is very unstable
and in turbulent conditions
at altitude, breaks into "cells
" of varying density
. These cells refract
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".