Stars are self luminous and can be seen at night, despite the absence of sunlight. They appear to us as point sources as they are at a very large distance from us. Now, as the density of the air increases along with lowering the height above the surface of the earth, the refractive index also increases. Starlight coming towards us travels from rarer medium to denser medium and constantly bends towards the normal, thus making the star appear to be higher in the sky as compared to its actual position. The motion of the atmospheric air as well as the changing air density and temperature result in changing of the apparent position of the star. This leads to the continuously changing refractive index of air. As a result, the position and brightness of the star also keep changing continuously and the star appears as if it is twinkling.
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