Sirius twinkling. Optical image of a trail of the star Sirius (Alpha Canis Majoris) showing the colour variations as it twinkles. This view was obtained by moving the camera during a long exposure. Sirius is the brightest star in the sky (excluding the Sun), and has a pure white colour. It appears to twinkle, as do all the other stars, as the Earth's atmosphere is never still, but has random variations of density, particulate matter and wind speed throughout its volume. This leads to constant and rapid changes in the refraction and absorption of the star's light, causing the colour and brightness variations seen here.