The twinkling of star is due to atmospheric refraction as the star light enters the atmosphere it undergoes repeated refraction before reaching us since the       astmosphere bends the starlight toward the normal the apparent position is different from actual position . apparent star is slightly higher than actual satr when viewed from horizon the apparant position is not sationary it keeps on changing because the earth physical condition keeps on changing. star is point source of light as the path of light from it keeps on changing it fluctuates and the amount of light in our eyes flickeres