The
Earth's
atmosphere is very
unstable and in
turbulent conditions at altitude, breaks into "
cells" of varying
density. These cells
refract the
starlight differently and the
net result is a
flashing sparkling star image. Each image is in effect a small spectrum or
rainbow of light which the atmosphere sweeps backward and forward across you eye giving rapidly changing colours. The
combined effect of many such cells in the line of sight towards a star gives it a very
complex twinkling of all the
colours of the rainbow. It is more noticeable for
stars at
low elevations as we are then looking through a greater path-length in the Earth's atmosphere.
When you look through a large telescope at the star image, the combined effect over a larger diameter path makes the star fuzz up into a blur. Astronomers refer to this as bad "seeing".