it don't matter the planet, they all should flicker based on anomalous refraction of light?

    How did they figure that out?
     
  5. Jun 30, 2015 #4

    Drakkith

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    Planets are much less likely to flicker because they are not quite 'point sources' like stars are. That means that when the light corresponding to one side of the planet is refracted, the other side is probably not refracted the same way, so there's no noticeable flicker. You could say that the flickering of the points on the disk (the image of the planet) tends to cancel themselves out. Instead, if you look at a planet through heavy turbulence, you will notice a substantial blurring and shimmering effect, similar to looking at a far away object on a hot day.

    Science!
     
  6. Jun 30, 2015 #5

    A.T.

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    Don't know who proposed it first, but meanwhile we have telescopes in space to avoid such atmospheric effects.
     
  7. Jun 30, 2015 #6
    So is the flicker of stars just because of solar flares and what not. I thought planets flicker more because I thought I was looking at mars and it was flickering heavy. But stars flickering because of life cycle or nature makes much more sense.

    Bonus question: when we see mars with the naked eye from earth is it red and brighter than stars.

    Much appreciated Drakkith
     
  8. Jun 30, 2015 #7
    anomoul
    Why would stars flicker more than planets if they are point sources.

    The luminosity is more for planets like Jupiter so the atmospheric effects should be greater?

    Changes in the brightness because of moving planets and refraction is what I was thinking but it is not noticeable.

    Why is a point source important
     
  9. Jun 30, 2015 #8

    russ_watters

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    What? Where did you get solar flares from what was suggested? No, it isn't solar flares, it is turbulent atmospheric refraction.
    As said, the amount of turbulent refraction is not typically enough to refract all of the light from a non-point source in the same way because they are bigger. Spreading-out the light from a point source makes it much dimmer whereas spreading-out the light from a non point source doesn't make it much dimmer because the amount of spreading is lessened. Consider the example of the moon -- it is so big you don't notice any twinkling.
     
  10. Jun 30, 2015 #9
    thank you
    idk where I got solar flares from i thought someone said it I definitely didn't guess.

    So the stars flicker more because their light has spread out so much in space that by the time it reaches earth the photon is weak enough such that the tiny atmospheric anomalies can mess with the stars luminosity from our perspective.

    But I still don't understand the significance of a point source or what it means, google and Wikipedia wont give me an easy answer, please answer
     
  11. Jun 30, 2015 #10

    russ_watters

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    It isn't the spreading-out in space, it is the spreading-out by the atmosphere. Bright stars are similar in apparent magnitude to planets, but will flicker more.

    Also, a photon is what it is: it does not get "weaker" due to travel through space (at least not for the purposes of this discussion).
    A point source is literally what it sounds like: a source of light that is so small it appears to be a point instead of a circular object. A point source has no identifiable size.
     
  12. Jun 30, 2015 #11
    So its the opposite of spreading out, its because the photon is like a pinpoint so it can be affected by the refraction.

    And all the that matters is the size of the light source.
     
  13. Jun 30, 2015 #12

    russ_watters

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    It is easier to see the effect of twinkling through a telescope. Here's what a normal star looks like on a calm day, with the size of the point limited by the size of the telescope (larger scopes yield tighter points):

    tip_tilt.jpg

    Here's a star obliterated by atmospheric twinkling, animated:

    STAR.gif

    Here's a comparison showing them getting progressively worse:

    Starcomp2.jpg

    And a link to the page explaining the issue:

    http://www.noao.edu/education/gsmt/seeing
     
    Last edited by a moderator: Apr 28, 2017
  14. Jun 30, 2015 #13
    Nice
    thank you, Russ my question is pretty much answered, great article
     
  15. Jun 30, 2015 #14

    Drakkith

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    Don't think in terms of photons. A photon is simply the energy of a single interaction of an EM wave with matter. The question you're asking is MUCH easier to understand in terms of classical EM waves.
     
  16. Jun 30, 2015 #15

    Drakkith

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    A point source is a source of light that, for all intents and purposes, has zero angular size. If you hold your finger right in front of your eye it blocks out most of your field of view and we say it has a very large angular size. As you move your finger away it becomes smaller and smaller, and we say its angular size is getting smaller. Angular size is measured in degrees, with 360 degrees being something that would surround you. The Moon is approximately 0.5 degrees across. Jupiter, at closest approach to Earth, is about 50 arc-seconds across, where 60 arc-seconds = 1 arc-minute and 60 arc-minutes = 1 degree. So Jupiter is about 0.014 degrees across at closest approach.

    In contrast, the star with the largest angular size when viewed from the Earth, other than the Sun, is about 0.06 arc-seconds, which is 10,000 times smaller than Jupiter. All other stars are even smaller than this. This is so small that, except for a couple of exceptions, we do not have telescopes with a high enough resolving power to see the star as an actual disk instead of a little dot. If you cannot resolve your target (meaning that your maximum resolution isn't high enough to see the target clearly) then we call that target a 'point source' and can treat it as if it has zero angular size for almost all purposes.

    The reason this is significant is because when we look at an object through the atmosphere, objects which are treated as point sources (stars or small objects here in the solar system) act as if all of their light comes from a single point in the sky, which means that turbulence affects most/all of the light the same way. So when most of the incoming light is refracted away from your eye, there is no other light from another nearby point in the sky and the star appears to dim. For planets, a slight dimming of the light from one point is barely noticeable because other nearby points are not refracted away from your eye or have extra light refracted towards your eye to make up for the light loss. This is actually a gross simplification, but I don't know how to explain it without getting into a lot of detail with optics.
     
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