Because of refraction of light....

As the stars are very far, they may 
sory ans.was not proper,as stars are very far they may be considered to be point of source of light. When star light passes the atmosphere it bend towards the normal till it enters our eyes due to refraction so its apparent potion is slightly higher than its actual potion.Due to mobility of an air and changes in temperature, the refractive index of atmosphere goes on changing continuously and randomly.Therefore apparent potion of star is not steady, but it changes slightly,so star twinkly