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Twinkle Twinkle
11/22/2002
Listen in RealAudio 
"Twinkle, twinkle little star..." How often have we heard that rhyme? We all know that stars
twinkle, but how?
Although stars are large masses of glowing gas, they are so far away that they appear to us as
bright, steady pin-points of light. While their light travels relatively straight and true
across the light-years of interstellar space, when it reaches Earth's atmosphere, its rays are
diverted from their direct line by changes in air density on their way toward the surface.
This is called refraction.
As the starlight penetrates into the atmosphere, the air becomes denser, but not uniformly so.
Atmospheric layers are mottled with pockets of varying density caused by rising and falling
air parcels and strong horizontal winds. When local air density changes rapidly with time, the
condition is termed "turbulent."
Starlight passing through a turbulent atmosphere has its path changed slightly but perceptibly
by refraction -- bending one way, one moment, slightly different the next. This constant,
random shifting makes the star's image jive and jiggle, fade in and out, and even change
colors. This stellar dancing causes the twinkle.
The greater the turbulence, the greater the twinkle effect. In fact, wildly twinkling stars
usually indicate strong winds in the atmosphere above.
Twinkling only affects objects whose visual size is smaller than the refractive shifts caused
by the atmosphere. Even though they may look like stars, planets don't twinkle because they
present a visually large disk compared to the level of turbulent refraction. A great way to
identify whether an object is a star or planet is to see if it twinkles.
Our show is produced by the Mount Washington Observatory, with support from Subaru, The Beauty
of All-Wheel Drive, and The National Science Foundation.
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