(i) Stars are self luminous and can be seen at night in the absence of sunlight.
(ii) They appear to be point sources as they are at a very large distance from us.
(iii) The apparent position of the star keeps changing a bit because of the motion of atmospheric air and changing air density and temperature.
(iv) Because of this, refractive index of air keeps changing continuously.
(v) So the position and brightness of the stars keeps changing and so the stars appear to be twinkling.
