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Why do stars twinkle?

Dear Cecil:

Why do stars twinkle?

Ben Schwalb, Laurel, Maryland

Cecil replies:

Ben, you amateur, stars don’t “twinkle.” They exhibit “stellar scintillation.” The Pentagon isn’t going to fund a damn twinkle study. Whatever you call it, it’s caused by turbulence in the atmosphere, which in turn is caused by convection — clumps of warm air rising through colder stuff. Air will refract (bend) light a varying amount depending on its temperature.

You can see this in exaggerated form in the waves, or striae, that ripple above a radiator, a sun-baked highway, or some other heated surface. Because of the bending, sometimes you see more starlight, sometimes less, and it looks like the star is, you know, twinkling. The planets and the moon don’t scintillate (as much, anyway) because their apparent size is so much larger that a little atmospheric refraction doesn’t greatly alter the amount of light that reaches the eye.

Cecil Adams

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