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Why do stars flicker?

  1. Aug 2, 2005 #1
    Stars are known t0 flicker.

    a) estimate the no. of photons entering the eye of an observer when he looks at the star of first apparent visual magnitude. Such a star produces flux on the surface of the earth of 10^-6 lumens/sq.meter. One lumen is 0.0016 watts. Star Aldebaran is an example.

    b)why do start flicker? Not the planets, or very little.
     
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  3. Aug 2, 2005 #2

    Danger

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    Only variable stars 'flicker', and that variation is over a minumum of several hours. Atmospheric aberations make any extraterrestrial light source appear unstable because the light is constantly being diffracted and reflected on the way down. This applies to the planets as well.
     
  4. Aug 2, 2005 #3

    Astronuc

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    Planets are much closer than stars, so the light has less chance of interacting with interstellar media, and

    Stars are more or less point sources, whereas planets are 'more' distributed, although the solid angle subtended by a planet is still rather small.

    I have also heard an argument that planets are cool bodies and reflect light, rather than a hot body like a star which generates light.
     
  5. Aug 2, 2005 #4

    Danger

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    On the face of it, that doesn't seem to make much sense. Or is it just because the planetary albedo limits the frequencies of light and therefore the prismatic effect of the atmosphere?
     
  6. Aug 2, 2005 #5

    turbo

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    Stars flicker because they have no apparent diameter to us. They are very distant, and because they appear point-like, ALL the light from each of them travels on a single path to our retinas. If this path is disturbed by variations in the atmosphere (which is a refractive medium), the stars will appear to shift positions, intensity, and color. Planets flicker, but to a much lesser extent, primarily because they have an apparent diameter as seen from Earth, no matter how tiny, and the light from them is reflected from all over their illuminated surfaces. The light from them comes to us on a multitude of paths, so the atmosphere has to be pretty unsteady to make them appear to flicker visibly. Even so, on nights when stars twinkle (observational astronomers call this atmospheric condition "poor seeing"), getting decent planetary photos on film is a lost cause. Digital detectors often do not need a very long exposure time, and so astronomers using these can take advantage of short periods of adequate seeing to get usable, if not fantastic, planetary photos.

    I hope this helps.
     
    Last edited: Aug 2, 2005
  7. Aug 2, 2005 #6

    DaveC426913

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    NOT variable stars and NOT interstellar media

    Nonsense. Flickering stars are due to scintiillation, the atmosperic phenomenon mentioned several times here, and has nothing to do with variable stars - or anything else outside our atmosphere.


    Nonsense. Flickering has nothing to do with interstellar media.

    Guys, if you're going to make guesses, at least say they're guesses. People here will take your answers as if you know what you're talking about...
     
  8. Aug 2, 2005 #7

    Danger

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    You might want to try reading my post before attacking, rather than just skimming without comprehension. The first sentence is in reference to physical 'flickering', and variables are the closest thing to it. The rest of the post deals with perceived 'flickering', which is exactly the scintillation effect.
     
  9. Aug 3, 2005 #8

    Chronos

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    I like the observational interpretation, like turbo pointed out. Stars usually appear to flicker because earth has an atmosphere. Variable stars [intrinsic flickers] are a whole different story.
     
  10. Aug 3, 2005 #9

    DaveC426913

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    I did. That other parts were true does not negate the fact that the part I quoted was nonsense.

    While your answer may have been a well-meaning attempt to round out the answer, I'm sure upon reflection, you'll recognize that it merely muddles what is an obvious, simple answer to the OP's question.
     
    Last edited: Aug 3, 2005
  11. Aug 4, 2005 #10

    Danger

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    Obvious to you, perhaps, but the OP didn't specify whether he was referring to variables or to scintillation. Since his first statement was that 'stars are known to flicker', where in fact they don't, opened up the possibility that he was referring to magnitude variations. The cyclic time that I pointed out should have disillusioned anyone of the possibility that perceived flickering was what I was referring to in that sentence. Therefore the part that you quoted was not 'nonsense'; it was a disclaimer. And on a cosmic time-scale, the cycles of Cepheid or RR Lyrae variables are flickers.
     
    Last edited: Aug 4, 2005
  12. Aug 4, 2005 #11

    DaveC426913

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    "the OP didn't specify whether he was referring to variables or to scintillation."

    "Caveat scisco": "Let the asker beware".

    A novel philosophy in a forum intended to educate.
     
    Last edited: Aug 4, 2005
  13. Aug 4, 2005 #12
    Stars are far off-bodies as compared to the planets.They appear as point-objects as compared to the planets which can be seen as discs while looking through the telescopes.So approximatingly, a single light ray in straightline comes out eyes , the emdium between out eyes and the star is the atmosphere.So when the light rays coming from the star pass through different layers of different densities/temperatures , the light ray suffers refraction as different rays from different medium reach us,and we perceive the 'twinkling' , as the poets put it.
     
  14. Aug 4, 2005 #13

    Astronuc

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    Dave, thanks for pointing that out. I was being careless with the term interstellar media - in which I was lumping all medium including the atmosphere.

    On Earth, the atmosphere has a particle density of about 2.4e19 particles/cc near the surface (at 1 atm of pressure), and several hundred km away from the surface it falls to about 1e12, and on the moon, the atmoshpere is something like 200,000 molcules/cc. Then way out of the solar system the particle density is something like 1-100 particles/cc.

    The twinkling on earth is certainly due to the earth's atmosphere where density variations and particulate matter scatter the light. The light from the star as a point source or line of photons is affected by the variation in the optical properties of the atmosphere, while the sun, moon and planets which occupy a solid angle (larger distributed surface) are not affected by the minute variations in optical properties.

    I hope that clears things up. :smile:
     
  15. Aug 4, 2005 #14
    So if I leave earths orbit, stars will not flicker, unless some other object crosses the beam. What a beautiful sight that must be.
     
  16. Aug 5, 2005 #15

    Danger

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    For the benefit of those who either don't or shouldn't teach for a living, it's called 'covering the bases'. Other than brandishing the term 'scintillation' while trying ineffectually to demean Astronuc and me, you seem to have contributed nothing to answering the original question. And if I may be so bold as to point it out, I was the first one to mention atmospheric abberations. So much for guesswork.

    It would appear to me that your only slip was in saying 'interstellar' rather than 'interplanetary' in reference to the medium. Easily done, and it's not as if you were lacking in educational background on that subject. The interplanetary particle density will also vary with solar and cometary activity, so any firm prediction of interactions with starlight is impossible.

    You just have to leave the atmosphere, not the orbit. That's the whole point of the Hubble and other space-borne observatories. Get away from our immensely disruptive atmosphere to obtain clear pictures. As Astro pointed out, there is still some distortion from particles inherent to interplanetary space, but it's just about negligible in comparision to what we live with down here.
     
  17. Aug 27, 2011 #16
    maybe, now im guessing... but maybe it blinks instead of flickers because the time it took for a planet to go one revolution around the start contracts to a few seconds.

    we know that at close to the speed of light, time and length would contract. thus the length of lights wavelength in this period of time might contract making the light seem to blink by the time it reaches our eyes.

    the rate of which the length might contract might be exponential. which would explain why some stars have a longer period of which the star stays shining without blinking.
     
  18. Aug 27, 2011 #17

    HallsofIvy

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    It is not necessary to leave the earth's orbit. As soon as you get out of the earth's atmosphere that happens. You see unflickering stars against a perfectly black background.
     
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