Q:

Why does the star Sirius sometimes appear to change colors rapidly, red-green-white, several times per second?

A:

The Earth's atmosphere acts like a prism to split (refract) the light from bright stars into the various colors of the rainbow (spectrum). Different colors of light are bent by different amounts, so as turbulent eddies in the atmosphere move through your line of sight to the star, you many seen more red, yellow, green or blue light, depending on the properties of the atmosphere. This light show is most noticeable when the star is near the horizon, so you are looking through more of the Earth's atmosphere, or when there is a lot of turbulence or smog in the atmosphere, and sometimes leads to reports of UFOs.
(See http://chandra.harvard.edu/chronicle/080999/cappella.html)

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