Stars appear to twinkle because of atmospheric refraction when the star light enters the the earths atmosphere it undergoes repeated refraction before reaching the earth . the atmospheric refraction occurs in a medium where gradual change of refractive index takes place the atmosphere bends the star light towards the normal the apparent position is different from actual position the apparent star is slightly higher than the actual star. when viewed from horizon . the apparent position is not station nary because the physical condition of earth's atmosphere is not stationary the stars are very far so they are point source of light the path of light coming from the stars keep on changing i fluctuates and the amount of light entering our eye flickers