* Sirius is closer: Sirius is about 8.6 light-years away from Earth, while Pollux is about 34 light-years away. Being closer means the light from Sirius reaches us more intensely.
* Sirius is intrinsically brighter: Even if they were the same distance away, Sirius would still be brighter than Pollux. Sirius is a much more massive and luminous star than Pollux. It is classified as an A-type main-sequence star, while Pollux is a K-type giant star.
While Pollux is larger than Sirius, size isn't the only factor determining brightness. The temperature of the star plays a crucial role. Sirius is much hotter than Pollux, and hotter stars emit more light.
Therefore, the combined effect of Sirius being both closer and intrinsically brighter than Pollux makes it appear significantly brighter in the night sky.
