Why do Stars Twinkle?

The twinkling of a star is due to atmospheric refraction of starlight. The  starlight on entering the earth’s atmosphere undergoes refraction  continuously before it reaches earth. The atmospheric refraction occurs  in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from that of its actual position. The star appears slightly higher(above) than its actual position, when viewed near the horizon. Further, this apparent position of the star is not stationary, but keeps on changing slightly, since the physical conditions of the earth’s atmosphere are not stationary. Since the stars are very distant, they approximate point-sized sources of light. As the path of light rays coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers i.e. the star sometimes appears brighter and at some other time, fainter, which is the twinkling effect.