What is the reason for the twinkling of stars?
Light from the stars has to come through a thick layer of the atmosphere. We know that the density of atmosphere keeps on changing as gravitation increases. So more dense layer of the atmosphere will have a greater refractive index and hence will bend light more. Since the physical condition of the atmosphere is not constant and keeps on changing, the amount of starlight entering our eye keeps on changing. Thus sometimes they appear bright while sometimes faint-twinkling of stars'.