When Sirius, the brightest star in the nighttime sky, rises in the southeast about 7 p.m., people comment on its fierce twinkling. Stars twinkle because their thin beams of light are bent and twisted while traveling through turbulant layers of our atmosphere. The light is also refracted into its many colors. Being so bright, Sirius twinkles more than other stars. Its twinkling slowly ceases as it gains altitude.
Source: John Mosley, Griffith Observatory