Why Star Twinkling

Stars are far far away from us ( some light years) , the light comes from star to us in very small intensity.

The twinkling of stars is due to atmospheric refraction of star-light. The refraction of light caused by the earth's atmosphere having air layers of varying optical density is called atmospheric refraction. The physical conditions of the atmosphere keep on changing continuously due to which density of air in different layers of atmosphere also keeps on changing, As a result of this, the refractive index of the various layers of atmosphere also keeps on changing continuously. So, light coming from stars suffers multiple refraction and the amount of starlight reaching the eye also keeps changing and so, due to fluctuation of perceived brightness of the star, they appear like they are twinkling.