The twinkling of a star is due to atmospheric refraction of starlight. The atmospheric refraction occurs in a medium of the graduallv changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. This apparent position of the star is not stationary but keeps on changing slightly, as the physical conditions of the earth's atmosphere are not stationary. Since the stars are very distant, they approximate point-sizes sources of light. As the path of rays of light corning from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers-the star sometimes appear brighter, and at some other time, fainter, which is the twinkling effect.
