When the light coming from a star enters the earth's atmosphere, it undergoes refraction due to the varying optical densities of air at various altitudes. The atmosphere is continuously changing. The continuous changing atmosphere refracts the light from the stars by different amounts from one moment to the next. When the atmosphere refracts more starlight towards us, the star appears to be bright and when the atmosphere refracts less starlight, then the star appears to be dim. In this way, the starlight reaching our eyes increases and decreases continuously due to atmospheric refraction and the star appears to twinkle at night.